Hygroscopic Water Content As a Surrogate for Clay Percentage in Proximal Soil Sensing.

Geophysical techniques of proximal soil sensing provide non-invasive, rapid, cost effective and spatially exhaustive measurements of soil properties to understand plot scale soil heterogeneity. This is useful in precision agriculture, soil fertility, and soil and environmental contamination management. The benefits of rapid proximal sensing in reconnaissance surveys, however, can be weakened by the cost and tedious requirements of soil sampling and analysis of properties such as clay percentage required for the calibration of the geophysical signal. We hypothesized that hygroscopic water content whose determination does not require any special equipment and is time and cost efficient, can act as a pedotransfer function of clay percentage for efficient interpretation of soil spatial patterns based on proximal sensing. Using standard clays and soils of varying mineralogies from tropical (Trinidad, West Indies) and temperate (Arizona, USA) regions, we determined an accepted relative humidity value and re-equilibration time for the determination of hygroscopic water content in the laboratory. We found the relative humidity of 50 % (RH⁵⁰) as the convenient point at which the change of hygroscopic water content with humidity is minimum and 2.5 days as the minimum re-equilibration time for the range of mineralogies. Hygroscopic water content measured at RH⁵⁰ showed a strong linear dependence with clay percentage, suggesting that it has good potential to act as a pedotransfer function to estimate clay percentage for efficient reconnaissance surveys.