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SOIL FERTILITY

Potassium Fertilization Effects on Alfalfa in a Mediterranean Climate

UdL-IRTA, Av. Rovira Roure 177, 25198, Lleida, Spain

Corresponding author (jaume.lloveras{at}irta.es)

Potassium fertilization rates for alfalfa (Medicago sativa L.) have been increasing with intensive cropping systems or decreasing with policies that generally lead to reduced fertilizer inputs. In this case, nutrient buildup or maintenance of high soil test levels may not be desirable and drawdown of K reserves may be beneficial in the short term. The objective of this research was to evaluate the effects of potassium fertilization of alfalfa in areas of high soil exchangeable K levels and long growing seasons. A field experiment was established under irrigation from 1993 to 1997 in the Mediterranean environment of the Ebro Valley (Spain) on a silty clay loam soil. The treatments were five annual rates of K (0, 41.5, 83, 166, and 332 kg K ha^-1) and two rates of K (166 and 332 kg K ha^-1) applied prior to seeding on two alfalfa cultivars. The average annual dry matter (DM) yield was 21.5 Mg ha^-1 and showed a small linear response to K fertilization (Pr > F = 0.0589). Total K removal in the herbage increased linearly with each rate of K and reached 1728 kg K ha^-1 with the application of 332 kg K ha^-1 yr^-1, compared with 1546 kg K ha^-1 without K fertilization. At the end of the experiment, soil ammonium acetate extractable K (K_e) increased little with K rates, and the differences were observed only in the first 30 cm of depth. Despite the uptake of 1546 kg K ha^-1, soil K_e values did not change appreciably, suggesting that much of the K uptake was derived from the fertilizer and from nonexchangeable soil K fractions. Although K fertilization slightly increased alfalfa DM yields in this high testing Mediterranean soil, the economic benefit of this limited response does not justify the expense.

Abbreviations: DM, dry matter • K_e, soil ammonium acetate extractable potassium