Rasel Parvej, Matt Foster, David Moseley, Andre Reis and Syam Dodla

Nitrogen (N) is one of the most essential nutrients required for crop growth, development and reproduction. It is the building block of proteins, amino acids, chlorophyll and DNA. Plants require more nitrogen than any other mineral nutrient. Generally, the aboveground portion of the plant contains 3% to 4% nitrogen. Therefore, fertilizing with nitrogen sources is often required for maximizing crop yield and profit.

Nitrogen is the most yield limiting nutrient for corn. Nitrogen management in corn production is one of the biggest concerns for corn producers. A 200-bushel corn crop requires about 200 to 250 pounds of fertilizer nitrogen per acre, which is roughly 1 to 1.25 pounds of nitrogen per bushel of corn harvested. The lower range is for sandy to silt loam soils and the upper range is for clayey soils. Corn production in clayey soils requires more fertilizer nitrogen than sandy or silty soils because the high cation exchange capacity (CEC) of clayey soils fixes more applied fertilizer nitrogen (ammonium-nitrogen, NH₄-N) between clay layers, and the high microbial community of clayey soils immobilizes more applied nitrogen into a biologically unavailable form than sandy or silty soils. In addition, clayey soils have a more complex nitrogen uptake route from the source (fertilizer nitrogen) to the sink (plant roots) due to the presence of micropores, and some of the fertilizer nitrogen gets lost from this complex route during uptake process.

It is recommended to apply nitrogen in two to three splits from planting to tasseling because nitrogen is highly prone to environmental loss. However, many corn producers in Louisiana apply the total nitrogen fertilizer in a single application through side-dressing at or a few weeks after corn emergence. In most years, a significant amount of this single applied nitrogen can be lost during the growing season through volatilization, denitrification, leaching and/or runoff, resulting in corn yield loss. Volatilization, the loss of nitrogen to the atmosphere as ammonia (NH₃) gas, is high in hot and humid climates such as in Louisiana. It is also high in places with alkaline soils — a pH more than 7.0 — if nitrogen fertilizer is not incorporated within a few days after application. While volatilization loss is high for urea fertilizer, if not incorporated, it can also be the case for urea ammonium nitrate (UAN) fertilizer as it contains 50% urea. Denitrification, a microbial conversion of nitrate-nitrogen (NO₃-N) into nitrous oxide (N₂O) and nitrogen (N₂) gases, is the main concern in poorly drained soils, but it can occur in any soil with excessive rainfall that creates waterlogged anaerobic conditions. Leaching loss is prevalent in high rainfall areas, especially in sandy soils with a low cation exchange capacity (less than 10 milliequivalents/100 grams).

In most years in Louisiana, excessive rainfall often occurs in the lower Mississippi Delta during the early growing season, resulting in saturated soils for several days, which accelerates nitrogen losses via denitrification, leaching, and/or runoff, and reduces corn yield potential. Although researchers from the Midsouth have shown that it is possible to maximize corn yield by a single nitrogen application during the growing season in both silt loam and clay soils, for this to occur, the growing season must be ideal with moderate temperature and adequate and evenly distributed rainfall, which rarely occurs in Louisiana.

In general, a 200-bushel corn crop in silt loam soils uptakes about 250 pounds per acre nitrogen from both soil and fertilizer nitrogen sources. These include:

• Approximately 10% (25 pounds) from planting to V6 stage (six visible collar leaves and the plant is about 12-18 inches tall).
• Approximately 60% (150 pounds) from V6 to R1 (silking) stage.
• Approximately 30% (75 pounds) from R1 to R6 (maturity) stage.

Corn initiates ear shoots and sets yield components, such as kernel rows per ear, from the V1 stage to V6 stage; potential kernels per ear row shortly after V6 to pre-tasseling (PT) stage; and harvestable kernels per ear row from PT to R3 (milk) stage. Considering both nitrogen requirements and yield component development at different growth stages of corn, it seems that a nitrogen management plan for 200-bushel corn should include nitrogen applications in three splits, with a small amount (15%) of nitrogen at planting, the majority (60%) of the nitrogen at around V6 stage and another small amount (25%) at the pre-tassel stage. However, most research shows that two applications (one-quarter at planting and three-quarters from V6 to V8 stages) are good enough to maximize corn yield under normal conditions in most soils with medium to high cation exchange capacity (greater than 10 milliequivalents/100 grams).

Regardless of the split application number, applying a small amount of nitrogen at or before planting would provide the corn plant with enough nitrogen for setting the maximum number of kernel rows per ear from the V1 to the V6 stage. It would also provide a wide window of opportunity to make the main side-dress nitrogen application during the growing season from the V6 to the V8 stage. For instance, having a pre- or at-planting nitrogen application would allow the producers to delay their side-dress application if missed at V6 stage due to rainfall and wet soil conditions. Unfortunately, pre- or at-planting nitrogen applications are not very common in Louisiana corn production. Rather, most corn producers in Louisiana often use in-furrow starter fertilizer (ammonium polyphosphate 10-34-0 or 11-37-0 at a rate of 5 gallons per acre) to compensate for corn nitrogen requirements during the early growing season. However, this starter fertilizer is not adequate to meet the early season nitrogen demand of about 30 pounds nitrogen per acre because a 5-gallon starter fertilizer (10-34-0) per acre provides 19.8 pounds phosphorus (P₂O₅) but only 5.8 pounds nitrogen.

Corn nitrogen demand during the early season can only be met by applying nitrogen as broadcast (urea; 46-0-0) followed by incorporation before planting or as banding (2 by 2, which is 2-inches deep and 2-inches to the side of the seed row), side-dressing or dribbling (urea ammonium nitrate; 32-0-0) at planting. Note that the pre- or at-planting nitrogen rate should not exceed 45 pounds per acre for silt loam and 60 pounds per acre for clay soils and should not be applied in the seed furrow due to the likelihood of potential salt injury from ammonium-nitrogen.

Sometimes a third application of nitrogen (around 45 to 50 pounds per acre) at the pre-tassel stage (V12 to V14; about one week prior to tassel) is beneficial especially for coarse-textured, low cation exchange capacity soils (less than 10 milliequivalents/100 grams) as well as for poorly drained soils that are very prone to waterlogged conditions. This also helps protect corn yield losses in years with excessive rainfall during the early corn growing season, which increases nitrogen losses. Including a pre-tassel application in the nitrogen management program can help reduce nitrogen losses and ensure adequate nitrogen supply during the maximum nitrogen uptake period from V10 to grain-filling stage. Many land-grant university studies, including LSU AgCenter trials, showed that a pre-tassel nitrogen application can increase corn yield when part of the pre-plant and side-dress nitrogen is lost due to excessive rainfall during the early growing season. However, the need for a third nitrogen application at or before tasseling should be based on crop growth, rainfall amount, soil type and soil conditions during the growing season as well as yield potential, environmental forecasts, reference strips and leaf tissue nitrogen testing. An application of nitrogen-stabilizer treated nitrogen fertilizers during side-dressing around V6 stage also helps in minimizing potential nitrogen losses and eliminates the need for additional split application at pre-tassel stage.

To determine the need for a pre-tassel (V12 to V14) nitrogen application, a nitrogen reference strip can easily be established by applying 1.5 to 2 times the normal rate of the total nitrogen in one corner of the corn field. The normalized difference vegetation index (NDVI) readings can be taken by crop sensor from both reference strip and standard production areas and input to the LSU AgCenter sensor-based nitrogen calculator app, which would be able to determine whether the corn field requires an additional nitrogen application.

To accurately determine the corn nitrogen need before tasseling stage, leaf tissue samples can be collected from V10 to the tasseling stage and analyzed for total nitrogen concentration. For tissue nitrogen testing, the uppermost fully developed leaf with a visible collar below the whorl from 10 to 12 plants should be collected, put into a labeled bag and sent immediately to the lab for total nitrogen concentration. For a large field, several composite tissue samples from different parts of the field should be collected to better understand corn nitrogen status and follow-up management. Leaf nitrogen concentration below 3.1% is considered deficient (additional nitrogen is needed for maximizing yield) and above 3.1% is considered sufficient (no additional nitrogen is needed). Care should be taken in collecting leaf tissue sample and interpreting nitrogen concentration because leaf nitrogen concentration can be high due to insufficient plant growth (low dilution) associated with drought, diseases, and pest infestation. Once decided, pre-tassel nitrogen (around 45 to 50 pounds per acre) can be applied either by using dry urea or liquid urea ammonium nitrate fertilizer. Urea can easily be flown by airplane, but liquid urea ammonium nitrate should be dibbled on soil surface near corn roots but not onto plant foliage as a foliar application because a high rate of urea ammonium nitrate can result in severe foliage burn (salt injury).

Overall, an ideal nitrogen management program for over 200-bushel corn yield should include 30 to 45 pounds nitrogen at planting and the remainder at V6 to V8 stage with or without 45 to 50 pounds of nitrogen before tasseling based on NDVI readings from reference strips or leaf nitrogen concentration. By implementing these nitrogen best management practices, producers can increase their chances to be profitable and sustainable.

Rasel Parvej is an assistant professor at the Scott Research, Extension, and Education Center. Matt Foster is an assistant professor at the Scott Research, Extension, and Education Center. David Moseley is an assistant professor at the Dean Lee Research Station. Andre Reis is an assistant professor at the Dean Lee Research Station. Syam Dodla is an associate professor at the Red River Research Station.

This article appears in the winter 2023 edition of Louisiana Agriculture.

Young corn plants grow in a south Louisiana corn field. Photo by Jessie Hoover

Soybeans and Nitrogen

Soybeans require a lot of nitrogen to grow and reproduce. For every soybean bushel, the plants require approximately 4 to 5 pounds of nitrogen. Although this requirement per bushel is higher than corn, nitrogen applications are usually not required to produce soybeans. Soybeans are legume plants, which uptake nitrogen from biological fixation through a symbiotic relationship with the Bradyrhizobium japonicum bacterium. The bacterium converts N2 to plant-available NH4+, and the plant supplies food from photosynthesis to the bacteria. If soybeans have been grown in a field within the previous few years, there should be enough viable bacteria to support high yields. In fields not recently planted to soybeans, it is important to inoculate the soybean seed with Bradyrhizobium japonicum. In addition, results from research trials at the LSU AgCenter Red River Research Station have indicated that inoculating seed with Bradyrhizobium japonicum in recent soybean production fields can increase soybean yields by 3% to 7%. For nitrogen fixation, low pH soil (acidic soils) is detrimental because of less availability of molybdenum. In this case molybdenum should be added. However, molybdenum should not be added directly with the Bradyrhizobium japonicum inoculum unless it is directly before planting because molybdenum can decrease the number of viable bacteria. The plants can acquire approximately 70% to 75% of their nitrogen requirement through nitrogen fixation. The remaining 25% to 30% of the requirement comes from soil organic matter and crop residue breakdown. Nitrogen fixation and soil nitrogen content will generally supply enough nitrogen to produce more than 80 bushels of soybeans per acre. Therefore, nitrogen is usually not a limiting yield factor. In fact, adding additional nitrogen can have an inverse reaction on nitrogen fixation resulting in no net gain in yield.

Unfortunately, the nitrogen fixation process can fail due to various environmental factors. The roots should be scouted early in the vegetative growth stages for healthy nodules formed by the Bradyrhizobium japonicum bacteria. By the third true leaf growth stage (V3), there should be at least approximately seven nodules (Figure 1) with a pink or red cross-section (Figure 2). If a deficient number of healthy nodules continues and the leaves continue showing symptoms of nitrogen deficiency, such as light green to yellow leaves, supplemental nitrogen may be necessary. Other maladies may mimic nitrogen deficiency, so a tissue test should be taken to see if nitrogen is deficient. If supplemental nitrogen is added, a granular or dry nitrogen formulation should be applied to prevent burning the leaf material. Applying supplemental nitrogen early in the reproductive stages will help maximize vegetative and reproductive growth potential.

A cross-section view of a healthy Bradyrhizobium japonicum bacteria soybean nodule. Photo by David Moseley

Soybean root nodules formed by Bradyrhizobium japonicum bacteria. Photo by David Moseley