Soil acid and alkaline phosphatase activity as pH adjustment indicators

Soil fertility and crop production are affected by biological processes and these processes, including enzyme activites, are influenced by pH. We investigated the potential of using alkaline phosphatase (AlkP) and acid phosphatase (AcdP) activities, for determining the optimum soil pH for crop production and the amount of lime required to achieve this optimum. Five acid soils, which varied widely in selected properties, were treated with CaCO3 at rates of 0, 0.2, 0.5, 1.0 and 2.0× the soil's lime requirement needs. To remove soil variations in absolute enzyme activity values, an AlkP/AcdP activity ratio was used to test soil response. The ratios of AlkP/AcdP responded immediately to the changes in pH caused by CaCO3 additions and an AlkP/AcdP ratio of approximately 0.5 divided soils into those with appropriate pH adjustment and those still needing additional lime treatment. However, incubation of the lime-treated soils for 67 days followed by treating the soils with organic amendments (which included finely ground chicken manure and alfalfa residues) increased the AlkP/AcdP ratios to approximately 3.0. For cropping systems that rely heavily on natural biological processes to maintain productivity, measuring the AlkP/AcdP ratio may be preferable to chemical approaches for evaluating effective soil pH and liming needs.