In 1999, widespread crop potassium deficiencies were documented. Many deficiencies occurred on fields that received potassium fertilization. Many problem fields had medium to high soil test potassium levels. In the September 1999 Today's Farmer, I attributed these problems to soil compaction caused by planting into wet soils. The problems were noticed relatively late in the growing season (July-August) last year.

As of this writing (mid-June), widespread potassium deficiencies are back. However, this year, the environmental conditions, timing, affected areas and causes for poor crop potassium nutrition are completely different. Both years had dry April weather that allowed early planting, especially corn. The difference occurred in that 1999 had wet soils at planting, compared to dry soil planting conditions in 2000. Also, 1999 had good rainfall in May, whereas virtually no rain fell during May 2000. The extremely dry, early season this year caused poor root growth, especially where seed was planted shallow. Greater than 75 percent of all corn problems I observed this year have been related to shallow planting. The limited root growth near the dry soil surface has caused a myriad of problems, including poor potassium uptake.

Potassium is relatively nonmobile in soils. It may move an inch through sandy soils, but with silty or clayey soils the distance is much smaller. This means, crop roots must intercept potassium. This interception will not occur under extremely dry soil conditions. Under the drought conditions experienced this spring, crop roots did not grow enough to intercept soil potassium. During the past two weeks (mid-June), most dry areas have received significant rainfall. During this time, I noticed that many corn potassium deficiency symptoms are disappearing. However, with corn approaching tasseling, how much yield have we lost? Currently, I have not seen soybean potassium deficiency symptoms disappearing from affected crops. The soybean taproot may not be as efficient at nutrient uptake recovery response after rainfall compared to the fibrous corn root system.

Potassium deficiency symptoms in corn and soybeans are similar and easy to diagnose. Unlike its nonmobility in soils, potassium is very mobile within plants. This results in potassium deficiency symptoms occurring first on the lower, older leaves. Younger plant tissue is a priority sink for energy and nutrients. In other words, younger leaves steal potassium from older leaves. Potassium deficiency symptoms in corn and soybeans occur as a yellow/brown discoloration at the leaf margins. Severe plant stunting is also associated with potassium deficiency.

Unlike last year when we saw many potassium deficiencies on medium- to high-testing soils that had received adequate fertilization, most problems occurred this year on low-testing soils and/or where potassium fertilizer was applied at low rates or omitted completely. Many producers, who reduced inputs because of low grain prices, started by "cutting back" on potassium fertilizer. Given the early season drought conditions, this decision probably cost them considerable crop yield.

The two most common management-induced, potassium-deficiency situations this year were on rented land and where continuous soybeans are grown. Many renters will not invest in long-term crop production inputs such as maintaining high soil nutrient levels. This management will cause reduced yield and reduced farm income in years like 1999 and 2000 where low-soil nutrient availability conditions exist.

There is a myth that corn responds better to potassium fertilization than soybeans. This philosophy is common and demonstrated often by many producers who use potassium fertilizer on their corn but not their soybeans in the crop rotation. This philosophy has apparently spilled into continuous soybean production. Many of the potassium deficiencies observed this year have been on continuous soybeans that received little or no potassium fertilizer in recent years.

Cyst nematode activity negatively affects potassium uptake and movement in soybean plants. Cyst nematode problems are always worse in continuous compared to rotated soybeans. Therefore, limited potassium fertilization of continuous soybeans can also increase damage caused by soybean cyst nematode.

Soybeans actually require more potassium than corn. Corn grain removes 0.3 to 0.4 pounds of potassium per bushel. A bushel of soybeans removes 1.4 to 1.5 pounds of potassium. Therefore, 150-bushels- per-acre corn would remove 45 to 60 pounds of potassium compared to 63 to 68 pounds of potassium removed from a 45-bushels-per-acre soybean crop.

The last two field seasons have demonstrated that potassium nutrition can become limited over a wide range of environmental and management conditions. Weather conditions cannot be controlled, but through proper planting, crop rotation and potassium fertilization you can greatly reduce the risk of them causing problems in your fields. With an early harvest predicted, this fall will be an excellent time to address your crop potassium fertilization needs.