AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Spring 1998, No. 2
Corn is receiving increased interest as a rotational crop with cotton where nematodes and other pests present challenges for cotton. Rotating corn with soybeans often increases soybean yields as well. As growers plant corn instead of cotton on some of the less productive soils, there is a common misperception that potassium needs will be less for corn than for cotton. Failure to adequately provide the potassium needs for corn can prove just as costly as potassium deficiency in cotton. Medium to high soil test potassium levels will also be needed by corn to produce good yields. For example, a 180 bushel per acre crop will take up 240 pounds of K2O. If adequate potassium is not provided: stalks can be weak and lodging can occur, plants can become more susceptible to drought stress, nitrogen use efficiency can decrease, and nitrate-nitrogen can accumulate in the soil profile and threaten water quality.
If corn leaf tips and margins begin to yellow and then turn brown near the bottom of the plant, potassium deficiency is indicated, and yield loss has probably already occurred. This potassium deficiency symptom is often confused with a nitrogen deficiency symptom. Nitrogen deficiency appears first on older leaves near the bottom of the plant and starts at the leaf tip, but moves back toward the stalk along the midrib, and not the leaf margins. In nitrogen-deficient young stands, the entire plant may become pale green and have a spindly stalk. Deficiencies of both nitrogen and potassium progress up the plant as the deficiencies become more severe.
About 50 pounds of K2O are removed in the harvest of 180 bushels of corn compared to about 40 pounds in 1,000 pounds of cotton lint. During the maximum uptake period, corn may absorb 7 to 8 pounds of K2O per acre per day. This rapid uptake period for corn occurs from about the knee-high to tassel stages (38 to 60 days after emergence), for 20 to 25 days. Maximum potassium uptake rates for cotton usually occur between 40 to 100 days after planting.
The higher total potassium uptake demand by 180 bushels of corn, compared to 1,000 pound yields of lint cotton, as well as a higher peak daily demand by corn, calls for close attention to potassium needs for corn. Soil tests should be used to determine potassium needs, with upward adjustments for high yield goals. Increased potassium rates may also be needed to compensate for inefficiencies associated with compaction, soil acidity, or inadequate moisture. In-season leaf tissue monitoring can be used to evaluate the fertilization program and potassium sufficiency. Optimum levels for corn are from 2.5 to 4.0 percent potassium in whole plants less than 12 inches tall; from 2.0 to 2.5 percent potassium in the leaf below the whorl, prior to tasseling; and from 1.7 to 2.5 percent potassium in the ear leaf at silking.
Plan to produce more corn in 1998 with optimum potassium fertilization, in balance with good nitrogen management. If converting from cotton or soybeans, pay close attention to the corn potassium demand. You will be increasing your profit potential, decreasing the lodging risk, improving water-use efficiency, increasing nitrogen-use efficiency, and leaving less nitrogen in the soil to be subjected to leaching.