NEWS for North Dakotans
Agriculture Communication, North Dakota State University
7 Morrill Hall, Fargo, ND 58105-5665

February 26, 1998

Farmers Growing Soybeans in New Areas May Need a New Approach

Getting acceptable yields on new soybean acres planted in northwest Minnesota and northern North Dakota may require a very different management approach from the one used in traditional soybean regions, according to Dave Franzen, soil specialist for the North Dakota State University Extension Service.

In traditional regions to the south, soybeans may flourish on their own nitrogen, and on phosphorus and potassium scavenged from the overdose given to the previous year's corn crop. In the new, less soybean-friendly areas, they may not.

One problem in these northern areas, says Franzen, is that soybeans are under unusual stress because of cool soils, high levels of free lime and carbonate minerals, high levels of salt and low iron availability. As a result, the soybean may not be able to easily start generating its own nitrogen.

"The relationship between the nitrogen-fixing bacteria and the soybean is very intricate," says Franzen. "The young soybean root gives off a chemical signal which is sensed by bacteria, which are then attracted to the root and become cemented to its surface. The bacteria give off a growth-regulator chemical that makes the small root-hair curl, and that opens the interior up to invasion—or infection, as we call it.

"Inside, the bacteria produce a tumor that we call a nodule, making a home for many colonies of bacteria called bacteroids, each containing thousands of bacteria. These bacteroids within the nodules are bathed in plant nutrients and begin to fix nitrogen for the soybean.

"But if the soybean is under stress, it is not as likely to release the signal to the bacteria to encourage infection."

That's why, says Franzen, growers in these new, marginal areas may need to add nitrogen to the soil if they wish to get the best yield. Studies at Crookston, Minn., showed that soybean yields were increased significantly when a small amount of nitrogen fertilizer was present at seeding.

Generally, if nitrate-N soil test levels are below 50 pounds per acre to a depth of 2 feet, application of nitrogen fertilizer to reach the 50 pound-per-acre level should increase yields, says Franzen. But he cautions that this small amount merely serves as a stimulant to get the soybean crop to produce its own nitrogen. For good yields, the soybeans will need 200 to 300 pounds of nitrogen per acre, most of which the soybean plants will need to produce themselves—which is why the crop absolutely must be inoculated with nitrogen-fixing bacteria.

Here are the steps to follow to make certain a soybean crop grown in the new areas to the north will have sufficient nutrients to allow a maximum yield:

• Add nitrogen sufficient to bring the level to 50 pounds per acre.
• Inoculate the seed at planting with proper bacteria.
• Place no fertilizer with the seed, for the seed is very sensitive to the salt and ammonia in most fertilizers. Recent tests show that even the safest seed-placed fertilizer may reduce soybean stands by more than 30 percent.
• Phosphorus should definitely be applied if soil test levels are less than medium. If you do apply phosphorus fertilizer, banding offers no advantage and broadcast application may even be better. At medium and higher levels the phosphorus recommended is intended to maintain soil test levels, not provide large yield increases, so it may not be practical to hire an applicator to apply 20 to 30 pounds per acre of phosphorus fertilizer alone. Consider front-loading the previous crop or tagging some extra phosphorus on to the succeeding crop to maintain soil test levels.
• If soil potassium levels are high, there should be no need for additional. An exception would be in ridge-till systems, but there are only a handful of these in North Dakota.
• No micronutrients, such as zinc or copper, are generally needed, even when they are in the soil at low levels. It is true that soybeans can respond to added sulfur if soil sulfur is at a low level, but that usually only occurs in sandy areas in upland landscapes, such as the beach areas east and west of the Red River Valley.
• Plant the most iron-chlorosis-resistant variety available for the maturity zone. Iron deficiency commonly appears in soils high in free lime which, when wet, produces bicarbonate that apparently inhibits iron uptake. Seed treatment or foliar sprays of the iron chelate FeEDDHA have been shown to increase yield but the treatment costs $20 or $30 an acre and results are inconsistent.

Some people, says Franzen, have suggested that if fertilizer were applied to soybeans just at the time when leaves stop sending nutrients to the roots and begin sending those nutrients instead to the developing pods and seeds, yields might increase. The theory is that when activity of the nitrogen-fixing bacteria drops off for lack of nutrients, added fertilizer might kick in and give an added boost to the plant. So far, however, research seems to indicate that this practice works only rarely.

"Because of the low frequency of yield increases, late-season fertilizer application to soybeans is not recommended," says Franzen.

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Source: Dave Franzen (701) 231-8884

Editor: Barry Brissman (701) 231-7866