Making the Corn Starter Fertilizer Decision

Mike Rankin
Crops and Soils Agent
University of Wisconsin - Extension

        Using starter fertilizer for corn has been a standard practice in Wisconsin for many years. This is not the case in some other states where all plant nutrients are often applied as a broadcast application in the spring or fall. Each year, questions arise as to whether fertilizer needs to be applied at planting. The major disadvantage to such a practice is that it slows down the planting operation because time is spent filling dry or liquid fertilizer holding units on the planter. Also, many fields already have excessively high levels of phosphorous (P) or potassium (K) and the need for adding more nutrients as a starter fertilizer is justifiably questioned.

        To help answer these starter fertilizer questions for corn, University of Wisconsin soil scientists initiated a large research project that involved 100 replicated starter fertilizer comparisons on farms throughout the state. Essentially, fertilizer treatments were compared on each farm where a starter fertilizer was used or was not used. Most of the locations involved fields with high or excessively high P and K levels. Depending on the year, positive economic response to starter fertilizer occurred 32 to 48 percent of the time. Corn yield responses ranged from -10 to +42 bushels per acre.

        Results from these on-farm trials showed that corn hybrid maturity, planting date, and soil test K levels were the major site-specific factors affecting corn yield response to starter fertilizer. Contrary to conventional thinking, the probability for profitable yield responses was highest at later planting dates for any given corn hybrid relative maturity (R.M.), especially when soil test K levels were less than 140 ppm. Any combination of circumstances that pushed the limit on corn maturity (e.g. excessively long R.M. corn planted anytime but especially late) increased the odds for positive economic response. Planting a short-season hybrid early in the spring resulted in the least likely scenario for positive response from using a starter fertilizer.

        Site-specific management decisions need to be made. For sites with excessively high levels of P and K, we still recommend to apply a minimum rate of starter fertilizer of about 10-20-20 (N-P-K) pounds per acre. It's highly unlikely that a yield response will be obtained by applying nutrients beyond this level. Even at this rate, early planting of a short season hybrid is unlikely to result in additional yield. For fields having optimum or high soil test levels, higher than minimum starter fertilizer rates are needed to provide the entire crop P and K requirement, or use the minimum starter fertilizer rate at planting and supply the additional needed nutrients with a broadcast application. For heavily manured fields, apply only the minimum 10-20-20 rate.