Nodulation Responses of Soybeans to Added Phosphorus, Potassium, and Calcium Salts1

Abstract

AbstractLarge and highly significant curvilinear responses in number, weight, and leghemoglobin content of nodules of 5 soybean Plant Introductions to application of P were observed in outdoor pot experiments with 3 nutrient variables and uniform mixing of soil and fertilizers. P × Ca interaction effects were sometimes significant for nodule number and weight. Ca had a strong negative effect on leghemoglobin content. Significant varietal differences in number and weight of nodules and differential responses of nodule weight due to P, P2, and P × Ca were found at 2 stages of development. Between 2 stages of growth, the nodule weight of 1 line among 3 changed from lightest to heaviest, indicating a significant variety × time interaction. Maximum nodulation required very high levels of applied P and K salts. At rates of 400 to 500 pp2m P and 600 to 800 pp2m K, the predicted nodule weight at the end of flowering was tripled. Maximum nodule activity required even higher levels of applied P. The findings reported in this paper not only confirm the dominant role of P fertility for optimum nodulation of soybeans but give a better idea of how high the levels need to be and of the number and weight of nodules which may be produced under good growing conditions.