Source: International Plant Nutrition Institute

As corn growers prepare to supplement their pre-plant programs with side-dressed N, an understand­ing of several important points may help improve the efficiency of crop response and N uptake.

  • Prior to the 6th or 7th leaf (~25 to 30 days after emergence, or V6-V7), corn roots tend to grow nearly paral­lel to the soil surface, and in well-drained soils, more than 70 percent of the roots may be present in the top 10 in. Nitrogen shortage at this time can reduce the number of kernel rows on the ear.

  • By 7 to 8 weeks after emergence (after V12), roots extend completely to the row middle and 4 ft. below the base of the corn stalk.

  • Research on sandy, irrigated soils has shown consistently higher corn yields with side-dress applications than with N applied before planting.

  • About 40 to 50 percent of the total N uptake usually occurs after tasseling (VT), but uptake accelerates rapidly be­tween V8 and V10 (~30 to 40 days after emergence). Any limitation of root activity and nutrient uptake dur­ing this critical period can seriously short-change growth (e.g. leaf area expansion) and cause yield losses (especially the number of kernels along the length of the ear).

  • In many systems, applying N to alternating row middles has proven as efficient as application to each row middle — provided that soil compaction, leaching, and water-logging are not limiting effective N use in the ap­plication row middle.

  • Placing urea-containing products (e.g. urea, urea-ammonium nitrate solutions) into the soil, beneath surface residues, helps minimize the gaseous loss (volatilization) of ammonia from urea-containing N fertilizers. Yield benefits of 8 percent or more compared to broadcast or surface dribble applications have been measured in research.

Many farmers strive to place side-dressed N near the actively-feeding roots of young plants, especially where plants have not achieved much top growth. Where plants may be 14 to 16 in. tall (i.e. after V6-V7), place­ment of N mid-way between rows should be adequate. All too often, in an effort to stimulate greater root uptake, application rigs get within 6 to 8 in. of the corn stalks. This "too-close-to-the-row" application may be injuring roots and taking a toll on plant vigor ... and yield. When making side-dress N applications, unnecessary root injury can be avoided by spacing application knives or coulters closer to the centers between rows.

With increased N costs, reports of some regional supply/distribution challenges affecting access to N sources, early-season crop stresses ... and the current opportunities to capture desirable corn market prices ... can one risk impairing nutrient and water uptake by allowing unnecessary root injury and root pruning by N application equipment? Growers should adjust the spacing of coulters and knives on N applicators before side-dressing this season to improve N use efficiency and effectiveness.