The physical properties of compost are key factors in determining its quality as a growing medium. Physical properties are of great concern, because in contrast to the nutritional properties, they cannot easily be changed when inadequate. In horticultural greenhouse systems water is often applied in a short period of time such as in ebb-flood systems or locally as when using drip irrigation. Thus, horticultural media must be able to absorb water fast and be able to distribute the water in the medium. The physical quality of a substrate is related essentially to its ability to adequately store and supply air and water to plants grown in closed containers. The storage and supply of air and water are controlled by pore size abundance, tortuosity and continuity (Allaire et al., 1996). In general, particle size, water holding capacity and air-filled porosity is used as measures of the physical properties of growing media, and an ideal substrate must have a total pore space of more than 85% (Gruda and Schnitzler, 2004). In compost based on plant materials pore size was found to be above 90% (Dresboll and Thorup-Kristensen, submitted B).
Gas exchange properties are important as well. The air-filled porosity does not provide sufficient information about movement and availability of gasses, and this is regulated not only by the particle size but also by the geometries and pore space continuity (Nkongolo and Caron, 1999), why visualisation of the particles could enhance the understanding of how to produce a suitable growing medium (Dresboll and Thorup-Kristensen, submitted B). Gas diffusitivity or oxygen diffusion rate in growing media is thus thought to be a much better index of aeration than air content or air-filled porosity (Allaire et al., 1996; Caron and Nkongolo, 1999). Similarly, the water holding capacity reveal how much water the media can hold and to some extent the drainage capabilities, but does not bring sufficient information on how easily the media absorbs water. During experiments where the size of bark particles in a growing medium was increased, Nkongolo and Caron (1999) found that the air-filled porosity of the media remained the same while pore tortuosity increased and gas relative diffusivity decreased. It was concluded that when assessing the physical suitability of media for plant growth, the evaluation should not be limited to measurements of storage-related physical properties like air-filled porosity, bulk density and water-holding capacity alone, but should also include gas exchange characteristics such as pore tortuosity and gas relative diffusivity (Nkongolo and Caron, 1999).
The fraction of particles in the size range 0.1-0.5mm is responsible for the plant-available water holding capacity of the media. These particles are expected to produce pores in the range of 30-300^m between them, being optimal for water retention in growing media (Payne, 1988). In green waste compost an even spread between 0.1 and 5 mm was seen in the particle size distribution (Spiers and Fietje, 2000). The largest proportion of particles was also found in this range when examining plant-based composts (Dresboll and Thorup-Kristensen, submitted B). The finer the material, the higher is the water holding capacity per unit volume;
however a reduction of the particle size leads to a decrease of air capacity and might also decrease the availability of water (Gruda and Schnitzler, 2004).
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