Methodologically controlled variations in laboratory and field pH measurements in waterlogged soils

We have tested the reliability and consistency of conventional pH measurements made on water-soil mixtures with respect to sieving, drying, ratio of water to soil, and time of shaking prior to measurement. The focus is on a waterlogged soil where the preservation potential of archaeological artefacts is critical. But the study includes agricultural and forest soils for comparison. At a waterlogged site, laboratory results were compared with three different field methods: calomel pH probes inserted in the soil from pits, pH measurements of soil solution extracted from the soil, and pH profiles using a solid-state pH electrode pushed into the soil from the surface. Comparisons between in situ and laboratory methods revealed differences of more than 1 pH unit. The content of dissolved ions in soil solution and field observations of O-2 and CO2 concentrations were used in the speciation model PHREEQE in order to predict gas exchange processes. Changes in pH in soil solution following equilibrium in the laboratory could be explained mainly by CO2 degassing. Only soil pH measured in situ using either calomel or solid-state probes inserted directly into the soil was not affected by gas exchange processes. Variations on the order of 0.2-0.5 pH unit in different laboratory methods could not be explained by degassing and seem to be soil-type specific and strongly influenced by drying and shaking. Further attention should be given to standardization of pH measurements, particularly before pH measurements from different soil types are compared.