Alfalfa cultivar responses to phosphorus and potassium deficiency: Biomass

Soil phosphorus (P) fertility needs for alfalfa (Medicago sativa L.) in the intermountain western United States have been a concern since the inception of irrigation during the late 19th century. Soil potassium (K) fertility needs have received much less attention because K deficiencies, related to long term use of high quality, low‐K irrigation water, have developed only in recent years. The objectives of this research were to compare the responses of five commercial alfalfa cultivars (CVs) to phosphorus (P) and K deficiency and to examine the interactions between CVs and P and K soil fertility. An experiment was established on a highly calcareous Redfield silt loam soil (Fine‐silty carbonatic, 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 plot design where CV was the whole plot. There was a significant CV x year interaction in biomass production that was not related to diseases or insects. There was also a significant CV x P interaction within and among years. Response to K fertility, though significant, was not prominent in this trial. Alfalfa CV performance trials should segregate CVs best adapted to short term (2–3 year) rotations from those best adapted to long term (4–5 year) rotations. Fertilizer effects on low‐K calcareous soils are short lived; moderate rates of K fertilizer should be applied each spring. Soil P requirements of alfalfa can be met by applying small rates annually or by applying enough for a 4‐ to 5‐year rotation. Phosphorus‐use efficiency by alfalfa may be improved by plant breeding and selection for this trait.