Tissue Testing Helps Determine In-Season Nitrogen Loss And Pre-Tassel Nitrogen Requirements In Corn
DR. RASEL PARVEJ
BATON ROUGE, LOUISIANA
Core Ideas:
- Leaf tissue testing is one of the best indicators of determining N losses during the growing season.
- Leaf tissue testing before tasseling indicates the pre-tassel N requirement for maximizing corn yield.
- Leaf tissue sampling should be done from V10 (10 collar leaf stage) to R1 (silking) stage.
- The sufficient leaf-N concentration from V10 to R1 (silking) stage should be over 3.0%.
In the lower Mississippi Delta, excessive rainfall is common during the early growing season, leading to saturated soils for several days. This condition accelerates nitrogen (N) losses through denitrification, leaching, and runoff, thereby reducing corn yield potential. Consequently, the LSU AgCenter recommends applying N in at least two splits for silt loam and clayey soils, and in three splits for sandy soils. This approach involves a small application at planting (either 2x2 or dribbled on top of the bed), followed by the majority at the V5-6 stage (when the plant has 5-6 leaves with visible collars and is about 12 to 18 inches tall) as a sidedress for silt loam and clayey soils, and an additional small amount before tasseling, especially for sandy soils. For instance, for a 200-bushel corn crop on silt loam soils, 200 lbs of N per acre (1 lb N per bushel of corn yield per acre for sandy to silt loam soils and 1.25 lbs N for clayey soils) should be split into approximately 30 to 40 lbs at planting and 160 to 170 lbs at the V5-6 stage. While research from the mid-South states indicates that maximizing corn yield with a single N application during the growing season is possible in both silt loam and clay soils, such a strategy requires ideal growing conditions with moderate temperatures and evenly distributed rainfall, which rarely occur in Louisiana. Therefore, relying on a single N application is typically a risky management plan for corn production in most years in Louisiana.
At any given stage, it is challenging to measure how much N has been lost from corn fields because N losses due to excessive rainfall depend on various factors such as soil type, drainage, and cation exchange capacity (CEC). Although N can be lost through different mechanisms, denitrification is the primary concern during periods of excessive rainfall, especially in poorly drained soils. However, denitrification can occur in any soil that becomes waterlogged and anaerobic due to excessive rainfall. In well-drained corn fields without waterlogged conditions, denitrification is less of a concern, though N leaching can still be an issue with high rainfall. Leaching is particularly common in sandy soils with low CEC (<10). Fortunately, N leaching primarily affects nitrate-N (NO3-N) fertilizer. Since urea ammonium nitrate (UAN; 32-0-0, 30-0-0-2S, or 28-0-0-5S) is the most commonly used N fertilizer for corn production in Louisiana, and it contains only 25% nitrate-N, the maximum potential leaching loss from UAN is limited to 25% of the total N applied.
Corn tissue testing is a crucial tool for assessing the N status of corn during the growing season and determining if a pre-tassel N application is needed to maximize yield. To evaluate N losses due to excessive rainfall after sidedressing, producers should wait until the V10 stage (10-collar leaf stage) to take tissue samples. Sampling can be done from the V10 to the R1 (silking) stage, but it is preferable to sample earlier (at V10) if the fields have experienced several days of waterlogging. For tissue testing, collect the uppermost fully developed leaf with a visible collar below the whorl (Figure 1) from 10-15 plants and send the samples immediately to the lab for total N concentration analysis. In large fields, collect several composite tissue samples from different areas to better understand the overall N status and guide subsequent management decisions.
The critical leaf nitrogen (N) concentration for corn from the V10 to R1 stage is 3.0% (Figure 2). A leaf N concentration below 3.0% is considered deficient, indicating that additional N is needed to maximize yield. Conversely, a concentration above 3.0% is sufficient, meaning no additional N is required. When collecting and interpreting leaf tissue samples, care must be taken because high leaf N concentrations can result from insufficient plant growth (low dilution) due to drought, diseases, or pest infestations. If the leaf N concentration is near the critical level, a pre-tassel N rate of 15 to 25% of the total N applied, or approximately 40 to 60 lb N/acre (87 to 130 lb urea/acre), is recommended. However, if the tissue N concentration is significantly low, around 2.5% or lower, a higher pre-tassel N rate is needed. For tissue concentrations between 2.0% and 2.5%, at least 100 lb N/acre should be applied.
Producers have the option to use either dry (urea, 46-0-0) or liquid (UAN, 32-0-0, 30-0-0-2S, or 28-0-0-5S) nitrogen (N) sources. Urea can be easily applied via airplane, while UAN should be applied as a surface treatment because high rates of undiluted UAN as a foliar application can cause severe foliage burn. Alternatively, UAN can be applied through a pivot irrigation system, if available, as fertigation. Regardless of the N source, it is preferable to place the N fertilizer close to the plant base, if possible, using a high-clearance applicator with “Y-drops” to facilitate rapid uptake, minimize N losses, and avoid foliage damage. Applying N before an anticipated rain (0.25-0.5 inch) or pivot irrigation is recommended to incorporate the N and minimize volatilization loss. Additionally, using N stabilizers (urease inhibitors) can further reduce volatilization loss. Experiments conducted by the LSU AgCenter showed that when urea was surface applied at late growth stages, the use of N stabilizers reduced ammonia volatilization losses by 74% and increased corn grain yield by 12 to 25% compared to uncoated urea. Overall, producers should consider rainfall amounts following sidedress N application, field conditions, crop growth, yield potential, and tissue testing to evaluate N losses and determine pre-tassel N application. The effectiveness of pre-tassel N application depends on accurate tissue sampling at the correct growth stage, proper interpretation of leaf N concentration, and applying the right N rate at the optimal time and method. ∆
DR. RASEL PARVEJ: Assistant Professor and State Soil Fertility Specialist, LSU AgCenter