Controlled-release fertilizers enhance yields and efficiency, but at a cost
By Renee Stern
Controlled-release fertilizers can improve efficiency, boost yield and quality, and reduce nitrate leaching, but whether the gains pay for the materials' higher costs depends on the crop and growing conditions, researchers say.
Polymer coatings on the fertilizer pellets break down gradually and release urea and other nutrients into soil, allowing growers to reduce applications to a single early-season pass through their fields or orchards.
That advantage disappears with short-season crops that already rely on one-time fertilizer applications, says Russ Wallace, vegetable and weed specialist at Texas A&M University's Agricultural Extension Center in Lubbock.
Tests of Georgia-Pacific's Nitamin 30L and Nitamin 43G in cantaloupes, watermelons, chili peppers and processing snap beans show the longer-season cantaloupe and chili peppers gain more potential benefit, Wallace says.
Robert Revels, owner of Robert Revels Farm in Hastings, Fla., has included 20 of his 500 acres of chipping potatoes in University of Florida field tests of various controlled-release fertilizers for the past two years. Current costs make their use throughout his operation prohibitive, he says.
"But one advantage is it's a one-shot deal," Revels says. "It costs money, too, to put out fertilizer in two or three applications for fuel, labor and machinery wear."
With supplies expanding while costs of fuel, labor and conventional fertilizers increase, the price differential is narrowing and making such controlled-release fertilizers as Scotts Company's CitriBlen more attractive, says Tom Obreza, professor of soil and water science at the University of Florida in Gainesville.
Obreza has focused on citrus crops, where cutting fertilizer applications can simplify grove management when younger replants that require additional nutrients are mixed among more mature trees.
Tests also show less nitrate leaching with the use of controlled-release fertilizers compared with both fertigation or conventional dry fertilizer, he says. For growers in areas where leaching is a significant concern, that benefit may tip the balance.
Benefits for growers on sandy soils
Controlled-release fertilizers are a good pick for growers working with sandy soils, says Don Genrich, county agricultural Extension agent in Adams, Wis. Models indicate that without some change, nitrate concentrations in local groundwater eventually will reach an unacceptable level of 30 to 40 parts per million.
"In those years where we have very large rainfall events, these products are superior," Genrich says. "In years where we don't have any, the results are comparable."
Not losing nutrients to leaching means growers can achieve their yield goals using less fertilizer. Trials with Agrium Inc.'s ESN Smart Nitrogen controlled-release fertilizer produced a bushel of corn with a 0.5 pound of nitrogen, compared with the 1.2 pounds per bushel recommended for fine soils, he says.
Nitrate leaching is also a concern in Florida's sandy soils, where a state water management program offers some cost-sharing to potato growers who adopt best management practices, says Chad Hutchinson, a UF assistant professor of horticultural sciences.
"We've been very successful," Hutchinson says.
Controlled-release fertilizers can cut nitrogen application rates from the 200 pounds per acre suggested in the BMPsitself already lower than the local average of 250 pounds per acreto 150 pounds per acre. "That's a huge savings," he says.
Putting pencil to paper
The final equation shows controlled-release fertilizers roughly equivalent to or slightly more expensive than conventional fertilizers, Hutchinson says.
Using Scotts' PotatoBlen product, for example, might run an extra $10 to $15 per acre.
Controlled-release fertilizers provide growers with a sense of security that their fertilizer program is safe no matter how much rain falls, Hutchinson says. With conventional fertilizers, soil tests can pinpoint nutrient levels and how much fertilizer remains, but often too late to correct any shortfall.
And for crops where uneven growing conditions produce stress that creates quality defects, the steady release of nutrients can pay off in more valuable harvests, he says.
In corn and other grain crops, doling out nitrogen in a steady stream reduces the unwanted vegetative growth that can appear with sudden, large nutrient bursts from conventional fertilizers, Genrich says.
Even growth means better quality
For potato growers, evening out growth spurts should decrease such quality defects as hollow heart and internal heat necrosis, Hutchinson says.
Revels says his fields in the controlled-release tests have produced fewer defects as well as more uniformly sized potatoes--both important factors for processing markets.
"That's one of the first things I noticed in the first year," he says.
Test fields also had higher yields the first year, but drier weather might have caused a drop-off in the second year, he says.
Controlled-release fertilizers are more sensitive to moisture levels because they rely on moisture and soil temperature to break down protective coatings.
Wet and dry cycles may slow or interrupt the process, Obreza says. At the same time, too much irrigation could hasten nitrogen release.
Wallace's Nitamin tests showed a 14 percent yield gain for chili peppers over conventional fertilizers, as well as a boost in marketable fruit. Processing beans also showed a 4 percent gain in desirable sizes, increasing marketable yields.
But, he says, high temperatures during the 2006 growing season reduced watermelon, cantaloupe and snap bean growth while encouraging chili pepper growth, potentially skewing yield differences.
Wallace suggests growers conduct their own trials before adopting controlled-release fertilizers to determine how much benefit their operations might see.
"Every grower will have a different reason for using or not using these types of fertilizers," he says.
Despite the higher price, Revels hasn’t ruled out the controlled-release products.
“The program has potential," Revels says. "The key is to improve the environment and still be able to farm and make a living at it."
University of FloridaControlled-release fertilizer opportunities and costs for potato production in Florida: