Hydrological Behavior of Peat- and Coir-Based Substrates and Their Effect on Begonia Growth

The physical-hydraulic properties of eight substrate mixtures based on sphagnum peat and coir were determined and their effect on the growth of Begonia xelatior was studied. The particle size distribution, water retention curve, saturated hydraulic conductivity, and pore size distribution of the substrates were determined. All substrates exhibited high total porosity, satisfactory water retention capacity, and high saturated hydraulic conductivity. Increasing the percentage of perlite in the mixtures contributed to the reduction of water retention capacity and the increase of large pores. Unsaturated hydraulic conductivity estimated by the Mualem-van Genuchten model showed a sharp decrease within a range of water pressure heads (0 to -50 cm) observed between two successive irrigations. To assess aeration and water retention capacity, total porosity; airspace; and easily, and nonavailable water, as well as the bulk density of the substrates, were determined and concomitantly compared with the ideal substrates determined by De Boodt and Verdonck. The comparative results showed that substrate porosity alone is not efficient to create ideal plant growth conditions, but dynamic parameters are also needed. Plants grown in a substrate classified as nonideal showed significantly greater growth than the plants grown in most of the other substrates studied.