Should starter fertilizer rate be adjusted by initial soil test P concentration?

Starter fertilizers containing phosphorus (P) are applied to increase corn (Zea mays L.) early growth and ultimately grain yield. This study determined the rate of starter P needed to increase corn early plant growth and grain yield at differing starting soil test phosphorus (STP) concentrations with or without broadcast P application. Field trials were established at 10 site-years in Minnesota using a split-plot design. Main blocks consisted of 0 or 59 kg P ha⁻¹ broadcast pre-plant. Sub-plots consisted of liquid starter fertilizer (10-15-0 N-P-K): 0, 29, 58, and 87 kg ha⁻¹ applied on the corn seed. Analysis was conducted across sites after classifying each block per site (low, medium, high, and very high STP) according to University of Minnesota guidelines. Corn plant mass and P uptake at V5–V7 increased linearly as the rate of starter P regardless of where broadcast P was applied and initial STP concentration. Corn yield was increased by P when STP was in the low or medium STP classification, and application of starter P alone did not maximize grain yield in low P soils. The data indicate that 29 kg ha⁻¹ of the starter applied in this study is sufficient to increase early plant growth and corn yield compared to broadcast P only when STP was medium or higher. Broadcast P is needed to maximize yield when STP was low, and varying starter rates more than 29 kg ha⁻¹ will not result in a greater yield potential across sites versus broadcast P alone regardless of STP concentration.