This article is the fourth in a six-part companion series to the Nebraska Extension NebGuide G2365, “In-Season Nitrogen Management for Irrigated Corn.” Each article will explore critical concepts from the guide that growers have expressed a need for greater guidance on, offering additional insights and practical recommendations to help optimize nitrogen use and profitability.
When deciding how much nitrogen (N) should be applied for a corn crop, growers utilize a variety of resources: their own experience, advice from retailers and crop advisors, and university extension recommendations. These decisions often rely on quantitative data: soil test values, last year’s yield level, irrigation water tests, rates and timing of manure application, and fertilizer and crop prices. The rate may also be influenced by factors such as application timing, use of inhibitors (urease or nitrification), or use of biological products. These factors may be used in prediction equations that attempt to develop an Economic Optimum Nitrogen Rate ahead of the growing season.
Yet some — perhaps most, if not all — of the N needed by a corn crop can come from sources other than fertilizer. The challenge for farmers is knowing how much N credit to assign for these sources in order to determine the most profitable fertilizer rate. Guidance comes primarily from universities and industry research, as well as the farmer’s personal experience.
Legume Credits
Soybean is the most common legume in Nebraska crop rotations. Soybean meets most of its N needs from symbiotic N fixation from the atmosphere but is also efficient at utilizing inorganic N from soil. Consequently, residual soil N after soybean is usually quite low, and the crop does not add significant N to the soil, as is sometimes assumed. Instead, soybean residue has a low carbon to nitrogen (C:N) ratio, meaning that decomposition will immobilize less mineralized soil N than corn residue, leaving more mineralized soil N available for the following crop.
Alfalfa, clover and other legume hay crops do add significant N to the soil when terminated by tillage or herbicide application. In many cases, these credits are large enough that little or no fertilizer is needed for corn in the first year after such crops.
The actual N credit assigned for a preceding legume crop is based on previous N rate response research. In these studies, corn yield at various fertilizer N rates was compared to studies without a preceding legume crop, over a wide range soil types and weather conditions. While these trials provide estimated legume credits, the actual value can vary from field to field and year to year.
Soil Residual Inorganic N
Plants primarily use inorganic N in soil (nitrate [NO3-N] and ammonium [NH4-N]). These forms of N in soil are produced by mineralization from more complex, carbon-based (organic) compounds in organic matter, crop residues and manure. Quantities of NO3-N and NH4-N can vary in soil, so crediting for these inorganic N forms can reduce the amount of fertilizer N needed.
In Nebraska, past research found NO3-N and NH4-N levels in soil were highly correlated, so current recommendations call for testing only NO3-N. More recent findings suggest NH4-N levels may vary compared to NO3-N, which could change this guidance in the future. Both NO3-N and NH4-N levels will vary in soil with weather conditions and availability of compounds that mineralize into inorganic N. As with legume N credits, credit for inorganic N is based on past research comparing crop response to varying N rates across a range of inorganic N levels, soils and weather conditions.
Organic Matter
All soils have some level of organic matter, which contains nitrogen. Through microbial activity, organic matter continuously breaks down in a process called mineralization, releasing inorganic N. At the same time, inorganic N may be converted into organic forms — also through microbial activity — eventually forming stable organic matter. Accounting for the release of inorganic N from organic matter — known as the mineralization rate — is challenging because many factors influence this rate. The level of organic matter and weather conditions (moisture and temperature) are the primary drivers.
As with other N credits, the credit allowed for organic matter mineralization is derived from research evaluating crop response to different fertilizer N rates across a wide range of soil organic matter levels over many years. In practice, the actual credit for soil organic matter mineralization will vary from year to year, field to field, and even within fields, as organic matter varies spatially across fields.
Manure
From a soil management perspective, manure includes a wide range of materials — feedlot manure, composted manure, poultry litter, swine slurry, biosolids and industrial wastes. These materials often contain N in both organic and inorganic forms, and their availability to crops following land application can vary widely depending on the material’s properties, application rates, soil chemical and physical properties, and weather conditions (see Nebraska Extension NebGuide G1335, “Determining Crop Available Nutrients from Manure”). Depending on the C:N ratio, some materials may even immobilize and reduce soil N availability for a time. Given all the factors influencing availability, predicting N credit from manure and other similar materials is challenging.
As with other sources, N credits for manure and similar products are based on previous research and conservative estimates of N availability given typical weather conditions. Beyond nitrogen, manure also supplies most primary, secondary, and micronutrients, along with carbon that improves soil tilth and water-holding capacity.
Irrigation Water Nitrate-N
In many parts of Nebraska, groundwater contains elevated nitrate-N levels. While detrimental from a drinking water perspective, the N in groundwater can benefit crops. The amount of N credit provided from groundwater will vary with the NO3-N concentration and the amount and timing of irrigation. For example, NO3-N in irrigation water applied late in the season — past the period of significant N uptake by the crop — provides little benefit.
As with other sources of N credit, the amount allowed is derived from previous research evaluating crop response to different fertilizer N rates at varying irrigation water NO3-N levels, across a range of soils and weather conditions. In practice, the actual credit for a field may differ from the prediction due to year-to-year variability.
Cover Crops
In Nebraska, no credit is currently allowed for cover crops, though this remains an active area of research. Cover crop residues — particularly legume residues — may contribute significant N to the following corn crop. However, cover crop residues can also immobilize soil and fertilizer N, sometimes increasing the need for fertilizer N.
Summary
A substantial portion of N needed for crop growth comes from sources other than fertilizer; however, each credit carries uncertainty in both amount and timing of N availability. Past research provides estimates of how much N credit to allow for these sources, but each credit has some range of uncertainty, largely due to weather impacts — especially moisture and temperature — on the amount and timing of these credits.
Reactive, sensor-based in-season N management eliminates the need to predict N credits. Sensors simply assess whether the crop is adequately supplied with N during the growing season from all sources; if not, N fertigation through the irrigation system is triggered. This approach helps farmers to more efficiently utilize N fertilizer, applying it only if needed and taking full advantage of N supplied by the soil.
