Soil tests are widely used to predict the probability of crop responses to application of fertilizers, particularly phosphorus P , potassium K , and in some instances manganese Mn , copper Cu , zinc Zn , and iron Fe and application of lime. Soil-test levels at which no response is obtained are defined as critical soil-test levels that have been determined by greenhouse and field experiments. The commonly used soil-test extractants for P in the United States are the Bray-1 Midwest , Mehlich 1 and 3 extractants southeastern United States , and the Olsen extractant calcareous soils.
The Mehlich 3 extractant is being used by many laboratories because it is suitable for measuring soil-test P over a wide range of soil properties and also is a multi-element extractant. Because nitrogen N can be a very mobile element, most laboratories do not routinely run a N soil test. Nitrogen recommendations are made on the basis of yield goals for a given crop. Where nitrate-leaching potential is at a minimum, the amount of residual nitrate in the soil profile before planting has been related to the need for fertilizer N.
In certain humid regions, nitrate levels of the soil have been measured before N side dressing of corn, and the values interpreted as to the amount of fertilizer N to apply. Using soil testing to identify the potential for an environmental impact may have value, but only if a comprehensive approach is taken. Response parameters for other uses of soil testing have not been so well defined, and linkages often remain intuitive or based on the best professional judgment of a team of scientists.
Progress in soil testing is facilitating assessment of soils likely to act as sources of nutrients for surface and ground water. Extractable soil concentrations of nutrients or of nonessential elements, organics, etc. In the late s, soil testing became an important practice for determining the need for lime and fertilizer in the United States.
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This change was brought about by several factors. Mechanization led to increased farm size; and the large-scale use of synthetic N fertilizers led to increased crop yields. As crop nutrient removal increased with these higher yields, soil reserves of certain plant nutrients, particularly P and K, began to be depleted, resulting in nutrient deficiencies and lower yields. In response to these problems, there was a concerted effort to develop soil-testing methods that could identify P and K deficiencies in different regions of the United States.
