Since the mid-'90s, potassium (K) deficiency in corn has become more noticeable in major production states in the Corn Belt, says Antonio Mallarino, Iowa State University (ISU) agronomist.

If left unchecked, extreme cases of the problem can easily cut yields by as much as 50% or more per acre and affect grain quality.

A recent survey by the Potash & Phosphate Institute (PPI) reports that nationally about 43% of soils were potassium deficient.

The survey found that a higher percentage of lower-testing soils occurred in the Midwest, as well as in the East and South. Soils in the Great Plains and the West tended to exhibit naturally higher potassium levels.

But putting your finger on the root cause of this deficiency problem isn't always easy since it may involve several factors — individually or in combination.

“In some cases you may face a fertility issue where there simply isn't enough available K in the soil in most areas of a field or its availability may be low only in a few select areas within the field,” explains Mallarino. “In other cases, the deficiency may be induced by dry topsoil while roots still can get water from the subsoil. Or, it may be caused by soil compaction, inadequate placement depth of the potassium (more often for ridge-till and no-till systems), corn rootworm or other pest problems.”

What are some signs to look for? Potassium deficiency symptoms in corn appear first on the lower leaves because K is easily moved within the plant from older to younger leaves, Mallarino says.

Leaf symptoms are a yellowing to necrosis (brown dead tissue) of the outer leaf margins. These symptoms begin at the leaf tip and progress down the margin toward the leaf base.

He also cautions not to confuse symptoms of potassium deficiency with nitrogen deficiency, which first appears as yellowing on the lower (older) leaves, but is located from the leaf tip down the midrib in a V-shape.

Dry weather-induced K deficiency symptoms may not include necrosis (dying back) of leaf tips and lower leaf margins. For this type of deficiency, look for leaves in the middle of the plant to turn yellow at the tip as they become chlorotic (lose chlorophyll). The entire leaf eventually turns yellow, which produces a yellowish sheen on the plant except for newly emerged leaves that are a normal green color.

Following a good soil testing program still remains your best defense against potassium deficiency, especially in suspected areas of fields, says Scott Murrell, north central director of the Potash & Phosphate Institute in Woodbury, MN.

“In crop production you obviously don't get something for nothing,” says Murrell. “Soybeans can remove substantial amounts of potassium — more than corn in many areas. But forage crops, like corn silage and alfalfa, remove the most per harvest unit by far.”

Much of the potassium in the plant is found in vegetative parts of the plant. When these are harvested, substantial amounts of potassium are also removed. Failing to account for these losses and under-fertilizing with potassium can lead to problems.

“If you suspect potassium deficiency, you may want to soil sample more often, such as every other year instead of every four years, and on a grid system of 21½2 acres or less,” says Mallarino. “If you're using a global positioning system, you can record where more deficient spots are and for the next crop you can apply a higher potassium rate to those areas, using a variable-rate fertilizer application with relatively good accuracy.”

If you're using ridge-till or no-till practices, Mallarino also suggests paying close attention to the placement of the potassium.

In field tests conducted on farms in Iowa, Mallarino found that in most cases deep banding potassium with coulter knives at depths of 5-7 in. promoted better utilization compared to shallower depths.

Mallarino also admits that in past years, the recommended application rates for potassium may have been on the conservative side.

Now, with higher yielding corn varieties and based on more recent and extensive soil testing research, he says ISU changed the soil test potassium interpretation classes and now recommends maintaining a higher soil potassium level and slightly higher application rates for the new low-testing categories.

“However, it's still important to base those rates on a good soil testing program,” says Mallarino. “It would be very difficult to solve a potassium deficiency problem with a one-size-fits-all mentality.”

Maintaining adequate potassium levels can also influence the efficiency of nitrogen (N) utilization in crop production, according to Murrell.

“Applying more nitrogen can't compensate for the drop in yield on potassium-deficient soils,” Murrell says. “Adequate K fertility levels help a crop make better use of the N that is applied. Higher yields and greater returns to N investments are possible when crops are well fed. More efficient use of N by the crop means less N is left to move to unwanted places in the environment.”

For more information on potassium deficiency, you can access these Web pages at www.ipm.iastate.edu/ipm/icm/2002/7-1-2002/cornleaf.html or www.extension.umn.edu/distribution/crop.

You can also get more information from the PPI Research Database web site at www.ppi-ppic.org/far/farguide.nsf. In the left-hand column, go to “North American Projects” and click on “By Topics” where you can scroll down to the subject of potassium.