Soil fertility—the ability to support plant growth—depends on various factors, including the soil's structure or texture; its chemical composition, esp. its content of plant nutrients; its supply of water; and its temperature. Agriculture necessarily lowers soil fertility by removing soil nutrients incorporated in the harvested crops. Cultivation, especially with heavy machinery, can degrade soil structure. Agricultural soils are also vulnerable to mismanagement. Exposure of soils to wind and rain during cultivation encourages erosion of the fertile surface. Excessive cropping or grazing can depress soil-nutrient levels and degrade soil structure.
Soil conservation techniques have been developed to address the range of soil management issues. Various methods of cultivation conserve soil fertility (see cover crop; rotation of crops). Minimum-tillage systems, often entailing herbicide use, avoid erosion and maintain soil structure. Soil fertility and agricultural productivity can also be improved, restored, and maintained by the correct use of fertilizer, either organic, such as manure, or inorganic, and other soil amendments. Organic matter can be added to improve soil structure. Soil acidity can be decreased by addition of calcium carbonate or increased by addition of sulfuric acid.