POTASSIUM MANAGEMENT IN ALFALFA: A SUMMARY OF EIGHT YEARS OF RESEARCH IN AN ARID ENVIRONMENT

Alfalfa is an important forage and cash crop in the Western U.S. In 1995, a long-term research program was initiated at Utah State University to evaluate phosphorus (P), potassium (K), and sulfur (S) fertilizer recommendations for irrigated alfalfa. The purpose of this paper is to summarize results from the K research. Experiments were conducted at three locations with low soil test K levels (49 to 105 mg/kg). Potassium was applied at rates ranging from 0 to 300 or 400 lb K2O/acre primarily in single spring applications although some split spring/in-season applications were made. Hay yields and soil test and tissue K were measured. Yield responses to K were nil or linear. Linear responses suggested a return of 1 ton of hay per 350 to 960 lb K2O applied, depending on location and year. Soil test K was not increased above 123 mg/kg at any location; therefore, additional studies with rates up to 600 lb K2O/acre were conducted. Responses to the higher rates were linear for two site-years and quadratic for two site-years. Splitting applications of the 600 lb K2O/acre rate resulted in higher yields than single applications at the sites with quadratic responses. The relationship between soil test K and relative alfalfa yield suggested a critical soil test K of 150 mg/kg for maximum yield. While not presented in this paper, tissue concentration did not relate well to relative alfalfa yield due to variations in tissue K concentration among harvests. Additional sampling showed that the time of year and depth at which soil samples were collected significantly affect the soil test value and resulting fertilizer recommendation. Once depleted, relatively high rates of K are necessary to rebuild low K-testing soils. Soil testing can be a reliable tool to diagnose a deficiency; however, to ensure accurate results, sampling and analytical methods must be the same as those used to develop the recommendation database