Absence of Air or Water Is Injurious In any growing medium, the space not occupied by solids constitutes pore space which is utilized partially for holding water and partially to provide air so plant roots can funtion normally. An absence of either is injurious to the growing plant. The higher the total porosity, the greater the potential to provide water and air. The smaller the pores,the greater the water retenton and the lower the air supply. Conversely, the larger the pores, the less water retained and the greater the amount of air space in the mixture.
Peat moss is well known for high porosity and perlite has a similar property.
The effect of perlite grade is one of increasing porosity as the gradation becomes coarser. Another consistent trend is for total porosity to increase modestly as the proportion of peat moss increases regardless of perlite grade. Differences are not great but indicate slightly higher total porosity for the peat mosses used compared to the perlite sources. Moisture Retention is Substantial The quantity of water held in a growing medium is best measured on a volume percentage basis. The general trend is for moisture retention to decline as perlite particle sizes become coarser and to increase as peat moss percentages increase.