Among the major nutrients (nitrogen, phosphorus and potassium), phosphorus (P) has the least mobility, writes Paulo Pagliari, Nutrient Management Specialist, University of Minnesota Extension. As the fertilizer granule dissolves, most of the P in the fertilizer will likely only move a couple eights of an inch away from the granule, primarily by diffusion. The dissolved P will then start to react with cations in solution such as calcium, aluminum, and iron, and will start to adsorb onto soil particles.
In general, tie up of P as calcium phosphates is a concern when the soil pH exceeds 7.3. Soils will become more acidic over time if they are not limed. With the decrease in pH, the availability of P will change. When the pH of soils range between 4.8 and 5.5, P is more reactive with aluminum in the soil and is tied up as aluminum phosphates that are not available to the plants. Liming of the soil can help to increase P availability from Fe and Al bound forms. The reaction of sorption and precipitation will limit P availability to plants.
Phosphorus will be most available to the plant within a few day s to two weeks after fertilizer addition, slowly dropping as time goes on. When applied in the fall, P will stay in the soil for as long as four to six months before plant uptake. This is a long time for P to sit in the soil unused by the plants, which can minimize the fertilizer’s effectiveness to supply plants with adequate levels of P. However, in Minnesota the colder temperatures in the fall and winter keep P reactions from happening, which minimizes P sorption and precipitation in soils. The P that is tied up in the soil is not measured by routine soil test procedures. Some P that is tied up can return back to plant available forms depending on the solubility of the phosphate compound formed in the soil. Calcium-bound forms of P vary in their solubility. Dicalcium phosphate dihydrate is the most soluble followed in order of decreasing solubility by dicalcium phosphate, octocalcium phosphate, tricalcium phosphate and primary phosphate containing minerals like apatite. Acidification of soils to release Ca bound P forms is not feasible in Minnesota.
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