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Air-filled porosity (AFP) affected the suppressiveness of organically based plant growth media against Pythium ultimum infection. AFP also interacted with microbial activity, moisture levels altered by covering/uncovering media and water severity of pythium damping-off. It has not been possible to determine a narrow range of AFP required to achieve consistently suppressive media, as AFP has been found to interact with many factors such as microbial activity, temperature shock of media and plants and moisture levels modified by covering and uncovering media, or using different pot heights to modify WHC. The use of different batches of compost and ageing of compost in the seven experiments reported here may also account for some of the different effects of AFP on the severity of Pythium disease. It is not recommended that AFP be used for controlling Pythium damping-off, unless certain conditions, such as microbial activity, can be controlled to a range of specifications also. Variability of compost is one of the principal factors limiting its widespread use, so therefore, improving the quality control during the composting process would be the principal factor which should be investigated in future.