Heavy Rainfall is Making Nitrogen Management a Challenge

Amy Shober, Extension Specialist – Nutrient Management and Environmental Quality, University of Delaware, ashober@udel.edu, Sydney Riggi, Extension Agent – Nutrient Management, University of Delaware, sydney@udel.edu; Gurpal Toor, Extension Specialist – Nutrient Management and Water Quality, University of Maryland College Park, gstoor@umd.edu; Bob Kratochvil, Extension Specialist – Agronomy, University of Maryland College Park, rkratoch@umd.edu; Jarrod Miller, Extension Specialist – Agronomy, University of Delaware, jarrod@udel.edu

Continued rainfall in our region has kept many growers out of the field again this week. These wet weather patterns are frustrating and serve as a reminder about the uncertainty and unpredictability of weather. Over the last four days, weather stations in Newark, Dover, and Georgetown report between 0.03 to 1.28 inches of rainfall and over the last month the Georgetown weather station has recorded 10.22 inches of rain. Not only has this excess rainfall affected the timely planting of corn (and even beans at this point), but has the potential to affect the crops that are already planted.

Nitrogen (N) management is one of the major challenges faced due to the wet conditions this spring. This type of wet weather is one of the main reasons we heavily advocate split applications of N to corn, despite the convenience of applying all N prior to or at planting. Early in the season, N uptake by corn will be about 1 lb of N per day per acre and N uptake will increase to approximately 3 lb N per day per acre during later vegetative stages. As such, if you applied 200 lb of N per acre prior to or at planting, uptake by the corn crop may be 30 lb per acre in the first 30 days; this leaves 170 lb of N per acre in the soil. This spring especially, N losses via leaching (movement of N below the root zone), runoff (movement of N in surface water flow), or denitrification (loss of N gas to the atmosphere) from fields that received all N prior to or at planting will be excessive, as shown in the below image.

In fields where only a small amount of N was applied prior to or at planting, the risk for early season N losses are reduced. This is because the bulk of the N is applied in a sidedress (or even one or more fertigation events) closer to the time that corn enters into rapid growth phase and the bulk of the N uptake occurs. The period of rapid N uptake occurs when corn is approximately 10 to 20 inches tall, which usually occurs sometime around 35 days after emergence. However, the N applied prior to sidedress, as well as residual N from past manure applications, cover crop biomass, or previous legume crops was susceptible to N losses this spring.

There is no way to know exactly how much N was lost from fields, as this requires precise measurement. While the extent of N loss will vary depending upon soil type, location, and amount of rainfall received, we can make some general assumptions about N losses. If we consider that 1 inch of rain adds 27,154 gallons of water to an acre and weighs about 113 tons, this 1 inch of rain will penetrate 6 to 15 inches in soil (depending on soil texture). This rain water will flush (leach) nitrate, which is the most common form of N in soils and is soluble and highly mobile (due to negative charge that prevents binding with soils), deeper into the soil profile. Since we received several inches of rain, we expect that nitrate has been leached below the current depth of plant roots. In addition, we have had several events where rainfall amounts exceeded the infiltration capacity of soils, resulting in ponding or even surface runoff. When runoff occurs, nitrate and N attached to soil particles is lost if you have erosion. In fields with ponding of water, conversion of nitrate to N gas, which will escape to the atmosphere, is highly likely.

Going forwards, growers should consider conducting a pre-sidedress nitrate test (PSNT) to determine if there is a need to add more N fertilizer. It is important to remember that the PSNT is most appropriate for fields where manure or other organic N sources were applied. Regardless of whether fields received all N up-front or a split application was planned all along, growers should contact their nutrient consultant to assure that in-season N application is in compliance with their nutrient management plan. As in season adjustments are made, Delaware nutrient consultants can write a justification for additional fertilizer applications. A copy of the addendum should be included with the original nutrient management plan before the end of the calendar year and will be essential if the farm is audited.