Importance of Nitrogen Timing and Methods for Southern Corn

Mississippi’s warm, wet climate can pose considerable issues with nitrogen fertilization, particularly for crops like corn which are very responsive and demand high amounts for optimal productivity.  The South’s warm, high rainfall climate greatly increases potential nitrogen loss during the season through volatilization, denitrification and leaching, compared to drier and colder climates.  Of course, the corn growing season is very long, encompassing considerable time during the spring and early summer when rainfall is normally plentiful, leading to extended soil saturation and significant nitrogen loss. Likewise, irrigation may also compound issues associated with nitrogen management. Therefore, optimizing your nitrogen fertilizer dollars should involve considerably more planning and management than simply applying a given fertilizer rate.

Your primary goal for improving corn response to nitrogen fertilizer should be to minimize nitrogen exposure to wet conditions, while still meeting crop demand.  During early corn vegetative stages, plant nitrogen needs are very low, while rainfall is normally high.  Therefore, applying only a small portion of total nitrogen near or shortly following emergence can greatly reduce your nitrogen exposure to unfavorable conditions promoting loss. Subsequent “split” or “topdress” applications should contain the bulk of the crop seasonal nitrogen needs.  The corresponding graph represents corn seasonal nitrogen uptake relative to key growth stages and serves as a basis for optimal application timing.

For example, although corn doesn’t require much nitrogen during early vegetative growth stages, you need to apply at least 30 to 60 lbs/a of nitrogen (10 – 20% of seasonal total) by the time it gets 4 to 6 inches tall, or slightly later if you applied a starter fertilizer.  Corn begins rapid growth and nitrogen uptake at the V6 growth stage. This is the key stage for your primary ground-applied nitrogen operation or side-dress.  A practical goal for side-dressing or injecting liquid nitrogen is when corn reaches V4 – V6 growth stage (8 to 18 inches tall).  A pre-tassel application can also be incorporated as simply another split in this system to improve nitrogen efficiency.

Top-dressing urea is also becoming more popular method for growers in the Mid-South. However, there are inherent drawbacks with this method that increase risk, compared to the traditional side-dress method of injecting liquid nitrogen solution in the soil. Top-dressing urea requires rainfall or over-head irrigation to incorporate nitrogen and prevent volatility loss. This presents considerable risk, particularly when we experience a lengthy dry spell encompassing key application time, like we did last May. This is why reliance on top-dress urea fertilizer is not common in drier regions of the U.S. We strongly recommend using a urease inhibitor containing the active ingredient NBPT, such as Agrotain, Aborite AG, N-Veil, NITRAIN, NitroGain UI to limit volatility of top-dressed urea.  Of course, timing urea application prior to forecast rainfall is suggested to help aid incorporation and reduce potential volatility. Additionally, avoid application to saturated or wet soils which promote loss. In order to minimize leaf burn associated with top-dress application, we generally suggest applying no more than 150 lbs/a of urea per application on corn more than three feet tall.  Thus, multiple top-dress applications (likely 4 to 6) are required to meet total seasonal N demand. After the initial fertilizer is applied shortly after crop emergence, we suggest timing subsequent applications beginning at the V5 growth stage (12-inches tall).