Reducing phosphorus release from paddy soils by a fly ash-gypsum mixture

A mixture of fly ash and phospho-gypsum (50:50, wt wt−1) was selected to study its potential to supply Ca and Si to rice while reducing B toxicity. We expected that the high Ca content in this mixture might convert water-soluble P to less soluble forms and thereby reduce the loss of soil P to surface runoff. The mixture was applied at rates of 0, 20, 40, and 60 Mg ha−1 in two paddy soils of contrasting textures (silt loam in Yehari and loamy sand in Daegok). The mixture significantly reduced water-soluble phosphate (W-P) in the surface soils by shifting from W-P and iron bound-P (Fe-P) to calcium bound-P (Ca-P) and aluminum bound-P (Al-P) during rice cultivation in both soils. Lancaster and Mehlich 3 extractable P increased significantly with application rate due to high contents of P and Si in the mixture. Mixtures of fly ash and phospho-gypsum should reduce P loss from rice paddy soils and increase soil fertility.