Wheat and Forage Sorghum Response to Residual Phosphorus in Blackland Soils 1

Phosphorus fertilizers are generally applied for crops grown on Blackland Prairie soils of Texas, but the crop yield response due to residual P from single applications of P is unknown. Studies were conducted over a 5‐yr period (1977 to 1981) on Austin silty clay (fine‐silty, carbonatic, thermic Entic Haplustolls) and Dalco clay (fine, montmorillonitic, thermic Udic Pellusterts) soils to evaluate the residual effects of P on subsequent crops. Fertilizer rates of 0, 25, and 50 kg P ha −1 were applied in 1977,1978, and 1979 at three sites with soil P levels of 6,10, and 13 mg kg −1 NaHCO 3 ‐extractable P (0 to 15 cm depth). Following the initial crop year, plots were split to include 25 kg P ha −1 with the seed. The other half received no fertilizer P other than the initial P application. The test crops were wheat ( Triticum aestivum L.), forage sorghum ( Sorghum bicolor L. Moench), and sweet clover ( Melilotus albus Lam.). A significant yield increase due to initial P application was obtained at each site. Crop yield increases due to residual effects of the 50 kg P ha −1 rate ranged from 0 to 20%, while yield increases due to 25 kg P ha −1 at planting ranged from 19 to 43%. Percent yield increase from residual P was greater on soil with 6 mg kg −1 soluble P than on soil with 13 mg kg −1 soluble P. Phosphorus concentrations in wheat plants were increased by residual P, but were much higher in plants receiving P at planting. These data indicate that some yield increase can be expected from residual P from a single application of moderate amounts of P. For more efficient use of P and high yields, P should be applied to each crop if initial NaHCO 3 ‐extractable P levels are between 6 and 13 mg kg −1 .