Physical Properties of Ground Parboiled Fresh Rice Hulls Used as a Horticultural Root Substrate

Abstract

Fresh parboiled rice hulls ground in a hammer mill and screened through a 1.18-mm screen and collected on a 0.18-mm screen (RH3) and particles with a specific diameter of 0.5 to 1.0 mm had total pore space (TPS), air-filled pore space (AFP), and water-holding capacity (WHC) similar to that of Canadian sphagnum peat (peat). However, RH3 had more available water, a higher bulk density (BD), and a higher particle density (PD) than peat. When blended with 20% to 40% perlite or 1 cm aged pine bark, RH3-based substrates had lower TPS, similar AFP, and lower WHC than equivalent peat-based substrates. The RH3-containing substrates had higher BD and average PD than equivalent peat-based substrates. When blended with parboiled rice hulls (PBH), RH3-based substrates had lower TPS than equivalent peat-based substrates. When blended with 20% to 40% PBH, RH3-based substrates had lower AFP than equivalent peat-based substrates. RH3-based substrates containing up to 20% PBH had lower WHC than equivalent peat-based substrates. RH3-based substrates containing 40% PBH had a higher WHC than equivalent peat-based substrates. When blended with PBH, all RH3-based substrates had higher BD and average PD than equivalent peat-based substrates. The addition of 40% RH3 to a peat-based substrate containing 20% perlite decreased substrate TPS, whereas the addition of 10% to 40% decreased AFP. The addition of 10% to 30% RH3 increased WHC. The addition of 30% RH3 to a peat-based substrate containing 20% 1 cm aged pine bark decreased substrate TPS and the addition of 20% to 40% RH3 decreased AFP. The addition of 10% RH3 increased WHC, but the addition of 20% or more RH3 did not affect WHC. The addition of 30% RH3 increased the BD, but the addition of RH3 had no effect on average PD. The addition of 20% or more and 30% or more RH3 to a peat-based substrate containing 20% PBH decreased substrate TPS and AFP, respectively. The addition 20% RH3 decreased WHC. The addition of 10% to 40% RH3 increased BD. Overall, RH3 was the ground rice hull product that had physical properties most similar to peat. Peat-based substrates in which up to 40% of the peat was replaced with RH3 had physical properties that, although different from peat controls, were within commonly recommended ranges for substrates used to grow greenhouse crops.