Empirical relationship between conventionally determined pH and in situ values in waterlogged soils

Abstract

Soil pH is a key factor in several chemical processes occurring in the pond bottom environment. Conventionally, pH determinations are made on 1:1 dry soil:distilled water paste (pH 1:1). However, such pH data might not be relevant for flooded soil conditions, conditions under which several pH modifying processes take place. A better estimate for the in situ pH is obtained when the soil is incubated under flooding conditions for a prolonged period (e.g. ca. 3 weeks). In this study, the correlation between ΔpH (pH difference between in situ pH and pH 1:1) was evaluated in 106 soil samples (experimentally determined in our laboratory and literature data) from different systems including aquaculture ponds in USA, Israel, Thailand, Honduras, Peru, and Kenya; lowland rice soils from the Philippines, Taiwan, Vietnam, China, and Louisiana; and agricultural upland soils from Israel, Germany, Columbia, and Ireland. The regression for all the data points was: ΔpH=5.72−0.83× pH 1:1; R 2=0.78 Removing five samples with pH<4.0 improved the model and was: ΔpH=7.11−1.03× pH 1:1; R 2=0.89 The use of this regression to evaluate the in situ pH of flooded soils may provide better and more reliable predictions.