Fertilizer Placement Effects on Soybean Seed Yield, N2 Fixation, and 33P Uptake1

Various recommendations on placement of fertilizer for soybeans [Glycine max (L.) Merrill] have been made (banded near the seed or midway between the rows and surface broadcast and left on the surface or incorporated). Because P is an immobile element in the soil, placement may be critical. This study was planned to evaluate the efficiency of P fertilizer utilization by field‐grown soybeans with different fertilizer placements using 33P labelled fertilizer (35 kg P/ha) on a Waukegan silt loam soil (Typic Hapludoll) low in plant available N and P. In addition, urea (30 kg N/ha) labelled with 15N was added to measure the effect of P fertilizer placement on N uptake and N2 fixation using the ‘A’ value concept. The ‘A’ value is a mathematical expression for calculating the amount of available nutrient element in a soil in terms of a fertilizer standard provided that the proportion of the nutrient in the plant derived from this standard is determined (usually by isotope labelling). Fertilizer placement was evaluated in the field by using unlabelled urea N and P (concentrated superphosphate) in plot areas 2.25 × 10 m; a sub‐plot within the larger plot received applications of 15N labelled urea and 33P labelled concentrated superphosphate. Total plant samples for 15N and 33P analysis were obtained with special precautions to recover all leaves. Analysis of 15N was performed with photospectrometry methods; 33P was measured using a liquid scintillation technique. Soybean seed yield and total P uptake were increased significantly by adding P fertilizer with no differences among the fertilizer placements (seed yield ranged from 3,811 to 4,035 kg/ha compared to 3,200 kg/ha for the control). The 33P labelled fertilizer provided a direct, quantitative measure of the efficiency of plant use of the added P and provided a basis of evaluating the fertilizer placements without the hazards associated with 33P over the 149‐day experiment. The ‘AN’ value concept (based on ‘A’ value procedures using a nodulating and nonnodulating crop) provided a reliable estimate of N2 fixation which agreed with Kjeldahl measurements.