On March 6, 2015, revised guidelines for fertilizing corn grown on irrigated sandy soils were released by the University of Minnesota. Minnesota has about 500,000 acres of irrigated sandy soils. Corn is grown on about half of these acres in any one year. With the use of irrigation and fertilizer, sandy soils are very productive.

In 2000, the guideline for nitrogen (N) application for sandy soils was around 230 lb N/acre. In 2006, the University of Minnesota joined several North Central Land Grant Universities to develop a common method of developing N guidelines for corn. This method was called the Maximum Return to Nitrogen (MRTN). The goal of this process was to improve the predictability, involve economics, include some adjustment for the user attitude towards risk, and to use a similar method for developing N guidelines across the region.

To make the MRTN method work well, a database with a large number of corn responses to N fertilizer was needed. Minnesota had a large number of response information for highly productive non-irrigated soils, but did not have adequate information for corn grown on irrigated sandy soils at the time the MRTN was first developed.

Because of a lack of data specific to irrigated soils, a decision was made to use the guideline for corn grown on non-irrigated highly productive soils for corn grown on irrigated sandy soils. This was not a good decision. The MRTN economic involve the ratio of the price of N per pound to the price of corn per bushel.

At the common 0.10 ratio ($0.50 N to $5.00 corn) the 2006 guideline was 140 lb N/A with a range of 120 to 165 lb N/A for corn grown after corn or group 2 crops. This was too low.

New studies were established starting in 2006 and data was collected from 22 irrigated sandy soil corn N response studies over the past 8 years. The sites were fertilized with urea with seven to eight N rates. Nitrogen Best Management Practices (BMPs) were followed for application timing. The N fertilizer was split into two equal applications, the first at planting and the second at growth stages V3 or V4.

Eighteen of those sites were used in the MRTN calculation. When the data was analyzed, there was no relationship between the optimum N rate and corn grain yield. This reinforces the nonuse of a grain yield goal in the guideline determination. The MRTN was calculated and also the range around the MRTN that would result in a loss of $1 per acre.

The new N guidelines based on the MRTN are reported in Table 1. At the common 0.10 price ratio the new MRTN is 209 lb N/acre. This compares to 140 lb N/acre from the 2006 guidelines. The new MRTN is similar to the irrigated sandy soil N guidelines from the University of Wisconsin.

Table 1. Guidelines for use of N fertilizer for corn grown on irrigated sandy soils.

N price/Crop value ratio MRTN Acceptable range
  ———-lb N per acre———-
0.05 233 214 – 252
0.10 209 192 – 225
0.15 191 177 -206
0.20 177 164 – 190

What are the environmental consequences of this change? There is little field data to document the effect of N application rate from the 2006 guideline to the new guideline in Minnesota. We are currently gathering this information from experiments at the Rosholt Irrigation Farm near Westport.

We know that increasing N rate beyond the optimum for grain yield will dramatically increase the amount of N available for loss. The corn plant uses nitrogen applications efficiently up to the point where optimum grain yields occur. Because of this the amounts lost are relatively small.

For these new N guidelines for corn grown on irrigated sandy soils, the following conditions must be met:

  1. Use the MRTN.
  2. Apply N fertilizer as at least a 2 time split.
  3. No pre-plant N
  4. Count N in starter and P fertilizer sources.
  5. Count N in irrigation water if the N concentration in it is above 10 ppm.
  6. Use irrigation scheduling. (do not over water).
  7. Guard against volatilization – use a proven product if needed.
  8. Take credit for previous crop and manure application. ie. Soybean is a 30 lb N/A credit.
  9. Do not over apply!