As stated in an SDSU Soil/Water Research Progress Report from several years ago, alfalfa is often overlooked as a crop to fertilize. This may be because alfalfa is a well-known legume crop, and it's fairly common knowledge that legume crops can supply their own nitrogen needs by fixing the nutrient from the abundant supply in the atmosphere. That leaves phosphorus and potassium as the major fertilizer products of concern.

An alfalfa yield of 4 Tons/Acre is considered to be similar in terms of dry matter removal to a 150 Bu/Acre corn yield. This level of dry matter removal removes a significant amount of plant nutrients. Each ton of alfalfa contains 12 lbs of phosphorus and 50 lbs of potassium. For most of western South Dakota, potassium levels are well above the very high level, and there is no need for fertilizer K. A much different situation exists for phosphorus and many soils in the central and western part of the state test low or very low unless the landowner has been building soil test levels by adding more than crop removal.

When planting a tract of land to alfalfa is an excellent time to apply phosphorus fertilizer. Adequate available phosphorus is a key to establishing a vigorous stand and maintaining high productivity throughout the life of the stand. Collect soil samples long enough before seeding to allow time for laboratory analysis and fertilization plans. If you are tilling the soil, broadcasting the product and incorporating it in the seedbed preparation process would be ideal. If you are establishing the crop via no-till, it will still be beneficial to broadcast the fertilizer. Although phosphorus is not water soluble, and tends to remain very close to the soil surface, it can still be beneficial to broadcast the product before planting no-till. The process of no-till planting would slightly incorporate some of the phosphorus fertilizer and make it more available to the plant roots than if it were lying on top of the soil. Even fertilizer granules that land on the soil surface will be stirred into the top inch or so of soil as precipitation events occur, machinery travels across the field, etc.

Broadcasting phosphorus fertilizer and not incorporating it with tillage is effective on perennial crops because the roots can grow very close to the soil surface. This phenomenon makes it effective to apply phosphorus fertilizer to alfalfa after the crop is established as well. There are two schools of thought regarding phosphorus application rates for alfalfa, collect soil samples and fertilize according to laboratory test results or replace the amount removed by the crop. Particularly if you collected soil samples before establishing the alfalfa crop, replacing crop removal will likely work well for the time the field will be in alfalfa. Savings on application costs can be realized by applying two years' crop removal every other year. The question was recently asked, is spring a good time to apply phosphorus to alfalfa? A quick consultation with Ron Gelderman, SDSU Extension Soils Specialist, says yes, the likelihood of precipitation in the spring makes it an effective time to make the application. F

–SDSU Extension