SHORT-TERM EFFECTS OF MOISTURE CONTENT ON SOIL SOLUTION pH AND SOIL EH.

This laboratory study examined the short-term effects of soil moisture on the soil solution pH (pHss). Within minutes after adding water to dry soil, soil solution pH changed. As moisture increased, pH increased, whereas redox potential (Eh) decreased, and consequently, soil Eh and pHss were negatively correlated (r2 = 0.90). Soil pH and Eh changes in relation to moisture content fit sigmoidal curves well (r2 > 0.96). Altered pH and Eh response to moisture levels in chloroform treatments suggested that the observed pH changes were microbially mediated. We suggest that the rapid pH decrease observed in soil incubated at "field capacity" is due to proton generation associated with nitrification. Increased pH in saturated soils seems to be due to denitrification reactions. Addition of a nitrification inhibitor to soils resulted in smaller pH decreases. It seems that conditions conducive to denitrification occur at water contents much below saturation. In saturated Yolo silt loam, soil solution pH increased rapidly in the first 8 h and was nearly constant for the next 2 days. Proton consumption in saturated soils and proton production in soils maintained at field capacity fit as a power function very well. Based on the rapid changes in pHss in several agricultural soils, we conclude that soil pH as commonly measured (soil/water ratio 1:1-1 h of equilibration) may not reflect the pHss at longer time periods or be a good measure of soil pH in the field. The effects are especially pronounced in weakly buffered coarse textured soils.