This experiment was designed to characterize the physical, chemical and microbial
properties of a standard commercial horticultural, greenhouse container, bedding plant
medium (Metro-Mix 360), that had been substituted with a range of increasing concentrations
(0%, 5%, 10%, 25%, 50% and 100% by volume) of pig manure vermicompost and to relate
these properties to plant growth responses. The growth trials used tomatoes (Lycopersicon
esculentum Mill.), grown in the substituted media for 31 days under glasshouse conditions,
with seedling growth recorded in 20 pots for each treatment. Half of the tomato seedlings
(10 pots per treatment) were watered daily with liquid inorganic fertilizer while
the other half received water only. The percentage total porosity, percentage air
space, pH and ammonium concentrations of the container medium all decreased significantly,
after substitution of Metro-Mix 360 with equivalent amounts of pig manure vermicompost;
whereas bulk density, container capacity, electrical conductivity, overall microbial
activity and nitrate concentrations, all increased with increasing substitutions of
vermicompost. The growth of tomato seedlings in the potting mixtures containing 100%
pig manure vermicompost was reduced, possibly as a result of high soluble salt concentrations
in the vermicompost and poorer porosity and aeration. The growth of tomato seedlings
was greatest after substitution of Metro-Mix 360 with between 25% and 50% pig manure
vermicompost, with more growth occurring in combinations of pig manure vermicompost
treated regularly with a liquid fertilizer solution than in those with no fertilizer
applied. Some of the growth enhancement in these mixtures seemed to be related to
the combined effects of improved porosity, aeration and water retention in the medium
and the high nitrate content of the substrate, which produced an increased uptake
of nitrogen by the plant tissues, resulting in increased plant growth. When the tomato
seedlings were watered daily with liquid inorganic fertilizer, substitution of Metro-Mix
360 with a very small amount (5%) of pig manure vermicompost resulted in a significant
increase in the growth of tomato seedlings. Such effects could not be attributed solely
to the nutritional or physical properties of the pig manure vermicompost. Therefore,
it seems likely that the pig manure vermicompost provided other biological inputs,
such as plant growth regulators into the container medium, that still need to be identified
fully.