SOME THOUGHTS ON THE PHYSICAL PROPERTIES OF SUBSTRATES WITH SPECIAL REFERENCE TO AERATION
In assessing the physical fitness of a substrate to serve as a growing-medium for ornamental plants, the following two critical points on its pF curve are of interest:
- The point coinciding with the critical air percentage (v/v). This point represents the minimum pF together with the concomitant maximum moisture percentage (v/v) to be maintained, when growing plants on this particular substrate. As the levels of these date are limiting to the water conditions, these data are of decisive value. In this regard it should be borne in mind that the availability of soil water is inversely related to the sum of Sm and osmotic suction Ss (Wadleigh & Ayers, 1945). Ss in turn is inversely related to the moisture percentage (v/v) of the substrate, Moreover, keeping pF low implicitly favours water transport in the substrate, capillary conductivity being inversely related to matric suction. Thus the lower the pF and the higher the moisture percentage at the critical air percentage are, the better the water conditions to be possibly obtained in that substrate.
- The point at 20% (v/v) desorption starting from Point 1. This point is included in the evaluation so as to allow for a certain water depletion. The chosen value of 20% (v/v), corresponding with the approximate daily transpiration loss of an almost mature potted chrysanthemum (Arnold Bik, 1969), is rather arbitrary. Nevertheless, Point 2 is indicative of the substrate's water buffering-power. As to the relevance of both pF and moisture percentage at Point 2 to the water conditions in the substrate, the same rule holds true as for Point 1.
Additionally, it may be noted that for potted plants, including those grown on capillary benches, the range between pF 1.0 and 2.0 is considered the most important pF section. Arnold Bik (1969) showed that growth of gloxinia in a peat-sand mixture was markedly decreased by increasing the pF level to which daily was watered, from 1.4 to 1.9.
The above method of interpreting a pF curve is exemplified in Fig. 1, showing the pF curves of three widely differing substrates.
Assuming the critical air content to be 20% (v/v) and the air content at