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

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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.
Funding Information
  • Tennessee Valley Authority