Soil is the natural medium in which crops grow. The nutrient content of a soil and the availability of the nutrients to crops are important factors that determine a soil's productivity. Soil nutrients are mostly present in forms not immediately available to plants, such as being adsorbed onto or as constituents of soil mineral particles, or in organic matter. They become available through slow processes, such as biological decomposition of organic matter called mineralization, chemical reactions on soil minerals called weathering, and release from soil particles.
Soil nutrients in agricultural land may be gradually lost with the removal of harvested products, such as grain and straw, which take with them considerable amounts of all nutrients. In addition, nutrients may be lost from the soil through leaching and erosion. Therefore, it is common for field crops to suffer from nutrient deficiencies. On the other hand, certain soils may present the crops with problems of mineral toxicity, that is, excess amounts of particular elements. To maintain healthy growth of crops, it is necessary to correct these problems.
Soil testing, visual diagnosis of the plant and plant tissue analysis can all be used to evaluate the fertility status of the soil and detect deficiencies and toxicities in soil nutrients. The results of such tests provide the basis for recommendations on the application of fertilizer and soil amendments.
See also: Coping with Nutrient Deficiencies, Overcoming Nutrient Toxicities
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