Fertilization Of Alfalfa Stands

DR. DIRK PHILLIP

FAYETTEVILLE, ARK.
   Alfalfa stands can produce high yields with several cuttings per year, but the right conditions have to be present to use the full potential of this forage cash crop. Fertilization is one of the most important aspects in crop production, as the high yields generated require nutrient imports on a large scale. Apart from following soil test recommendations, there are several factors that affect long-term nutrient use efficiency.
   The first step is selecting the proper site for a planned alfalfa field. Soils should be well drained, and soil samples should be taken after ground work is done to determine fertilizer and liming needs. NRCS soil maps and data should be consulted to find out how water infiltration and drainage rates might look. Even slight drainage problems can result in waterlogging during certain times of the year that leave uneven stands with weakened spots where weeds can encroach. This won’t be a problem with herbicide-resistant alfalfa, but bare spots with no soil cover will nevertheless remain. It is advisable to obtain an early soil test to judge liming requirements before any tillage or disking is done. Low pH values cannot be corrected in the short term. It will take at least six months or longer before changes in pH will be noticeable. The good thing is that with enough planning, lime can be incorporated while doing the tillage work and that can take place months in advance. Proper seed planting techniques are a must, obviously including the use of certified inoculated seed material adapted to your climatic region.
   Once the alfalfa is established, the quality of management, including fertilization, will determine longevity and yield. Here are few comments regarding the most important macro- and micronutrients:
   Nitrogen
   Alfalfa obviously fixes nitrogen, and if using inoculated seeds, N-fixation should occur reliably and at high rates. Research has shown that alfalfa is among the legumes with the highest N-fixing potential, and even very high-yielding stands apparently derive all N needed from N-fixation. Likewise, most research has shown that deliberately applying N will not increase yields and will sometimes even reduce yields. However, there are instances where record yields were achieved under high N fertilization and high irrigation amounts. For the eastern U.S., this scenario is of hypothetical value only.
   Potassium and Phosphorus
   Alfalfa removes large amounts of both P and K, about 15 and 60 pounds/ton harvested dry matter. What should not be overlooked is the fact that if soil test P and K are relatively low, high yields may not be achieved with just supplying predicted plant uptake amounts over the entire season. In soils with low P and K tests, plant uptake may be hindered because some of the fertilizer applied may be converted to unavailable forms in the soil.
   There are a few additional factors that affect nutrient uptake – soil pH, temperature, soil compaction, soil oxygen and general root health. It is well known that low or high pH makes P less available or even unavailable to plants. Low soil temperatures can lower P uptake; also important is that high soil compaction will obviously restrict root growth with the result that roots will not be able to reach beyond the immediate root zone to obtain additional P. If soil oxygen is low, P-uptake is impeded, but this risk can be minimized with proper site selection. This is also important for K, as plant-availability of this mineral is usually improved with improved drainage and aeration.
   Micronutrients
   The two minerals to which alfalfa responds the most are boron and copper. Other than P and K, it is not useful, however, to “build up” these elements in the soil, as only small amounts are needed by the alfalfa plant. It should be noted that micronutrient availability is reduced above a pH of 7, so monitoring pH values will save money in the long run. Also, soil test results may not exactly indicate what the plant actually needs. Some practitioners, therefore, recommend plant tissue analysis to pin down the actual need for micronutrients. Also, correcting deficiencies of micronutrients will be difficult if macronutrients are not supplied to alfalfa in the required amounts. Regardless of the size of your dairy operation, well-organized calf management will optimize the health and productivity of your calves. ∆
  DR. DIRK PHILIPP: Assistant Professor, University of Arkansas