Most hobbyist greenhouse growers have their own preferred soil recipe or pre-packaged soil mix. Because each plant variety has its own unique requirements and growers have their own personal growing techniques, some soils will work better than others. When gardeners find what works for them they rarely stray from that mix. There are, however, a large number of media beyond the plethora of standard potting soils that are being used by commercial growers and hobbyists alike. A closer look at some of these alternative media will give a horticulturalist heightened understanding of the grow media available to them.
Soilless potting mix is essentially a very weak, over-aerated soil. Soilless mixes are usually comprised of peat moss and/or coconut fiber and a lot of perlite. Some companies will add very minute amounts of compost and/or other organic amendments. The abundance of perlite ensures quick drainage and high oxygen holding c
apacity. Soilless mixes can be used as an alternative to soil for just about any plant variety as long as the gardener is willing to water and fertilize more often. This can be an advantage or disadvantage depending on the crop that is being grown. For most ornamentals there is little advantage to using a soilless mix. It requires more work from the grower and the fruits of their labor (pun intended) would be minimal. A gardener growing annual vegetable crops, like tomatoes or cucumbers, however, could capitalize on the benefits of a soilless mix. The quick drainage of a soilless medium brings oxygen to the root mass faster than a standard potting soil. This increased amount of oxygen has been shown to accelerate root production and growth for virtually all plants. Growers could also tailor their feeding regiment to maximize growth with the supplemented fertilizers. With specific feeding programs tailored to their crop, gardeners can supercharge growth and optimize their return. This is one of the main reasons soilless mixes are so popular in commercial greenhouses.
Coconut fiber (a.k.a coir fiber or coco-coir) is comprised of coconut husks. This product is a by-product of the coconut industry which makes it a great choice for the environmentally conscious grower. Coconut fiber has an incredible capability of holding moisture and oxygen at the same time. Coconut fiber can be used in both potting containers and hydroponic systems. This versatility has contributed to its continually expanding popularity. Commercial growers are taking advantage of coconut fiber’s relatively low price and unique ratio of water to air holding capabilities. Coconut fiber is almost entirely inert; meaning it has virtually no nutritional value. This requires all of the plant’s nutrition to be supplied by the horticulturalist. Much like a hydroponic system, horticulturalists growing in potting containers filled with coco fiber have 100% control over what is fed to their plants and when it is fed. Many fertilizer companies have coco specific fertilizers that can simplify things for the grower. While commercial growers are utilizing coco fiber as their exclusive medium, hobbyist gardeners are taking advantage of coconut fiber as a soil amendment. When added to a potting soil or soil bed, coconut fiber can increase oxygen retention and will dramatically help growers that tend to love their plants to death by overwatering. Coconut fiber is available in bales, compressed blocks, or loose (pre-shredded). There are also many pre-made coconut fiber blocks, fused with natural resins, designed specifically for hydroponic systems (usually top feed or ebb and flow systems). Due to its relatively low cost, environmentally friendly nature, and overall versatility, coconut fiber’s popularity amongst both hobbyist and professional growers will continue to rapidly expand.
Stonewool is a man-made substance comprised of melted, spun basaltic rock. Stonewool spinning machines
are like giant, souped-up cotton candy machines. These huge machines melt the basaltic rock, spin it, add a binder, and then compress it into the desired slab. By adjusting the amount of pressure used in the compression process, the density of the medium can be customized. Some of the spun fibers are made into a shredded product that can be handled similarly to peat or shredded coco fiber. This type of stonewool is usually referred to as “flocked”. Companies can further customize stonewools’ ability to hold moisture by adding wetting agents to the product. Although stonewool generally requires some initial pH adjustments, once it has been stabilized the pH will be the same as the nutrient solution added. Many hydroponic growers experience large fluctuations in their pH caused by the plant’s root nutrient absorption. Due to the inert nature of stonewool this can be quickly corrected by watering with a pH balanced nutrient solution. A substrate that can produce good root growth is a blessing for many large scale tomato and pepper producers.
Clay pebbles sold for horticultural applications are super-fired which creates their porous consistency. These are most commonly used as a growing substrate in hydroponic systems. Clay pebbles can also be amended to aerate the soil, although usually a less expensive option is chosen. Clay pebbles are an inert medium that has a neutral pH and can hold a lot of oxygen while retaining some moisture. Clay pebbles have a very clean, modern look that can be used for aesthetic purposes. A gardener can add a nice finishing touch to potted plants by covering just the very top of the potting soil with clay pebbles.
Diatomite or diatomaceous earth shows tremendous potential as a grow medium. Diatomite is comprised of the fossilized skeletal remains of microscopic single-celled aquatic plants known as diatoms. These tiny creatures developed a silicious protectant throughout their lives which was also fossilized. It is this silica that makes diatomite so special in terms of horticultural use. Silica has been proven as a beneficial element to plant growth, root development, and stimulating systemic acquired resistance. Although the majority of the silica contained within diatomite is insoluble, there is a small but significant amount of soluble silica. This soluble silica is released slowly and directly to the plant’s roots. Diatomite is naturally porous and has high water retention. Diatomite’s silica content combined with its water holding capabilities makes it a great hydroponic substrate or soil additive.
Perlite is a very common potting soil amendment but can also be used as a stand-alone grow medium. Perlite is expanded volcanic glass that has a neutral pH, excellent wicking action, high oxygen capacity, and is extremely light weight. This manipulated mineral has virtually replaced pumice in the horticultural industry because of its low cost due to its light weight (less shipping cost). Virtually every potting soil available contains perlite as an amendment. Its unique characteristics combined with its relatively low cost has its popularity as a stand-alone medium rising steadily amongst commercial growers.
This medium is designed specifically for, you guessed it, orchids. Orchid bark is almost always comprised of mostly fir or pine bark. Many orchid barks are actually fir or pine bark amended with nutrients specific to the needs of orchids. Always read the label of a packaged orchid bark to find out the exact composition of the mix. Although most mixes work for growing orchids, there may be a large variance in overall quality, nutrient composition and water retention between different manufacturers. Orchid bark can be used as a soil amendment or as a hydroponic medium although most gardeners choose a less expensive approach.
Some grow media may offer advantages over standard potting soil while others are more suited for hydroponic growing systems. Many of these media can be used to amend an existing soil so a gardener may gain some of its unique qualities. I urge us all, as horticulturists, to step out of our comfort zone every now and then so we can continue to experiment and learn. Try a new fertilizer, experiment with hydroponics; amend your soil with a new medium. As we experiment we share our trials and tribulations. It is from these trials and tribulations, experienced by our family, friends and neighbors that we collectively learn. Even the seemingly meaningless experimentation of the greenhouse hobbyist is just as instrumental in the future of horticulture as the next great technology. As we grow, we learn.
Eric Hopper resides in Michigan’s beautiful Upper Peninsula where he enjoys gardening and pursuing sustainability. He is a Garden & Greenhouse contributing editor and may be contacted at Ehop@GardenAndGreenhouse.net.