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Variable‐rate (VR) technology can be used to vary fertilization rates within a field. The objective of this study was to compare VR and uniform‐rate (UR) P fertilization for corn ( Zea mays L.)–soybean Glycine max (L.) Merr. rotations. Grid soil sampling (0.2‐ to 1.7‐ha cells), differential global positioning systems (DGPS), and grain yield monitors were used in strip trials established on six fields (12 site‐years). Three replicated P fertilization treatments were a control (no P) and a single application of the P requirement of the 2‐yr rotation based on soil‐test P (STP) using UR or VR. Measurements were plant dry weight (DW), P concentration (PC), and P uptake (PU) when crops were 15 to 20 cm tall; grain yield; and STP after crop harvest. Phosphorus increased grain yield ( P ≤ 0.05) of five crops, and the fields had initial mean STP ≤ 16 mg kg −1 (Bray‐P 1 or Mehlich‐3). Phosphorus increased plant DW, PC, and PU of five, six, and seven crops, respectively. Within each field, yield responses were observed only in areas with STP < 20 mg kg −1 and (or) areas with Clarion soil (fine‐loamy, mixed, mesic, Typic Hapludoll). The responses of plant DW, PC, and PU were not related to STP or soil series. The fertilization method did not influence ( P ≤ 0.05) crop responses to P. However, VR fertilization resulted in better P fertilizer management because it applied 12 to 41% less fertilizer and reduced STP variability compared with the traditional UR fertilization method.
Wittry et al. (Thu,) studied this question.