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Beware Nutrient Deficiencies in Soybean Fields

Lack of potassium, manganese can wreck yields.

During the Kansas Soybean Expo each year, farmers can obtain practical research that may help boost productivity, or discover trends that affect production efficiency.

The 2008 version was no different. Kansas State University agronomists and researchers provided a wealth of information pertaining to this increasingly-valuable crop. Read on for more:

Potassium deficiency in southeast Kansas

A bushel of soybeans contains between 1.2 and 1.4 pounds of potassium, making that nutrient an important element in soybean production.

"In most of Kansas, potassium deficiency isn't an issue because the soils and subsoils are inherently high in potassium. But south of the Kansas River, east of the Flint Hills, the soils are generally low in potassium and we're seeing potassium deficiencies that have to be impacting yield," explains Dave Mengel, soil fertility specialist at K-State.

Potassium deficiency is more apparent later in the season, during vegetative growth and as the pods begin to fill. "You'll see marginal necrosis, with the leaf turning yellow at the leaf margins at the edge," he says. A potassium level of less than 130 parts per million is considered critical, and the shortage must be corrected prior to planting. Foliar applications don't allow the nutrient to get to the root zone, where it is needed. Potassium does not move in the soil. Interestingly, the problem seems to be more prevalent in dryland, no-till practices.

Mengel began a research project in 2007 to study the issue. Weather-related issues hampered the 2007 research, but he plans to continue analysis in 2008 and will broaden the project to include corn, and applying potassium as a starter fertilizer and in strip-till applications, he says.

Manganese deficiency in RR soybeans

Farmers planting Roundup Ready soybeans may be able to boost yields with a dose of manganese, says Barney Gordon, agronomist in charge at the North Central Kansas Experiment Field in Republic County.

Manganese is responsible for the degradation of fixed nitrogen as the nutrient is transported from roots to shoots. But studies show that the Roundup Ready gene inhibits this process; furthermore, glyphosate application may interfere with manganese metabolism within the plant.

Gordon's research, conducted in 2005-06, includes several corrective measures, including applications of 2.5, 5.0 and 7.5 pounds of magnesium sulfate (MnSO4) banded at planting. As the rate of MnSO4 increases, the yield of conventional soybeans fell from 77 to 71 bushels per acre; the yield of RR beans rose from 70 to 77 bushels per acre.

Further research indicates that two 0.3 pound-per-acre applications of liquid manganese - banding at planting plus a foliar application at the V4 stage – provided the most bang for the buck.

Gordon cautions, however, that manganese may not pay off for everyone.

"This fits well in a program managed for yields above 60 bushels per acre. It may not help when yields average 30- to 40-bushels per acre," he says.

Farms with soil pH levels of 6.5 and higher, with sandier soils are also more likely to show a response, he adds.

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