INFLUENCE OF CALCIUM ON AVAILABILITY OF OTHER SOIL CATIONS

An 8-year study on 20 important agricultural soils of New Jersey, in which use was made of the roots of alfalfa plants as cation-extracting agents, has led to the following conclusions: conditions approach the optimum for the cation nutrition of alfalfa when 65% of the exchange complex of the soil is occupied by Ca, 10% by Mg, 5% by K, and 20% by H. Ca is the dominant cation in the soil and K in the alfalfa plant. The plant tends to take up more K than it needs unless the Ca content of the soil is maintained at a relatively high level. When any one of the nutrient cations is deficient in the soil, more of the others move into the alfalfa, but the sum of the cation equivalent in the plant per unit weight of dry tissue tends to be a constant. The tendency of alfalfa to absorb more K than it requires makes it necessary to consider how this can be avoided. K is a much more expensive element than the Ca which it replaces. Furthermore K is of much less importance, quantitatively, than Ca in the nutrition of animals. In soils of low exchange capacity, or in those that are maintained at low pH values for the benefit of acid-soil crops, it would be desirable to apply moderate amounts of K from time to time during the crop season rather than a large amount at the time of planting only. Mg deficiency tends to be widespread on sandy and acid soils because the liberal applications of K that are normally used under such conditions enable the plant to absorb an excess of K in place of part of the needed Mg. Liming acid soils to the point where virtually all the exchange H in the entire volume of plowed soil has been replaced by Ca tends to cause a deficiency of Mn in crops other than the deep-rooted perennials.