The peaks and valleys of phosphorus fixation
Understanding the peaks and valleys of phosphorus fixation in the soil, as affected by the soil pH, is one of the critical steps to manage this essential plant nutrient.
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.
Figure 1. Peaks and valleys of phosphorus fixation as influenced by soil pH
Another source not represented in this graphic is the P fixed in organic matter. Generally, organic P accounts for 30 to 50 percent 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.