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Fluid Journal : Fluid Journal 2002-2004
Early Spring 2004 Keep Close Watch on NPKs Fertilization of Alfalfa Dr. Richard Koenig Summary: Few studies have found positive effects of nitrogen (N) fertilization on established alfalfa. Situations where alfalfa may respond to N include poor or ineffective nodulation due to use of a wrong, old or damaged inoculant, and stresses that can reduce nodulation or bacterial activity. Phosphorus (P) is the most common fertilizer input for alfalfa across the Western U.S. It is essential for optimum alfalfa production. A long history of high- yielding alfalfa production has frequently depleted native soil potassium (K) levels to the extent that most Western states now encourage testing for K. Yield responses to sulfur (S) in alfalfa of up to 300 percent have been reported where there is severe soil S deficiency. lfalfa is the most common legume forage crop grown in the Western States. As recently as 1997, over 32 million tons of alfalfa was produced on approximately 7.4 million Western acres. In addition to its economic importance, alfalfa also fills an important rotational niche in Western agriculture. Being a deep- rooted crop, alfalfa can scavenge nutrients remaining after the growth of less efficient, shallow rooted crops. Determining when applications of NPKs are needed is important to ensure adequate alfalfa yield and quality. Over-generalizations made about the occurrence of NPKs deficiencies are dangerous. Western U.S. climates, soil Some form of soil and tissue testing must be used to identify dificiencies and monitor nutrient status. Fluid Journal A types, historical nutrient use patterns and alfalfa production systems are highly variable and directly impact the need for NPKs. Therefore, some form of soil and tissue testing must be used to identify deficiencies and monitor the nutrient status of alfalfa. We'll briefly review some of the criteria to use in determining what nutrient to use and at what rate. Nitrogen Nitrogen application in established alfalfa stands is still controversial. Application of larger amounts of N during establishment is thought to inhibit bacterial colonization of the root system and may reduce growth of mature alfalfa plants. Few studies have found any positive effects of N fertilization on established alfalfa; however, enough studies have shown a yield and/or protein response to N fertilization to warrant consideration. Situations in which alfalfa may respond to N fertilization appear to include poor or ineffective nodulation due to use of the wrong, old, or damaged inoculant, and stresses that can reduce nodulation or bacterial activity. If producers suspect low rates of N fixation in alfalfa stands they should first try to identify the cause and then consider on-farm testing to determine whether or not fields will respond to N fertilization. Phosphorus Phosphorus is essential for alfalfa production and likely represents the most common fertilizer input for this crop across the Western U.S. Phosphorus recommendations in western states with alkaline-calcareous soils are based on a sodium bicarbonate (NaHCO3) extractant or "Olsen" test. Correlations between soil test P and alfalfa yield are normally very good for the Olsen P test. These relationships are used to identify the critical soil test P above which fertilizer recommendations are not made. Figure 1. Effect of Fluid P on alfalfa yield at two locations. P applied at rate of 150 or 250lbs/A of P2O5. 1
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