An important physical property of root substrates is air-filled pore space, which allows for drainage and gas exchange between the root environment and the outside atmosphere. Various materials are used to at least partially provide for air-filled pore space in substrates, with one of the most common being perlite. In addition to the high cost of perlite, it produces a siliceous dust in its dry state that irritates the eyes and lungs.
Parboiled fresh rice hulls (PBH) are a rice-milling co-product obtained as a result of a steaming process and are therefore free of viable weed seed. PBH can also be obtained for about half the price of perlite. Previous studies have shown that PBH could be successfully used as an alternative to perlite in root substrates, but the physical properties of PBH amended sphagnum peat-based substrates compare to those amended with perlite have not been reported.
The objectives of this study were to determine and compare total pore space, air-filled pore space, water-holding capacity and bulk density of sphagnum peat-based substrates amended with various amounts of PBH or perlite and to determine how the amount of PBH or perlite affects these physical properties.
Rice hulls were bagged immediately after parboiling and drying without being stored outdoors. Ten substrates were formulated with either 20, 30, 40, 50 or 60 percent perlite or PBH, with the remainder being sphagnum peat.
The root substrates were air-dried in a greenhouse at 89-95° F until they no longer lost weight over a 24-hour period. The samples were re-wetted then placed into sealed plastic bags and allowed to equilibrate for a day to attain moisture uniformity.
Total pore space in perlite-containing substrates ranged from 71.5 percent to 79.4 percent, while total pore space in PBH-containing substrates ranged from 82.1 percent to 87.7 percent (Figure 1). All substrates containing PBH had higher total pore space than substrates containing an equivalent amount of perlite.
The decrease in total pore space of perlite-containing substrates could be attributed to perlite having a total pore space of 74 percent, which was lower than that for sphagnum peat at 84 percent.
Likewise, the small change in total pore space with increasing amounts of PBH could be attributed to PBH having a total pore space of 82 percent, which was similar to the total pore space of sphagnum peat.
Air-filled pore space ranged from 9.5 percent to 12.7 percent for perlite-containing root substrates and 11.5 percent to 37.8 percent for PBH-containing substrates (Figure 2). The air-filled pore space was not different between substrates containing either 20 percent perlite or PBH. However, the air-filled pore space was higher in PBH-containing root substrates than in equivalent perlite-contain substrates when the amount of PBH or perlite was at least 30 percent.
As the amount of perlite increased, the air-filled pore space decreased. As the amount of PBH increased, the air-filled pore space increased. Perlite had an air-filled pore space of 54 percent while PBH had an air-filled pore space of 69 percent. The air-filled pore space of PBH-containing substrates also increased at a higher rate compared to perlite-containing substrates. The difference in the rate of change in air-filled pore space may also have been partially due to the elongated shape of PBH (in contrast to perlite granules, which were generally spherical), which allowed the individual hulls to cross connect and create more, larger pores in substrates containing high concentrations of PBH.
Water-holding capacity ranged from 59 percent to 68.9 percent for the perlite-containing substrates and 45.1 percent to 71.7 percent for the PBH-containing substrates (Figure 3). The 20 percent PBH-containing substrate had a higher water-holding capacity than the 20 percent perlite-containing substrate. However, at 30 percent or higher PBH, the PBH-containing root substrates had a lower water-holding capacity than equivalent perlite-containing substrates. As the percent perlite increased, the water-holding capacity decreased. As the percent PBH increased (Figure 3), the water-holding capacity decreased.
As expected, water-holding capacity was generally inversely related to air-filled pore space. As air-filled pore space increased, the water-filled pores, and thus water-holding capacity decreased. The properties of PBH, namely particle size and shape, that resulted in a higher air-filled pore space, higher rate of change in air-filled pore space with increasing amounts, and a lower water-holding capacity and higher rate of decline in the water-holding capacity as PBH concentration increased as compared to equivalent perlite-containing substrates.
The differences in bulk densities were not great enough to be of practical significance and were all similar to that of sphagnum peat (0.11 g/cm3) or within an acceptable range. Unlike materials such as calcined clay, bulk density of PBH would be acceptable and not add to shipping costs of the substrates or plants grown in the substrates.
The inclusion of PBH provided for increased and air-filled pore space and drainage in the sphagnum peat-based substrates. Furthermore, increasing the amount of PBH in the substrate resulted in a greater increase in air-filled pore space and a greater decrease in the water-holding capacity than an equivalent amount of perlite. Therefore, PBH served a similar role in the substrate as perlite, but less PBH would be required in a substrate to provide the same air-filled pore space and water-holding capacity as perlite when more than 20 percent perlite or PBH was used.