Although, alfalfa production was limited this year, because of the lack of irrigation water, it is still important to maintain proper soil nutrient levels to prevent any further stress on the plants. Determining when applications of nitrogen (N), sulfur (S), potassium (K) and phosphorus (P) fertilizers are needed is also important to maintain alfalfa yield and quality. The Western U.S. is diverse in terms of climate, soil types, irrigation water need and availability, historical nutrient use and alfalfa production systems. Therefore, few generalizations hold regarding the timing and location of N, S, K and P deficiencies.
Soil and tissue testing are the most reliable ways to assess alfalfa nutrient status and determine fertilizer needs. Growers must understand what soil test methods (mainly sampling depth and chemical extractants) are appropriate for their area and use interpretations (recommendation tables) developed for those methods. Tissue testing procedures and interpretations are more similar throughout the West, but growers must still match tissue sampling protocols (mainly growth stage and plant part) with the interpretation data.
• Nitrogen (N). Alfalfa is a legume and through a symbiotic relationship with bacteria obtains N required for growth from the atmosphere. Most references acknowledge that, during establishment and before bacterial symbiosis develops, a small amount of N is beneficial (20-40 lb. N/acre). Nitrogen application in established alfalfa stands is still controversial. Few of the studies found any positive effects of N fertilization on established alfalfa.
• Sulfur (S). Sulfur is an important component of several amino acids and has been shown to influence the yield, protein content, stand density and stand life of alfalfa. Yield responses of up to 300% have been reported under severe S deficiency conditions; however, more typical yield responses to S fertilization are in the range of 15% to 25%.
Some soil deficiencies that have been attributed to lack of phosphorus may actually have been cases of sulfur deficiency. If poor crop growth was corrected by an application of single superphosphate, the benefit may have been due to the gypsum contained in this material. Critical soil test values for sulfur are less than 25 ppm of SO4 or less than 8 ppm of SO4-S.
• Potassium (K). More K is removed than any other nutrient except N by alfalfa. A long history of high-yielding alfalfa production has depleted native soil K levels to the extent that most Western States now encourage testing for K and make K fertilizer recommendations.
Recent research from Utah State University showed alfalfa yield responses to relatively high K application rates on low K-testing soils. 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. Critical soil test values for potassium are less than 150 ppm K.
• Phosphorus (P). Phosphorus is essential for alfalfa production and likely represents the most common fertilizer input for this crop across the Western U.S. Correlations between soil test P and alfalfa yield are normally very good for the Olsen P test.
Various P fertilizer sources are available in the Western U.S. Research has generally found that conventional dry (11-52-0, 0-45-0) and liquid (10-34-0, phosphoric acid) fertilizer sources are equally effective for alfalfa. Selection of a P fertilizer source should be based on availability and cost per pound of P2O5.
For many years, the recommended time to apply P and K fertilizer was one application in the fall. The thinking was that most of the potassium would be taken up by the roots before winter and the phosphorus needed sufficient time in the soil to become plant-available. By spring, the phosphorus would be ready to be used by the alfalfa plants. This system works and has worked very well for a number of years.
But as alfalfa yields keep getting pushed higher and higher, some fields are running out of P and K after a few cuttings. As stated earlier, many times it isn’t bad enough for the classic deficiency signs to appear, but there is a drop in yield that can be raised by a late spring application (especially of potassium).
When you consider that each ton of hay removes 6 pounds of phosphorus and about 50 pounds of potassium, you can imagine that fields fertilized just enough to get them to the desired level by the first cut in the spring will be low after just a couple of cuttings. Another thing worth mentioning is that since alfalfa is a luxury consumer of potassium, even if you apply a year’s worth in the fall, the plants will take up more than they need by early spring, and the potassium could leave the field in the first cutting or two.
These two factors have led to the recommendation of splitting P and K into two applications, one after the last cut in the fall and one between first and second cutting in the spring. The fall application will drive yield in the first two cuttings in the spring, and the late spring application will carry through the rest of the growing season.
Dairy farms that haul manure throughout the year are getting their split applications on if they haul manure to a field more than once in the growing season. The exact ratio of how much should be applied in the first or second application isn’t totally settled, so most people split the applications evenly.
On-farm testing is one way in which alfalfa growers can refine fertility programs for specific situations. An on-farm test might involve a trial to determine whether a higher rate of P or K results in higher yields. When conducting an on-farm test, treat several strips in a field with the new fertility practice and alternate strips with the standard practice. Where possible, treat several test strips in a field and only change one variable (such as nutrient type or rate) for each trial. Keep individual records of fertilizer applications, soil and tissue test values, and alfalfa yield for the trial to assess whether the new practice improved yield or changed soil test values.
As always, when it comes to soil fertility, field history is critical. No matter if it is phosphorus, potassium or even nitrogen applied to your alfalfa, the fertility needs of a field that gets frequent manure applications will not be the same as the hay field that you can’t quite remember if you fertilized last year. Soil testing is the only way to know for sure what is needed.