Theoretical equilibrium considerations explain the failure of the maleic-itaconic copolymer to increase efficiency of fertiliser phosphorus applied to soils

Abstract

Efficiency of use of various phosphate fertilisers is affected by water-insoluble reaction products such as Fe–P, Al–P, and Ca–P in either discrete precipitates or surface-adsorbed forms. A product, maleic–itaconic acid copolymer, has been marketed for increasing efficiency of use of phosphate fertilisers. Field trials have been conducted to evaluate the agronomic effectiveness of this product with no measured positive effects. This absence of positive effects can be explained if fundamental principles of chemistry are considered. The negative logarithm of the equilibrium constant (pK) of CaHPO4.2H2O (DCPD) is higher than that of Ca–maleic acid and Ca–itaconic acid complexes. Therefore, the copolymers cannot prevent DCPD formation in calcareous soils. Likewise, because of the pK values of Fe–maleic, Fe–iticonic acid, Al–maleic acid, and Al–itaconic acid are lower than those values for AlPO4.2H2O, and FePO4.2H2O, the copolymer cannot block or prevent formation of AlPO4.2H2O and FePO4.2H2O in acid soils. The results of agronomic greenhouse and field trials can be explained by the considerations of the solubility-product constant or the dissociation constant of cation-chelate for various reactions. Therefore, the copolymer marketed has little value for increasing P efficiency from phosphate fertilisers as claimed by the manufacturer.