P & K in Soil (Are They Really Immobile?)

Phosphorus (P) and Potassium (K) are primary nutrients used by plants in large amounts. We generally think about these nutrients as being “immobile in the soil. When phosphorus fertilizer is applied to the soil it precipitates with elements like aluminum in acidic soils and calcium in soils with pH higher than 7.0. These compounds have very low solubility, so the P does not move much in the soil. When K fertilizer is applied to the soil it doesn’t move much because potassium has a positive charge (K+) and is held by the soil particles which have a negative charge.

This past year AGVISE did a demonstration project where we applied various rates of P & K fertilizer on several sites. All sites were in conventional tillage systems. This spring we sampled these same sites to a depth of 0-6” and 6-12”. We wanted to confirm what we all learned in college, that P & K do not move very much in the soil.

The data in the tables below shows that the P & K soil test levels increased in the 0-6” soil profile with higher rates of fertilizer, but test levels in the 6-12” profile changed very little. Even the sandy loam site showed very little movement of P & K into the 6-12” profile at high rates of P & K. It is important to note that all of these sites are in dryland production.

Recent research from Carl Rosen at the University of Minnesota shows that Potassium can move in the soil profile when you have very sandy soil which is irrigated and very high rates of K fertilizer are applied. So we can’t say that K is always immobile in the soil, but it does take a certain set of circumstances for this to happen. (See bottom figure.)