Soybean Growth and Yield Responses to Starter Fertilizers 1

On highly fertile soils, increases in plant growth resulting from starter fertilizers, applications generally do not occur unless root growth is restricted or temperatures are cool. The use of some conservation-tillage systems may result in restricted root growth and cooler soils at planting. The purpose of this 3-yr field study was to determine if growth and yield of soybeans [Glycine max (L.) Merr.] planted in a conservation-tillage system would be increased by starter fertilizers, and if interactions occurred between residual fertility levels and starter fertilizers. Treatments included residual fertility levels (low P and K, low P and high K, high P and low K, and high P and K) and starter fertilizers (no starter, N, P, K, N-P, N-K, and N-P-K). The N, P, and K starter fertilizer rates were 16, 19, and 44 kg ha⁻¹, respectively. Raising both residual P and K to the high soil test rating improved soybean yields 43%. The N-containing starter fertilizers, especially those also containing P, increased early season plant root and top weights, but the K starters did not. However, the P, K, N-K, and P-K starter fertilizers had the greatest effect on yields; when compared to the control, they increased yields 46% when residual P, K, or both were low, and 26% when the residual P and K were high. The N, N-P, and N-P-K starter fertilizers increased yields 21% when residual P was low, but had no effect on yields when P was high. Seed quality did not vary among residual fertility levels, but was improved by the starter fertilizers, especially those containing K. The P and K concentrations in the leaves just prior to pod set corresponded to differences in yields among residual fertility levels, but not to differences among starter fertilizers.