Six Years of Phosphorus and Potassium Fertilization of Irrigated Alfalfa on Calcareous Soils

Long-term irrigated alfalfa (Medicago sativa L.) field experiments at two locations in Colorado [Keith clay loam (Aridic Argiustolls) and Ravola loam (Typic Torrifluvents)] were established to (i) determine the magnitude and frequency of P and K fertilization required to maintain initial soil tests, and (ii) evaluate the residual value of soil and fertilizer P and K. Ten P and K treatments (3 annual, 1 triennial, and 1 initial application) were used. Alfalfa yield, P and K uptake, NaHCO₃-extractable P, and NH₄C₂H₃O₂-exchangeable K were measured over 6 production years. Soil test P declined in both soils to levels where significant yield responses to fertilizer P were expected, but none were observed. Organic P mineralization and/or an extensive mycorrhizal association were suggested as contributing factors to P availability. Generally, application of 50 kg P ha⁻¹ maintained initial NaHCO₃-P at both locations, while soil P declined below the initial level for all other P rates. Phosphorus uptake was significantly increased by P fertilization on the Keith soil, due to lower NaHCO₃-P level, but not on the Ravola soil. Soil test K remained at high levels in both soils and no yield responses to fertilizer K were observed. Application of 380 and 5 kg K ha⁻¹ were calculated to maintain initial NH₄C₂H₃O₂-K in the Keith and Ravola soils, respectively. The low maintenance requirement of the Ravola soil was due apparently to a high nonexchangeable K release rate which buffered exchangeable K near 100 mg kg⁻¹. Potassium uptake was significantly increased by K fertilization in the Ravola soil, due to a lower exchangeable K level, but not in the Keith soil. The results indicate very large P and K reserves exist in both soils and unless improved forage quality influences the economic benefit from P and K fertilizer application, fertilization of these soils would not be recommended.