There is a fine balance between water retention for plant uptake between irrigations and maintaining optimal air space of 25-50% to promote root respiration in a container. There are a number of factors that can influence these parameters. The components of a substrate are the most commonly attributed factor when considering water holding capacity. Often there is a large focus on particle size, aggregate incorporation rate, and substrate composition. In general peat and coconut coir have a greater water holding capacity when compared to wood fiber, however, the particle size of each component must also be considered. Additionally, aggregates such as perlite, bark, and wood chips, can be incorporated to increase drainage within a substrate. Smaller particle sizes will allow the substrate to hold onto more water when compared to larger particle size substrates due to the smaller pores between the particles. However, when utilizing a substrate that consists of a wide variety of components growers must consider how all parts of a substrate blend interact with the various aggregates and particle sizes of each component. While substrate composition and particle size distribution are important factors in water retention and drainage container height is also an important factor to consider.
Factors such as container capacity and air space within the substrate vary depending on container height and volume (Fonteno, 1988). However, total porosity may remain the same across containers of various sizes as long as the bulk density stays the same. When the container height increases the air space will increase which can result in a lower container capacity (the amount of water a substrate can hold after gravitational drainage). Taller containers also help avoid a post-irrigation perched water table that is common with shorter containers. When using a shorter container with a substrate composed of fine particle sizes, insufficient airspace may be observed, potentially resulting in increased occurrence of root rot or plant desiccation.
Recently when visiting a grower, we observed marigolds grown in a 4" pot but that was considerably (5.0" tall) taller than a traditional 4" pot (3.75" tall). While this taller pot does have greater volume resulting in more substrate being required compared to a shorter pot of the same diameter, the available airspace is much greater. Based on the pairing of container geometry and substrate utilized 2 growers must modify their growing practices to accommodate for the difference in air space available and where the roots are within the pot.
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