Hydraulic Conductivity of Rootzone Mixtures with High Peat to Sand Ratios

Sand and peat mixtures have been widely used as constructed rootzones in golf course putting greens and sports fields, horticultural potting materials, and in water filtering systems. Water holding and water conductivity of the mixtures are very important properties in those applications. Direct measurement of hydraulic properties often is time consuming. Many models for estimation of hydraulic properties do not include organic matter (OM) content as a predictor. When OM is considered, very often it is treated as clay-sized particles. However, peat and other organic materials used in sand root zone mixtures are fibrous rather than layer-silicates. Previously, a step-wise multi-linear regression model (MLR) was developed to predict saturated hydraulic conductivity (Kₛₐₜ) of sand-based root zone materials from readily available soil properties including bulk density, capillary porosity, clay content, and particle size distribution. However, OM was not significant enough to be included in the model because of the low content. Organic matter tends to accumulate as the sand-based root zones aging and a robust model need to be to be established to account for this fact. Also, horticultural pot mixes and water filtering systems use very high percentage of OM. One of the objectives of this study was to test if the saturated water flow is laminar and obey Darcy’s law in a porous medium of sand that conform to the USGA specifications mixed with a wide range of peat content (0 to 25% w/w) and of different peat types (reedsedge, woody sphagnum, and sphagnum). Another objective was to evaluate the test MLR models for sand-based rootzones with a wider range of OM content. The results showed that Darcy’s law prevailed in the range of peat ratios used in this study at a hydraulic pressure gradient up to 3. A stepwise MLR model was developed as: Log₁₀ (Kₛₐₜ) = 5.0213 − 0.97ρb − 1.148CP − 0.0543OM + 0.0872Φ₅ − 0.5743Φ₁₀ + 1.1628Φ₁₆ − 0.3859Φ₈₄ + 0.2323Φ₉₅, (R² = 0.53), where ρb is bulk density(g/cm³), CP is capillary porosity (%), OM is organic matter content (%), and Φ₅, Φ₁₀, Φ₁₆, Φ₈₄, Φ₉₅ values from the particle size distribution curve for grain size in phi (Φ) unit. Briefly, Φₓ = log = (2, = d), with x representing the percentage of weight passing particle size d in a traditional particle size distribution curve. The model showed a negative correlation between Kₛₐₜ and OM content. This result is in agreement with the pedo-transfer models by Nemes et al. (2005) which include OM as one of the predictors negatively correlates Kₛₐₜ.