Fertilizing Established Alfalfa Stands This Fall

Robert Mullen and Mark Sulc, OSU Extension

As final harvests of alfalfa are collected, fertilization of potassium should be considered if it is not a part of your program. Alfalfa that has adequate levels of potassium accumulates more carbohydrates in their root system which improves their over-wintering ability and vigor early the next spring. Fall fertilization should be done as soon as possible so that the plant can take advantage of the added nutrients before the onset of winter.

Application of potassium should be considered if soil test levels are less than 300 lb/acre. Potassium application rates can be determined by computing how much potassium next year’s crop will remove (base it on this year’s yield or a known “average”). Generally, for every ton of alfalfa forage removed approximately 50 lb of K2O is removed. Thus a 6 ton/acre crop will remove 300 lb K2O/acre. If you wish to maintain current potassium levels, add 300 lb K2O/acre.

Application can be split between this fall and after the first harvest in the spring (applying half in the fall and half in the spring will minimize luxurious uptake of potassium by the crop early next year). The earlier potassium is applied this fall the greater the opportunity for the crop to take it up and prepare for the winter. Soil test values well above 300 lb/acre probably do not warrant any application at all. Check soil test levels next fall to monitor potassium needs for the next year.

Phosphorus can also be applied in the fall after the final cutting. If soil test levels are below 100 lb P/acre, apply additional phosphorus. Crop removal can also be used to determine the rate of application (a ton of alfalfa removes approximately 13 lbs P2O5). If the soil test value is well above 100 then additional phosphorus is not necessary. Providing adequate nutrition for your alfalfa crop will pay dividends next year and extend the life of the stand.