On farm evaluation of preplant soil test P and K in double-crop soybeans

Many producers in the southern Great Plains choose to plant summer crops immediately following the harvest of winter crops to increase farm revenue. The practice, referred to as double cropping, is often done with limited inputs to reduce economic risk. The study aims to evaluate soil testing as an indicator of a nutrient response in double-crop (DC) soybean [Glycine max (L.) Merr.] across Oklahoma. Nutrient-rich strips of phosphorus (P) and potassium (K) were applied at 49 sites across Oklahoma in 2016 and 2017. Soil test values across all sites of extractable P and K ranged from 3 to 96 and 56 to 374 mg kg(-1), respectively. At crop maturity, subplots were hand harvested from each strip as well as from the farmer practice strip (FPS) collected outside of the plot area. Over half (58%) of the total comparisons throughout this study were noted as those that could have positive yield response: 44 locations for P and 13 for K. Soil test results and critical values were most accurately able to correctly identify the locations that would not respond to the additional nutrient inputs, as out of 98 total comparisons, only seven unexpectedly yielded a significant response. Predicting response was found to be especially difficult, as only six of the 57 comparisons expected to be responsive yielded a response. Unpredictability of responses could be attributed to low-yielding environments as well as the complexity of soil nutrient pool and plant availability relationship. This work highlights the challenges in predicting the response to nutrients in a DC system based upon soil test analysis.