Nitrogen, Sulfur, Potassium and Phosphorus Fertilization in Alfalfa When Are They Necessary?

Determining when applications of nitrogen (N), sulfur (S), potassium (K) and phosphorus (P) fertilizer are needed is important to maintain alfalfa yield and quality. The Western U.S. is diverse in terms of climate, soil types, irrigation water need and availability, historical nutrient use and alfalfa production systems. Therefore, few generalizations hold regarding the timing and location of N, S, K and P deficiencies. Soil and tissue testing are the most reliable ways to assess alfalfa nutrient status and determine fertilizer needs. Soil test extractants and interpretations vary throughout the West. Growers must understand what soil test methods (mainly sampling depth and chemical extractants) are appropriate for their area and use interpretations (recommendation tables) developed for those methods. Tissue testing procedures and interpretations are more similar throughout the West, but growers must still match tissue sampling protocols (mainly growth stage and plant part) with the interpretation data. In this paper, summary information on critical soil and tissue test levels is presented, as well as recent alfalfa fertilizer response data from the Western U.S.