AGRI-BRIEFS
  AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Summer 2002, No. 4

HAVE YOU FED YOUR SOYBEANS TODAY?

Soybeans need potassium,and lots of it. The common practice is to apply fertilizer ahead of corn in the corn-soybean rotation and depend on residual, or carryover, nutrients to meet the needs of the soybean crop. It is easy to forget that soybeans remove large amounts of nutrients. Relative to corn, they are especially heavy feeders on potassium, removing 1.4 pounds of K2O per bushel of yield. A 60-bushel soybean yield removes 84 pounds of K2O per acre, or the equivalent of 140 pounds of 0-0-60. If grown in a rotation with 180-bushel corn, an additional 52 pounds of K2O, or 87 pounds of 0-0-60 are needed, for a total of 227 pounds of 0-0-60 per acre every two years just to replace what is removed. Additional amounts are needed to account for efficiency factors and to meet any buildup needs. Standard application rates of 200 pounds of 0-0-60 per acre commonly reported by Midwest farmers are not sufficient to sustain productivity at those yield levels. They are mining their fields of potassium reserves and stealing potential from future crop years. Take soil tests, measure crop yields, and be sure potassium rates are adequate to meet crop needs.

Potassium deficiency in soybeans has been observed more frequently over the past several years. Usually, it appears as chlorosis, or yellowing, of the upper leaves (in contrast to the traditional expectation of yellowing on lower leaves), beginning with the leaf margins and spreading to the centers. These yellow areas eventually die and turn brown. This deficiency occurs as the plants are shifting toward high nutrient demand during the filling of the pods. If the roots are restricted, potassium soil tests are inadequate, or the soil is dry, symptoms will be more prominent. They also may be seen more frequently under ridge planting, where nutrients have a tendency to concentrate between the rows.

If visible deficiency symptoms develop,this means that yields have already been penalized. However, hidden hunger...that is deficiency that reduces growth, but doesn’t show visible symptoms...will cut yields and profits before any symptoms are seen. The 2001 Potash & Phosphate Institute Soil Test Summary Update for North America showed potassium soil tests have declined across much of the soybean production region, and especially in the Midwest, where one-half to two-thirds of the samples analyzed by commercial and university labs showed a need for buildup potassium applications to get the soils into the most productive range.

Potassium deficiency makes the soybean crop more susceptible to plant diseases and insect damage. Plants with inadequate potassium are also much less efficient in using water and will suffer greater yield losses in dry weather. Areas of the field with insufficient potassium will have more stalk lodging when corn is grown in the rotation. Plant tissue tests can be used to help verify that potassium deficiency is present. It is best to send samples of both deficient and normal plants to the laboratory, so the nutrient values can be compared. The preferred time for taking leaf samples is at the early podset stage. Get specific sampling instructions from the laboratory that will be handling the analysis. Soil samples should be taken from both the deficient and normal areas of the field. These plant and soil test results will help verify that potassium deficiency is the proper diagnosis and will guide corrective action.

Sometimes the potassium deficiency is caused by root restrictions,such as soil compaction or drought. Correction then depends on eliminating the source of the problem. Be sure to assess the conditions in the field before drawing a final conclusion. If potassium levels are truly low, corrective action in-season is difficult to achieve. But adjustments should be made in the recommendations for potassium fertilizer before the next crop season.
—HFR—

For more information, contact Dr. Harold F. Reetz, Jr., Midwest Director, PPI, 111 E. Washington Street, Monticello, IL 61856-1640. Phone: (217) 762-2074. E-mail: hreetz@ppi-far.org
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