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Variable-rate technology could be key nutrient management tool

If EPA mandates the use of nutrient management plans (NMPs) on farms under the Clean Water Act, one result could be more use of variable-rate technology, especially for nitrogen, according to a Louisiana crop consultant participating in a panel discussion on NMPs during the 2005 Beltwide Cotton Conferences in New Orleans.

But some components of NMPs — such as how to best steward soil fertility — could be complex and would require more knowledge than can currently be applied, according to Harold Lambert, a crop consultant based in Innis, La.

The panel members described how they currently manage their fertility programs, then discussed changes they might make if NMPs were implemented.

“Currently, I try to do more soil testing than is done in the community in general, especially on light- to medium-textured soils,” Lambert said. “Our heavier-textured soils are Sharkey clay and Sharkey silty clay loams, and frequent soil testing is not going to be quite as important.”

Lambert usually recommends additional phosphorus, zinc and Contsulfur, “especially on our lighter soils that are commonly very low in organic matter. I do have soil test data that indicates these elements are needed.”

On the other hand, with the use of the strong Bray phosphorus extraction, (a test which LSU uses), “it's difficult to get a phosphorus recommendation on soils where we've seen a benefit, especially with corn production, by supplementing the phosphorus status. I'm no expert on it, but it's the most rigorous extraction I've seen.

“We need some research to get a better phosphorus recommendation. When we apply outside the best available information from the university, I have a question as to whether that's a violation of best management practices, and what might be the ramifications on the producer in that situation.”

Lambert says that as a general rule, “there are probably too many blanket applications of phosphate and potash in my area. We make some variable-rate nitrogen applications, using zone management based on soil conductivity. This, combined with yield history, allows us to address rank cotton potential on strong soils. If we have ample rainfall, we can't apply enough plant growth regulator.

“On fields that have excellent yield history no matter what crop we produce, I frequently recommend phosphate and potash based on removal rates. If we are producing a 60-bushel wheat crop followed by a 50-bushel soybean crop, we remove a substantial amount of potassium.

“I recommend slightly less nitrogen for corn on our lighter soils because of how that impinges on the following year's cotton crop.”

Lambert says his first response when asked how he would change his fertility management under the Clean Water Act, “is to do nothing. We're doing about as well as we can right now. But that's not really the answer.

“We definitely need to do more variable-rate nitrogen applications, which would save us costs in both nitrogen and plant growth regulators.

“If we are conservative with our nitrogen rates on cotton on our strong land, then once the growing season is under way and there is the potential for high yields, we may have to rely on a little more foliar-applied nitrogen to help us make a good crop.”

Variable-rate lime is another option, according to Lambert. “Using zone management is the way we need to go. We have enough variability in a lot of fields that we can drop rates in the heavier clay in the back third of the field that is just not going to need the lime rate.”

In addition, good soil stewardship might mean increasing an applied rate where fertility is low. “How we evaluate that might affect a nutrient management plan for factors such as non-point source pollution.

“It's something I need to learn. We have a lot of soils that are definitely testing low in a number of nutrients, and it's going to take a number of years for the fertility to be added to them.

“We're going to have to look more at grass buffer strips and riparian buffer strips to address runoff. But if we have torrential rainfall, some of these grass waterways could take a licking with glyphosate runoff.

“We have a good track record on crop rotation, but we're going to need to do more of it,” Lambert said. “Some of our rotational crops have had their share of problems, like aflatoxin in corn. We need to do more fall rebedding, which is going to mean extra effort and expense for a lot of people. We could have a better approach to winter cover cropping.”

Southeast Arkansas crop consultant Charlie Guy says if the future mandates NMPs, technology will play an important role. “Currently, we're not doing the variable-rate applications that we need to be doing.”

Shep Morris, a central Alabama cotton producer and Southeast High Cotton award winner, already has nutrient plans in place on fields where he applies poultry litter as fertilizer. “We probably are going to shift our application to the spring on our corn to better use our nitrogen. We also need to do a better job of record-keeping.”

The Clean Water Act, which was put in place by Congress in 1972 and modified several times since, establishes the basic structure for regulating discharge of pollutants into U.S. waters. Under the act, EPA is empowered to implement pollution control and set water quality standards.

There are two key points for agriculture: establishing total maximum daily loads (TMDLs) and developing management programs that describe methods to reduce non-point-source pollution nutrients from agriculture.

State organizations in many states have already begun the process of establishing nitrogen and phosphorus levels that will be acceptable by Clean Water Act standards in various bodies of water.

The next step is for each state department of environmental quality to present a package to its state legislature for enacting the new standards into law. Many states in the Mid-South are either at that step or close to it.


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