Monocalcium phosphate monohydrate concentration in soil suspension amended with organic matter

The effect of humic substances, Fe$^{3+}$, Al$^{3+}$, and soil clay mineralogy on the availability of monocalcium phosphate monohydrate added at pH5 were investigated. Both solution and suspension experiments showed that humic matter chelates phosphorus and prevents the formation of less soluble forms of phosphorus than monocalcium phosphate. However, Fe$^{3+}$ and Al$^{3+}$ ions in the solution lead to the precipitation of Fe-P and Al-P, less soluble compounds. Organic matter, by its chelating power for Ca$^{2+}$, Fe$^{3+}$ and Al$^{3+}$ions, explains the availability of phosphorus in solution at pH5. Clay minerals, especially smectites, induced an increase in solution phosphorus content because of their adsorption properties for Ca$^{2+}$, Fe$^{3+}$ and Al$^{3+}$. Soil organic matter should be maintained at a sufficient level in order to get enough phosphorus in soil solution for plant uptake