In considering this, we can look at the nutrients removed when alfalfa is harvested. Tim McDermott, MSU soil microbiologist, once said that next to nitrogen, potassium is the most important nutrient for alfalfa. Potassium is thought to aid rapid regrowth after a cut, and stand decline is often associated with depleted soil potassium. Surface application of potassium is the preferred method. Phosphorus is also important.
All legumes readily respond to phosphorus fertilization.
Their symbiotic association with rhizobia bacteria is the basis for the higher phosphorus requirement. Virtually every aspect of the symbiosis is enhanced by phosphorus. This includes traits such as nodule number, nodule weight, plant dry matter production, plant nitrogen concentration and nitrogen fixation. Therefore, maintaining adequate phosphorus is important to stand establishment and maintenance. Best results are obtained when applied at planting, but annual broadcast is the usual method of phosphorus application after establishment and will result in enhanced yield.