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The water retention of Sphagnum peat substrate as used in the production of containerized forest tree seedlings was determined at increasing matric potentials from saturation. Samples of commercially available peat substrate were taken and compressed to correspond to a wet bulk density of 0.2 g cm-3 — the value of the substrate used in practice in the nursery. The samples were saturated (-0.1 kPa) and the water content determined after applying successive matric potentials of -1, -10 and -100 kPa by means of a pressure plate apparatus. The effect of loosening the substrate was investigated, as was also the effect of applying the higher potentials directly to saturated samples. The volumetric water retention varied between 88-93% at -0.1 kPa, 72-85% at -1 kPa, 29-30% at -10 kPa and 18-25% at -100 kPa. Loosening the substrate between applying successively higher potentials to the same samples and, in the case of the higher potentials, determining retention directly from saturated samples gave the lower retention values. It was concluded that the most natural and practical way to determine water retention is to use the same samples, successively applying higher potentials without loosening between applications. Such a procedure includes the effect of increasing compaction and changes in pore size distribution of the substrate upon drying.
Keywords: Compaction, matric potential, shrinkage, water content
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Heiskanen,
The Finnish Forest Research Institute, Suonenjoki Research Station, SF-77600 Suonenjoki, Finland
Sähköposti:
ei.tietoa@nn.oo
