Slow‐release of potassium from fused potassium silicate fertilizer

(pp. 805–810) K2Ca2Si2O7, the major component of fused potassium silicate fertilizer, released potassium (K), calcium (Ca) and silicon (Si) in a slow manner. The 10% of K in K2Ca2Si2O7 was quickly solubilized in water. Further K dissolution was very slow. The amounts of dissolved Ca and Si in water were much smaller than that of K. The decrease of the Ca and Si concentration in water indicated the occurrence of a re-deposit of Ca and Si at a later stage. The surface imaging method was used in order to study the slow-release process of K2Ca2Si2O7 in water. The surface analysis of K2Ca2Si2O7 particles after dissolution in water for 30 min showed that there were Ca- and Si-rich particles stuck on the fertilizer surface. The results of the analysis of the cross section of K2Ca2Si2O7 particles showed that the K content was lower than those of Ca and Si in the surface boundary layer. On the other hand, the mole ratio of K, Ca and Si was same inside the fertilizer particle. In the portion between the inside and the surface of the fertilizer particle, the content of K and Ca was lower than that of Si. These results indicated that the order of dissolution of fertilizer components from K2Ca2Si2O7 particle was first K, then Ca, and Si last. From the results mentioned above, the process of slow-release K was speculated to be as follows: 1) K on the particle surface was released quickly by an ion exchange reaction with hydrogen ions in water. 2) K inside the particle was released slowly because of dissolution through Si-O-Si bonds.