Fall Fertilization of Forage Crops
Posted: August 28, 2012
The principal nutrients of concern for legume forages are P and K. An alfalfa crop, for example, removes approximately 15 lb P2O5 and 50 lb K2O per ton of hay equivalent yield and at a typical yield of 5 ton/acre, it will remove/require a total of 75 lbs P2O5 and 250 lbs K2O /acre, respectively. This removal is built into the soil test recommendations. However, if yields are higher than anticipated when the soil test was run, even if the recommended nutrients have been applied earlier in the year, additional maintenance applications of P and K may need to be made to replace the higher removal. Fall is an excellent time to make this adjustment. In the late summer and fall, K especially becomes a nutrient of agronomic concern. Potassium in the plant is largely found in the sap serving as a regulator of numerous metabolic processes. A major benefit of sufficient K in the soil is winter hardiness provided it is available in time for uptake by the crop before dormancy occurs. Thus, if soil test levels are in the low optimum or below optimum range, K should be applied in the fall because this should help to improve winter survival for the long term benefit of the stand.
Legumes are not generally considered to be a good place for manure application mainly because the N in the manure is wasted on legumes so this reduces the economic return to the manure application. Also, there are concerns with stand injury and the possibility of increased weed pressure. However, if it is necessary to apply manure to legume fields, fall is the best time to do it. This is a way to supply the additional P and K that might be required, there is less chance of stimulating weeds, and there is good cover in these fields in the fall, over the winter and early spring to minimize nutrient losses to the environment. Usually we give priority to older stands for manure application because they typically are more depleted in nutrients and if there are some negative effects from the manure application, there will be less long term impact on a stand that will soon be rotated compared to a new seeding.
We want to make sure that we replace the removal of K if needed, but not overdo it because alfalfa is a luxury consumer of K. This means that if there is extra K available because soils are already high in K or unnecessary K applications are made, the crop will take it up whether it needs it or not. This can result in unnecessary fertilizer expense, high K levels in the forage, and less K available for future uptake.
If the field has not been soil tested recently, this would be an excellent time to take a sample. Sampling now will indicate whether soil K levels have been depleted to the point where a fall fertilizer application is necessary. On the other hand, regardless of higher yield, if the soil reserves are still adequate the soil test can help reduce the cost of an unnecessary fertilizer application. Soil test kits from the Penn State Agricultural Analytical Services Laboratory are available from all Penn State Extension offices.