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PHYSICAL PROPERTIES OF MIXTURES OF PEATS AND MINERALS OF DIFFERENT PARTICLE SIZE AND BULK DENSITY FOR POTTING SUBSTRATES
The minerals used included perlite, vermiculite, calcined clay and sand or grit.
Total pore space (TPS) was inversely related to bulk density (BD) and there was no interaction between the particle size of the minerals and TPS. Mixtures of fine grades of minerals and peat had lower air-filled porosity (AFP) values than those of either the separate minerals or the peats.
Coarse grades of minerals increased the AFP of the mixes.
Easily available water (EAW) was not significantly affected by using a fine grade of mineral.
The coarse grade of minerals however reduced the EAW, especially when used at >25% by volume in the mix.
The volume of water retained after the substrates had drained decreased as the BD of the mix increased.
Mixes made with the coarse grade of minerals also retained less water than those with the fine grades.
The effective TPS of coarse and fine grades of perlite were lower by 14 and 10% respectively than values obtained by the usual method.
Measurement of the AFP and EAW of peat gave a better assessment of its physical potential than particle analysis by sieving.
If an AFP of 10–15% is required for plants grown in shallow (10 cm) containers, then fine grades of minerals should not be added to any of the peats used in this investigation.
If it is desired to increase either the BD or the wettability of the mix then only coarse and not fine grades of minerals should be used.