Impacts of Variable‐Rate Phosphorus Fertilization Based on Dense Grid Soil Sampling on Soil‐Test Phosphorus and Grain Yield of Corn and Soybean

Most agricultural fields have high soil‐test phosphorus (STP) variability. Variable‐rate (VR) technology facilitates application of different P rates over a field and could improve nutrient application and crop yield. Replicated strip trials (6–12 ha) were established at six Iowa fields and were evaluated during 4 yr to compare VR and fixed‐rate (FR) P fertilization for corn ( Zea mays L.)–soybean ( Glycine max L. Merr.) rotations. Fields had median Mehlich‐3 STP ≤ 20 mg P kg −1 , although minimum and maximum values within each field were 4 to 18 and 22 to 62 mg P kg −1 , respectively. Treatments replicated at least three times were a control, VR based on STP from grid sampling (0.06‐ to 0.08‐ha grids), and FR based on median STP. Treatments were applied with commercial spreaders and grain was harvested with combines equipped with yield monitors and global positioning systems (GPS). Phosphorus increased yield in 13 site‐years and application methods differed in 1 site‐year, when FR increased yield further. On average, VR applied 12.4% less P and reduced STP variability in five fields compared with FR. Semivariograms and SD showed that fertilization, especially VR, often reduced yield variability and seldom increased it. High STP variability at a small scale and P recommendations to maximize yield and buildup STP in low‐testing soils might explain a lack of yield differences between application methods. Although VR did not increase yield compared with FR, it managed P better and showed potential for reducing excess P loss from fields through reduced P application to high‐testing field areas.