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Phosphorus and Potassium Fertilization of Irrigated Alfalfa on Calcareous Soils: I. Soil Test Maintenance Requirements¹

ABSTRACT

Field experiments with irrigated alfalfa (Medicago sativa L.) were established in far eastern Colorado on a Keith soil and in far western Colorado on a Ravola soil in 1976. Concentrated superphosphate was applied at rates of 25 and 50 kg P/ha annually for 3 years or 75 kg P/ha in a single application at seeding. Muriate of potash was applied at rates of 140 and 280 kg K/ha annually for 3 years or 420 kg K/ha in a single application at seeding. Soil profile changes in NaHCO₃-P and exchangeable K were determined. At a location having low to medium available-P status (crop response to P expected), 2.2 times the P removed by the alfalfa was required to maintain the initial NaHCO₃-P level. At a location with medium to high available-P status (no P response expected), only 1.4 times the removed P was required for maintenance. The two locations required a fertilizer K rate of 0.75 and 0.22 times the K removed by alfalfa uptake to maintain their respective initial exchangeable-K levels. The low maintenance requirement of the latter soil was apparently due to the K minerals present which buffered the exchangeable K near 100 mg·kg^–1.

¹ Published as Scientific Series Paper no. 2704 of the Colorado State Univ. Exp. Stn. This research was supported in part by the Soils and Fertilizer Research Branch, Tennessee Valley Authority, Muscle Shoals, AL 35660.

² Former Graduate Research Assistant and former Professor, Dep. of Agronomy, Colorado State Univ., Fort Collins, CO 80526. Authors are presently Assistant Professor, Dep. of Pl. Sci., South Dakota State Univ., Brookings, SD 57007, and Western Director, Potash & Phosphate Institute, Davis, CA 95616.

Received for publication December 10, 1981. Accepted for publication September 16, 1982.

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