Potassium fertilization effects on alfalfa in a Mediterranean climate

Potassium fertilization rates for alfalfa (Medicago saliva L.) have been in creasing with intensive cropping systems or decreasing with policies that g enerally lead to reduced fertilizer inputs. In this case, nutrient buildup or maintenance of high soil test levels may not be desirable and drawdown o f K reserves may be beneficial in the short term. The objective of this res earch 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 Medite rranean environment of the Ebro Valley (Spain) on a silty clay loam soil. T he treatments were five annual rates of K (0, 41.5, 83, 166, and 332 kg K h a(-1)) and two rates of K (166 and 332 kg K ha(-1)) applied prior to seedin g 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-c) increased litt le 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-c values did not c hange appreciably, suggesting that much of the K uptake was derived from th e fertilizer and from nonexchangeable soil K fractions. Although K fertiliz ation slightly increased alfalfa DM yields in this high testing Mediterrane an soil, the economic benefit of this limited response does not justify the expense.