This is a supplement to the recent CropWatch article, Nutrient Management Issues for 2019. It addresses nitrogen application options for corn and availability of P and K from land-applied grain and fodder that was damaged by flooding.
If the soil is dry enough to bear equipment traffic without causing much compaction, applying anhydrous ammonia may be the best option. If the soil profile has high water content, especially in sandy loam soil and soils of higher sand content, a well-validated nitrification inhibitor containing nitrapyrin or DCD should be used. Nitrate-N is highly mobile with soil water percolation, while ammonium-N is relatively immobile. The nitrification inhibitor delays the conversion of ammonium-N to nitrate-N.
Most downward movement of nitrate-N typically occurs in the spring (approximately May 1 to June 5) when the soil water content is high and the corn crop is not using much water or N. The inhibitor can delay conversion of 50% of applied ammonium-N to nitrate-N by approximately two weeks if average soil temperature is above 70°F and by three weeks if average soil temperature is less than 60°F. Therefore, the timing of the anhydrous ammonia application, even if treated with a nitrification inhibitor is important, to minimize leaching loss.
Application of urea or UAN can be done with less traffic compaction and soil disturbance as compared with application of anhydrous ammonia, but there is greater risk of N loss. A few may opt for aerial application of approximately 50 lb/ac urea-N to supply early crop growth with the intent of a later sidedress or fertigation application, preferably with rates based on the pre-sidedress nitrate test (see Using the PSNT for Spring Testing of Nitrogen Availability) or crop canopy sensing (Site-specific Nitrogen Management for Irrigated Corn). Unless there is a high probability of sufficient rainfall to take the urea into the soil within five days, treatment of the urea with a well-validated urease inhibitor (products containing NBPT or NBPT + NPPT) is advised to prevent excessive volatilization loss. Volatilization is increased with heavy crop residue cover, a soil pH > 7, and damp conditions. If leaching or denitrification with water-logged soil as well as N volatilization loss is of concern, the urea can be treated with a well-validated product that inhibits both urease and nitrification to protect against volatilization and to delay nitrification.
Nutrient Availability from Applied Organic Material
The 2019 availability of nutrients from land-applied, flood-damaged feedstuffs is not easily estimated. A ton of corn grain on a dry weight basis contains on average about 12 lb P2O5, 9 lb K2O, and 2 lb S. However, the nutrient content of flood-damaged feedstuffs vary and samples of such material should be analyzed by a laboratory before application.
Estimating how much of the applied nutrients will be available to the 2019 crop is a greater challenge. Unlike most of the P and K in feedlot manure, the P and other nutrients will be organically bound in the feedstuffs. Most of the K should become available during the season, but less than 50% of the P may become available in 2019 and little will be available before the V12 growth stage in corn. If soil test results indicate a need for P, K, or Zn, 50% of the recommended fertilizer rate should be applied in 2019. The organic material could be expected to meet late-season growth needs. The soil should be tested for the availability of these nutrients again in 2020 to fully credit the supply from the applied organic material.