The role of organic matter in soil productivity (with special reference to tropical and arid regions).

arid zones crop rotation crops cultivos fertilidad del suelo fosfatos (sales) humus nutrientes nutrients nutrition des plantes oferta offre organic matter phosphate (sel) phosphates (salts) plant nutrition plante de culture productividad productivity rotation culturale soil soil chemicophysical properties soil fertility sol substance nutritive suelo sulfate sulfatos sulphates supply zone aride