Cotton Fertility
The Need for Nitrogen,
Some Things Never Change

By Henry Gantz
hgantz@meistermedia.com

From bat guano to anhydrous ammonia to variable-rate applications of liquid nitrogen ...
That sounds like a long, strange trip, but the basic fact remains: Cotton needs nitrogen; only the sources and methods of application have changed.

Based on research from Mississippi State University, a general guideline is that 50-60 pounds of nitrogen are needed to produce one bale of cotton on light soils; 60-70 pounds for medium-textured soils, and 70-80 pounds on clay and clay-loam soils.

Just the opposite might be true in the Southeast, says North Carolina Extension cotton specialist Dr. Keith Edmisten. "Our soil-type recommendations vary from 45-90 pounds," he says. "In Mississippi, they recommend higher rates on heavier soils because of denitrification. But here, we recommend the higher rates on our sandier soils because we tend to have a lot of leaching. So here, the higher rate - 90 pounds - would be for the sandy soils."

In the Mid-South, a cotton/corn rotation is standard. In the Southeast, it could be cotton/peanuts or cotton/soybeans. Being legumes, peanuts and soybeans add nitrogen to the soil.

"Our studies show that for Norfolk sandy loam, if you are following peanuts or a good soybean crop, it would be very unlikely to get a yield response with over 70 pounds of nitrogen," Edmisten says.

On all soil types, Edmisten recommends that one-fourth to one-third of total nitrogen be put down preplant and the remainder put out a week or two pre-bloom. "It needs to be put out when the plants need it," he explains. "You can lose nitrogen several ways. Primarily for us it is leaching, but it can be denitrification of wet soils. It’s sort of like leaving a $20 bill on the table; the longer it lays there, the more likely it is for someone to come along and take it."

Soil testing recommendations can also vary by region.

In the Southeast, Edmisten says, "It is not necessary to take soil tests every year for nitrogen. The reason is that nitrogen is pretty mobile, so to do a good job, you would have to take samples as deep as the root system, and that is not practical."

A baseline is set based on soil type, previous crops, organic matter and how much nitrogen those things can contribute. "We calibrate for a two-bale yield," says Edmisten.

Maury Knowlton, president and CEO of Cleveland, MS-based InTime does use soil samples to establish a baseline, but it starts with aerial imagery - which InTime’s Vari-N product provides - of the previous year’s crop. The imagery shows nitrogen uptake at full bloom and comparative zones are created within the field.

"The point of the Vari-N image is to create the zones. We generally use 3-4 zones for this product," he explains. "We can usually get by with two soil samples per zone. The samples indicate the residual soil nitrogen available to the crop. The producer can then decide by zone how much more nitrogen should be applied."

From that data, variable-rate applications are made with each zone.

Too much nitrogen is costly later in the season. Says Edmisten: "If we are facing a year when the crop doesn’t have much potential, too much nitrogen will cause tremendous regrowth that nothing will work on as far as defoliation."
Petiole analysis is one way to prevent over fertilization because it will show exactly what the plant needs.

"We encourage growers to take petiole samples during the season to see if there is a particular seasonal affect on nitrogen availability," says Edmisten.

Says Knowlton: "The best way to determine nitrogen needs through the year is by tissue samples during the year. That will help you determine what the plant needs. We will create a new image as often as a farmer thinks he may need nitrogen."