Using starter fertilizers for corn, grain sorghum, and soybeans

Table 1. Salt index comparisons for commonly used starter fertilizer products, expressed as pounds of salt effect per gallon and relative to 10-34-0.

Product	Salt index, lb/gal	Value relative to 10-34-0
Ammonium polyphosphate 10-34-0	2.28	1.00
7-21-7	3.04	1.33
Urea ammonium nitrate 28-0-0	6.75	2.96
Urea ammonium nitrate 32-0-0	7.78	3.41
Ammonium thiosulfate 12-0-0-26	30.90	13.55
From J.J. Mortvedt, http://www.spectrumanalytic.com/support/library/ff/salt_index_calculation.htm (Accessed March 2 2006).

Starter fertilizer is applied in addition to nutrients applied to correct soil nutrient deficiencies. It should not be confused with fertilizer placed in a band near the row to correct a soil nutrient deficiency. Band application is a recommended way to correct P and other deficiencies and can result in early growth response as well.

In Nebraska, starter fertilizer typically contains some N and P such as with 10-34-0. The optimum N : P₂O₅ ratio varies with soil N and P availability with relatively less P applied as soil test P increases above Bray-P1 of 15 ppm. Zinc should be included when the zinc level in the soil is marginally adequate. Sulfur should be added for sandy soils low in organic matter.

The primary benefit of starter fertilizer for corn and grain sorghum is an increase in early growth and crop uniformity. In much of Nebraska soybeans seldom respond to starter fertilizer although response of short season varieties has been documented in western Nebraska (Ogallala) and further north in Brookings, S.D.

Early growth response often does not result in a grain yield increase. Grain yield increase from starter fertilizer containing phosphorus is most likely on soils low in phosphorus (Bray-P1 of 15 ppm or less). Grain yield increase on sandy soil is commonly due to nitrogen and/or sulfur in the starter fertilizer.

Greater response to starter fertilizer might be expected with no-till than with tilled conditions. In recent research conducted in Nebraska, starter fertilizer use commonly resulted in increased grain yield, with an average increase of 13.5 bu/ac for no-till irrigated corn on medium texture soils when Bray-P1 was less than 15 ppm. Starter fertilizer application did not result in increased yields of no-till dryland corn in eight trials conducted in eastern Nebraska, but Bray-P1 was above 15 ppm for seven of these trials. Placing the starter fertilizer over the row, in the furrow, and to the side and below the seed was equally effective.

Table 2. The amount of 10-34-0 that can be safely applied per acre for corn and grain sorghum in 30-inch rows as influenced by distance from the seed and soil texture**. Determine safe application rates for other fertilizers by dividing the “value relative to 10-34-0” given in Table 1 into the amounts listed below.

Placement	Sandy soils	Non-sandy soils
	10-34-0 (gal/acre)*
With the seed (pop-up)	5	5
¼ to ½ in. from the seed	10	10
1 in. from the seed	20	40
2 in. or more from seed	20+	40+
* The safe application rate for soybeans is one-half of these values.
**For narrower row-widths, the application rate may be increased. For 22 inch rows, multiply values by 1.36 and for 15 inch rows multiply by 2.

With starter fertilizer on grain sorghum early growth was increased and time to flowering was decreased by three days; however, yield was increased in only one of 18 no-till trials and the mean increase over the 18 trials was 2 bu/ac for N+P starter fertilizer. Yield was not increased by adding sulfur. Row cleaning before or at planting to remove crop residues from the crop row also resulted in early enhanced growth but did not increase no-till grain sorghum yield. Starter fertilizer and row cleaning generally did not result in lower grain moisture at harvest time.

Starter fertilizer placed near the seed has a salt effect that can damage germination and early plant development. The starter fertilizer rate that can be applied safely depends on the salt content or salt index value of the fertilizer (Table 1), the distance between the fertilizer and the seed, and the soil texture. Salt index is often estimated as the sum of N + K + 0.5*S, but this underestimates the salt index for some products, especially for ammonium thiosulfate. The limits for rates of starter fertilizer application to avoid salt damage are listed for 10-34-0 in Table 2. Rates for other fertilizers are determined relative to the rates for 10-34-0. These application rates are determined to provide safe conditions for all years with very rare exceptions.

Gary W. Hergert
Extension Soils Specialist
West Central REC
Charles S. Wortmann
Extension Soils Specialist, Lincoln

Copyright 2006 by the University of Nebraska Board of Regents. All rights reserved.

Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture.

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