Effects of Phosphorus and Potassium on Soybean Nodules and Seed Yield

Much of the nutritional research on soybean [Glycine max (L.) Merrill] nodulation has been under greenhouse or laboratory conditions. Additional field data are needed on the influence of nutrients on nodulation and other plant characteristics. The purpose of this investigation was to determine under field conditions the effects of P and K fertilization on number and weight of soybean nodules; the chemical composition of leaves, nodules, and seed; the number of pods per plant; and seed yield. Annual P rates of 0, 15, 30, and 60 kg/ha were applied with 0 and 112 kg K/ha in one field experiment. In another, annual K rates of 0, 28, 56, and 112 kg/ha were applied with ~0 and 60 kg P/ha. Either P or K applied alone increased the number of nodules per plant and per unit volume of soil. Applied K increased the number of nodules, total and individual weight of nodules, and the number of pods per plant more than P, but increases were largest when both P and K were applied. Applied K increased the K concentration of nodules, but had little influence on N, P, Ca, Cu, Fe, Mn, and Zn concentration, and decreased Mg; applied P without K increased the P concentration. There was a good correlation between the relative concentrations of P and K in leaves sampled for each of four different sampling dates and the concentration of these elements in the seed. Applied P in excess of 15 kg/ha did not increase soybean yields. Each additional increment of K up to the maximum used (112 kg/ha) resulted in seed yield increases when P was also applied, but with no added Pseed yield responses to more than 28 kg K/ha were not significant. These results showed that application of both P and K individually increased nodulation and pod formation with more response from K than P. Maximum response was obtained when both elements were added. Even though the soil was very low in available P when the experiment was initiated there was no seed yield response to more than 15 kg P/ha. A good yield response was obtained from 28 kg K/ha and to increasing rates of K when 60 kg P/ha was added, thus indicating the higher requirement of soybeans for K than for P.