ABSTRACT
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.
ACKNOWLEDGMENTS
We are very grateful to Atatürk University Scientific Research Projects Foundation for their generous financial support for this study (Project Number BAP 2005/33).