Cation-equivalent constancy in alfalfa

Consideration has been given to the cation content of alfalfa plants that were grown on 20 very different New Jersey soils under as nearly optimum conditions as our knowledge of these soils at the outset of the study permitted. The data indicate that high yielding-capacity of these soils for alfalfa over a period of sufficient duration to permit of eight cuttings was associated with a high content of available Ca in the soil at the start of the test. Under conditions in which all the fertilizer was applied before seeding, the alfalfa accumulated large amounts of K in the first few crops with the result that most of the soils were rapidly depleted of this element. This tendency toward excessive absorption of K by plants might be expected from the position of this element in the electromotive series. The equivalents of K per 100 grams dry weight of alfalfa tended to decrease from the second to the eighth crop, whereas those of Ca and mg tended to increase. The sum of the equivalents of Ca, Mg, and K per unit of plant material tended to be a constant for the produce of any given harvest, this constant having a value approaching 170 M.E. per 100 grams dry matter in the first crop and 187 M.E. in the eighth. The evidence supports the belief that each of these cations has at least two functions in the plant, one specific and the other or others of the type that can be performed interchangeably by all three cations. Once the supply of each cation is adequate to meet the specific need for it, there can be a wide range in ratios in the remaining quantities that are absorbed by the plant to meet its total cation needs. Whether the problem of growing alfalfa is considered from the point of view of economy in its production or that of its mineral value to the animal to which it is fed, it would appear that the soil on which it is to be grown should be fortified with an abundance of Ca and Mg in preparation for seeding, but that the K applications should be governed by the specific annual needs of the plant, a suitable application being made at seeding time and additional quantities being supplied each year the crop is allowed to continue on the same land. Because of alfalfa's tendency to accumulate K in excess of its critical need for it, difficulty is experienced in maintaining an adequate supply of this element in the soil. The annual application of K must be sufficient to maintain the K content of the plant at not less than 1%, but it should not be so large as to effect a substitutlon of K for Ca and Mg in the functions that are common to all three cations in the plant. The Na content of the alfalfa was too small to be of any significance in connection with the conclusions reached in this paper, the largest amount found being 2.04 M.E. per 100 grams dry matter in the produce of the Whippany silt loam soil.