Sol – Wikipédia

mélange de matière organique, de minéraux, de gaz, de liquides et d’organismes qui, ensemble, soutiennent la vie

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Sol est un mélange de matière organique, de minéraux, de gaz, de liquides et d’organismes qui, ensemble, soutiennent la vie. Le corps du sol de la Terre, appelé pédosphère, a quatre fonctions importantes:

Toutes ces fonctions, à leur tour, modifient le sol et ses propriétés.

Le sol est aussi communément appelé Terre ou alors saleté; certaines définitions scientifiques distinguent saleté de sol en restreignant le premier terme spécifiquement au sol déplacé.

La pédosphère s’interface avec la lithosphère, l’hydrosphère, l’atmosphère et la biosphère.^[1] Le terme pédolithe, couramment utilisé pour désigner le sol, se traduit par terre Pierre dans le sens pierre fondamentale, du grec ancien πέδον terre, Terre. Le sol se compose d’une phase solide de minéraux et de matière organique (la matrice du sol), ainsi que d’une phase poreuse qui contient des gaz (l’atmosphère du sol) et de l’eau (la solution du sol).^[2][3] En conséquence, les pédologues peuvent envisager les sols comme un système à trois états de solides, de liquides et de gaz.^[4]

Le sol est le produit de plusieurs facteurs: l’influence du climat, du relief (élévation, orientation et pente du terrain), des organismes et des matériaux d’origine du sol (minéraux d’origine) interagissant avec le temps.^[5] Il subit continuellement un développement par le biais de nombreux processus physiques, chimiques et biologiques, qui comprennent l’altération et l’érosion associée. Compte tenu de sa complexité et de sa forte connectivité interne, les écologistes des sols considèrent le sol comme un écosystème.^[6]

La plupart des sols ont une densité apparente sèche (densité du sol prenant en compte les vides à l’état sec) comprise entre 1,1 et 1,6 g / cm³, alors que la densité des particules du sol est beaucoup plus élevée, de l’ordre de 2,6 à 2,7 g / cm³.^[7] Peu de sol de la planète Terre est plus ancien que le Pléistocène et aucun n’est plus ancien que le Cénozoïque,^[8] bien que les sols fossilisés soient préservés d’aussi loin que l’Archéen.^[9]

La science du sol a deux branches d’étude de base: l’édaphologie et la pédologie. Édaphologie étudie l’influence des sols sur les êtres vivants.^[10]Pédologie se concentre sur la formation, la description (morphologie) et la classification des sols dans leur environnement naturel.^[11] En termes d’ingénierie, le sol est inclus dans le concept plus large de régolithe, qui comprend également d’autres matériaux meubles qui se trouvent au-dessus du substrat rocheux, comme on peut le trouver sur la Lune et sur d’autres objets célestes.^[12]

Processus[[[[Éditer]

Le sol fonctionne comme une composante majeure de l’écosystème terrestre. Les écosystèmes du monde sont profondément affectés par les processus mis en œuvre dans le sol, avec des effets allant de l’appauvrissement de la couche d’ozone et du réchauffement climatique à la destruction de la forêt tropicale et à la pollution de l’eau. En ce qui concerne le cycle du carbone de la Terre, le sol agit comme un important réservoir de carbone,^[13] et c’est potentiellement l’un des plus réactifs aux perturbations humaines^[14] et le changement climatique.^[15] À mesure que la planète se réchauffe, il a été prédit que les sols ajouteront du dioxyde de carbone à l’atmosphère en raison d’une activité biologique accrue à des températures plus élevées, une rétroaction positive (amplification).^[16] Cette prédiction a cependant été remise en question en tenant compte des connaissances plus récentes sur le renouvellement du carbone du sol.^[17]

Le sol agit comme un milieu d’ingénierie, un habitat pour les organismes du sol, un système de recyclage des nutriments et des déchets organiques, un régulateur de la qualité de l’eau, un modificateur de la composition atmosphérique et un milieu pour la croissance des plantes, ce qui en fait un fournisseur extrêmement important de services écosystémiques. .^[18] Étant donné que le sol a une vaste gamme de niches et d’habitats disponibles, il contient la majeure partie de la diversité génétique de la Terre. Un gramme de sol peut contenir des milliards d’organismes, appartenant à des milliers d’espèces, pour la plupart microbiennes et encore largement inexplorées.^[19][20] Le sol a une densité procaryote moyenne d’environ 10⁸ organismes par gramme,^[21] alors que l’océan n’a pas plus de 10⁷ organismes procaryotes par millilitre (gramme) d’eau de mer.^[22]Le carbone organique contenu dans le sol est finalement renvoyé dans l’atmosphère par le processus de respiration effectué par des organismes hétérotrophes, mais une partie substantielle est retenue dans le sol sous forme de matière organique du sol; le travail du sol augmente généralement le taux de respiration du sol, conduisant à l’épuisement de la matière organique du sol.^[23] Puisque les racines des plantes ont besoin d’oxygène, l’aération est une caractéristique importante du sol. Cette ventilation peut être réalisée via des réseaux de pores du sol interconnectés, qui absorbent et retiennent également l’eau de pluie, la rendant facilement disponible pour l’absorption par les plantes. Étant donné que les plantes ont besoin d’un approvisionnement en eau presque continu, mais que la plupart des régions reçoivent des précipitations sporadiques, la capacité de rétention d’eau des sols est vitale pour la survie des plantes.^[24]

Les sols peuvent éliminer efficacement les impuretés,^[25] tuer les agents pathogènes,^[26] et dégrader les contaminants, cette dernière propriété étant appelée atténuation naturelle.^[27] En règle générale, les sols maintiennent une absorption nette d’oxygène et de méthane et subissent un rejet net de dioxyde de carbone et d’oxyde nitreux.^[28] Les sols offrent aux plantes un soutien physique, de l’air, de l’eau, une modération de la température, des nutriments et une protection contre les toxines.^[29] Les sols fournissent des nutriments facilement disponibles aux plantes et aux animaux en convertissant la matière organique morte en diverses formes de nutriments.^[30]

Composition[[[[Éditer]

Un sol typique contient environ 50% de solides (45% de matières minérales et 5% de matière organique) et 50% de vides (ou pores) dont la moitié est occupée par l’eau et l’autre moitié par le gaz.^[31] Le pourcentage de contenu minéral et organique du sol peut être traité comme une constante (à court terme), tandis que le pourcentage de teneur en eau et en gaz du sol est considéré comme très variable, de sorte qu’une augmentation de l’un est simultanément compensée par une réduction de l’autre.^[32] L’espace poreux permet l’infiltration et le mouvement de l’air et de l’eau, qui sont tous deux essentiels à la vie dans le sol.^[33]Le compactage, un problème courant avec les sols, réduit cet espace, empêchant l’air et l’eau d’atteindre les racines des plantes et les organismes du sol.^[34]

Avec suffisamment de temps, un sol indifférencié fera évoluer un profil de sol qui se compose de deux ou plusieurs couches, appelées horizons de sol. Celles-ci diffèrent par une ou plusieurs propriétés telles que leur texture, leur structure, leur densité, leur porosité, leur consistance, leur température, leur couleur et leur réactivité.^[8] Les horizons diffèrent grandement en épaisseur et manquent généralement de frontières nettes; leur développement dépend du type de matériau d’origine, des processus qui modifient ces matériaux d’origine et des facteurs de formation du sol qui influencent ces processus. Les influences biologiques sur les propriétés du sol sont les plus fortes près de la surface, tandis que les influences géochimiques sur les propriétés du sol augmentent avec la profondeur. Les profils de sols matures comprennent généralement trois horizons maîtres de base: A, B et C. Le solum comprend normalement les horizons A et B. La composante vivante du sol est en grande partie confinée au solum, et est généralement plus proéminente dans l’horizon A. Il a été suggéré que le pédon, une colonne de sol s’étendant verticalement de la surface au matériau d’origine sous-jacent et suffisamment grande pour montrer les caractéristiques de tous ses horizons, pourrait être subdivisée en humipédon (la partie vivante, où résident la plupart des organismes du sol, correspondant à la forme d’humus), les copedon (en position intermédiaire, là où la plupart des altérations des minéraux ont lieu) et le lithopédon (en contact avec le sous-sol).^[36]

La texture du sol est déterminée par les proportions relatives des particules individuelles de sable, de limon et d’argile qui composent le sol. L’interaction des particules minérales individuelles avec la matière organique, l’eau, les gaz via des processus biotiques et abiotiques amène ces particules à floculer (coller ensemble) pour former des agrégats ou des peds.^[37] Lorsque ces agrégats peuvent être identifiés, un sol peut être considéré comme développé et peut être décrit plus en détail en termes de couleur, de porosité, de consistance, de réaction (acidité), etc.

L’eau est un agent critique dans le développement du sol en raison de son implication dans la dissolution, la précipitation, l’érosion, le transport et le dépôt des matériaux qui composent un sol.^[38] Le mélange d’eau et de matières dissoutes ou en suspension qui occupent l’espace poreux du sol s’appelle la solution du sol. Étant donné que l’eau du sol n’est jamais de l’eau pure, mais qu’elle contient des centaines de substances organiques et minérales dissoutes, elle peut être appelée plus précisément la solution du sol. L’eau est essentielle à la dissolution, à la précipitation et au lessivage des minéraux du profil du sol. Enfin, l’eau affecte le type de végétation qui pousse dans un sol, ce qui à son tour affecte le développement du sol, une rétroaction complexe qui est illustrée dans la dynamique des modèles de végétation en bandes dans les régions semi-arides.^[39]

Les sols fournissent aux plantes des nutriments, dont la plupart sont maintenus en place par des particules d’argile et de matière organique (colloïdes)^[40] Les nutriments peuvent être adsorbés sur des surfaces minérales argileuses, liés dans des minéraux argileux (absorbés) ou liés dans des composés organiques en tant que partie des organismes vivants ou de la matière organique du sol mort. Ces nutriments liés interagissent avec l’eau du sol pour tamponner la composition de la solution du sol (atténuer les changements dans la solution du sol) lorsque les sols se mouillent ou se dessèchent, que les plantes absorbent les nutriments, que les sels sont lessivés ou que des acides ou des alcalis sont ajoutés.^[41][42]

La disponibilité des éléments nutritifs des plantes est affectée par le pH du sol, qui est une mesure de l’activité des ions hydrogène dans la solution du sol. Le pH du sol est fonction de nombreux facteurs de formation du sol et est généralement plus bas (plus acide) là où l’altération est plus avancée.^[43]

La plupart des éléments nutritifs des plantes, à l’exception de l’azote, proviennent des minéraux qui composent la matière mère du sol. Une partie de l’azote provient de la pluie sous forme d’acide nitrique dilué et d’ammoniac,^[44] mais la majeure partie de l’azote est disponible dans les sols en raison de la fixation de l’azote par les bactéries. Une fois dans le système sol-plante, la plupart des nutriments sont recyclés par les organismes vivants, les résidus végétaux et microbiens (matière organique du sol), les formes liées aux minéraux et la solution du sol. Les organismes vivants du sol (microbes, animaux et racines de plantes) et la matière organique du sol sont d’une importance cruciale pour ce recyclage, et donc pour la formation et la fertilité des sols.^[45] Les enzymes microbiennes du sol peuvent libérer des nutriments à partir de minéraux ou de matière organique destinés à être utilisés par les plantes et autres micro-organismes, les séquestrer (les incorporer) dans des cellules vivantes, ou provoquer leur perte du sol par volatilisation (perte dans l’atmosphère sous forme de gaz) ou par lessivage.^[46]

Formation[[[[Éditer]

La formation du sol, ou pédogenèse, est l’effet combiné de processus physiques, chimiques, biologiques et anthropiques agissant sur le matériau d’origine du sol. On dit que le sol se forme lorsque la matière organique s’est accumulée et que les colloïdes sont lavés vers le bas, laissant des dépôts d’argile, d’humus, d’oxyde de fer, de carbonate et de gypse, produisant une couche distincte appelée horizon B. Il s’agit d’une définition quelque peu arbitraire car des mélanges de sable, de limon, d’argile et d’humus soutiendront l’activité biologique et agricole avant cette date.^[47] Ces constituants sont déplacés d’un niveau à un autre par l’eau et l’activité animale. En conséquence, des couches (horizons) se forment dans le profil du sol. L’altération et le mouvement des matériaux dans un sol provoquent la formation d’horizons de sol distinctifs. Cependant, les définitions plus récentes du sol englobent les sols sans aucune matière organique, tels que les régolithes qui se sont formés sur Mars.^[48] et des conditions analogues dans les déserts de la planète Terre.^[49]

Un exemple de développement d’un sol commencerait par l’altération du substrat rocheux coulé de lave, qui produirait le matériau d’origine purement minéral à partir duquel se forme la texture du sol. Le développement du sol se ferait le plus rapidement à partir de la roche nue de flux récents dans un climat chaud, sous des pluies abondantes et fréquentes. Dans ces conditions, les plantes (dans un premier temps les lichens et cyanobactéries fixateurs d’azote puis les plantes épilithiques supérieures) s’établissent très rapidement sur la lave basaltique, même s’il y a très peu de matière organique.^[50] Les plantes sont soutenues par la roche poreuse car elle est remplie d’eau nutritive qui transporte les minéraux dissous des roches. Les crevasses et les poches, la topographie locale des roches, contiendraient des matériaux fins et hébergeraient des racines de plantes. Les racines des plantes en développement sont associées à des champignons mycorhiziens altérant les minéraux^[51] qui aident à briser la lave poreuse, et par ces moyens la matière organique et un sol minéral plus fin s’accumulent avec le temps. Ces étapes initiales de développement du sol ont été décrites sur les volcans,^[52] inselbergs,^[53] et moraines glaciaires.^[54]

Le déroulement de la formation du sol est influencé par au moins cinq facteurs classiques qui sont étroitement liés dans l’évolution d’un sol. Ce sont: le matériau d’origine, le climat, la topographie (relief), les organismes et le temps.^[55] Lorsqu’ils sont réorganisés selon le climat, le relief, les organismes, le matériau d’origine et le temps, ils forment l’acronyme CROPT.^[56]

Propriétés physiques[[[[Éditer]

Les propriétés physiques des sols, par ordre d’importance décroissante pour les services écosystémiques tels que la production végétale, sont la texture, la structure, la densité apparente, la porosité, la consistance, la température, la couleur et la résistivité.^[57] La texture du sol est déterminée par la proportion relative des trois types de particules minérales du sol, appelées sol sépare: le sable, le limon et l’argile. À la prochaine plus grande échelle, les structures du sol appelées peds ou plus communément agrégats de sol sont créés à partir du sol se sépare lorsque les oxydes de fer, les carbonates, l’argile, la silice et l’humus enrobent les particules et les font adhérer à des structures secondaires plus grandes et relativement stables.^[58] La densité apparente du sol, lorsqu’elle est déterminée dans des conditions d’humidité normalisées, est une estimation du compactage du sol.^[59] La porosité du sol se compose de la partie vide du volume du sol et est occupée par les gaz ou l’eau. La consistance du sol est la capacité des matériaux du sol à adhérer ensemble. La température et la couleur du sol sont auto-définies. La résistivité fait référence à la résistance à la conduction des courants électriques et affecte le taux de corrosion des structures métalliques et en béton enfouies dans le sol.^[60] Ces propriétés varient selon la profondeur d’un profil de sol, c’est-à-dire à travers les horizons du sol. La plupart de ces propriétés déterminent l’aération du sol et la capacité de l’eau à s’infiltrer et à être retenue dans le sol.^[61]

Humidité du sol[[[[Éditer]

Humidité du sol fait référence à la teneur en eau du sol. Il peut être exprimé en volume ou en poids. La mesure de l’humidité du sol peut être basée sur in situ sondes (p. ex. sondes capacitives, sondes à neutrons) ou méthodes de télédétection.

L’eau qui pénètre dans un champ est éliminée d’un champ par ruissellement, drainage, évaporation ou transpiration.^[62] Le ruissellement est l’eau qui coule à la surface jusqu’au bord du champ; le drainage est l’eau qui coule à travers le sol vers le bas ou vers le bord du champ sous terre; la perte d’eau par évaporation d’un champ est la partie de l’eau qui s’évapore dans l’atmosphère directement à partir de la surface du champ; la transpiration est la perte d’eau du champ par son évaporation de la plante elle-même.

L’eau affecte la formation, la structure, la stabilité et l’érosion du sol, mais est une préoccupation majeure en ce qui concerne la croissance des plantes.^[63] L’eau est essentielle aux plantes pour quatre raisons:

1. Il constitue 80 à 95% du protoplasme de la plante.
2. C’est essentiel pour la photosynthèse.
3. C’est le solvant dans lequel les nutriments sont transportés vers, dans et à travers la plante.
4. Il fournit la turgescence grâce à laquelle la plante se maintient en bonne position.

De plus, l’eau modifie le profil du sol en dissolvant et en re-déposant des solutés et colloïdes minéraux et organiques, souvent à des niveaux inférieurs, un processus appelé lessivage. Dans un sol limoneux, les solides constituent la moitié du volume, le gaz un quart du volume et l’eau un quart du volume dont seulement la moitié sera disponible pour la plupart des plantes, avec une forte variation selon le potentiel matriciel.^[65]

Un champ inondé drainera l’eau gravitationnelle sous l’influence de la gravité jusqu’à ce que les forces adhésives et cohésives de l’eau résistent à un drainage supplémentaire, point auquel on dit qu’il a atteint la capacité du champ. À ce stade, les plantes doivent appliquer une aspiration pour puiser de l’eau dans un sol. L’eau que les plantes peuvent puiser dans le sol s’appelle l’eau disponible. Une fois que l’eau disponible est épuisée, l’humidité restante est appelée eau non disponible car la plante ne peut pas produire une aspiration suffisante pour aspirer cette eau. À 15 bar d’aspiration, point de flétrissement, les graines ne germent pas, les plantes commencent à se flétrir puis meurent à moins qu’elles ne le soient. capable de récupérer après le réapprovisionnement en eau grâce à des adaptations spécifiques à l’espèce.^[71] L’eau se déplace dans le sol sous l’influence de la gravité, de l’osmose et de la capillarité.^[72] Lorsque l’eau pénètre dans le sol, elle déplace l’air des macropores interconnectés par flottabilité et brise les agrégats dans lesquels l’air est emprisonné, un processus appelé extinction.^[73]
La vitesse à laquelle un sol peut absorber l’eau dépend du sol et de ses autres conditions. Au fur et à mesure qu’une plante grandit, ses racines éliminent d’abord l’eau des plus grands pores (macropores). Bientôt, les pores plus grands ne retiennent que l’air, et l’eau restante se trouve uniquement dans les pores de taille intermédiaire et de plus petite taille (micropores). L’eau dans les plus petits pores est si fortement maintenue à la surface des particules que les racines des plantes ne peuvent pas l’arracher. Par conséquent, toute l’eau du sol n’est pas disponible pour les plantes, avec une forte dépendance à la texture.^[74] Lorsqu’il est saturé, le sol peut perdre des nutriments à mesure que l’eau s’écoule.^[75] L’eau se déplace dans un champ drainant sous l’influence de la pression où le sol est localement saturé et par capillarité tirant vers les parties les plus sèches du sol.^[76] La plupart des besoins en eau des plantes sont fournis par l’aspiration causée par l’évaporation des feuilles des plantes (transpiration) et une fraction inférieure est fournie par l’aspiration créée par les différences de pression osmotique entre l’intérieur de la plante et la solution du sol.^[77][78] Les racines des plantes doivent chercher de l’eau et pousser de préférence dans des microsites de sol plus humides,^[79] mais certaines parties du système racinaire sont également capables de réhumidifier les parties sèches du sol.^[80] Une eau insuffisante endommagera le rendement d’une culture.^[81] La majeure partie de l’eau disponible est utilisée dans la transpiration pour attirer les nutriments dans la plante.^[82]

L’eau du sol est également importante pour la modélisation du climat et la prévision numérique du temps. Le Système mondial d’observation du climat a spécifié l’eau du sol comme l’une des 50 variables climatiques essentielles (ECV).^[83] L’eau du sol peut être mesurée in situ avec un capteur d’humidité du sol ou peut être estimée à partir de données satellitaires et de modèles hydrologiques. Chaque méthode présente des avantages et des inconvénients, et par conséquent, l’intégration de différentes techniques peut réduire les inconvénients d’une seule méthode donnée.^[84]

Rétention d’eau[[[[Éditer]

L’eau est retenue dans un sol lorsque la force adhésive d’attraction des atomes d’hydrogène de l’eau pour l’oxygène des particules du sol est plus forte que les forces de cohésion ressenties par l’hydrogène de l’eau pour les autres atomes d’oxygène de l’eau. Lorsqu’un champ est inondé, l’espace poreux du sol est complètement rempli d’eau. Le champ se drainera sous la force de la gravité jusqu’à ce qu’il atteigne ce qu’on appelle la capacité du champ, à quel point les plus petits pores sont remplis d’eau et les plus grands d’eau et de gaz.^[86] La quantité totale d’eau retenue lorsque la capacité du champ est atteinte est fonction de la surface spécifique des particules de sol.^[87] En conséquence, les sols à haute teneur en argile et à haute teneur organique ont des capacités de champ plus élevées.^[88] L’énergie potentielle de l’eau par unité de volume par rapport à l’eau pure dans les conditions de référence est appelée potentiel d’eau. Le potentiel hydrique total est une somme du potentiel matriciel qui résulte de l’action capillaire, du potentiel osmotique pour un sol salin et du potentiel gravitationnel lorsqu’il s’agit de la direction verticale du mouvement de l’eau. Le potentiel hydrique dans le sol a généralement des valeurs négatives, et par conséquent il est également exprimé en succion, qui est définie comme le moins du potentiel hydrique. L’aspiration a une valeur positive et peut être considérée comme la force totale requise pour tirer ou pousser l’eau hors du sol. Le potentiel d’eau ou l’aspiration est exprimé en unités de kPa (10³pascal), bar (100 kPa) ou cm H₂O (environ 0,098 kPa). Logarithme commun d’aspiration en cm H₂O s’appelle pF.^[89] Par conséquent, pF 3 = 1000 cm = 98 kPa = 0,98 bar.

Les forces avec lesquelles l’eau est retenue dans les sols déterminent sa disponibilité pour les plantes. Les forces d’adhérence retiennent l’eau fortement sur les surfaces minérales et humiques et moins fortement sur elle-même par des forces cohésives. La racine d’une plante peut pénétrer un très petit volume d’eau qui adhère au sol et être initialement capable d’aspirer de l’eau qui n’est que légèrement maintenue par les forces de cohésion. Mais au fur et à mesure que la gouttelette est aspirée, les forces d’adhérence de l’eau pour les particules du sol produisent une aspiration de plus en plus élevée, jusqu’à 1500 kPa (pF = 4,2).^[90] À une aspiration de 1500 kPa, la quantité d’eau du sol est appelée point de flétrissement. À cette aspiration, la plante ne peut pas soutenir ses besoins en eau car l’eau est toujours perdue de la plante par la transpiration, la turgescence de la plante est perdue et elle se flétrit, bien que la fermeture stomatique puisse diminuer la transpiration et ainsi retarder le flétrissement en dessous du point de flétrissement, en particulier. sous adaptation ou acclimatation à la sécheresse.^[91] Le niveau suivant, appelé séchage à l’air, se produit à une aspiration de 100 000 kPa (pF = 6). Enfin, la condition de séchage à l’étuve est atteinte à une aspiration de 1 000 000 kPa (pF = 7). Toute eau en dessous du point de flétrissement est appelée eau indisponible.

Lorsque la teneur en humidité du sol est optimale pour la croissance des plantes, l’eau des pores de grande et moyenne taille peut se déplacer dans le sol et être facilement utilisée par les plantes.^[74] La quantité d’eau restant dans un sol drainé à la capacité du champ et la quantité disponible sont des fonctions du type de sol. Le sol sableux retiendra très peu d’eau, tandis que l’argile en retiendra le maximum.^[88] L’eau disponible pour le limon limoneux pourrait être de 20% alors que pour le sable, elle pourrait être de seulement 6% en volume, comme indiqué dans ce tableau.

Point de flétrissement, capacité au champ et eau disponible de différentes textures de sol (unité:% en volume)^[93]

Texture du sol	Point de flétrissement	Capacité sur le terrain	Eau disponible
Le sable	3,3	9,1	5,8
loam sableux	9,5	20,7	11.2
Terreau	11,7	27,0	15,3
Limon limoneux	13,3	33,0	19,7
Terreau d’argile	19,7	31,8	12,1
Argile	27,2	39,6	12,4

Les valeurs ci-dessus sont des valeurs moyennes pour les textures du sol.

L’écoulement de l’eau[[[[Éditer]

L’eau se déplace dans le sol sous l’effet de la gravité, de l’osmose et de la capillarité. À une aspiration de zéro à 33 kPa (capacité du champ), l’eau est poussée à travers le sol à partir du point de son application sous la force de gravité et le gradient de pression créé par la pression de l’eau; c’est ce qu’on appelle un débit saturé. À une aspiration plus élevée, le mouvement de l’eau est tiré par capillarité d’un sol plus humide vers un sol plus sec. Ceci est causé par l’adhésion de l’eau aux solides du sol et est appelé écoulement insaturé.^[95]

L’infiltration et le mouvement de l’eau dans le sol sont contrôlés par six facteurs:

1. Texture du sol
2. Structure du sol. Les sols à texture fine et à structure granulaire sont les plus favorables à l’infiltration d’eau.
3. La quantité de matière organique. La matière grossière est préférable et si elle est en surface, elle aide à prévenir la destruction de la structure du sol et la création de croûtes.
4. Profondeur du sol jusqu’aux couches imperméables telles que les dures ou le substrat rocheux
5. La quantité d’eau déjà présente dans le sol
6. La température du sol. Les sols chauds absorbent l’eau plus rapidement tandis que les sols gelés peuvent ne pas être capables d’absorber selon le type de gel.

Les taux d’infiltration d’eau varient de 0,25 cm par heure pour les sols à haute teneur en argile à 2,5 cm par heure pour le sable et les structures de sol bien stabilisées et agrégées. L’eau traverse le sol de manière inégale, sous la forme de soi-disant «doigts de gravité», en raison de la tension superficielle entre les particules d’eau.^[98][99]

Les racines des arbres, vivants ou morts, créent des canaux préférentiels pour l’écoulement des eaux de pluie dans le sol,^[100] grossissant les taux d’infiltration de l’eau jusqu’à 27 fois.^[101]

Les inondations augmentent temporairement la perméabilité du sol dans les lits des rivières, aidant à recharger les aquifères.^[102]

L’eau appliquée à un sol est poussée par des gradients de pression depuis le point de son application où elle est saturée localement, vers des zones moins saturées, comme la zone vadose.^[103][104] Une fois que le sol est complètement mouillé, toute eau supplémentaire se déplace vers le bas ou s’infiltre hors de la gamme des racines des plantes, transportant avec elle de l’argile, de l’humus, des nutriments, principalement des cations et divers contaminants, y compris des pesticides, des polluants, des virus et des bactéries, pouvant causer contamination des eaux souterraines.^[105][106] Par ordre de solubilité décroissante, les nutriments lessivés sont:

• Calcium
• Magnésium, soufre, potassium; selon la composition du sol
• Azote; généralement peu, à moins que l’engrais nitrate n’ait été appliqué récemment
• Phosphore; très peu car ses formes dans le sol sont de faible solubilité.

Aux États-Unis, l’eau de percolation due aux précipitations varie de presque zéro centimètre juste à l’est des montagnes Rocheuses à cinquante centimètres ou plus par jour dans les Appalaches et sur la côte nord du golfe du Mexique.

L’eau est tirée par action capillaire en raison de la force d’adhérence de l’eau aux solides du sol, produisant un gradient d’aspiration du sol humide vers un sol plus sec^[109] et des macropores aux micropores.^{[[[[citation requise]} L’équation dite de Richards permet de calculer le taux de changement temporel de la teneur en humidité dans les sols en raison du mouvement de l’eau dans les sols non saturés.^[110] Fait intéressant, cette équation attribuée à Richards a été initialement publiée par Richardson en 1922.^[111] L’équation de la vitesse de l’humidité du sol,^[112] qui peut être résolu en utilisant la méthode d’écoulement de zone de vadose à teneur en eau finie,^[113][114] décrit la vitesse de l’écoulement de l’eau à travers un sol non saturé dans le sens vertical. La solution numérique de l’équation de Richardson / Richards permet de calculer le débit d’eau insaturée et le transport de soluté à l’aide d’un logiciel tel que Hydrus,^[115] en donnant au sol les paramètres hydrauliques des fonctions hydrauliques (fonction de rétention d’eau et fonction de conductivité hydraulique insaturée) et les conditions initiales et aux limites. Un écoulement préférentiel se produit le long des macropores interconnectés, des crevasses, des canaux de racine et de ver, qui drainent l’eau par gravité.^[116][117]
De nombreux modèles basés sur la physique du sol permettent maintenant une représentation de l’écoulement préférentiel comme un double continuum, une double porosité ou une double perméabilité, mais ceux-ci ont généralement été «boulonnés» à la solution de Richards sans aucun fondement physique rigoureux.^[118]

Absorption d’eau par les plantes[[[[Éditer]

Les moyens par lesquels les plantes l’acquièrent et leurs nutriments sont tout aussi importants pour le stockage et le mouvement de l’eau dans le sol. La plupart de l’eau du sol est absorbée par les plantes comme absorption passive causée par la force de traction de l’eau qui s’évapore (transpire) de la longue colonne d’eau (flux de sève de xylème) qui mène des racines de la plante à ses feuilles, selon la théorie de la cohésion-tension .^[119] Le mouvement ascendant de l’eau et des solutés (élévation hydraulique) est régulé dans les racines par l’endoderme^[120] et dans le feuillage végétal par conductance stomatique,^[121] et peut être interrompu dans les vaisseaux du xylème des racines et des pousses par cavitation, également appelée embolie au xylème.^[122] De plus, la forte concentration de sels dans les racines des plantes crée un gradient de pression osmotique qui pousse l’eau du sol dans les racines.^[123] L’absorption osmotique devient plus importante pendant les périodes de faible transpiration de l’eau causée par des températures plus basses (par exemple la nuit) ou une humidité élevée, et l’inverse se produit à des températures élevées ou à une faible humidité. Ce sont ces processus qui provoquent respectivement la guttation et le flétrissement.^[125]

L’extension des racines est vitale pour la survie des plantes. Une étude d’une seule plante de seigle d’hiver cultivée pendant quatre mois dans un pied cube (0,0283 mètre cube) de sol limoneux a montré que la plante développait 13 800 000 racines, soit un total de 620 km de longueur et 237 mètres carrés de surface; et 14 milliards de racines de cheveux d’une longueur totale de 10 620 km et d’une superficie totale de 400 mètres carrés; pour une surface totale de 638 m². La superficie totale du sol limoneux a été estimée à 52 000 mètres carrés. En d’autres termes, les racines n’étaient en contact qu’avec 1,2% du sol. Cependant, l’extension des racines doit être considérée comme un processus dynamique, permettant à de nouvelles racines d’explorer un nouveau volume de sol chaque jour, augmentant considérablement le volume total de sol exploré sur une période de croissance donnée, et donc le volume d’eau absorbé par la racine. système sur cette période.^[127] L’architecture racinaire, c’est-à-dire la configuration spatiale du système racinaire, joue un rôle prépondérant dans l’adaptation des plantes à l’eau du sol et à la disponibilité des nutriments, et donc dans la productivité des plantes.^[128]

Les racines doivent chercher de l’eau car l’écoulement insaturé de l’eau dans le sol ne peut se déplacer qu’à un rythme allant jusqu’à 2,5 cm par jour; en conséquence, ils meurent et grandissent constamment car ils recherchent des concentrations élevées d’humidité du sol.^[129] Une humidité insuffisante du sol, au point de provoquer un flétrissement, causera des dommages permanents et les rendements des cultures en souffriront. Lorsque le sorgho-grain a été exposé à une aspiration du sol aussi faible que 1300 kPa pendant l’émergence de l’épi aux stades de floraison et de germination, sa production a été réduite de 34%.

Consommation et efficacité d’utilisation de l’eau[[[[Éditer]

Seule une petite fraction (0,1% à 1%) de l’eau utilisée par une usine est conservée dans l’usine. La majorité est finalement perdue par transpiration, tandis que l’évaporation de la surface du sol est également importante, le rapport transpiration: évaporation variant selon le type de végétation et le climat, atteignant un sommet dans les forêts tropicales humides et plongeant dans les steppes et les déserts.^[131] La transpiration et la perte d’humidité du sol par évaporation sont appelées évapotranspiration. L’évapotranspiration plus l’eau contenue dans la plante correspond à une consommation consommatrice, ce qui est presque identique à l’évapotranspiration.^[132]

L’eau totale utilisée dans un champ agricole comprend le ruissellement de surface, le drainage et la consommation. The use of loose mulches will reduce evaporative losses for a period after a field is irrigated, but in the end the total evaporative loss (plant plus soil) will approach that of an uncovered soil, while more water is immediately available for plant growth.^[133]Water use efficiency is measured by the transpiration ratio, which is the ratio of the total water transpired by a plant to the dry weight of the harvested plant. Transpiration ratios for crops range from 300 to 700. For example, alfalfa may have a transpiration ratio of 500 and as a result 500 kilograms of water will produce one kilogram of dry alfalfa.

Soil gas[[[[edit]

The atmosphere of soil, or soil gas, is very different from the atmosphere above. The consumption of oxygen by microbes and plant roots, and their release of carbon dioxide, decrease oxygen and increase carbon dioxide concentration. Atmospheric CO₂ concentration is 0.04%, but in the soil pore space it may range from 10 to 100 times that level, thus potentially contributing to the inhibition of root respiration.^[135] Calcareous soils regulate CO₂ concentration by carbonate buffering, contrary to acid soils in which all CO₂ respired accumulates in the soil pore system.^[136] At extreme levels CO₂ is toxic.^[137] This suggests a possible negative feedback control of soil CO₂ concentration through its inhibitory effects on root and microbial respiration (also called ‘soil respiration’).^[138] In addition, the soil voids are saturated with water vapour, at least until the point of maximal hygroscopicity, beyond which a vapour-pressure deficit occurs in the soil pore space.^[33] Adequate porosity is necessary, not just to allow the penetration of water, but also to allow gases to diffuse in and out. Movement of gases is by diffusion from high concentrations to lower, the diffusion coefficient decreasing with soil compaction.^[139] Oxygen from above atmosphere diffuses in the soil where it is consumed and levels of carbon dioxide in excess of above atmosphere diffuse out with other gases (including greenhouse gases) as well as water.^[140] Soil texture and structure strongly affect soil porosity and gas diffusion. It is the total pore space (porosity) of soil, not the pore size, and the degree of pore interconnection (or conversely pore sealing), together with water content, air turbulence and temperature, that determine the rate of diffusion of gases into and out of soil.^[140]Platy soil structure and soil compaction (low porosity) impede gas flow, and a deficiency of oxygen may encourage anaerobic bacteria to reduce (strip oxygen) from nitrate NO₃ to the gases N₂, N₂O, and NO, which are then lost to the atmosphere, thereby depleting the soil of nitrogen.^[142] Aerated soil is also a net sink of methane CH₄^[143] but a net producer of methane (a strong heat-absorbing greenhouse gas) when soils are depleted of oxygen and subject to elevated temperatures.^[144]

Soil atmosphere is also the seat of emissions of volatiles other than carbon and nitrogen oxides from various soil organisms, e.g. roots,^[145] bacteria,^[146] fungi,^[147] animals.^[148] These volatiles are used as chemical cues, making soil atmosphere the seat of interaction networks^[149][150] playing a decisive role in the stability, dynamics and evolution of soil ecosystems.^[151] Biogenic soil volatile organic compounds are exchanged with the aboveground atmosphere, in which they are just 1–2 orders of magnitude lower than those from aboveground vegetation.^[152]

We humans can get some idea of the soil atmosphere through the well-known ‘after-the-rain’ scent, when infiltering rainwater flushes out the whole soil atmosphere after a drought period, or when soil is excavated,^[153] a bulk property attributed in a reductionist manner to particular biochemical compounds such as petrichor or geosmin.

Solid phase (soil matrix)[[[[edit]

Soil particles can be classified by their chemical composition (mineralogy) as well as their size. The particle size distribution of a soil, its texture, determines many of the properties of that soil, in particular hydraulic conductivity and water potential,^[154] but the mineralogy of those particles can strongly modify those properties. The mineralogy of the finest soil particles, clay, is especially important.^[155]

Chemistry[[[[edit]

The chemistry of a soil determines its ability to supply available plant nutrients and affects its physical properties and the health of its living population. In addition, a soil’s chemistry also determines its corrosivity, stability, and ability to absorb pollutants and to filter water. It is the surface chemistry of mineral and organic colloids that determines soil’s chemical properties.^[156] A colloid is a small, insoluble particle ranging in size from 1 nanometer to 1 micrometer, thus small enough to remain suspended by Brownian motion in a fluid medium without settling.^[157] Most soils contain organic colloidal particles called humus as well as the inorganic colloidal particles of clays. The very high specific surface area of colloids and their net electrical charges give soil its ability to hold and release ions. Negatively charged sites on colloids attract and release cations in what is referred to as cation exchange. Cation-exchange capacity (CEC) is the amount of exchangeable cations per unit weight of dry soil and is expressed in terms of milliequivalents of positively charged ions per 100 grams of soil (or centimoles of positive charge per kilogram of soil; cmol_c/kg). Similarly, positively charged sites on colloids can attract and release anions in the soil giving the soil anion exchange capacity (AEC).

Cation and anion exchange[[[[edit]

The cation exchange, that takes place between colloids and soil water, buffers (moderates) soil pH, alters soil structure, and purifies percolating water by adsorbing cations of all types, both useful and harmful.

The negative or positive charges on colloid particles make them able to hold cations or anions, respectively, to their surfaces. The charges result from four sources.

1. Isomorphous substitution occurs in clay during its formation, when lower-valence cations substitute for higher-valence cations in the crystal structure.^[159] Substitutions in the outermost layers are more effective than for the innermost layers, as the electric charge strength drops off as the square of the distance. The net result is oxygen atoms with net negative charge and the ability to attract cations.
2. Edge-of-clay oxygen atoms are not in balance ionically as the tetrahedral and octahedral structures are incomplete.^[160]
3. Hydroxyls may substitute for oxygens of the silica layers, a process called hydroxylation. When the hydrogens of the clay hydroxyls are ionised into solution, they leave the oxygen with a negative charge (anionic clays).^[161]
4. Hydrogens of humus hydroxyl groups may also be ionised into solution, leaving, similarly to clay, an oxygen with a negative charge.^[162]

Cations held to the negatively charged colloids resist being washed downward by water and out of reach of plants’ roots, thereby preserving the fertility of soils in areas of moderate rainfall and low temperatures.^[163][164]

There is a hierarchy in the process of cation exchange on colloids, as they differ in the strength of adsorption by the colloid and hence their ability to replace one another (ion exchange). If present in equal amounts in the soil water solution:

Al³⁺ replaces H⁺ replaces Ca²⁺ replaces Mg²⁺ replaces K⁺ same as NH⁴⁺ replaces Na^+[165]

If one cation is added in large amounts, it may replace the others by the sheer force of its numbers. This is called law of mass action. This is largely what occurs with the addition of cationic fertilisers (potash, lime).^[166]

As the soil solution becomes more acidic (low pH, meaning an abundance of H⁺, the other cations more weakly bound to colloids are pushed into solution as hydrogen ions occupy exchange sites (protonation). A low pH may cause hydrogen of hydroxyl groups to be pulled into solution, leaving charged sites on the colloid available to be occupied by other cations. This ionisation of hydroxyl groups on the surface of soil colloids creates what is described as pH-dependent surface charges.^[167] Unlike permanent charges developed by isomorphous substitution, pH-dependent charges are variable and increase with increasing pH.^[42] Freed cations can be made available to plants but are also prone to be leached from the soil, possibly making the soil less fertile.^[168] Plants are able to excrete H⁺ into the soil through the synthesis of organic acids and by that means, change the pH of the soil near the root and push cations off the colloids, thus making those available to the plant.^[169]

Cation exchange capacity (CEC)[[[[edit]

Cation exchange capacity should be thought of as the soil’s ability to remove cations from the soil water solution and sequester those to be exchanged later as the plant roots release hydrogen ions to the solution. CEC is the amount of exchangeable hydrogen cation (H⁺) that will combine with 100 grams dry weight of soil and whose measure is one milliequivalents per 100 grams of soil (1 meq/100 g). Hydrogen ions have a single charge and one-thousandth of a gram of hydrogen ions per 100 grams dry soil gives a measure of one milliequivalent of hydrogen ion. Calcium, with an atomic weight 40 times that of hydrogen and with a valence of two, converts to (40/2) x 1 milliequivalent = 20 milliequivalents of hydrogen ion per 100 grams of dry soil or 20 meq/100 g. The modern measure of CEC is expressed as centimoles of positive charge per kilogram (cmol/kg) of oven-dry soil.

Most of the soil’s CEC occurs on clay and humus colloids, and the lack of those in hot, humid, wet climates, due to leaching and decomposition, respectively, explains the apparent sterility of tropical soils.^[171] Live plant roots also have some CEC, linked to their specific surface area.^[172]

Cation exchange capacity for soils; soil textures; soil colloids

Soil	State	CEC meq/100 g
Charlotte fine sand	Florida	1.0
Ruston fine sandy loam	Texas	1.9
Glouchester loam	New Jersey	11.9
Grundy silt loam	Illinois	26.3
Gleason clay loam	California	31.6
Susquehanna clay loam	Alabama	34.3
Davie mucky fine sand	Florida	100.8
Sands	——	1–5
Fine sandy loams	——	5–10
Loams and silt loams	—–	5–15
Clay loams	—–	15–30
Clays	—–	over 30
Sesquioxides	—–	0–3
Kaolinite	—–	3–15
Illite	—–	25–40
Montmorillonite	—–	60–100
Vermiculite (similar to illite)	—–	80–150
Humus	—–	100–300

Anion exchange capacity (AEC)[[[[edit]

Anion exchange capacity should be thought of as the soil’s ability to remove anions (e.g. nitrate, phosphate) from the soil water solution and sequester those for later exchange as the plant roots release carbonate anions to the soil water solution. Those colloids which have low CEC tend to have some AEC. Amorphous and sesquioxide clays have the highest AEC,^[174] followed by the iron oxides. Levels of AEC are much lower than for CEC, because of the generally higher rate of positively (versus negatively) charged surfaces on soil colloids, to the exception of variable-charge soils.^[175] Phosphates tend to be held at anion exchange sites.^[176]

Iron and aluminum hydroxide clays are able to exchange their hydroxide anions (OH⁻) for other anions.^[177] The order reflecting the strength of anion adhesion is as follows:

H₂PO₄⁻ replaces SO₄²⁻ replaces NO₃⁻ replaces Cl⁻

The amount of exchangeable anions is of a magnitude of tenths to a few milliequivalents per 100 g dry soil. As pH rises, there are relatively more hydroxyls, which will displace anions from the colloids and force them into solution and out of storage; hence AEC decreases with increasing pH (alkalinity).^[178]

Reactivity (pH)[[[[edit]

Soil reactivity is expressed in terms of pH and is a measure of the acidity or alkalinity of the soil. More precisely, it is a measure of hydrogen ion concentration in an aqueous solution and ranges in values from 0 to 14 (acidic to basic) but practically speaking for soils, pH ranges from 3.5 to 9.5, as pH values beyond those extremes are toxic to life forms.^[179]

At 25 °C an aqueous solution that has a pH of 3.5 has 10^−3.5moles H⁺ (hydrogen ions) per litre of solution (and also 10^−10.5 mole/litre OH⁻). A pH of 7, defined as neutral, has 10⁻⁷ moles of hydrogen ions per litre of solution and also 10⁻⁷ moles of OH⁻ per litre; since the two concentrations are equal, they are said to neutralise each other. A pH of 9.5 has 10^−9.5 moles hydrogen ions per litre of solution (and also 10^−2.5 mole per litre OH⁻). A pH of 3.5 has one million times more hydrogen ions per litre than a solution with pH of 9.5 (9.5–3.5 = 6 or 10⁶) and is more acidic.^[180]

The effect of pH on a soil is to remove from the soil or to make available certain ions. Soils with high acidity tend to have toxic amounts of aluminium and manganese.^[181] As a result of a trade-off between toxicity and requirement most nutrients are better available to plants at moderate pH,^[182] although most minerals are more soluble in acid soils. Soil organisms are hindered by high acidity, and most agricultural crops do best with mineral soils of pH 6.5 and organic soils of pH 5.5. Given that at low pH toxic metals (e.g. cadmium, zinc, lead) are positively charged as cations and organic pollutants are in non-ionic form, thus both made more available to organisms,^[184][185] it has been suggested that plants, animals and microbes commonly living in acid soils are pre-adapted to every kind of pollution, whether of natural or human origin.^[186]

In high rainfall areas, soils tend to acidify as the basic cations are forced off the soil colloids by the mass action of hydrogen ions from the rain against those attached to the colloids. High rainfall rates can then wash the nutrients out, leaving the soil inhabited only by those organisms which are particularly efficient to uptake nutrients in very acid conditions, like in tropical rainforests.^[187] Once the colloids are saturated with H⁺, the addition of any more hydrogen ions or aluminum hydroxyl cations drives the pH even lower (more acidic) as the soil has been left with no buffering capacity.^[188] In areas of extreme rainfall and high temperatures, the clay and humus may be washed out, further reducing the buffering capacity of the soil.^[189] In low rainfall areas, unleached calcium pushes pH to 8.5 and with the addition of exchangeable sodium, soils may reach pH 10.^[190] Beyond a pH of 9, plant growth is reduced.^[191] High pH results in low micro-nutrient mobility, but water-soluble chelates of those nutrients can correct the deficit.^[192] Sodium can be reduced by the addition of gypsum (calcium sulphate) as calcium adheres to clay more tightly than does sodium causing sodium to be pushed into the soil water solution where it can be washed out by an abundance of water.^[194]

Base saturation percentage[[[[edit]

There are acid-forming cations (e.g. hydrogen, aluminium, iron) and there are base-forming cations (e.g. calcium, magnesium, sodium). The fraction of the negatively-charged soil colloid exchange sites (CEC) that are occupied by base-forming cations is called base saturation. If a soil has a CEC of 20 meq and 5 meq are aluminium and hydrogen cations (acid-forming), the remainder of positions on the colloids (20-5 = 15 meq) are assumed occupied by base-forming cations, so that the base saturation is 15/20 x 100% = 75% (the compliment 25% is assumed acid-forming cations or protons). Base saturation is almost in direct proportion to pH (it increases with increasing pH).^[195] It is of use in calculating the amount of lime needed to neutralise an acid soil (lime requirement). The amount of lime needed to neutralize a soil must take account of the amount of acid forming ions on the colloids (exchangeable acidity), not just those in the soil water solution (free acidity).^[196] The addition of enough lime to neutralize the soil water solution will be insufficient to change the pH, as the acid forming cations stored on the soil colloids will tend to restore the original pH condition as they are pushed off those colloids by the calcium of the added lime.

Buffering[[[[edit]

The resistance of soil to change in pH, as a result of the addition of acid or basic material, is a measure of the buffering capacity of a soil and (for a particular soil type) increases as the CEC increases. Hence, pure sand has almost no buffering ability, while soils high in colloids (whether mineral or organic) have high buffering capacity.^[198] Buffering occurs by cation exchange and neutralisation. However, colloids are not the only regulators of soil pH. The role of carbonates should be underlined, too.^[199] More generally, according to pH levels, several buffer systems take precedence over each other, from calcium carbonate buffer range to iron buffer range.^[200]

The addition of a small amount of highly basic aqueous ammonia to a soil will cause the ammonium to displace hydrogen ions from the colloids, and the end product is water and colloidally fixed ammonium, but little permanent change overall in soil pH.

The addition of a small amount of lime, Ca(OH)₂, will displace hydrogen ions from the soil colloids, causing the fixation of calcium to colloids and the evolution of CO₂ and water, with little permanent change in soil pH.

The above are examples of the buffering of soil pH. The general principal is that an increase in a particular cation in the soil water solution will cause that cation to be fixed to colloids (buffered) and a decrease in solution of that cation will cause it to be withdrawn from the colloid and moved into solution (buffered). The degree of buffering is often related to the CEC of the soil; the greater the CEC, the greater the buffering capacity of the soil.

Nutrients[[[[edit]

Plant nutrients, their chemical symbols, and the ionic forms common in soils and available for plant uptake

Element	Symbol	Ion or molecule
Carbon	C	CO2 (mostly through leaves)
Hydrogen	H	H+, HOH (water)
Oxygen	O	O2−, OH −, CO32−, SO42−, CO2
Phosphorus	P	H2PO4 −, HPO42− (phosphates)
Potassium	K	K+
Nitrogen	N	NH4+, NO3 − (ammonium, nitrate)
Sulfur	S	SO42−
Calcium	Ca	Ca2+
Iron	Fe	Fe2+, Fe3+ (ferrous, ferric)
Magnesium	Mg	Mg2+
Boron	B	H3BO3, H2BO3 −, B(OH)4 −
Manganese	Mn	Mn2+
Copper	Cu	Cu2+
Zinc	Zn	Zn2+
Molybdenum	Mo	MoO42− (molybdate)
Chlorine	Cl	Cl − (chloride)

Seventeen elements or nutrients are essential for plant growth and reproduction. They are carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), magnesium (Mg), iron (Fe), boron (B), manganese (Mn), copper (Cu), zinc (Zn), molybdenum (Mo), nickel (Ni) and chlorine (Cl).^[205] Nutrients required for plants to complete their life cycle are considered essential nutrients. Nutrients that enhance the growth of plants but are not necessary to complete the plant’s life cycle are considered non-essential. With the exception of carbon, hydrogen and oxygen, which are supplied by carbon dioxide and water, and nitrogen, provided through nitrogen fixation,^[205] the nutrients derive originally from the mineral component of the soil. The Law of the Minimum expresses that when the available form of a nutrient is not in enough proportion in the soil solution, then other nutrients cannot be taken up at an optimum rate by a plant.^[206] A particular nutrient ratio of the soil solution is thus mandatory for optimizing plant growth, a value which might differ from nutrient ratios calculated from plant composition.^[207]

Plant uptake of nutrients can only proceed when they are present in a plant-available form. In most situations, nutrients are absorbed in an ionic form from (or together with) soil water. Although minerals are the origin of most nutrients, and the bulk of most nutrient elements in the soil is held in crystalline form within primary and secondary minerals, they weather too slowly to support rapid plant growth. For example, the application of finely ground minerals, feldspar and apatite, to soil seldom provides the necessary amounts of potassium and phosphorus at a rate sufficient for good plant growth, as most of the nutrients remain bound in the crystals of those minerals.

The nutrients adsorbed onto the surfaces of clay colloids and soil organic matter provide a more accessible reservoir of many plant nutrients (e.g. K, Ca, Mg, P, Zn). As plants absorb the nutrients from the soil water, the soluble pool is replenished from the surface-bound pool. The decomposition of soil organic matter by microorganisms is another mechanism whereby the soluble pool of nutrients is replenished – this is important for the supply of plant-available N, S, P, and B from soil.^[209]

Gram for gram, the capacity of humus to hold nutrients and water is far greater than that of clay minerals, most of the soil cation exchange capacity arising from charged carboxylic groups on organic matter.^[210] However, despite the great capacity of humus to retain water once water-soaked, its high hydrophobicity decreases its wettability.^[211] All in all, small amounts of humus may remarkably increase the soil’s capacity to promote plant growth.^[209]

Soil organic matter[[[[edit]

Soil organic matter is made up of organic compounds and includes plant, animal and microbial material, both living and dead. A typical soil has a biomass composition of 70% microorganisms, 22% macrofauna, and 8% roots. The living component of an acre of soil may include 900 lb of earthworms, 2400 lb of fungi, 1500 lb of bacteria, 133 lb of protozoa and 890 lb of arthropods and algae.^[213]

A few percent of the soil organic matter, with small residence time, consists of the microbial biomass and metabolites of bacteria, molds, and actinomycetes that work to break down the dead organic matter.^[214][215] Were it not for the action of these micro-organisms, the entire carbon dioxide part of the atmosphere would be sequestered as organic matter in the soil. However, in the same time soil microbes contribute to carbon sequestration in the topsoil through the formation of stable humus.^[216] In the aim to sequester more carbon in the soil for alleviating the greenhouse effect it would be more efficient in the long-term to stimulate humification than to decrease litter decomposition.^[217]

The main part of soil organic matter is a complex assemblage of small organic molecules, collectively called humus or humic substances. The use of these terms, which do not rely on a clear chemical classification, has been considered as obsolete.^[218] Other studies showed that the classical notion of molecule is not convenient for humus, which escaped most attempts done over two centuries to resolve it in unit components, but still is chemically distinct from polysaccharides, lignins and proteins.^[219]

Most living things in soils, including plants, animals, bacteria, and fungi, are dependent on organic matter for nutrients and/or energy. Soils have organic compounds in varying degrees of decomposition which rate is dependent on the temperature, soil moisture, and aeration. Bacteria and fungi feed on the raw organic matter, which are fed upon by protozoa, which in turn are fed upon by nematodes, annelids and arthropods, themselves able to consume and transform raw or humified organic matter. This has been called the soil food web, through which all organic matter is processed as in a digestive system.^[220] Organic matter holds soils open, allowing the infiltration of air and water, and may hold as much as twice its weight in water. Many soils, including desert and rocky-gravel soils, have little or no organic matter. Soils that are all organic matter, such as peat (histosols), are infertile.^[221] In its earliest stage of decomposition, the original organic material is often called raw organic matter. The final stage of decomposition is called humus.

In grassland, much of the organic matter added to the soil is from the deep, fibrous, grass root systems. By contrast, tree leaves falling on the forest floor are the principal source of soil organic matter in the forest. Another difference is the frequent occurrence in the grasslands of fires that destroy large amounts of aboveground material but stimulate even greater contributions from roots. Also, the much greater acidity under any forests inhibits the action of certain soil organisms that otherwise would mix much of the surface litter into the mineral soil. As a result, the soils under grasslands generally develop a thicker A horizon with a deeper distribution of organic matter than in comparable soils under forests, which characteristically store most of their organic matter in the forest floor (O horizon) and thin A horizon.^[222]

Humus[[[[edit]

Humus refers to organic matter that has been decomposed by soil microflora and fauna to the point where it is resistant to further breakdown. Humus usually constitutes only five percent of the soil or less by volume, but it is an essential source of nutrients and adds important textural qualities crucial to soil health and plant growth.^[223] Humus also feeds arthropods, termites and earthworms which further improve the soil.^[224] The end product, humus, is suspended in colloidal form in the soil solution and forms a weak acid that can attack silicate minerals.^[225] Humus has a high cation and anion exchange capacity that on a dry weight basis is many times greater than that of clay colloids. It also acts as a buffer, like clay, against changes in pH and soil moisture.^[226]

Humic acids and fulvic acids, which begin as raw organic matter, are important constituents of humus. After the death of plants, animals, and microbes, microbes begin to feed on the residues through their production of extra-cellular soil enzymes, resulting finally in the formation of humus.^[227] As the residues break down, only molecules made of aliphatic and aromatic hydrocarbons, assembled and stabilized by oxygen and hydrogen bonds, remain in the form of complex molecular assemblages collectively called humus.^[219] Humus is never pure in the soil, because it reacts with metals and clays to form complexes which further contribute to its stability and to soil structure.^[226] While the structure of humus has in itself few nutrients (with the exception of constitutive metals such as calcium, iron and aluminum) it is able to attract and link, by weak bonds, cation and anion nutrients that can further be released into the soil solution in response to selective root uptake and changes in soil pH, a process of paramount importance for the maintenance of fertility in tropical soils.^[228]

Lignin is resistant to breakdown and accumulates within the soil. It also reacts with proteins,^[229] which further increases its resistance to decomposition, including enzymatic decomposition by microbes.^[230]Fats and waxes from plant matter have still more resistance to decomposition and persist in soils for thousand years, hence their use as tracers of past vegetation in buried soil layers.^[231] Clay soils often have higher organic contents that persist longer than soils without clay as the organic molecules adhere to and are stabilised by the clay.^[232]Proteins normally decompose readily, to the exception of scleroproteins, but when bound to clay particles they become more resistant to decomposition.^[233] As for other proteins clay particles absorb the enzymes exuded by microbes, decreasing enzyme activity while protecting extracellular enzymes from degradation.^[234] The addition of organic matter to clay soils can render that organic matter and any added nutrients inaccessible to plants and microbes for many years,^{[[[[citation needed]} while a study showed increased soil fertility following the addition of mature compost to a clay soil.^[235] High soil tannin content can cause nitrogen to be sequestered as resistant tannin-protein complexes.^[236][237]

Humus formation is a process dependent on the amount of plant material added each year and the type of base soil. Both are affected by climate and the type of organisms present.^[238] Soils with humus can vary in nitrogen content but typically have 3 to 6 percent nitrogen. Raw organic matter, as a reserve of nitrogen and phosphorus, is a vital component affecting soil fertility.^[221] Humus also absorbs water, and expands and shrinks between dry and wet states to a higher extent than clay, increasing soil porosity.^[239] Humus is less stable than the soil’s mineral constituents, as it is reduced by microbial decomposition, and over time its concentration diminishes without the addition of new organic matter. However, humus in its most stable forms may persist over centuries if not millennia.^[240]Charcoal is a source of highly stable humus, called black carbon,^[241] which had been used traditionally to improve the fertility of nutrient-poor tropical soils. This very ancient practice, as ascertained in the genesis of Amazonian dark earths, has been renewed and became popular under the name of biochar. It has been suggested that biochar could be used to sequester more carbon in the fight against the greenhouse effect.^[242]

Climatological influence[[[[edit]

The production, accumulation and degradation of organic matter are greatly dependent on climate. Temperature, soil moisture and topography are the major factors affecting the accumulation of organic matter in soils. Organic matter tends to accumulate under wet or cold conditions where decomposer activity is impeded by low temperature^[243] or excess moisture which results in anaerobic conditions.^[244] Conversely, excessive rain and high temperatures of tropical climates enables rapid decomposition of organic matter and leaching of plant nutrients. Forest ecosystems on these soils rely on efficient recycling of nutrients and plant matter by the living plant and microbial biomass to maintain their productivity, a process which is disturbed by human activities.^[245] Excessive slope, in particular in the presence of cultivation for the sake of agriculture, may encourage the erosion of the top layer of soil which holds most of the raw organic material that would otherwise eventually become humus.^[246]

Plant residue[[[[edit]

Cellulose and hemicellulose undergo fast decomposition by fungi and bacteria, with a half-life of 12–18 days in a temperate climate.^[247]Brown rot fungi can decompose the cellulose and hemicellulose, leaving the lignin and phenolic compounds behind. Starch, which is an energy storage system for plants, undergoes fast decomposition by bacteria and fungi. Lignin consists of polymers composed of 500 to 600 units with a highly branched, amorphous structure, linked to cellulose, hemicellulose and pectin in plant cell walls. Lignin undergoes very slow decomposition, mainly by white rot fungi and actinomycetes; its half-life under temperate conditions is about six months.^[247]

Horizons[[[[edit]

A horizontal layer of the soil, whose physical features, composition and age are distinct from those above and beneath, is referred to as a soil horizon. The naming of a horizon is based on the type of material of which it is composed. Those materials reflect the duration of specific processes of soil formation. They are labelled using a shorthand notation of letters and numbers which describe the horizon in terms of its colour, size, texture, structure, consistency, root quantity, pH, voids, boundary characteristics and presence of nodules or concretions.^[248] No soil profile has all the major horizons. Some, called entisols, may have only one horizon or are currently considered as having no horizon, in particular incipient soils from unreclaimed mining waste deposits,^[249]moraines,^[250]volcanic cones^[251]sand dunes or alluvial terraces.^[252] Upper soil horizons may be lacking in truncated soils following wind or water ablation, with concomitant downslope burying of soil horizons, a natural process aggravated by agricultural practices such as tillage.^[253] The growth of trees is another source of disturbance, creating a micro-scale heterogeneity which is still visible in soil horizons once trees have died.^[254] By passing from a horizon to another, from the top to the bottom of the soil profile, one goes back in time, with past events registered in soil horizons like in sediment layers. Sampling pollen, testate amoebae and plant remains in soil horizons may help to reveal environmental changes (e.g. climate change, land use change) which occurred in the course of soil formation.^[255] Soil horizons can be dated by several methods such as radiocarbon, using pieces of charcoal provided they are of enough size to escape pedoturbation by earthworm activity and other mechanical disturbances.^[256] Fossil soil horizons from paleosols can be found within sedimentary rock sequences, allowing the study of past environments.^[257]

The exposure of parent material to favourable conditions produces mineral soils that are marginally suitable for plant growth, as is the case in eroded soils.^[258] The growth of vegetation results in the production of organic residues which fall on the ground as litter for plant aerial parts (leaf litter) or are directly produced belowground for subterranean plant organs (root litter), and then release dissolved organic matter.^[259] The remaining surficial organic layer, called the O horizon, produces a more active soil due to the effect of the organisms that live within it. Organisms colonise and break down organic materials, making available nutrients upon which other plants and animals can live.^[260] After sufficient time, humus moves downward and is deposited in a distinctive organic-mineral surface layer called the A horizon, in which organic matter is mixed with mineral matter through the activity of burrowing animals, a process called pedoturbation. This natural process does not go to completion in the presence of conditions detrimental to soil life such as strong acidity, cold climate or pollution, stemming in the accumulation of undecomposed organic matter within a single organic horizon overlying the mineral soil^[261] and in the juxtaposition of humified organic matter and mineral particles, without intimate mixing, in the underlying mineral horizons.^[262]

Classification[[[[edit]

Soil is classified into categories in order to understand relationships between different soils and to determine the suitability of a soil in a particular region. One of the first classification systems was developed by the Russian scientist Vasily Dokuchaev around 1880.^[263] It was modified a number of times by American and European researchers, and developed into the system commonly used until the 1960s. It was based on the idea that soils have a particular morphology based on the materials and factors that form them. In the 1960s, a different classification system began to emerge which focused on soil morphology instead of parental materials and soil-forming factors. Since then it has undergone further modifications. The World Reference Base for Soil Resources (WRB)^[264] aims to establish an international reference base for soil classification.

Soil is used in agriculture, where it serves as the anchor and primary nutrient base for plants. The types of soil and available moisture determine the species of plants that can be cultivated. Agricultural soil science was the primeval domain of soil knowledge, long time before the advent of pedology in the 19th century. However, as demonstrated by aeroponics, aquaponics and hydroponics, soil material is not an absolute essential for agriculture, and soilless cropping systems have been claimed as the future of agriculture for an endless growing mankind.^[265]

Soil material is also a critical component in the mining, construction and landscape development industries.^[266] Soil serves as a foundation for most construction projects. The movement of massive volumes of soil can be involved in surface mining, road building and dam construction. Earth sheltering is the architectural practice of using soil for external thermal mass against building walls. Many building materials are soil based. Loss of soil through urbanization is growing at a high rate in many areas and can be critical for the maintenance of subsistence agriculture.^[267]

Soil resources are critical to the environment, as well as to food and fibre production, producing 98.8% of food consumed by humans.^[268] Soil provides minerals and water to plants according to several processes involved in plant nutrition. Soil absorbs rainwater and releases it later, thus preventing floods and drought, flood regulation being one of the major ecosystem services provided by soil.^[269] Soil cleans water as it percolates through it.^[270] Soil is the habitat for many organisms: the major part of known and unknown biodiversity is in the soil, in the form of invertebrates (earthworms, woodlice, millipedes, centipedes, snails, slugs, mites, springtails, enchytraeids, nematodes, protists), bacteria, archaea, fungi and algae; and most organisms living above ground have part of them (plants) or spend part of their life cycle (insects) below-ground.^[271] Above-ground and below-ground biodiversities are tightly interconnected,^[238][272] making soil protection of paramount importance for any restoration or conservation plan.

The biological component of soil is an extremely important carbon sink since about 57% of the biotic content is carbon. Even in deserts, cyanobacteria, lichens and mosses form biological soil crusts which capture and sequester a significant amount of carbon by photosynthesis. Poor farming and grazing methods have degraded soils and released much of this sequestered carbon to the atmosphere. Restoring the world’s soils could offset the effect of increases in greenhouse gas emissions and slow global warming, while improving crop yields and reducing water needs.^{[273][274][275]}

Waste management often has a soil component. Septic drain fields treat septic tank effluent using aerobic soil processes. Land application of waste water relies on soil biology to aerobically treat BOD. Alternatively, Landfills use soil for daily cover, isolating waste deposits from the atmosphere and preventing unpleasant smells. Composting is now widely used to treat aerobically solid domestic waste and dried effluents of settling basins. Although compost is not soil, biological processes taking place during composting are similar to those occurring during decomposition and humification of soil organic matter.^[276]

Organic soils, especially peat, serve as a significant fuel and horticultural resource. Peat soils are also commonly used for the sake of agriculture in nordic countries, because peatland sites, when drained, provide fertile soils for food production.^[277] However, wide areas of peat production, such as rain-fed sphagnum bogs, also called blanket bogs or raised bogs, are now protected because of their patrimonial interest. As an example, Flow Country, covering 4,000 square kilometres of rolling expanse of blanket bogs in Scotland, is now candidate for being included in the World Heritage List. Under present-day global warming peat soils are thought to be involved in a self-reinforcing (positive feedback) process of increased emission of greenhouse gases (methane and carbon dioxide) and increased temperature,^[278] a contention which is still under debate when replaced at field scale and including stimulated plant growth.^[279]

Geophagy is the practice of eating soil-like substances. Both animals and humans occasionally consume soil for medicinal, recreational, or religious purposes.^[280] It has been shown that some monkeys consume soil, together with their preferred food (tree foliage and fruits), in order to alleviate tannin toxicity.^[281]

Soils filter and purify water and affect its chemistry. Rain water and pooled water from ponds, lakes and rivers percolate through the soil horizons and the upper rock strata, thus becoming groundwater. Pests (viruses) and pollutants, such as persistent organic pollutants (chlorinated pesticides, polychlorinated biphenyls), oils (hydrocarbons), heavy metals (lead, zinc, cadmium), and excess nutrients (nitrates, sulfates, phosphates) are filtered out by the soil.^[282] Soil organisms metabolise them or immobilise them in their biomass and necromass,^[283] thereby incorporating them into stable humus.^[284] The physical integrity of soil is also a prerequisite for avoiding landslides in rugged landscapes.^[285]

Degradation[[[[edit]

Land degradation refers to a human-induced or natural process which impairs the capacity of land to function.^[286] Soil degradation involves acidification, contamination, desertification, erosion or salination.^[287]

Soil acidification is beneficial in the case of alkaline soils, but it degrades land when it lowers crop productivity, soil biological activity and increases soil vulnerability to contamination and erosion. Soils are initially acid and remain such when their parent materials are low in basic cations (calcium, magnesium, potassium and sodium). On parent materials richer in weatherable minerals acidification occurs when basic cations are leached from the soil profile by rainfall or exported by the harvesting of forest or agricultural crops. Soil acidification is accelerated by the use of acid-forming nitrogenous fertilizers and by the effects of acid precipitation. Deforestation is another cause of soil acidification, mediated by increased leaching of soil nutrients in the absence of tree canopies.^[288]

Soil contamination at low levels is often within a soil’s capacity to treat and assimilate waste material. Soil biota can treat waste by transforming it, mainly through microbial enzymatic activity.^[289]Soil organic matter and soil minerals can adsorb the waste material and decrease its toxicity,^[290] although when in colloidal form they may transport the adsorbed contaminants to subsurface environments.^[291] Many waste treatment processes rely on this natural bioremediation capacity. Exceeding treatment capacity can damage soil biota and limit soil function. Derelict soils occur where industrial contamination or other development activity damages the soil to such a degree that the land cannot be used safely or productively. Remediation of derelict soil uses principles of geology, physics, chemistry and biology to degrade, attenuate, isolate or remove soil contaminants to restore soil functions and values. Techniques include leaching, air sparging, soil conditioners, phytoremediation, bioremediation and Monitored Natural Attenuation (MNA). An example of diffuse pollution with contaminants is copper accumulation in vineyards and orchards to which fungicides are repeatedly applied, even in organic farming.^[292]

Desertification is an environmental process of ecosystem degradation in arid and semi-arid regions, often caused by badly adapted human activities such as overgrazing or excess harvesting of firewood. It is a common misconception that drought causes desertification.^[293] Droughts are common in arid and semiarid lands. Well-managed lands can recover from drought when the rains return. Soil management tools include maintaining soil nutrient and organic matter levels, reduced tillage and increased cover.^[294] These practices help to control erosion and maintain productivity during periods when moisture is available. Continued land abuse during droughts, however, increases land degradation. Increased population and livestock pressure on marginal lands accelerates desertification.^[295] It is now questioned whether present-day climate warming will favour or disfavour desertification, with contradictory reports about predicted rainfall trends associated with increased temperature, and strong discrepancies among regions, even in the same country.^[296]

Erosion of soil is caused by water, wind, ice, and movement in response to gravity. More than one kind of erosion can occur simultaneously. Erosion is distinguished from weathering, since erosion also transports eroded soil away from its place of origin (soil in transit may be described as sediment). Erosion is an intrinsic natural process, but in many places it is greatly increased by human activity, especially unsuitable land use practices.^[297] These include agricultural activities which leave the soil bare during times of heavy rain or strong winds, overgrazing, deforestation, and improper construction activity. Improved management can limit erosion. Soil conservation techniques which are employed include changes of land use (such as replacing erosion-prone crops with grass or other soil-binding plants), changes to the timing or type of agricultural operations, terrace building, use of erosion-suppressing cover materials (including cover crops and other plants), limiting disturbance during construction, and avoiding construction during erosion-prone periods and in erosion-prone places such as steep slopes.^[298] Historically, one of the best examples of large-scale soil erosion due to unsuitable land-use practices is wind erosion (the so-called dust bowl) which ruined American and Canadian prairies during the 1930s, when immigrant farmers, encouraged by the federal government of both countries, settled and converted the original shortgrass prairie to agricultural crops and cattle ranching.

A serious and long-running water erosion problem occurs in China, on the middle reaches of the Yellow River and the upper reaches of the Yangtze River. From the Yellow River, over 1.6 billion tons of sediment flow each year into the ocean. The sediment originates primarily from water erosion (gully erosion) in the Loess Plateau region of northwest China.^[299]

Soil piping is a particular form of soil erosion that occurs below the soil surface.^[300] It causes levee and dam failure, as well as sink hole formation. Turbulent flow removes soil starting at the mouth of the seep flow and the subsoil erosion advances up-gradient.^[301] The term sand boil is used to describe the appearance of the discharging end of an active soil pipe.^[302]

Soil salination is the accumulation of free salts to such an extent that it leads to degradation of the agricultural value of soils and vegetation. Consequences include corrosion damage, reduced plant growth, erosion due to loss of plant cover and soil structure, and water quality problems due to sedimentation. Salination occurs due to a combination of natural and human-caused processes. Arid conditions favour salt accumulation. This is especially apparent when soil parent material is saline. Irrigation of arid lands is especially problematic.^[303] All irrigation water has some level of salinity. Irrigation, especially when it involves leakage from canals and overirrigation in the field, often raises the underlying water table. Rapid salination occurs when the land surface is within the capillary fringe of saline groundwater. Soil salinity control involves watertable control and flushing with higher levels of applied water in combination with tile drainage or another form of subsurface drainage.^[304][305]

Reclamation[[[[edit]

Soils which contain high levels of particular clays with high swelling properties, such as smectites, are often very fertile. For example, the smectite-rich paddy soils of Thailand’s Central Plains are among the most productive in the world. However, the overuse of mineral nitrogen fertilizers and pesticides in irrigated intensive rice production has endangered these soils, forcing farmers to implement integrated practices based on Cost Reduction Operating Principles (CROP).^[306]

Many farmers in tropical areas, however, struggle to retain organic matter and clay in the soils they work. In recent years, for example, productivity has declined and soil erosion has increased in the low-clay soils of northern Thailand, following the abandonment of shifting cultivation for a more permanent land use.^[307] Farmers initially responded by adding organic matter and clay from termite mound material, but this was unsustainable in the long-term because of rarefaction of termite mounds. Scientists experimented with adding bentonite, one of the smectite family of clays, to the soil. In field trials, conducted by scientists from the International Water Management Institute in cooperation with Khon Kaen University and local farmers, this had the effect of helping retain water and nutrients. Supplementing the farmer’s usual practice with a single application of 200 kg bentonite per rai (6.26 rai = 1 hectare) resulted in an average yield increase of 73%.^[308] Other studies showed that applying bentonite to degraded sandy soils reduced the risk of crop failure during drought years.^[309]

In 2008, three years after the initial trials, IWMI scientists conducted a survey among 250 farmers in northeast Thailand, half of whom had applied bentonite to their fields. The average improvement for those using the clay addition was 18% higher than for non-clay users. Using the clay had enabled some farmers to switch to growing vegetables, which need more fertile soil. This helped to increase their income. The researchers estimated that 200 farmers in northeast Thailand and 400 in Cambodia had adopted the use of clays, and that a further 20,000 farmers were introduced to the new technique.^[310]

If the soil is too high in clay or salts (e.g. saline sodic soil), adding gypsum, washed river sand and organic matter (e.g.municipal solid waste) will balance the composition.^[311]

Adding organic matter, like ramial chipped wood or compost, to soil which is depleted in nutrients and too high in sand will boost its quality and improve production.^[312][313]

Special mention must be made of the use of charcoal, and more generally biochar to improve nutrient-poor tropical soils, a process based on the higher fertility of anthropogenic pre-Colombian Amazonian Dark Earths, also called Terra Preta de Índio, due to interesting physical and chemical properties of soil black carbon as a source of stable humus.^[314]

History of studies and research[[[[edit]

The history of the study of soil is intimately tied to humans’ urgent need to provide food for themselves and forage for their animals. Throughout history, civilizations have prospered or declined as a function of the availability and productivity of their soils.^[315]

Studies of soil fertility[[[[edit]

The Greek historian Xenophon (450–355 BCE) is credited with being the first to expound upon the merits of green-manuring crops: « But then whatever weeds are upon the ground, being turned into earth, enrich the soil as much as dung. »

Columella’s Of husbandry, circa 60 CE, advocated the use of lime and that clover and alfalfa (green manure) should be turned under,^[317] and was used by 15 generations (450 years) under the Roman Empire until its collapse. From the fall of Rome to the French Revolution, knowledge of soil and agriculture was passed on from parent to child and as a result, crop yields were low. During the European Middle Ages, Yahya Ibn al-‘Awwam’s handbook,^[319] with its emphasis on irrigation, guided the people of North Africa, Spain and the Middle East; a translation of this work was finally carried to the southwest of the United States when under Spanish influence.^[320]Olivier de Serres, considered as the father of French agronomy, was the first to suggest the abandonment of fallowing and its replacement by hay meadows within crop rotations, and he highlighted the importance of soil (the French terroir) in the management of vineyards. His famous book Le Théâtre d’Agriculture et mesnage des champs^[321] contributed to the rise of modern, sustainable agriculture and to the collapse of old agricultural practices such as soil amendment for crops by the lifting of forest litter and assarting, which ruined the soils of western Europe during Middle Ages and even later on according to regions.^[322]

Experiments into what made plants grow first led to the idea that the ash left behind when plant matter was burned was the essential element but overlooked the role of nitrogen, which is not left on the ground after combustion, a belief which prevailed until the 19th century.^[323] In about 1635, the Flemish chemist Jan Baptist van Helmont thought he had proved water to be the essential element from his famous five years’ experiment with a willow tree grown with only the addition of rainwater. His conclusion came from the fact that the increase in the plant’s weight had apparently been produced only by the addition of water, with no reduction in the soil’s weight.^[324][325]John Woodward (d. 1728) experimented with various types of water ranging from clean to muddy and found muddy water the best, and so he concluded that earthy matter was the essential element. Others concluded it was humus in the soil that passed some essence to the growing plant. Still others held that the vital growth principal was something passed from dead plants or animals to the new plants. At the start of the 18th century, Jethro Tull demonstrated that it was beneficial to cultivate (stir) the soil, but his opinion that the stirring made the fine parts of soil available for plant absorption was erroneous.^[325]

As chemistry developed, it was applied to the investigation of soil fertility. The French chemist Antoine Lavoisier showed in about 1778 that plants and animals must [combust] oxygen internally to live and was able to deduce that most of the 165-pound weight of van Helmont’s willow tree derived from air.^[328] It was the French agriculturalist Jean-Baptiste Boussingault who by means of experimentation obtained evidence showing that the main sources of carbon, hydrogen and oxygen for plants were air and water, while nitrogen was taken from soil.^[329]Justus von Liebig in his book Organic chemistry in its applications to agriculture and physiology (published 1840), asserted that the chemicals in plants must have come from the soil and air and that to maintain soil fertility, the used minerals must be replaced.^[330] Liebig nevertheless believed the nitrogen was supplied from the air. The enrichment of soil with guano by the Incas was rediscovered in 1802, by Alexander von Humboldt. This led to its mining and that of Chilean nitrate and to its application to soil in the United States and Europe after 1840.^[331]

The work of Liebig was a revolution for agriculture, and so other investigators started experimentation based on it. In England John Bennet Lawes and Joseph Henry Gilbert worked in the Rothamsted Experimental Station, founded by the former, and (re)discovered that plants took nitrogen from the soil, and that salts needed to be in an available state to be absorbed by plants. Their investigations also produced the superphosphate, consisting in the acid treatment of phosphate rock. This led to the invention and use of salts of potassium (K) and nitrogen (N) as fertilizers. Ammonia generated by the production of coke was recovered and used as fertiliser.^[333] Finally, the chemical basis of nutrients delivered to the soil in manure was understood and in the mid-19th century chemical fertilisers were applied. However, the dynamic interaction of soil and its life forms still awaited discovery.

In 1856 J. Thomas Way discovered that ammonia contained in fertilisers was transformed into nitrates,^[334] and twenty years later Robert Warington proved that this transformation was done by living organisms.^[335] In 1890 Sergei Winogradsky announced he had found the bacteria responsible for this transformation.^[336]

It was known that certain legumes could take up nitrogen from the air and fix it to the soil but it took the development of bacteriology towards the end of the 19th century to lead to an understanding of the role played in nitrogen fixation by bacteria. The symbiosis of bacteria and leguminous roots, and the fixation of nitrogen by the bacteria, were simultaneously discovered by the German agronomist Hermann Hellriegel and the Dutch microbiologist Martinus Beijerinck.

Crop rotation, mechanisation, chemical and natural fertilisers led to a doubling of wheat yields in western Europe between 1800 and 1900.

Studies of soil formation[[[[edit]

The scientists who studied the soil in connection with agricultural practices had considered it mainly as a static substrate. However, soil is the result of evolution from more ancient geological materials, under the action of biotic and abiotic processes. After studies of the improvement of the soil commenced, other researchers began to study soil genesis and as a result also soil types and classifications.

In 1860, in Mississippi, Eugene W. Hilgard (1833-1916) studied the relationship between rock material, climate, vegetation, and the type of soils that were developed. He realised that the soils were dynamic, and considered the classification of soil types.^[338] Unfortunately his work was not continued. At about the same time, Friedrich Albert Fallou was describing soil profiles and relating soil characteristics to their formation as part of his professional work evaluating forest and farm land for the principality of Saxony. His 1857 book, Anfangsgründe der Bodenkunde (First principles of soil science) established modern soil science.^[339] Contemporary with Fallou’s work, and driven by the same need to accurately assess land for equitable taxation, Vasily Dokuchaev led a team of soil scientists in Russia who conducted an extensive survey of soils, observing that similar basic rocks, climate and vegetation types lead to similar soil layering and types, and established the concepts for soil classifications. Due to language barriers, the work of this team was not communicated to western Europe until 1914 through a publication in German by Konstantin Glinka, a member of the Russian team.^[340]

Curtis F. Marbut, influenced by the work of the Russian team, translated Glinka’s publication into English,^[341] and as he was placed in charge of the U.S. National Cooperative Soil Survey, applied it to a national soil classification system.^[325]

See also[[[[edit]

Wikimedia Commons has media related to Soils.

Wikiquote has quotations related to: Soil

References[[[[edit]

Bibliographie[[[[edit]

Further reading[[[[edit]

External links[[[[edit]

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