Many of these articles would suggest increasing phosphorus fertilization rates and provide advice on fertilizer placement and timing. At the time these articles were written many phosphorus soil test levels in most farming regions were less than adequate and phosphorus was limiting crop production. Inadequate levels of plant-available phosphorus are encountered less frequently today because producers heeded this advice. For example the graph to the left shows that soil analyses conducted by the University of Maryland Soil Testing Laboratory during 1996 showed only 31 percent of the samples contained phosphorus levels low enough to potentially limit crop productivity. The diagram below illustrates what happens to phosphorus in the soil.
Crops take up phosphorus in a soluble form.
These clay particles release phosphorus very slowly. This capacity of soils to fix phosphorus is one reason why levels of phosphorus in soil solution are usually low. Crops take up phosphorus in a soluble form. These clay particles release phosphorus very slowly. This capacity of soils to fix phosphorus is one reason why levels of phosphorus in soil solution are usually low.