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Research fine tunes potassium in rice

Because of its shallow, dense root system and ability to grow in flooded conditions, rice is considered a potassium-efficient crop. That belief still holds, but some years test the faith.

“This season we've had problems with potassium in rice, especially in northeast Arkansas,” says Nathan Slaton, agronomist and professor with the University of Arkansas. “We've concentrated field research there and have had good response. Extension agents, specialists, farmers and consultants have visited through the year. They say, ‘Yeah, that's what we've seen. Now I know what's going on in our fields.’”

Of the potassium deficiencies Slaton and colleagues have seen, most occurred on silt loam soils west of Crowley's Ridge (Cross, Poinsett, Woodruff, Craighead and St. Francis counties). The area has a history of potassium troubles which usually show up mid- to late-season.

“I just visited a farmer who'd applied potassium to a moderately deficient field,” says Slaton, who spoke at the Rice Research and Extension Center field day outside Stuttgart, Ark. “Adding potassium changed the look of his crop for the better. Comparing an untreated area in the field with the rest was a revelation.

“Before the application, he asked — and this is the question we get most often — ‘I'm finding symptoms of potassium deficiency from the start of heading to two or three weeks away from heading. Will it pay to apply potassium?’”

Slaton doesn't yet have good answers. Potassium is difficult to work with, and many field tests provide limited responses.

“We're studying it, but getting a properly researched answer within two years is probably optimistic. The nature of fertility research requires a longer time for answers.”

In the past, Slaton has applied potassium early and heavily enough to carry plants through the season. He still believes that's the best approach.

“However, we must accurately identify the fields that need potassium. Then we must do a better job of calibrating fertilizer rates. We have seven potassium rate studies across the state. That may sound like a lot, but it may take 40 tests to get enough data to properly calibrate fertilizer tests. With that many tests, it could be years before we have the answers we need.”

Most Arkansas boron deficiencies show up north of I-40 and west of Crowley's Ridge. Thus, most research efforts are also focused there. Soil testing, however, hasn't been very beneficial in identifying boron-deficient soils.

“We know boron deficiencies will show up in high-pH soils. But these soils constitute 90 percent of the fields in the area being studied,” says Slaton.

Researchers are attempting to determine the rates of boron needed and when to apply it. They have found a fairly wide application window, but in less than 5 percent of fields, applying boron early gives the best benefit.

“Here's the deal: by the time beans are blooming on that 5 percent, they're already boron-deficient and losing yield,” says Slaton. “Based on our research and for farmers' convenience, we believe a preventive approach is best.”

There are several ways to buck up a field's boron levels. Granular boron blended in with phosphorus and potassium will cost $2.50 to $3 per acre. “The nice thing about that is there's no special application — producers are going to put out P and K anyway.”

An alternative that works “really well” is boron mixed with a herbicide. “You can use a soil-applied herbicide or, in Roundup Ready beans, mix liquid boron with Roundup.”

Another alternative — potentially the most costly and certainly risky — is the “wait-and-see approach,” says Slaton. “In this case, a farmer must scout heavily. If he finds boron deficiencies, he looks at market prices, determines how bad the crop is, and decides whether or not to fly on up to a half pound of boron per acre.”

There are several problems with the third approach, including an extra application charge.

Producers usually don't scout soybeans as well as they do “money crops” like cotton or rice. A boron deficiency may not be found and diagnosed for several weeks.

And there's a tipping point. “After a certain point, plants aren't going to respond if boron is applied.”

Slaton plants rice test plots each year to study phosphorus, “because it has a huge impact on our crop, and a deficiency is very hard to turn around. Phosphorus has been at the top of our research for the last decade. By now, we feel pretty comfortable with our recommendations.”

Slaton focuses on plants shortly after flooding, when most P deficiencies show up. “We look at soil factors and plant tissue phosphorus concentrations to determine where and how much to apply. We've come up with some very strong relationships.”

The hole in the plan is phosphorus impact. Timing sometimes has a huge impact on plants. But that's hard to get across in the real world, says Slaton.

“Fertilizer dealers often give good deals that are hard for growers to pass on. A dealer will say, ‘It's fall and our equipment is idle. If you put out fertilizer for beans and rice now, we'll give you a big break in price.’”

Phosphorus is similar to nitrogen in that a lot can happen between the time it's applied and the time the crop is ready to take it up. Nitrogen can “float off” in the air. Phosphorus, says Slaton, can become “fixed” in the soil, no longer available to plants.

“If you put out phosphorus too early, especially in a soil that really needs it, the crop will suffer. That's why we're working on when and where to apply phosphorus, and the best time to apply it. Is it best applied preplant? Pre-flood? A combination of the two?”

On most soils, a phosphorus application just before planting will benefit a crop, says Slaton. On a few soils, though, “we've found a shot of phosphorus with pre-flood nitrogen gives rice a boost.”


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