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Fluid Journal : Fluid Journal 2002-2004
Fertilizer guides from most western states recommend that P be applied and incorporated before establishing an alfalfa crop. Since P is relatively immobile, placement below the soil surface improves root access and prevents P from being stranded on the surface of dry soil. When P needs are identified for established alfalfa, application as soon as possible is recommended. The effect of P fertilizer on alfalfa yield versus control at two Utah locations is shown in Figure 1. Potassium Potassium deficiency is a relatively recent occurrence in the West. A long history of high-yielding alfalfa production has depleted native soil K levels to the extent that most western soil testing labs now encourage testing for K, and make K fertilizer recommendations. Soil test extractants for K include sodium bicarbonate ("Olsen" extract), ammonium bicarbonate, and ammonium acetate. Soil test procedures for K, like P, are reliable indicators of deficiencies and fertilizer requirements. Recent research at Utah State University shows alfalfa yield responses to relatively high K application rates on low K-- testing soils (Figure 2). Due to the high K requirements of alfalfa, frequent and relatively high rates of K fertilizer will be required to maintain high yields once native soil levels are depleted. Sulfur Sulfur is an important component of several amino acids and has been shown to influence the yield, protein Figure 2. Effect of K rate on alfalfa field. Split applications consisted of 200 lbs/A of K2O in April, plus 200 lbs/A after first and second cuttings (600 lbs/A of K2O total). content, stand density, and stand life of alfalfa. Yield responses of up to 300 percent have been reported under severe S deficiency conditions; however, more typical yield responses to S fertilization are in the range of 15 to 25 percent and highly profitable. Soils at many high elevation locations have been found to be low in S. High precipitation, coupled with low irrigation water S concentrations and a history of high-yielding alfalfa production, explains the occurrence of these deficiencies. A recent comparison of S applications at two locations in Utah showed responses on the order of 16 to 20 percent above an unfertilized control (Figure 3). Initial soil test SO4-S ranged from 3 to 3.5 ppm. Managing The role and importance of on-farm testing and record keeping cannot be overstated when developing an alfalfa fertility program. Different soils respond differently to fertilization in terms of the initial and long-term impacts on soil test levels and crop response. Also, different yield levels will require different amounts of fertilizer inputs over time. An on-farm test might involve determining whether or not alfalfa responds to N, or whether or not a higher rate of P or K will produce higher yields. Multiple strips are useful when comparing new practices in one strip and standard practices in another. Keep individual records of fertilizer applications, soil and tissue test values, and alfalfa yield to assess whether or not the new practice improved yield and quality, changed soil test values, and improved profitability. Dr. Koenig is extension soils specialist in the Department of Crops and Soils, Washington State University, Pullman, WA. Early Spring 2004 Fluid Journal 2 Figure 3. Effect of S on alfalfa yield at two locations, Cache and Garfield.
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