Rockwool and other Substrates
Rockwool is an inorganic substrate as are sand, gravel and perlite. The main features of this class of substrates are that they have little cation exchange capacity and they maintain their structure over a long period of time. In general, at field capacity, rockwool holds more water per unit volume than the other inorganic substrates and therefore has a greater buffering capacity. The larger reserves of a nutrient solution coupled with excellent drainage makes rockwool easier to handle as an inorganic sub strate.
Organic substrates, peat, sawdust, etc. can have excellent water holding and release characteristics. The major drawback of these materials is that they decompose over a period of time. As decomposition proceeds the water holding capacity of the substrate changes. Thus, the grower must be aware of not only the growth of the plant but the changes in the substrate. For growers who are not experienced with hydroponics or liquid feeding the organic substrates may be easier to learn on than the inorganic substrates. Organic substrates, especially peat, have some cation exchange capacity. This gives the grower some buffer against nutrient changes in the root zone. This may help the new grower manage nutrition. Another advantage of most organic media is that they are inexpensive. Of course the grower must always balance the cost with the effectiveness and yield.
The hydrocarbon-based substrates have not been widely used. The water holding capacity of these can be very good. Their longevity is generally better than the organic substrates, but is not as great as inorganic substrates. The lack of wide acceptance of these media may have to do with cost.
The use of the hydroponic substrate system in general and rockwool in particular for vine crop and some cut flower production has been proven worldwide. In Holland, a respected world leader in greenhouse vegetable and cut flower production, of 3550 hectares of vegetables, 2350 are on substrate and 1980 of that are on Grodan Rockwool. Considering cut flowers, of the 1600 hectares total, 160 hectares are on Grodan Rockwool. In North America the trend is toward growing on substrates. Ease of handling (labor savings), better control over nutrition and better disease management are all reasons for this trend. These factors all point to more yield at lower cost.
Michael F. Dowgert studied agriculture at the University of Massachusetts where he received his B.S. and at Cornell University where he received his Ph.D. He worked in production and research at GLIE Farms in New York where he was responsible for greenhouse and field production of culinary herbs. He also conducted research on preservation of herbs. He is currently responsible for research projects and coordination of information on greenhouse production techniques at Agro Dynamics.
Courtesy of the Hydroponic Society of America. Used by permission.