Do Soil Tests Predict Yield Response to Phosphorus in Modern Corn and Soybean Production?.

Modern corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production systems produce higher yields, and consequently, may remove more nutrients than fertilizer rates based upon existing soil test recommendations for these yields. With greater emphasis on soil test levels for nutrient stewardship strategies, soil tests need to also predict critical levels in which fertilized nutrient responses are likely. Current soil test recommendations for Illinois have negligibly changed since they were published in the 1960’s. Since then, there has been an advent of improved fertilizer application technologies such as banding fertilizer beneath the row, modern genetics that have prolonged nutrient uptake and are capable of higher yields, and new fertilizer technologies that provide season-long nutrient availability. Therefore, our objective was to determine if the existing soil test level recommendations for phosphorus (P) are adequate predictors of the yield response. Twenty nine trials were conducted in corn and 31 trials in soybean between 2011 and 2015 across multiple locations in Illinois using modern corn and soybean production systems. All corn and soybean trials received 112 kg and 84 kg P₂O₅ per hectare respectively employing an enhanced phosphate fertilizer MicroEssentials^® SZ^TM (12-40-10S-1Zn) banded 10-15 cm directly underneath the crop row prior to planting. Yield responses to fertilizer from the unfertilized controls were compared to soil test values. Soil test values ranged from 16 to 164 ppm and 14-126 ppm by Mehlich III extraction for corn and soybeans while yield responses ranged from 1-16% and 0-26% for each crop, respectively. Yield responses greater than 5% occurred more than 48% of the time in soybeans and 59% in corn regardless of the soil test level. Soil test values had poor correlations at predicting yield responses when modern P fertilizer application technologies were utilized.