98 plugs per sheet
Rockwool is an inorganic substrate as are sand, gravel and perlite. Rockwool is manufactured by melting basaltic rock and spinning the melt into fibers. Immediately following spinning, a binder is added to the fibers and they are compressed and cured into large slabs.
Rockwool has a tremendous capacity for absorbing nutrient solution while retaining plenty of oxygen for rapid plant growth.
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 substrate.
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 water holding capacity of these is very good. Their longevity is generally better than the organic substrates, but is not as great as inorganic substrates.