Inorganic growing media is derived from natural sources, and through refinement become very valuable and often reusable products. Due to the ease of use, ease of reuse, and physical longevity of the substrate, inorganic growing media has a permanent place in both horticulture and hydroponics.
One of the most common inorganic substrates is perlite. Perlite is super-heated to expand into lightweight, porous rocks that provide excellent aeration and improve the drainage and compaction of any medium such as soil, coco, or peat. Perlite will also wick water to your root zone and can be used as a medium for cloning shrubs, bushes, and woody stemmed plants. Perlite is inexpensive and comes in various sizes all smaller than or equal to the size of a marble.
Another common inorganic medium is expanded clay. Expanded clay is used in hydroponics and other applications where a strong, reusable medium is necessary. Expanded clay, such as Hydroton, is reusable, durable, and can withstand high volumes of water without compromising the medium. Expanded clay is porous, allowing more surface area for plant roots to colonize and will also retain moisture. This will allow your plants to absorb more water and nutrients over a longer period of time compared to other rock based growing media.
Rockwool is a volcanic rock that has been super-heated and spun like cotton candy to make small woven fibers that contain air pockets. These air pockets, when moistened, create an ideal air to water ratio for plant roots; oxygen easily travels in and out of the root zone, creating explosive root growth and a healthy aerobic root environment. Rockwool can be reused, however over long periods of time it will compress and lose the air pockets that make it so useful. We do not recommend using rockwool in hydroponic systems because dead root matter can be difficult to remove completely and can lead to harmful pathogens that may harm your plants. It can however be composted and will break down naturally over.
Next we have Growstones. This medium is made from recycled glass that has been heated and expanded into large porous rocks. These stones will perform similar to expanded clay, however are less dense and is more lightweight than expanded clay. Another difference between the two is the cleanliness of the stone itself. A mild rinsing of the stones will clean any debris off of them, whereas expanded clay tends to be more tedious to properly and thoroughly clean.
Another rock substrate common to horticulture is silica rock. Silica rock is used to provide slow release silicon into the plants cells and is also used as a versatile, reusable hydroponic medium. Silica rock, such as Higromite, is a slightly larger rock, yet performs very well in hydroponic systems because of its strong, dense structure. There is little to no breakdown of material over long periods of time with heavy exposure to water and sunlight. Finally, it will provide slow release, natural silicon to your plants throughout the entire life-cycle. This will strengthen your plants and increase their resistance to extreme environmental conditional while saving you money on silicon supplements.