Physical Properties of Ground Fresh Rice Hulls and Sphagnum Peat Used for Greenhouse Root Substrates

Ground fresh rice (Oryza sativa) hull materials were produced by grinding whole fresh rice hulls and passing the resulting product through a 1-, 2-, 4- or 6-mm-diameter screen to produce a total of four ground rice products (RH1, RH2, RH4, and RH6, respectively). The physical properties and water release characteristics of sphagnum peatmoss (peat) and the four ground rice hull products were evaluated. All of the ground rice hull products had a higher bulk density (Bd) than peat, and as the grind size of the rice hull particle decreased, Bd increased. Peat had a higher total pore space (TPS) than all of the ground rice hull products except for RH6. As grind size decreased, the TPS decreased. Peat had a lower air-filled pore space (AFP) than all of the ground rice hull products and as the grind size of the rice hull products decreased, AFP decreased. Peat had a higher water holding capacity (WHC) than all of the ground rice hull products. Grind sizes RH4 and RH6 had similar WHC, whereas RH1 and RH2 had a higher WHC than RH4 and RH6. Peat, RH4, and RH6 had similar available water content (AVW), whereas RH2 had higher AVW than these materials and RH1 had the highest AVW. However, peat had the lowest AVW and easily available water (EAW) as a percentage of the WHC. The ground rice hull products RH1 and RH2 had the highest AVW and EAW of the components tested. Peat had the highest water content at container capacity. As pressure was increased from 1 to 5 kPa, peat released water more slowly than any of the ground rice hull products. The RH1 and RH2 ground hull products released water at a significantly higher rate than peat, but RH4 and RH6 released the most water over these pressures. For all rice hull products, most water was released between 1 and 2 kPa pressure. The rice hull products RH1 and RH2 had physical properties that were within recommended ranges and were most similar to those of peat.