Abstract

Plant growth response to substrate physical properties may be influenced not only by storage (water and air contents), but also by the exchange properties of the substrate. We studied the effect of variably sized components on substrate storage and exchange properties and related plant growth to these properties. Four substrates were composed of 40% coniferous bark (CB), 50% peat and 10%  gravel. Peat and gravel particle size remained constant, but coniferous bark particle size was varied (1 - 2, 2 - 4, 4 - 8 and 8 - 16 mm). Dendrathemummorifolium Ramat plants were transplanted in aluminum cylinders measuring 9.6 cm in diameter and 10.1 cm in height. The substrates were subjected to three (-0, -1.6 and 3.2 kPa) water potentials. Pore tortuosity factor (t) increased linearly (P=0.0001) with increasing bark particle size while the relative gas diffusion coefficient (D_s/D_o) decreased (P=0.0068).  Air-filled porosity (¦_a) remained unaffected by bark particle size. D. morifolium Ramat growth parameters were correlated to substrate exchange properties with  correlation coefficients ranging from 0.24 to 0.91. This study confirms the existence of a causal relationship between plant growth and bark size that is likely due to a modification of the substrate exchange properties.  The results also suggest that the effect of the substrate exchange properties on plant response is prevalent even over a short period of growth.

Key words: Gas diffusivity, pore tortuosity, air-filled porosity, peat substrates, peat-lite mixes,Dendrathemum morifolium.