Le pH optimal d'un sol dépend de plusieurs facteurs, liés au sol lui-même ou aux plantes que l'on y cultive. Un sol de pH inférieur à 5,5 doit être impérativement corrigé. Mais remonter le pH jusqu'à la neutralité (pH 7) est coûteux et ne se justifie qu'en sol très limoneux. Dans les autres types de sols, on cherche à maintenir un pH voisin de 6,5. Les besoins annuels des cultures varient de quelques kilos de CaO (forme sous laquelle on estime les prélèvements, bien que ceux-ci aient lieu sous forme ionique) à l'hectare, à 350 kg pour la luzerne. De même, la tolérance des plantes aux valeurs de pH est variable : certaines (avoine, pomme de terre, graminées fourragères) peuvent tolérer un pH inférieur à 6 ; d'autres souffrent dès que le pH est inférieur à 7 (blé, maïs, trèfle) ; d'autres encore (betterave, luzerne) ont un pH optimal supérieur à 7.
Si le problème de la correction du pH ne se pose pas en sol calcaire (pH supérieur à 7,5), environ la moitié des sols cultivés en France sont situés sur des roches non calcaires ; il faut donc surveiller l'évolution de leur pH. Le choix d'un amendement dépend de son coût mais également du type de sol (on préférera les amendements à dissolution rapide dans les sols argileux et froids, un amendement contenant de la magnésie lorsqu'il faut également corriger une carence en magnésium). Le calcul de la dose à apporter devra tenir compte de l'écart de pH à combler mais également du type de sol : on pourra chauler massivement un sol argileux, tandis qu'il faudra fractionner les apports dans les terres légères. Dans ces dernières, on évitera de relever le pH de plus d'une unité à la fois ou d'apporter en une fois plus d'une tonne environ de CaO par hectare. En effet, un apport trop massif d'amendement calcique peut bloquer certains éléments et entraîner des risques de carence pour la culture suivante (bore sur betterave, magnésium ou zinc sur blé).
L'épandage est en général réalisé avant un labour. Une façon superficielle préalable au passage de la charrue améliore le mélange avec la terre. Les amendements sous forme de poudre ou de granulés ne posent pas de problème d'enfouissement. Pour les amendements naturels (marnes, craie tendre), il faut laisser les morceaux se déliter à l'air libre pendant quelque temps avant de les étaler et de les enfouir.
Roger-Estrade
chaume
Tige creuse des graminées, ou partie de la tige des céréales qui reste en place sur le sol après la moisson (le reste faisant partie de la paille). ancien nom : éteule.
La hauteur du chaume dépend du réglage de la hauteur de la barre de coupe de la moissonneuse-batteuse, déterminé par le volume de paille que l'on veut récolter. Les chaumes sont enfouis dans le sol, lors du labour ou du déchaumage. Par extension, on appelle également « chaume » un champ moissonné ni déchaumé, ni labouré, ni semé.
Roger-Estrade
chaux
1. Oxyde de calcium communément appelé chaux vive (formule CaO). 2. Mode d'expression de la teneur en calcium des engrais et amendements, quelle que soit la forme sous laquelle se trouve le calcium (carbonates, chaux...).
En présence d'eau, l'oxyde de calcium prend la forme d'hydroxyde de calcium de formule chimique Ca(Oh)2 communément appelé chaux éteinte ou chaux hydratée.Les chaux sont obtenues par calcination, à environ 1 000o C, de roches calcaires dont le carbonate de calcium est décomposé en gaz carbonique et en oxyde de calcium (CaO). Certains calcaires contenant du carbonate de magnésium permettent d'obtenir, de la même manière, de la magnésie.
Types de chaux.
Les chaux sont utilisées comme amendements minéraux basiques : elles représentent les produits cuits (classe III) de la norme NF U 44001. On les retrouve sous plusieurs dénominations : chaux vive agricole qui doit contenir au minimum 70 % de CaO et MgO avec une valeur neutralisante d'au moins 70 ; chaux éteinte agricole à teneur minimum de 50 % de CaO et MgO pour une valeur neutralisante minimum de 50 ; chaux magnésienne vive : minimum de 70 % pour l'ensemble (CaO + MgO) et 10 % pour la magnésie, valeur neutralisante d'au moins 77 ; chaux magnésienne éteinte : au moins 50 % de CaO + MgO et 12 % de magnésie. Sa valeur neutralisante ne peut être inférieure à 55 ; déchets ou cendres de chaux appelés communément cendrées qui sont les résidus de la fabrication normale des chaux tels qu'ils sont séparés à la sortie des fours. Ils doivent contenir au moins 40 % de CaO + MgO pour une valeur neutralisante minimum de 40 ; déchets ou cendres de chaux magnésiennes qui ont les mêmes caractéristiques de teneur totale que les précédents, mais qui ont une teneur en MgO au moins égale à 8 % et une valeur neutralisante supérieure à 43.
Utilisation.
Les chaux sont des amendements minéraux basiques à action rapide et à forte teneur en oxyde de calcium. Elles sont particulièrement recommandées pour un emploi en sols lourds et argileux. Elles peuvent être également utilisées pour leur action antiparasitaire (lutte contre le douve et les strongles dans les prairies, par exemple).
L'ensemble des produits cuits représente 12 % de l'ensemble du marché des amendements minéraux basiques.
Thomas
cheimatobie
Papillon gris brunâtre de 20 mm d'envergure dont les larves verdâtres, appelées chenilles arpenteuses, s'attaquent à de nombreuses essences fruitières ou forestières (pommier, cerisier), dévorant bourgeons, inflorescences entières et feuilles.
On peut lutter contre les cheimatobies par des traitements d'hiver, préventifs, avec des insecticides polyvalents qui tuent les œufs. A défaut, on peut traiter à l'époque du débourrement par des oléo-insecticides. Les traitements de printemps contre les chenilles avec des insecticides peuvent être parfois nécessaires en pré- ou en postfloraison. On peut également utiliser la bactérie Bacillus thuringiensis.
STREBLER/RAYNAL
chélateur
Composé organique complexe ayant la propriété de « séquestrer » - c'est-à-dire d'inclure à l'intérieur de son édifice moléculaire - des métaux polyvalents, de les protéger contre les risques d'insolubilisation dans le sol et de les laisser lentement revenir à une forme libre, utilisable par les plantes.
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