Grain stored in bins affected by flooding this spring can’t be fed or sold. In many cases the only option for disposing of adulterated grain is to apply it to the land. The fertilizer nutrient value of land-applied grain is worth considering, as it will help offset costs for producing the next corn or soybean crop in that field.

The Iowa Department of Natural Resources and the Iowa Department of Agriculture and Land Stewardship offer the online guidelines Proper Management of Flooded Grain and Hay.

The publication says, “Farms disposing of spoiled grain and feed on their own property can land-apply damaged grain at the following application rates: 146 bushels corn per acre and 50 bushels soybeans per acre.” The publication also states that “spoiled grain needs to be incorporated [or] disked into the ground the same day of application to prevent poisoning migrating waterfowl and other birds.”

Guidelines from ISU agronomists

The following information, written by Iowa State University Extension soil fertility specialists John Sawyer and Antonio Mallarino, discusses the implications for nutrient supply from land-applied corn and soybean grain.

Corn and soybean grain contain all of the plant essential nutrients. The nutrients most important to consider for crop production in Iowa are nitrogen, phosphorus and potassium. Estimates of the concentration of each nutrient in corn and soybean grain, and bushels per acre that will be applied, are needed to approximate the nutrient amounts that will be applied to the soil.

It is difficult to uniformly apply wet grain and to estimate its nutrient value. Nutrient composition should not change significantly when grain becomes wet from flooding.

Phosphorus and potassium

For P and K, the calculation to estimate crop-available amounts applied is straightforward. Multiply the estimated bushels per acre applied times the concentration per bushel of phosphorus pentoxide (P₂O₅) and potassium oxide (K₂O). For corn, that’s 0.32 pound P₂O₅ per bushel and 0.22 pound K₂O per bushel at 15% grain moisture content. For soybeans, it’s 0.72 pound P₂O₅ per bushel and 1.20 pound K₂O per bushel at 13% grain moisture.

One can simply assume that all the grain’s P and K will be available the year of application. If a crop isn’t planted, then the applied P and K nutrients can increase soil test levels and be used by crops in future years.

Assuming the maximum amount of grain allowed by IDNR and IDALS will be applied, the amounts of P and K per acre will be the following. With 146 bushels per acre of corn grain, it will be 47 pounds P₂O₅ and 32 pounds K₂O per acre. With 50 bushels per acre of soybean grain, it will be 36 pounds P₂O₅ and 60 pounds K₂O per acre. In many cases, those amounts of P and K could meet fertilization requirements for a following crop.

If soils are testing in the “very low” category for P and K, then some P and K fertilizer, or starter, may be applied in addition to the amounts applied with grain to ensure adequate early-season availability. A fertilizer application would be more important for P than K, as the K is present in grain as the plant available K⁺ ion, while the P is mostly contained in organic forms that need to be broken down before becoming available for crops.

Helping meet N need

For N, estimating the crop-available amount applied is more complicated. If applied before a soybean crop, there should be no need to estimate the N from corn or soybean grain as the soybean crop can fix N that it can’t obtain from the soil.

If grain is land-applied before corn, then an estimate of plant-available N is needed.

The C:N ratio of corn grain varies but is approximately 33:1. That C:N ratio means that microbial degradation of the corn grain will not provide a net amount of mineralized N; instead, it would be in an approximate balance with soil microbial demand as the corn grain is used as an energy (carbon) source. In other words, there would not be an expected release of plant-available N to a corn crop. Applying corn grain before soybeans would eliminate the need for estimating plant-available N.

The soybean grain N content (mostly in proteins) varies, but 3.1 pounds N per bushel is a reasonable estimate. For example, if the maximum amount of grain allowed by IDNR and IDALS will be applied, a soybean grain rate of 50 bushels per acre would contain approximately 155 pounds N per acre. Think of the soybean grain as fertilization with an organic N form. Therefore, an estimate is also needed regarding conversion of the grain N to plant-available inorganic N (ammonium and nitrate).

Not all N readily available

Not all of the soybean grain N will be available the first year. Although it isn’t known exactly what the availability will be, a study of corn N response in fields with hailed-out soybean seed provides some guidance.

In a study in 2002-03, where soybean seed was hailed out at harvesttime in the fall 2002 (estimated 35 to 45 bushels per acre soybean grain yield on the ground), the next-year corn crop had an economic optimum N rate at 50 pounds N per acre compared to 120 pounds N per acre where the soybean crop had been harvested.

That fertilizer N application that optimized corn yield indicated a first-year N availability from the hailed-out soybean grain of approximately 60% at minimum.

The Late-Spring Soil Nitrate Test (LSNT) values were higher where the soybean seed was hailed out, but not up to the optimal test level. Therefore, for a 50-bushel-per-acre soybean grain land application rate, the N available to a first corn crop would be approximately 90 to 100 pounds N per acre.

Using late spring N test

Soil sampling in corn for the LSNT, or crop N stress sensing, can help confirm a corn N fertilization need. The LSNT results may be lower than expected due to grain N mineralization later in the season after sampling. Creating an “N-rich” or “non-limiting” N reference strip or multiple non-limiting small areas can provide a comparison of crop N response.

If sidedressing is the intended method for N application in corn, a suggested management practice would be to apply part of the needed N (30 pounds N per acre or more as starter, weed-and-feed, or manure) before or at planting to offset any early-season delay in N availability or lower-than-expected N supply from applied grain.

Micronutrient applications

There seldom is a need for micronutrient application to Iowa soils, except perhaps for zinc in corn and sorghum on very sandy soils, or specific soils that are eroded and have high pH with free lime. However, micronutrients will be supplied with the grain application.

Concerning the secondary nutrients calcium, magnesium and sulfur, only S deficiency has been an issue on some Iowa fields, and there will be S applied with the grain that can help with a potential S deficiency. On average, corn has 0.04 pound of sulfur per bushel and soybeans have 0.10 pound.

Overall, when adulterated grain from flooding is applied to fields, the importance of the nutrients contained in the grain added to the soil should not be underestimated. Despite some application rate and nutrient supply uncertainty, considering the nutrient value of applied grain will help offset costs to produce the next corn or soybean crop.