Influence of physical properties of peat-based potting mixes substituted with parboiled rice hulls on plant growth under two irrigation regimes

Abstract

This study was conducted to investigate the effects of physical properties of peat-based substrate mixtures partially substituted with parboiled rice hulls (PRH) on plant growth and performance. Thirteen substrate mixtures were formulated by substituting 0, 20, 30, 40, 50, 60, or 70% peat with either parboiled ground rice hulls (GRH) or whole rice hulls (WRH) (by volume) in commercial potting mix (CPM) containing 70% peat, 20% perlite, and 10% vermiculite. All substrate mixtures consisted of mainly medium-sized (2.0–0.25 mm) particles. However, regardless of the mixing ratio, GRH significantly increased the medium-sized particles especially with diameters of 2.0–0.71 mm, while WRH increased both the medium- and coarse-sized (> 2.0 mm) particles. A higher mixing ratio of PRH increased air space and decreased container capacity but to a greater extent by WRH compared to GRH. Similarly, bulk density was increased with a higher mixing ratio of both types of PRH, but to a greater degree by GRH. Total porosity of GRH-containing substrates was similar to that of CPM within the range of 30–70%. The substrate mixtures containing PRH did not have the same physical properties as the CPM at any mixing ratio, but the values of some GRH-containing mixtures were within or close to the suggested ranges for greenhouse substrates. Considering the maximum substitution of PRH for peat, the substitution with 40% GRH (GRH-40) was selected for a plant growth study in comparison to CPM. Petunia (Petunia × hybrida) ‘Easy Wave Neon Rose’ and zinnia (Zinnia elegans) ‘Benary’s Giant Golden Yellow’ were grown in pots containing CPM and GRH-40 at two irrigation levels: high (25–30%, by volume) and low (20–25%). Overall, shoot DW of both plant species was reduced in GRH-40, but root DW was maintained similar to those in CPM. The low irrigation level more significantly reduced shoot DW of petunia grown in CPM than in GRH-40, relative to their counterparts with the high irrigation level (20 and 7%, respectively); however, root growth of plant species grown in GRH-40 was either maintained or tended to be maintained even at the low irrigation level compared to their high irrigation counterparts. The water use efficiency of plants grown in GRH-40 was not significantly different from those in CPM regardless of irrigation level. These results indicate that GRH-40 provides desirable physical properties and water release characteristics, allowing more available water for plants under low irrigation levels than CPM. We concluded that substituting peat with 40% GRH in a commercial potting mix is a sustainable approach to produce petunia and zinnia.