Slow-release fertilizers promising

Last year, researchers were able to cut fertilizer rates using slow-release materials on vegetables without sacrificing yields. They also walked on a myth that has kept growers away from less-expensive chloride-based fertilizers.

Both situations have come to the forefront in vegetable production as the landscape has changed in the fertilizer industry.

Last year, Georgia Pacific introduced Nitamin, a 30 percent, liquid nitrogen material that releases slowly over time. “The cost for slow-release fertilizer is almost always going to be higher and applications and timing will have to be adjusted,” says Terry Kelley, University of Georgia horticulturist, who presented the research at the Southeast Regional Fruit and Vegetable Conference in Savannah, Ga.

“But there are advantages that could make it worth while for a producer, provided the cost is in line with the benefits.”

In tests on cabbage and onions, the slow-release material produced yields as good or better than conventionally applied fertilizer at 75 percent of the recommended rate of nitrogen.

Yields dropped off when the rate went below 50 percent of the recommended nitrogen rate. While the results weren't as conclusive on peppers, yields did increase.

One of the biggest advantages of the slow-release nitrogen, Kelley says, is it's a one-time, at-planting application.

In drip-irrigated crops, fewer applications mean the fertilizer is available to the crop as needed. “You can have fewer applications through drip irrigation on plastic,” Kelley says. “And you're not constantly leached out of the root zone. On bare-ground crops, such as cabbage, growers don't have to wait on the right field conditions to apply fertilizer during the wettest months of the season.”

There are nuances to consider when using slow-release fertilizers. “All crops don't react the same to slow-release fertilizers,” Kelley says.

For example, higher soil temperatures could cause a flush of fertilizer sooner in the season. “Getting the blends with all slow-release fertilizers may not be easy.

“The decision is ultimately one of cost,” Kelley says. “The reduction in rate may make up for the increase in cost.”

Four hurricanes in 2004 prevented Kelley from harvesting data on how slow-release fertilizer and tomatoes, carrots, snap beans and cucumbers. He plans to continue the research in 2005.

While testing a new product, Kelley also attacked the widely held assumption that chloride-based fertilizers hurt quality and shelf-life of vegetable crops.

He attributes the belief to a time when most tobacco fertilizers were chlorine-based and doubled as fertilizers on vegetables.

Kelley notes that vegetable producers on the West Coast use chlorine-based fertilizers. “Chloride-based potassium sources are cheaper,” Kelley says.

Stay away from using chloride-based fertilizers on Irish potatoes and root crops such as turnips. “Excess chloride can affect the specific gravity of Irish potatoes.”

“There's just no evidence to support the myth that chlorine-based materials decrease the quality and shelf-life of vegetables,” Kelley says.

Since chlorine-based materials are cheaper than others, “there's nothing wrong with using them as part of a routine vegetable fertility program,” Kelley says. “There's no reason to be afraid of chlorine-based fertilizers, as long as the amount supplied is along the same lines as crop demand.”

The chances of chlorine toxicity are almost nil, he adds.

e-mail: [email protected]

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