Soybean planting date affects the relationships between soil test values and grain yield

Many soybean [Glycine max (L.) Merr.] growers in the US Midwest rely on soil test values for evaluating the crop's fertilizer needs. However, threshold values for Illinois were calibrated to soybean yield in the 1960s when the production systems and yield potential were much different than today. The objective of this study was to determine which and how well soil test values predict yield of unfertilized soybean. Preplant soil samples, collected from 133 trials across Illinois from 2014 to 2021, were analyzed for 14 chemical attributes and compared to unfertilized soybean grain yields from those same studies. Pearson correlation coefficients (r), principal factor analysis, and latent variable regression models were used to determine those soil attributes most closely associated with grain yield and yield components. The association of planting date and yield (r = −0.56) led to dividing the data set into five planting date groups. Soil fertility levels resulted in a strong correlation with yield for Late or Very‐late planting groups, but not for the Early or Very‐early groups. A factor analysis of soil attributes largely resulted in retention of two factors, identified as Soil Organic Charge and Soil Fertility, across the planting‐date groups. Regression of these factors with yield confirmed that soil fertility had a greater influence on grain yield for late‐planted soybeans than early‐planted and that these differences were associated with average seed weight. Therefore, positioning late‐planted soybeans in higher fertility fields and early‐planted soybeans in lower‐fertility fields could reduce the need for supplemental fertilization.