Use of the evaporative method for determination of soilless substrate moisture characteristic curves

- Substrate moisture characteristics can be measured with the evaporative method.
- We hypothesize the extraction method results in loss of connectivity in porous substrate.
- The evaporative and pressure extraction method does not measure substrate porosity with accuracy.
- The evaporative and pressure extraction methods are useful in substrate research.

Historically, substrate science has utilized the pressure extraction method to measure soilless substrate moisture characteristic curves, albeit with published discrepancies. Recently, a device utilizing the evaporative method to generate moisture characteristic curves by measuring water potential as volumetric water content decreases via evaporation, known as a Hyprop, has become available. This research compares and contrasts moisture characteristic curves developed over a 2-week period using both the pressure extraction and the evaporative methods for two-component greenhouse (Sphagnum peat and perlite) and nursery (aged pine bark and sand) soilless substrates. The pressure extraction method was conducted between water potentials of 0 and −300 hPa (10 data points used in conventional methodology for allotted time), while the evaporative method measurements continued until the tensiometers cavitated (≈ −500 to −700 hPa) and provided higher data density (100 data points) within the two week period. The evaporative method was found to produce repeatable results, with subsequent measurements of each substrate providing analogous measurements (P > 0.9000 and P > 0.3700 for the peat and bark substrate, respectively). There was little variation between the two methodologies for the peat substrate (0.004% difference in the area under the curves from 0 to −300 hPa). However, differences were observed between the methodologies for the bark substrate, with the percentage difference increasing with increasing water potential (9.6% at −100 hPa; 23.7% at −300 hPa). Additionally, the evaporative method measured a continued decrease in volumetric water content of the aged pine bark and sand substrate with increasing water potentials throughout the range of measurements, unlike the pressure extraction method, which has documented issues with loss of hydraulic connectivity between the sample and the plate in coarse highly porous organic substrates. Therefore, the pressure extraction method ceases to decrease in volumetric water content (≤ −65 hPa) resulting in a divergence in curves generated by the two methods. Both methods were found to have limitations while measuring substrate water content near saturation, with the pressure plate resistance to free drainage of water influencing measurements and the evaporative method continually underestimating the saturation point. As a result, both methods provided decreased volumetric water content measurements near saturation than when static physical properties were directly measured; therefore, moisture characteristic curves should be used collectively with static properties to correct for underestimation of total porosity and to better yield an understanding of the hydrophysical properties of a soilless substrate.