Time-resolved in-situ pH measurement in differently treated, saturated and unsaturated soils

The pH-value is of utmost relevance for soil properties and functioning. Hence, a time-resolved in-situ measurement is mandatory but lacking. As an alternative, a two-probe pH electrode with gel-covered reference electrode was newly constructed and tested for a continuous, in-situ pH recording in saturated and unsaturated soil. This was done using samples from a set of 14 soils with different composition and pH(CaCl2) ranging from 3.5 to 7.5 in batch and repacked soil column experiments. In the latter, changes in pH and redox potential were monitored upon transport of citrate-phosphate buffer and pig slurry through the soil columns. The pH measurements were largely stable even upon substantial shifts in soil moisture content down to air-dry conditions. The results of the pH measurements agreed with standard methods using settled soil suspensions in electrolyte solutions and the conventional combination (single-probe) pH electrode. Testing the suspension effect, it was found that measuring pH directly in the soil is recommended. The pH measured in-situ was closest to pH values determined in 0.01 M CaCl2 suspensions according to DIN ISO 10390 (DIN, 2005). The transport of citrate buffer and pig slurry as pH active substances through soil induced strong effects on the pH and in part on the redox potential; the reversible effects lasted over days, which may affect the mobility and speciation of nutrients and pollutants as well as microbial processes.\