Soybean Yield Response to Phosphorus Fertilization in an Oxisol under Long-Term No-Till Management

Phosphorus (P) fertilizer recommendations for crops must be improved to optimize crop yields and minimize the environmental impact of P. We evaluated the effects of continuous annual fertilization (double crops) on soil-P availability by Mehlich-1 (PM-1) and resin (P-resin) plant uptake, P responsive concentrations in the soil and soybean trifolioliate leaflets and the yields of two soybean (Glycine max L.) cultivars (BRS1010 and BRS1003) grown in a Brazilian Oxisol under 20 yr of no-till (NT) management. Four treatments representing a combination of annual P rates (0, 20, 40, and 50 kg P ha(-1)) with a fixed K rate (40 kg ha(-1)) were evaluated. Resin-extractable P was highly correlated with PM-1 (0.86, p < 0.05) but P-resin was nearly 2.5 times as much as PM-1. The soil extractable P was linearly correlated with P contents in trifoliolate leaflets with r of 0.72 and 0.66 (both at p < 0.05) for PM-1 and P-resin, respectively. Maximum responsive concentrations of soil P(M-1) and P-resin for the year studied were in the range of 13 +/- 2.2 to 20 +/- 5.7 and 30 +/- 5 to 59 +/- 19.2 mg P dm(-3),respectively. Soybean yields increased 29% as the rates of annual-P rate increased from 0 to 20 kg ha(-1) and additional P application did not further improve yields. The trifoliolate leaflet P concentrations of both cultivars were correlated (r = 0.66, p < 0.05) with soybean yields. The maximum responsive concentration of P in the soybean trifoliolate was 2.8 +/- 1.9 to 5.4 +/- 2.8 g kg(-1) at R2 stage. Our results confirm that soil test for plant available P and plant tissue test are important to guide P fertilizer recommendations.