Every spring Jeff McCutcheon, OSU Extension educator in Morrow County, fields questions from producers about fertilizing their pastures with nitrogen, phosphorous and potassium. "Many producers coming out of winter want to give their pastures a boost or they are fertilizing crop fields and figure they might as well do their pasture while they are thinking about it," he says. "Eventually I lead the conversation to the question, 'Is this really the best time to fertilize pasture?' "
McCutcheon says he is not opposed to fertilizing pastures in the spring. Applications of phosphorus and potassium should be made prior to establishing a new seeding based on soil test results. A light application of nitrogen, 20-40 lbs. N/ac. in March could be used to jump start spring growth and allow for earlier grazing. This could potentially give about two weeks of earlier grazing if environmental conditions are favorable. But the acreage covered by this N application should be limited. The spring flush is coming and most producers can't normally harvest it all with grazing animals. Why add to the amount of forage produced when you don't need it? An early nitrogen application also can increase the potential for grass tetany and excess nitrogen in the spring may possibly increase toxins in endophyte-infected tall fescue. Generally, one acre of pasture for every two cows should be fertilized with N in early spring and never more than a third of the total pasture acreage.
All applications of K should wait until the plants can utilize it better, McCutcheon says. "If we were to look at soil levels of potassium during the year we would find that it is in greater concentration during the spring due to mineralization of K during the winter. Plants have the ability to take up more potassium than they need."
This is called luxury consumption, McCutcheon says. Luxury consumption can occur when there are high soil levels of potassium, like what we see in spring. High concentrations of potassium can affect magnesium uptake by plants. This not only affects the plant physiology but can also cause metabolic imbalances in animals that consume mainly forages. The metabolic imbalance in animals is usually referred to as grass tetany. Why apply potassium at a time when more is already available and plants can take up more than they need?
So when is the best time to apply fertilizer to pastures? Research shows that if one application of N, P and K is being done, then fall is the best time for the application, he says. By applying P and K in September or October plants develop a healthier root system and improve winter survival. This results in plants better able to withstand drought the following year. If high rates of phosphorus and potassium are recommended by soil test, then there is an advantage to splitting the application. Some of the recommended fertilizer should be applied after the first hay harvest in early summer, with the balance being applied in the fall. This will help reduce the luxury consumption of potassium by the plants and improve the efficiency of K use.
"Fertilizing pasture in the spring is not the best use of your time or fertilizer dollar," McCutcheon says.