AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Spring 2000, No. 1
The North American soybean crop removes more than twice as much potassium as it receives in the form of fertilizer. Every bushel contains almost a pound and a half of potash. Planted following grain corn, the high removal has rarely been a problem, as corn leaves behind more than two-thirds of the potassium it takes up from the soil. The potassium left behind becomes available when rains between growing seasons wash it out of the stover and back into the soil. But more and more often, soybeans follow crops other than corn.
Potassium fertility needs more attention. It’s often observed that soil fertility determines only a small part of yield for a successful soybean crop. That’s true when soil fertility has been managed well for the corn. But as new rotations involve potassium-hungry soybeans more frequently, the importance of soil fertility will be increasing.
Conservation tillage systems also change potassium needs. In such systems, potassium builds up near the soil surface and is depleted further down. In addition, soils may become more compacted and cooler, restricting root growth. With less roots exploring the soil volume, potassium uptake can be reduced. In clay-textured soils, wet or flooded conditions can make potassium less available by increasing soil fixation.
While soybeans will respond to broadcast potassium, they respond more often when the fertilizer is placed in a band a short distance away from the seed. In Iowa and Minnesota, the most frequent responses have occurred with deep banding, placing the potash about six inches deep. In other areas, responses were more common when the band was two inches beside and two inches below the seed. Fertilizer in the seed furrow does not work well. Soybean seeds are sensitive to fertilizer salts, even with the sulfate sources that have a lower salt index.
Potassium increases plant resistance to pests and diseases that often reduce soybean yield and quality. When soybean plants have a low potassium status, their ability to assemble proteins from amino acids is reduced. The amino acids that accumulate make the plant more attractive to pests and disease organisms. Higher potassium levels can reduce pod rot and purple seed stain. Maintaining high soil test levels is part of an effective management strategy to reduce the impact of the soybean cyst nematode.
Deficiency symptoms are often confused with other crop stresses. Soybeans affected by ozone air pollution, by leafhoppers, or by cyst nematodes often show the same yellowing of leaf margins that potassium deficiency produces. Diagnose the real cause with soil and tissue tests for potassium.
Soybeans often respond to the residual potassium applied to previous crops. Even where soybeans do not respond directly to applied fertilizer, it’s worth considering for the crop that follows. Base your decision on up-to-date, site-specific soil tests, and don’t forget to include any diagnostic observations from last year’s crops.
Replacing the potassium that soybeans remove is a key component of long-term stewardship of soil fertility.
For more information, contact Dr. Tom Bruulsema, Eastern Canada and Northeast U.S. Director, PPI, 18 Maplewood Drive, Guelph, Ontario N1G 1L8, CANADA. Phone: (519) 821-5519; E-mail: email@example.com.
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