Fertilizing your pasture
First, test the soils in the pasture to determine how much fertilizer you will need. Your local cooperative can take the soil test for you, or you can send a sample into the University of Minnesota Soil Testing Lab. They can be reached at 612-625-3101. The soil test will determine how much nitrogen (N), phosphorus (P) and potassium (K) you will need, the three major elements that effect plant growth. If fertilizer is needed, a good fertilizer program to follow is to divide your recommended fertilizer amount in three and fertilize on the major summer holidays: Memorial Day, 4th of July and Labor Day. The later first application will slow the growth in the spring and boost productivity in the heat of summer. The application in September will help extend the grazing season and help the grass store up reserves for over-wintering and the following spring. This routine will help keep the grass from getting ahead of your horses in the spring, and supply more forage in heat of summer.
It is important to be careful in the timing of the fertilizer application. Applying N during a dry spell can burn the grass. Avoid fertilizing right before a large storm event, as this can increase the chance the fertilizer may be washed away before the grass can absorb it. The ideal time to fertilize is right before a gentle, soaking rain. You should keep your horses off of the pasture until the fertilizer has dissolved so they do not accidentally ingest the fertilizer while grazing.