Desert soils on the other hand may be rich in nutrients and can support crops if irrigated. Human agriculture thus depends on optimum rainfall. Depending on other factors, 20-30 inches of rainfall per year is the threshold where mineral loss begins to offset gains.

High levels of rainfall are not the only factor which reduces the mineral content of the soil, and hence its fertility. Agricultural use inherently reduces both mineral content and organic material in soils, if they are not deliberately replenished by human action. If overuse or exploitation of a previously fertile soil reduces its organic matter or humus content, the results can be quite dramatic. Instead of remaining friable and granular with sponge-like qualities, the soil becomes cloddy and hardens into dense, concrete-like masses when it dries. Without humus, the soil loses its capacity to retain water and atmospheric gases, on which its biological communities depend. Plant roots may no longer be able to penetrate the altered soil composition, and therefore cannot absorb the minerals which are diminishing in availability anyway, since biological and chemical processes in soil thus affected tend to diminish as well.

In valleys like the Nile and in Mesopotamia, annual flooding deposits fresh silt on the fields as well as provides water for irrigation. These natural systems effectively preserve the fertility of the soil. But these sites represent special cases, and agriculturalists elsewhere have had to "husband" the precious resource of soil carefully to sustain the life of human communities over many generations. Frequently, farmers have to allow fallow periods (when no crops are planted) to permit the level of organic material to recover in their soil. Temperate-zone farmers have traditionally used fallow periods to restore fertility and also plant but plow under "green manure" crops, such as alfalfa or other legumes, which increase both organic material and nitrogen content in the soil.