Steady Diet
March 1, 2008 
Few growers would argue that it’s critical to provide adequate nitrogen to a crop when it’s needed. But the problem is, how do you ensure that happens? For example, what if your fields are soaking wet, and you can’t get out there to apply fertilizer? James Wargo, a senior agronomist with Georgia-Pacific, believes he and his colleagues in the company’s Plant Nutrition Group have found an answer.
It’s a new slow-release fertilizer called Nitamin Nfusion®. Instead of providing nitrogen (N) for seven to 14 days, as a regular urea fertilizer would, this new product breaks down slowly over 60 to 90 days. “Growers can have the peace of mind knowing that the fertilizer they put out a couple weeks ago is still working for their crop,” he says. “They have insurance; nitrogen is out there and hasn’t leached away.”
The new product is a urea polymer-based fertilizer that contains 22% nitrogen, of which 94% is slowly available. The urea molecules are linked together, and then slowly broken down by soil microbes. Another advantage of this approach, says Wargo, is that as soil warms, microbial activity increases, and so does the available N. That works in harmony with the crop, because as soil temperatures increase in the spring, so do the crop’s nutritional needs. “You don’t want to force-feed the crop,” he says. “This is sort of like eating three meals a day instead of one giant meal.”
Blends Work Best
Most growers want to get their crop off to a good start, of course, which means that a certain amount of nitrogen needs to be made available right away. That’s why growers typically mix the product with a quick-release nitrogen, usually with anywhere from 20% to 50% of the slow-release fertilizer, says Wargo. However, that depends on a whole host of factors, especially the type of crop. With a short season crop like 60-day cucumbers, a grower would want much of the nitrogen readily available, so a blend with 20% slow-release might be used. At the other end of the scale, a 50/50 blend might be used on 120-day potatoes.
“The grower has a great deal of control,” Wargo says. “By ensuring a steady supply of nitrogen at peak nitrogen demand, you’re not limiting crop growth.”
Crops are responding favorably. Numerous field trials performed by university researchers across the country on a wide variety of crops, including potatoes, tomatoes, onions, and melons, show that growers are getting higher yields, says Wargo. Not only are total yields higher, but yields of larger size grades that generally provide growers with greater returns have increased. (See chart “More Big Spuds.”) “We’re seeing a shift in the size curve,” he says. “Yields are generally up 5% to 15%, though we’ve seen them higher.”
Timing is the key. For example, in potatoes it’s crucial to have adequate nitrogen available during the tuber initiation phase, about 35 to 45 days after planting, says Wargo. If there’s not enough nitrogen the plants will stress and tubers will not fully develop, cutting the number of harvested spuds. Later during the summer comes the tuber bulking phase, says Wargo. If enough nitrogen is not available at this time, the grower won’t harvest as many of the larger potatoes that attract higher returns. “What we’re trying to achieve is putting N in position to be used by the plant when it needs it,” he says.
Environmental Benefits
Though more nitrogen is made available to the plant, that doesn’t mean more nitrogen is being applied. In fact, growers can generally apply lower rates of a slow-release blend and still see greater yields than they would with higher rates of straight urea, says Wargo. One reason for that is that unlike field crops, such as wheat and field corn, a vegetable grower can’t just put more N on as an insurance policy. Too much nitrogen at the wrong time isn’t good for vegetables.
For example, if a potato grower puts on too much nitrogen shortly after planting, the plants will put on excessive top growth at the expense of tuber formation. “Even though they’re the biggest plants in the field,” he says, “they can be the least productive.”
In addition, quick-release forms of nitrogen moves with irrigation water down through the soil profile. Repeated irrigations can leach the negatively charged nitrate anions beyond the plants’ root zone, rendering it unavailable. Not only that, but government agencies across the country have begun to crack down on nitrate leaching. A drip irrigation trial by Virginia Tech University demonstrated that if quick-release and slow-release fertilizers were applied at the same rate weekly over a 12-week period, cumulative nitrate leaching was 55% lower with the slow-release fertilizer.
One other environmental benefit of slow-release fertilizer is a lower salt index. That can be especially advantageous in areas with high salinity soils, such as in Florida and parts of California’s San Joaquin Valley, Wargo says. While it has a salt index of just 1.7, calcium nitrate has an index of 65 and urea has an index of 75.
More Big Spuds
Treatments:
Check- no fertilizer applied
Nitamin Nfusion pre-emergence – 160 pounds. N from Nitamin Nfusion/UAN blend applied in one pre-emergence application on May 22nd
Nitamin Nfusion sidedress – 80 pound. N from urea applied pre-emergence on May 22nd; 80 pounds N applied from Nitamin Nfusion/UAN blend on July 2nd.
Urea low – 80 pounds N from urea applied pre-emergence on May 22nd, and 80 pounds N as urea sidedressed in three applications 10-14 days apart beginning on July 2nd.
Urea high – 120 pounds N as urea applied pre-emergence on May 22nd, and 120 pounds N as urea sidedressed in three applications 10-14 days apart beginning on July 2nd.
Notes: Planting occurred on April 27, 2007. Plants were vine killed on Sept. 10th, and harvested on Sept. 28th. Applications of nutrients other than N were applied according to soil test and University of Idaho fertilizer recommendations. Soil type was a loamy sand with 1.0% organic matter, pH 8.0 and 4%-8% lime content.
Source: Bryan G. Hopkins, Ph.D., Brigham Young University
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