A little potash can go a long way in boosting corn yields.

But potash will help corn yields only if the soil lacks potassium, according to Tim Wagar, crops and soils educator with the University of Minnesota Extension Service.

"Nitrogen is the most important fertilizer input for corn and will usually provide the highest return on investment when operating funds are tight," says Wagar. "However, in southeast Minnesota even if the crop has enough nitrogen available, a shortage of potassium can hold down yields. Potassium is short when a soil test shows 80 parts per million or less."

Wagar says 29% of soil samples from southeast Minnesota submitted to the University of Minnesota Soil Testing Laboratory, from 1990 to 1997, tested low for potassium. In 1998 and 1999, 28% of the samples from southeast Minnesota were low in potassium.

"Low potassium soil test levels are due primarily to the removal of large amounts of potassium at harvest, especially with corn silage, alfalfa haylage and alfalfa hay," says Wagar. "The type of clay in silt loam soils that are predominant in the area also reduces the availability of potassium for plant uptake. Other factors that may affect corn potassium uptake are soil moisture, compaction and reduced tillage."

Symptoms of potassium deficiency can be hard to distinguish from nitrogen deficiency or herbicide damage, says Wagar. Areas short on potassium have uneven growth patterns and the plants are stunted, yellowish green and the lead margins of the lower leaves have a scorched appearance.

"Low potassium soil tests should be a concern to farmers growing corn in southeast Minnesota," says Wagar. "Field trials the past few years have shown significant yield increases for corn grown on soils testing low in potassium when the crop received recommended potash rates from starter or broadcast fertilizer."

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