Potassium (K) is a primary nutrient and used by plants in large amounts. We generally think that K does not move in the soil because K+ is a cation with a positive charge and is held on soil particles which have a negative charge. The different charges should hold the K in place and prevent it from moving in the soil profile. However, if a soil has a coarse texture and a low CEC (<10), K has been shown to move somewhat in the soil profile. Recent research from Carl Rosen from the U of Minnesota is an example of how K can move in the 0-24” soil profile on an irrigated loamy sand soil.
The data in the table shows the K soil test levels in the soil profile after the potatoes were harvested. A wide range of K rates were applied pre-plant and incorporated 3-4” deep before potatoes were seeded. The initial K soil test was very low at about 30 ppm. You can see that when the rates of K exceeded 180 lb/a, the K soil test level in the 12-24” soil profile started increasing. This would indicate K was being moved from the topsoil down through the profile when rates exceeded crop need on this type of soil. If you look at the rate of 180 preplant plus 180 at hilling, the 12-24” K level was not affected as much as when the 360 lb/a rate was applied in one preplant application. The K test level in the topsoil increased quickly after the rate exceeded 180 lb/a. Increasing the topsoil K level to the medium range will allow future K rates to be more modest.
In this study, if the soil texture had been heavier (loam, clay loam etc.) and had a higher CEC, we would not expect K to move as much in the soil profile. In 2015, AGVISE did a demonstration project where we applied K rates at 50, 100, 200 and 1000 lb/a K2O at four locations. This year we will test the K level in the soil profile (0-6, 6-12, 12-24) on these sites to document any movement of K in the soil profile. We will report on this project at our seminars next year.