Comparing Hydraulic Properties of Soilless Substrates with Natural Soils: A More Detailed Look at Hydraulic Properties and Their Impact on Plant Water Availability

Moisture release curves are often used when assessing plant-water relationships in soilless substrates.  However, differences between natural soils and soilless substrates make traditional assumptions about plant available water potentially invalid.  If soilless substrates are supposed to be treated like natural soils; why do plants begin wilting at very low water potentials (–10 to –30 kPa) and there is anywhere between 20% to 40% water left (on a volumetric basis) in the soil (Abad et al., 2005; Arguedas et al., 2006; Ristvey et al., 2008). We hypothesize that the fault lies in the methods used and the assumption that water potential is the only limiting factor in water availability to plants. Hydraulic properties, including the relationships that exist between plant available water, water content, and hydraulic conductivity of soilless substrates have traditionally been characterized using instrumentation such as pressure plates, hanging water columns, and tempe cells.  These approaches typically take a months and only provide data on select segments of the soil moisture release curve, and in the case of pressure plates and hanging water columns hydraulic conductivity is ignored and not very well understood.  Using the Wind/Schindler Evaporation method, more detailed measurements of these hydraulic properties can be measured in a less than a week.  A more detailed look at the hydraulic properties of soilless substrates and how they compare with natural soils may give us more insight into soil–plant–water–relations and what limits availability of water to plants.  Soil moisture release curves and hydraulic conductivity curves of different soil-less substrates were compared with curves from typical agriculture soils to give insight into how these properties compare.  Results of the soil moisture release curves showed that some soilless substrates had comparable moisture release curves to agricultural soils while others had bi-modal curves indicating gap-gradation in the pore size distribution.  These soils that showed this non-typical curve had hydraulic conductivities that dropped very low (500 times lower than agricultural soils) at low water potentials (around 10 kPa).  This dramatically lower hydraulic conductivity could lead to zones of depletion around the roots hindering plant water uptake.