The pH, electrical conductivity, and primary macronutrient concentration of sphagnum peat and ground parboiled fresh rice hull substrates over time in a greenhouse environment

Abstract

The primary objective of this research was to compare the pH, electrical conductivity (EC), and primary macronutrient status of three ground parboiled fresh rice hull (PBH) products to sphagnum peat when used as a root substrate over 56 days in a greenhouse environment. The three grades of ground rice hull products were produced by grinding PBH and passing the ground product through different screens. One grade (P3) was passed through a 2.00-mm screen and captured on a 1.00-mm screen. The second grade (P4) was passed through a 1.00-mm screen and captured on 0.50-mm screen. A third ground rice hull product (RH3) was a commercially available, ground PBH material that was ground in a hammer mill until it passed through a screen with 1.18-mm-diameter openings and was collected on a screen with 0.18-mm openings. The pH of sphagnum peat ranged from 3.4 to 3.7 across time. The pH of RH3 and P3 increased from 4.7 to 7.1 on day 5 and 14, respectively, before decreasing to 6.3 and 6.7, respectively, on day 56. The pH of P4 increased from 4.8 to 6.9 on day 6 before decreasing to 6.6 on day 56. The P4 had an EC of 1.2 dS·m-1, which was higher than that of peat, RH3, and P3, which had similar EC of 0.7 to 0.8 dS·m-1 regardless of time. The ammonium (NH4+) concentration was unaffected by time. Peat had an NH4+ concentration of 6.4 mg·L-1, which was lower than that of the ground rice hull products. The P3 had an NH4+ concentration of 14.6 mg·L-1, which was higher than that of RH3 and P4. The RH3 and P4 had similar NH4+ concentrations of 11.8 and 10.8 mg·L-1, respectively. The nitrate (NO3-) concentration was unaffected by time. The RH3 had a NO3- concentration of 8.2 mg·L-1, which was significantly higher than that of peat, P3, and P4, which had similar NO3- concentrations of 0.5 mg·L-1. The phosphorus (P) concentration in peat ranged from 1.3 to 2.5 mg·L-1 across the sampling times, and peat had a lower P concentration than all rice hull products, which ranged from 57.4 to 104.4 mg·L-1. The potassium (K) concentration in peat ranged from 2 to 5 mg·L-1 across the sampling times and was always lower than that of the rice hull products, which had a K concentration ranging from 195 to 394 mg·L-1. Because pH, P, and K concentrations were above recommended concentrations, ground rice hull products would not be suitable as a stand-alone substrate but might be amended with materials such as elemental sulfur or iron sulfate to adjust the pH or blended with other components to reduce the P and K concentrations to within recommended concentrations.