The effect of humic substances, Fe3+, Al3+, 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,
Fe3+ and Al3+ ions in the solution lead to the precipitation of
Fe-P and Al-P, less soluble compounds. Organic matter, by its chelating
power for Ca2+, Fe3+ and Al3+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 Ca2+, Fe3+ and Al3+. Soil organic matter should be maintained at a sufficient level in order to
get enough phosphorus in soil solution for plant uptake