Abstract

Aeration properties are key characteristics of growing media. On top of air filled porosity, pore tortuosity (t) and gas diffusivity (D_s/D_o) have been suggested to better characterize the aeration process. A two-year nursery study was conducted to investigate how D_s/D_o of growing mediawere affected by particle size and how plant growth responded to substrates with the same air storage, but different gas exchange properties. Four substrates were made of the same proportion (on a volume basis) of sphagnum peat (50%), composted coniferous wood bark (40%) and gravel (10%). Different particle sizes of coniferous bark (1 - 2, 2 - 4, 4 - 8 and 8 - 16 mm) were used. However, air-filled porosity (¦_a) remained unaffected by this change in bark particle size. The factor t increased linearly with increasing bark particle size while D_s/D_odecreased. Such substrates were used to grow Prunus cystena sp and Spirae japonica for two years in 5 L pot, at three different water potentials (-0.8, -1.2, -1.6 kPa) Increasing coniferous wood bark particle size decreased plant growth significantly for nearly all parameters studied. Prunus x cistena sp and Spiraea japonica shoot and root dry weights were very significantly correlated with both t and D_s/D_o with determination coefficients ranging from 0.43 to 0.71. This study suggests a causal-effect relationship between plant growth and substrate gas exchange characteristics.

Key words: Gas diffusivity, pore tortuosity, air-filled porosity, peat substrates, peat-lite mixes,Prunus x cistena, Spiraea japonica.