April's Ag Advice By April Borders Soil test levels for potassium have over the years shown to be decreasing. This is especially true in the eastern third of our state and can be seen especially on coarse textured or eroded soils. Why are we starting to see this problem? The most common cause is potassium applications fall well below what is removed from the field at crop harvest. Soybeans can remove substantial amounts of potassium, and so can corn. You can see in the graph below how much potassium is removed by the crop.
Forage crops, like corn silage and alfalfa, by far remove the most. The reason for this is that much of the potassium in the plant is found in vegetative parts, like stems and leaves. When these are removed from the field, substantial amounts of potassium are also removed. Failing to account for these losses and under-fertilizing with potassium can lead to problems.
A soil test level of 160 parts per million (ppm) is considered satisfactory and no potassium is recommended for crop production at that level. In the past few years, there have been many soil tests below the 160 ppm level. This would indicate that potassium should be added to the soil. According to Dr. Jim Gerwing, SDSU Extension Soil Specialist, the new recommendation for corn (that will be made in the new fertilizer recommendation guide) will be to recommend a minimum of 60 pounds of potassium (K2O) per acre if the soil test indicates a low test result.
Potassium deficiencies are generally in specific areas of the field. They usually are not over the entire field. Soil compaction, root-
soil contact and the hybrid type can all influence potassium.
Corn is more sensitive to a potassium deficiency than some of our other crops. If you had a lot of stalk rot in your corn last season, you might consider testing for potassium. Low levels of potassium (and high levels of nitrogen) are usually associated with stalk rots.
Soils that are very low in potassium reduce yields and quality. A real problem for many is the effect low potassium has on nitrogen efficiency. If only nitrogen is applied on potassium deficient soils, overall yields can be reduced far below where the same amount of nitrogen is applied on soils with adequate potassium. This reduces the amount of grain or forage produced per pound of nitrogen. In addition, inadequate potassium can also substantially change the application rate of nitrogen that is economically optimum. So, unless potassium deficiencies are rectified, nitrogen management becomes very uncertain.
Therefore you need to do a soil test on the areas that had potassium deficiency symptoms and yields were lower than the rest of the field. Gerwing recommends soil testing to determine if you need to add potassium for the following crop that will be planted. He also recommends that you add potassium to meet crop yield needs.
If you are trying to raise the potassium soil test levels, you will need to add seven pounds of potassium to raise the soil test level by one ppm. If you add manure to your fields, the potassium levels will usually be higher in those areas as compared to the rest of the field.
Soil test bags and information sheets are available at the Clay County Extension Office. For information, stop in or call me at 677-7111.