Factors of plant nutrient availability relevant to soil testing

Summary In most arable soils the nitrate availability depends mainly on the quantity of nitrate present in the rooting zone at the beginning of the growing season. Easily mineralizable organic N and the release of non-exchangeable NH₄ from clay minerals may in addition control the nitrogen availability during a season. In flooded soils, ammonium is the major form of nitrogen absorbed by plants. Ammonium dynamics in these soils is similar to that of potassium. The availability of both is controlled mainly by the intensity and buffering power for ammonium or potassium, respectively. Basically, intensity of the supply and buffering power for phosphate are the main factors determining the phosphate availability. The determination of the phosphate buffer power, especially in the root zone, however, remains to be difficult. Soil test methods should take into consideration the major factors and processes relevant to the availability of a particular plant nutrient.