In order to examine whether the electrolyte concentration in the soil solution
can be estimated by time domain reflectometry (TDR) measured bulk soil
electrical conductivity, column leaching experiments were performed using
undisturbed soil columns during unsaturated steady-state water flow. The
leaching experiments were carried out on 2 soils with contrasting pedological
structure. One was the strongly structured Ramiha silt loam, and the other the
weakly structured Manawatu fine sandy loam. Transport parameters obtained from
the effluent data were used to predict the transient pattern in the resident
electrolyte concentration measured by TDR. The electrolyte concentration was
inferred from the TDR-measured bulk soil electrical conductivity using 2
different calibration approaches: one resulting from continuous solute
application, and the other by direct calibration. Prior to these, calibration
on repacked soil columns related TDR measurements to both the volumetric water
content and the electrolyte concentration that is resident in the soil
solution. The former calibration technique could be used successfully to
describe solute transport in both soils, but without predicting the absolute
levels of solute. The direct calibration method only provided good estimates
of the resident concentration, or electrolyte concentration, in the strongly
structured top layer of the Ramiha soil. This soil possessed no immobile
water. For the less-structured layer of the Ramiha, and the weakly structured
Manawatu soil, only crude approximations of the solute concentration in the
soil were found, with measurement errors of up to 50%. The small-scale
pattern of electrolyte movement of these weakly structured soils appears to be
quite complex.
Site-Specific Management Zones Based on Soil Electrical Conductivity in a Semiarid Cropping System