Variable-rate applications of fertilizer nitrogen on cotton may one day help green up grower pocketbooks, provided producers have a good handle on soil type, soil residual nitrogen, total nitrogen requirements and nitrogen levels in growing plants.

But today, the process is far from perfect, according to Jac Varco, Mississippi State University professor of plant and soil sciences. “It's going to take some time to finetune the nitrogen decisions for growers. We've jumped into all this (variable-rate) technology without having a lot of recommendations to back it all up.”

There is more certainty about the pitfalls of misapplied nitrogen. For example, nitrogen deficiency in cotton limits yields and lowers quality, while excess nitrogen results in rank growth, boll rot, difficulty in harvesting and increased need for growth regulators, insecticides and defoliants.

That's not to mention the escalating costs of nitrogen — which at 25 cents to 40 cents per pound of nitrogen, depending on fertilizer source — are nearly double the 13 cents to 22 cents or so of the early 1990s.

Varco, speaking at InfoAg Mid-South 2007 in Starkville, Miss., noted that current university nitrogen recommendations are based on various factors such as soil texture, field properties, soil test nitrogen, tissue testing, in-season rainfall, irrigation, variety, yield potential, economics, previous crop, tillage, cover crop, and history of rank growth.

“But I don't think economics — shooting for maximum profitability — is emphasized enough.”

Two things can help growers achieve this goal, according to Varco. One is more research to determine nitrogen rates based on maximizing profits instead of yield. Second, the use of precision agriculture techniques to map variability and apply the appropriate rate where it's needed.

Varco says variable-rate applications of nitrogen are feasible when there is spatial variability in yield potential, available water, nitrogen and soil physical properties, such as changes from sandy loams to clay loams.

The process begins with gathering information through grid soil sampling, yield mapping, and soil electrical conductivity mapping to determine existing field level variability.

According to Varco's research, Mid-South fields show nitrogen variability across fields and across seasons. For example, tests on one field to determine the availability of soil nitrogen showed that in 2002, there was a range of 14 pounds to 93 pounds of nitrogen within a 2-foot depth. The range went to 23 pounds to 125 pounds of nitrogen in the soil profile in 2003. The average was around 50 pounds in 2002 and 62 pounds in 2003.

Studies also show that as silt content increases in soils, “we get increasing available soil nitrogen. This means our best cotton soils, the silts to silt loams, have some of the highest available nitrogen.”

If you have the soil residual nitrogen data for a field, then it's a matter of figuring out how much you need to add to maximize profits, notes Varco.

It's not an exact science, although current university recommendations hold some keys to developing nitrogen rates by soil type, according to Varco. For example, Mississippi fertilizer nitrogen recommendations are 50 pounds to 60 pounds per bale on light textured soils; 60 pounds to 70 pounds on medium textured soils; and 70 pounds to 80 pounds on clay to clay loam soils.

But keep yield potential in mind, Varco says. “You're not going out to that clay loam soil or sandier areas prone to drought and try to apply 160 pounds of nitrogen to make 2 bales if the actual yield potential is only 500 pounds to 750 pounds.”

Varco adds that MSU studies relating available soil nitrogen to yield indicate, “If we have anywhere from 80 pounds to 100 pounds of nitrogen in the soil profile to a 3-foot depth around planting time, we can make 90 percent of our potential yield.

“But in zones of a field where I already have 100 pounds of soil nitrogen and I put 100 pounds to 120 pounds on top of it, yield may suffer and other issues such as excessive plant height, boll rot, and difficulty in defoliation become more prominent.”

Variation in the optimum rate may surprise you, notes Varco. On a farmer's field in Bolivar County, Miss., three years of tests conducted on six locations with variable soil types found the profit maximizing nitrogen rate varied from 52 pounds to 104 pounds per acre.

“The highest yielding area did not require the highest fertilizer nitrogen rate,” Varco said. “So there are always other factors involved. Also the optimum nitrogen rate over all the locations and years was 85 pounds. Keep in mind that the average Mississippi nitrogen rate reported for cotton in 2005 was 120 pounds per acre.”

After the crop is up and growing, tissue testing can provide a sort of fuel gauge for the plant, according to Varco.

A recent Mid-South study indicated “we need about 4.3 percent or greater leaf tissue nitrogen at early bloom and 4.1 percent leaf tissue nitrogen or greater at peak bloom to optimize yields.”

Crop reflectance imagery is another method for determining nitrogen variability in a field. “We're not necessarily just talking about growth habits, but rather the actual nitrogen status of the plant. We've seen that potassium and nitrogen concentrations can be predicted. We've also seen that vegetative indices from aerial imagery are most highly correlated with leaf nitrogen at peak bloom.”

On-the-go crop sensing is another way to gather crop reflectance information, according to Varco.

This method correlates with actual leaf tissue nitrogen, “although early season, the correlation is weak. This is because nitrogen levels in soils are adequate across all soils to grow cotton to a certain point. It's when the plants start to put more demand on the available soil nitrogen that we see differences.

“This information can be used to build algorithms (for variable-rate applications). For example, if we know that at a certain growth stage, we're shooting for 4.5 percent nitrogen in the leaf and we have an area that is sensing 3.5 percent, we know from plot research that to get to 4.5, we need 60 pounds of nitrogen.

“It's going to take some time to get more quality data. Years ago, we didn't design experiments like this. We can't predict it 100 percent, or 100 percent of the time. Even with a good algorithm for basing fertilizer rate decisions, I would still take some leaf tissue data to help calibrate and validate the system.”