Impacts of Thirty‐Seven Years of Stocking on Soil Phosphorus Distribution in Bermudagrass Pastures

Abstract

Pasture management affects soil properties, but information concerning the effects of long‐term stocking and fertility regimens on soil P in pastures is limited. This study investigated changes in extractable soil P concentrations from Coastal and common bermudagrass [Cynodon dactylon (L) Pers.] pastures stocked for 37 yr at different intensities on an Ultisol. From 1969 through 2004, pastures were continuously stocked during February to September to achieve high, medium, and low herbage mass. Annual fall‐applied P averaged 44 kg ha–1 from 1969 to1984 and 51 kg ha–1 from 1998 to 2004. Extractable soil P concentrations in the 0‐ to 15‐cm depth increased (P ≤ 0.0001) from 5 mg kg–1 (very low) in 1975 to 43 mg kg–1 in 2004 (high), and deeper depths showed 14 to 36% increased P levels. Elimination of P fertilizer for 13 yr, 1985 to 1997, decreased (P ≤ 0.0001) extractable soil P in the 0‐ to 15‐cm depth by 30 and 47% in common and Coastal bermudagrass pastures, respectively. An interaction between year and stocking rate was evident as extractable P levels increased after 20 yr at high stocking rates. After 37 yr, extractable P levels at the 0‐ to 120‐cm soil depth followed the order high > medium ≥ low stocking rate. Despite the effect of stocking rate on soil P, 37 yr of continuous stocking showed no evidence of excessive P accumulation at levels that may pose an environmental threat. Phosphorus recycled through the soil–forage–animal system sustained adequate forage and animal productivity.