By George Silva
Phosphorus (P) fixation happens when it is applied to soil, regardless of the fertilizer brand or chemical composition. Fixation occurs when P reacts with other minerals to form insoluble compounds and becomes unavailable to crops. So how do we know if the P is fixed or not? A good place to start is soil pH. There are three peaks of P fixation (Figure 1). The two highest peaks occur in the acid range of pH 4 and 5.5, where P precipitates with iron and aluminum. It is very difficult to supply sufficient P for crop needs when P solubility is being controlled by iron and aluminum.
Liming to correct the soil pH is critical for P availability. The valley (area of lowest fixation) occurs between pH 6.0 and 7.0. This is the ideal environment for soil P and optimum crop growth.
The third peak occurs in alkaline soils around pH 8.0 when P is precipitated primarily by calcium. This fixation is relatively weak and it is generally more economical to apply a few extra pounds of P fertilizer than adding amendments to acidify the soil.
Another source not represented in this graphic is the P fixed in organic matter. Generally, organic P accounts for 30 to 50% of the total P in most mineral soils. When soil warms up, microorganisms begin to mineralize and convert the organic P into inorganic plant available forms.
Soil testing is the key to determining P requirements of crops. Some cost effective solutions for farmers to increase P availability include band application of starter P fertilizer and incorporation of P fertilizer by tillage when severe nutrient stratification in the soil is evident.
For additional information on soil phosphorus, please read "Understanding soil phosphorus: An overview of phosphorus, water quality, and agricultural management practices" by University of Wisconsin Extension.
This article was published on MSU Extension News. For more information from MSU Extension, visithttp://news.msue.msu.edu. To contact an expert in your area, visit http://expert.msue.msu.edu, or call 888-MSUE4MI (888-678-3464).
Silva is with Michigan State University Extension