Physical and Chemical Properties of a Peat-based Substrate Amended with Parboiled Rice Hulls

Parboiled rice hulls (PRH) has been proposed as a viable alternative growing media component for the greenhouse and nursery industry. Although PRH can be used as a substitute to peat moss, there is limited information regarding the proper mixing proportion of PRH and peat moss in producing crops. Identifying appropriate proportion of PRH and peat moss helps formulate more sustainable growing mixes to growers and reduce production cost. This research was conducted to evaluate the physical and chemical properties of PRH when amended to a peat moss-based commercial potting mix (CPM, Farfard #2, peat moss:perlite:vermiculite = 70%:20%:10% by volume) and to determine the proper mixing ratio of the substrates as comparable as CPM. Either ground PRH (GRH, size >2 mm) or whole PRH (WRH, size >5 mm) was mixed with different portions of peat moss (0%, 20%, 30%, 40%, 50%, 60% or 70%) to replace a total volume of up to 70% of peat moss in the potting mix, while maintaining the same proportion of perlite and vermiculite.  Water holding capacity (WHC) was higher in CPM than most of the other mixes, however, WHC of GRH at mixing proportions as low as 40% was not significantly different from that of CPM. Incorporation of higher proportion of GRH or WRH increased air filled porosity by 2 to 3 times while decreasing water holding capacity by 10 to 20%. Water holding capacity and air-filled porosity were maintained high when substrate was amended with either 40% GRH (51.0% and 24.5%) or 40% WRH (38.4% and 39.3%) as compared to CPM (55.3% and 28.7%). Most of measured physical properties of these mixes were within the recommended ranges for growing media. Meanwhile, bulk density increased with higher proportion of GRH but was little affected by higher proportion of WRH compared to CPM. Regardless of the mixing ratio, the pH and EC were significantly lower when rice hulls were incorporated in the mixes compared to CPM. Higher proportion of rice hulls slightly increased the pH, but the EC was only 20 to 50% as that of CPM even with the highest proportion of rice hulls. Our results suggested that substrate amendments with either 40% WRH or 40% GRH may be viable options to substitute peat moss in commercial potting mix. Further research needs to be done to evaluate plant performance grown with rice hull containing mixes in comparison with commercial potting mix.