Hydraulic Properties of Peat-based Substrates: The Importance of Hydraulic Conductance

The availability of water to plants grown in soilless substrates is typically evaluated with substrate moisture release curves, which describe the relationship between substrate water content and substrate matric potential. Past studies have generally concluded that there is little or no plant available water left at substrate matric potentials (Ψ_m) of -30 kPa. However, plant water potential is typically much lower than -30 kPa and the substrate-to-plant water potential gradient should allow for continued water uptake. This suggests that plant water uptake may not be limited by substrate matric potential. We hypothesize that hydraulic conductivity may limit water movement in soilless substrates. To test this, we measured substrate water content, matric potential, and evapotranspiration from a peat-perlite substrate (80:20, v:v) simultaneously. These results were then used to determine substrate moisture release curves and hydraulic conductivity. The substrate moisture release curves showed a typical trend, with the pF declining from -0.8 (Ψ_m = 0.6 kPa) at a substrate water content of approximately 75% (by volume) to -2.9 (Ψ_m = -74 kPa) at a substrate water content of 21%. The hydraulic conductivity was approximately 3,000× higher at a substrate water content of 36% (0.098 cm/d) than at 21% (0.00004 cm/d). This dramatic decrease in hydraulic conductivity as the substrate dries out is consistent with our hypothesis that hydraulic conductivity may limit plant water uptake. As plants are transpiring and take up water from the substrate, they create a depletion zone around the roots. The low hydraulic conductivity of dry substrates may limit water flow into this depletion zone and limit the ability of roots to extract water from throughout the container, thus decreasing or limiting plant water uptake. To optimize water availability, the effect of substrate pore size distribution on hydraulic conductivity should be taken into account.