Phosphorus Management of Lucerne Grown on Calcareous Soil in Turkey

Lucerne or alfalfa (Medicago sativa L.) is grown as a forage crop on many livestock farms. In calcareous soils in eastern Turkey, lucerne production requires phosphorus (P) additions as the soils are naturally P deficient. Phosphorus sorption isotherms were used to estimate P fertilizer needs for lucerne grown for two years in a 3-cut system on a calcareous P deficient Aridisol in eastern Anatolia, Erzurum province, Turkey. Annual P applications ranged from 0-1200 kg P ha-1. The Langmuir two-surface adsorption equation was used to derive the maximum P sorption capacity of unamended soil and to determine soil solution P, maximum buffer capacity (MBC), equilibrium buffer capacity (EBC), and P saturation at the optimum economic P rate (OEPR) for dry matter (DM) production. Soils were tested for Olson P at the onset of the study and after two years of P applications. In both years, tissue was analyzed for P content at flowering prior to first cutting. The OEPR (2-year average) was 754 kg P ha-1 yr-1 corresponding with a soil solution P concentration of 0.30 mg L-1, a DM yield of 8725 kg DM ha-1, and $528 ha-1 annual profit. The P content of leaves at flowering increased linearly with P application beyond 100 kg P ha-1 and was 3.2 g kg-1 P at the OEPR. The unfertilized soil had an EBC, MBC, P saturation, and Xmax of 3304 mL g-1, 3401 mL g-1, 6%, and 1086 mL g-1, respectively, whereas two years of fertilization to the OEPR decreased EBC and MBC to 358 mL g-1 and 540 mL g-1, and increased P saturation and Olsen P to 56% and 32 mg kg-1, respectively. These results suggest a P saturation 50% or Olsen P 30 mg kg-1 are needed to maintain an optimum soil solution concentration of 0.30 mg L-1 in this calcareous Aridisol. Similar studies with different soils and initial soil test P levels are needed to conclude if these critical soil test values can be applied across the region.