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Juha Heiskanen

Two commercially produced growth media made of light, low humified sphagnum peat, were used to determine how filling into containers affects the particle size distribution and water retention characteristics of peat. It was shown that the filling procedure used broke up the peat particles, resulting in a significant increase in the proportion of particles < 1 mm (g·g-1). Due to the increased proportion of fine particles, the water retention of the peat media increased under wet conditions (-0.1 kPa matric potential), while the air-filled porosity decreased to nearly 0. Also, at matric potentials lower than -0.1 kPa, the reduction in air-filled porosity may restrict aeration and availability of oxygen to roots, thus reducing growth of plants.

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Juha Heiskanen

Various irrigation treatments were studied to determine their effects on the water and aeration conditions of peat-based growth media and evaporation from these media. Low-humified sphagnum peat and a mixture of this peat and perlite (in containers) were subjected to three contrasting irrigation treatments in which the container capacity was used as a target water content. The more frequent the reirrigations and the lower the vertical position within containers, the higher the matric potential of the growth medium averaged. Furthermore, the higher the water content of the growth medium, the higher the mean evaporation was from the containers. Within each irrigation treatment, slightly more water (on average) evaporated from the peat than from the peat–perlite mixture. Due to higher water retention and shrinkage during drying, a pure peat growth medium may increase the risk of waterlogging and, hence, O2 deficiency for containerized plants if plants are irrigated frequently to container capacity. To prevent waterlogging and high water loss by evaporation, low-humified sphagnum peat can be irrigated relatively infrequently (especially at low evaporation rates) and thoroughly within a short time (especially at high evaporation rates).