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Trials seek cotton nitrogen balance

Answers for greater returns and environmental stewardship for cotton growers in California's San Joaquin Valley may lie only a couple of feet below the surface of their fields.

While that may seem just too simple, it is the thrust of research for the balance of tailoring of optimum nitrogen needs for the crop and preventing escape of nitrates into groundwater supplies.

Bruce Roberts, Kings County farm advisor, and Robert Hutmacher, University of California cotton specialist at Shafter, are among UC scientists and cooperating growers in two closely-linked projects dealing with nitrogen management.

They talked about their work during the Fertilizer Research and Education Program (FREP) Conference at Tulare. Both projects are funded by FREP, a California Department of Food and Agriculture program devoted to environmentally sound and agronomic uses of commercial fertilizers.

Roberts, referring to observations begun in 1995 on site-specific rates of nitrogen on cotton, said the work takes the findings of basic nitrogen management research and combines data on seasonal uses of nitrogen by cotton. Leader of that work is Richard E. Plant, professor of agronomy at UC, Davis.

“We are also combining with that the advanced technology of remote sensing,” said Roberts, “to identify areas of a field and apply fertilizers accordingly at a variable rate based on input requirements that we can determine.”

SJV cotton, to stay competitive in the future, will have to be a very high quality crop, a specialty crop, in fact. “Even in something as common as cotton,” he said, “we still fill a specialty niche.”

Managed by zones

At the same time, sustained, if not increased, yields cannot be ignored, and they require management of fields by zones. Roberts says that has meant considering each field's physical and chemical limitations when applying fertilizers, generally more in weak, sandy streaks, while less in stronger, more productive portions.

The goal of the research is to trim the higher costs of the weaker areas with soil amendments and apply fertilizer more efficiently at less cost.

“Ideally, growers will begin to look at fields, whether 40 acres or 400, to find areas that drag down yield. We can do that from remote imaging by satellite or aircraft or yield monitoring to generate yield maps. We can also generate salinity maps from soil samples or salinity measuring equipment.”

Once the zones that produce less are identified, corrective steps can be taken. As a short cut, yield mapping reveals parts of the field where soil sampling can do the most good.

Roberts said several progressive growers have taken up the practice, and off-season application of soil amendments, followed by variable rate fertilizer application, has increased significantly in the past couple of seasons.

Among the Kings County cotton fields in the survey, Roberts said the major limitation was high salinity. Typically, when nitrogen is applied uniformly across such a field, salinity limits plant growth and in salty portions less nitrates are taken up by the crop, leaving the potential for them to accumulate, posing potential for contamination of shallow groundwater supplies.

Less nitrogen

Part of the corrective action, along with soil amendments, is to apply less nitrogen in those portions of the field, to prevent the accumulation. Over time, as the saline spots are corrected with irrigation and gypsum to displace sodium with calcium, the soil structure improves and less nitrogen is needed.

In one 150-acre parcel in the study, fertilizer inputs were reduced 53 percent, he said. “That's not to say we found that in every field, but we did see the savings go that high. It's on a field-by-field basis, depending on the different zones it has.”

Roberts said he anticipates more variable-rate trials with grower-cooperators and demonstrations to provide this information of growers this year and to encourage them to adopt the practices.

Hutmacher, in detailing the FREP project he heads on residual soil nitrogen management for Acala cotton, said sampling of the top two to three feet of soil for springtime residual nitrogen is a worthwhile component for their fertilizer management decisions.

“Much of this is based on the nitrogen studies we've been doing the past five to six years, and the sampling is best done within a couple of weeks after planting.”

Although the samples might be taken shortly before planting, Hutmacher said soil warming — and rainfall that may occur soon after planting — influence nitrogen activity and what is available to the plant.

Soil samples

“Using soil sampling can help avoid losses and potential environmental problems from excess nitrates. This sort of thing can be helpful to growers who use dairy waste, such as some on the east side of the SJV, as part of their fertilizer program. Many of them don't need to apply nearly as much nitrogen as they did before using dairy waste,” he said.

SJV cotton growers have already begun to cut back on the nitrogen for economic reasons. Those who might have used 150 to 180 pounds to the acre in the mid-1990s are now putting on 100 to 150 pounds.

Hutmacher said he believes many growers could save 25 to 50 pounds of nitrogen without a problem. A cautious route for reduction is to try 25 pounds less in strip trials. Then with more confidence and guidance from soil testing, the rate might be pushed to 50 pounds less.

“Depending on their crop rotation and fertilizer program, many might be surprised at what they can reduce. Growers with a traditional, small grain-cotton rotation, without dairy waste, are probably already pretty tight on their nitrogen use.

“But chances are good someone coming out of forage alfalfa, vegetables, tomatoes or melons, has a good load of residual nitrogen in their soil.”

However, he added, it's not all that simple. “If you do back off on the nitrogen, be sure to have a secondary battle plan with petiole sampling, or at least checking the fruit load. You need to have in mind an additional side-dress or water-run treatment when you have great yields like last year.”

Potential problem

Backing off too much, and not having enough nitrogen to capitalize on a heavy crop, can neutralize all the best intentions for economy.

Soil sampling, while common, has become something many cost-stressed growers avoid doing on an annual basis. “But from what I saw last year, the samples are also good to keep from developing a potassium or micronutrient problem. A composite soil sample from several parts of a field each year is still a good idea if you don't care to have yearly samples taken.”

Hutmacher reminded that no soil test is an absolute guarantee of a crop response. “It is only something to plug into your decision-making process. Sampling at the three-foot depth is far better than at one foot, and nitrogen is something you need to assess early-on for each crop.”

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