The water holding capacity and drainage characteristics of substrates vary. Grodan rockwool, when allowed to drain by gravitational pull, i.e. at field capacity contains 80% solution, 15% air pore space and 5% rockwool fibers. This ratio of solution to air promotes vigorous root growth. Plants growing on rockwool will remove solution and increase the ration of air-pore space to solution. Thus, if a higher proportion of air is desired in the root zone increasing the time between watering will increase the percent of air.
The tension required to remove solution from rockwool increases only slightly as rockwool dries. This means that it is as easy for a plant to remove solution from saturated rockwool as it is from rockwool that has given up 50% or even 70% of its moisture. Thus, plants grown in rockwool are not exposed to water stress until the rockwool is almost completely dry. Given that a standard rockwool slab used for tomato production holds 15 liters of water, the grower has tremendous flexibility with regard to watering and control over the air content of the root zone. However, the grower must be careful because the plants will not show signs of water stress until it is too late. A watchful eye on the conditions in the root zone is required.