ALFALFA CULTIVAR RESPONSES TO PHOSPHORUS AND POTASSIUM-DEFICIENCY - BIOMASS

Soil phosphorus (P) fertility needs for alfalfa (Medicago sativa L.) i n the intermountain western United States have been a concern since th e inception of irrigation during the late 19th century. Soil potassium (K) fertility needs have received much less attention because K defic iencies, related to long term use of high quality, low-K irrigation wa ter, have developed only in recent years. The objectives of this resea rch were to compare the responses of five commercial alfalfa cultivars (CVs) to phosphorus (P) and K deficiency and to examine the interacti ons between CVs and P and K soil fertility. An experiment was establis hed on a highly calcareous Redfield silt loam soil (Fine-silty carbona tic, mesic Xeric Torrifluvents) that was low in both P and K. The CVs, planted in 1988, were studied through 1993. A 3x3 complete factorial combination of P and K fertilizer rates was established in a split plo t design where CV was the whole plot. There was a significant CV x yea r interaction in biomass production that was not related to diseases o r insects. There was also a significant CV x P interaction within and among years. Response to K fertility, though significant, was not prom inent in this trial. Alfalfa CV performance trials should segregate CV s best adapted to short term (2-3 year) rotations from those best adap ted to long term (4-5 year) rotations. Fertilizer effects on low-K cal careous soils are short lived; moderate rates of K fertilizer should b e applied each spring. Soil P requirements of alfalfa can be met by ap plying small rates annually or by applying enough for a 4- to 5-year r otation. phosphorus-use efficiency by alfalfa may be improved by plant breeding and selection for this trait.