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Six Years of Phosphorus and Potassium Fertilization of Irrigated Alfalfa on Calcareous Soils¹

ABSTRACT

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.

¹ Contribution from the Dep. of Agronomy, Colorado State Univ. Exp. Stn., Ft. Collins, CO 80523, and published as Scientific Series Paper no. 2843. This research was supported in part by the Agric. Research Branch, Tennessee Valley Authority, Muscle Shoals, AL 35660 and the Colorado Exp. Stn.

² Former Graduate Research Assistant; Professor, Dep. of Agronomy, Colorado State Univ.; Associate Professor, Fruita Research Station, Colorado State Univ., respectively. Senior author is currently Assistant Professor, Univ. of Nebraska, Panhandle Station, Scottsbluff, NE 69361.

Received for publication June 8, 1983. Accepted for publication October 20, 1983.

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