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Soil Savvy – Understanding Soil Phosphorus

By Dr. Gurbir Dhillon 

The application of organic-rich amendments like manure and plant residues can help increase phosphorus availability in soils. Also, the microbial breakdown of these amendments helps slow release of phosphorus that plants can take up before it becomes fixed in the soil.  

The management practices that help in maintaining soil pH between 6 and 7 and enhance mycorrhizal symbiosis can help improve phosphorus deficiencies and its availability in soils.  

Phosphorus availability in soils is controlled by the solubility of phosphorus minerals and adsorption of phosphate ions on soil particles.  

Soil pH can play a strong role in regulating phosphorus fixation. The greatest degree of phosphorus fixation occurs in highly alkaline or highly acidic soils.  

In highly acidic soils, iron and aluminum-containing phosphorus minerals have very low solubilities. As the soil pH increases above 5, these minerals become more soluble. 

In highly alkaline soils, calcium-containing phosphorus compounds are least soluble and tend to increase in solubility as pH decreases from more than 8 to below 6 or 7. Generally, overall soil phosphorus fixation is at its lowest when soil pH is maintained between 6-7.  

The amount of clay content in soils and type of clay minerals can also affect the amount of phosphorus fixation. Soils high in organic matter tend to show relatively lower phosphorus fixation. The organic acids released through plant root exudates and microbiological decay of organic matter can compete with phosphorus ions for fixation sites in soils thereby reducing phosphorus fixation.