Where corn planting has been delayed due to wet soils or where replanting is necessary due to poor stand establishment, use of starter fertilizer is important to increase yields and profits with these plantings.  The increased probability of economic response to starter applies even on soils with excessively high P and K soil test levels. This is because starter fertilizer accelerates plant development.  Results from numerous on-farm studies with corn response to starter fertilizer in Wisconsin showed more frequent response to starter with later planting dates and longer season relative maturity (RM) hybrids.  Where the sum of the day of year of planting and the hybrid relative maturity exceeds about 235, the probability of response to starter fertilizer is about 50% or more.  This would be the case any time a 100-day RM hybrid is planted after May 15 (100 RM + day of year 135) = 235.  Table 1 shows the probabilities of response to starter fertilizer with various hybrid RM and planting date combinations and shows the increasing probabilities of economic response to starter fertilizer as planting dates become later.  For a May 30 planting date, the probability of response to starter fertilizer ranges from 45 to 65% across the range of hybrid RM appropriate for Wisconsin. On soils with very high or excessively high soil tests, the full yield benefit from starter applications can be achieved with about 10-20-20 (lb/ acre of N-P₂O₅-K₂O) in a 2-inch by 2-inch side placement relative to the seed.  On these soils, higher starter rates will not give additional yield benefits. Using lower application rates may not maximize yield response. Thus, if you are reducing your starter fertilizer application rate because of high fertilizer costs, you may want to reconsider this decision depending on the soil test P and K levels, planting date, and rate of nutrients applied with the starter. Broadcast applications of fertilizer are not likely to give the same benefits as a banded starter treatment. Using a starter fertilizer that contains potassium is important to ensure maximum benefits from the starter application.  Although some additional time may be needed to apply starter at planting, the beneficial effects of the starter treatment on yields becomes more likely with the later planting dates. For more information about starter fertilizer see Chapter 10 Starter Fertilizer in UWEX Publ. A2809 Nutrient application guidelines for field, vegetable, and fruit crops in Wisconsin, which can be found online at: www.soils.wisc.edu/extension/. Also search the presentations and proceedings papers of the Wisconsin Fertilizer, Aglime, and Pest Management Conference online at: www.soils.wisc.edu/extension/wfapmc/.

Table 1.    Probability of obtaining a positive economic return from starter fertilizer for several corn relative maturity ratings at various planting dates on soils with excessively high P and K levels. *

 * This table does not alter current recommendations for early planting and selection of corn hybrids with appropriate maturities for the production zone.