Phosphorus fertilization of alfalfa on Coastal Plain soils

Abstract

Grazing tolerant varieties of alfalfa (Medicago sativa L.) are being introduced to improve the quality of pastures in the southern Coastal Plain. 'Alfagraze' alfalfa was planted on eight soils near Overton, Texas to determine the P requirement of the alfalfa and to assess the suitability of the five Ultisols, two Alfisols and one Inceptisol for alfalfa production. In this study, 0, 56, 112, and 168 kg P₂O₅ ha⁻¹ were applied preplant and annually for three years on Bowie, Cuthbert, Darco, Keithville/Sawtown, Kirvin, Lilbert, Redsprings, and Thenas soils. Soil samples were collected to a depth of 15 cm and to 1.8 m in 30-cm increments prior to site establishment and annually to a depth of 15 cm. Alfalfa was mechanically harvested from the center one-third of each treatment plot three times in 1993, and five times in 1994 and 1995. Phosphorus addition resulted in significant increases in extractable soil P in two of the eight soils in 1993, four of the eight soils in 1994, and six of the eight soils in 1995. Phosphorus addition resulted in significant increases in alfalfa dry matter yield on three of the four soils originally lowest in NH₄OAc-EDTA extractable soil P. The critical extractable soil P concentration below which alfalfa yield increases would be expected to follow P addition was 19 mg P kg⁻¹. One set of soil samples taken to a depth of 15 cm in December 1994 was analyzed for extractable soil P by the Bray P1, Mehlich III, Olsen, Texas A&M, and two resin methods. Each of the six methods detected significant changes in extractable soil P in three of the four soils originally lowest in extractable soil P. Increasing alfalfa forage P concentration resulted in significant yield increases on one soil in 1993, two soils in 1994, and four soils in 1995. The estimated critical alfalfa tissue P concentration was 2.8 g P kg⁻¹. Total alfalfa dry matter yield for the three study years also varied significantly with the potential rooting depth to Al-toxic subsoil on the eight soils studied. Yields declined significantly as depth to Al-toxic subsoils decreased.