Healthy plants depend on their root systems to draw moisture and nutrients up to the rest of the plant. The roots also anchor the stem in place and form a foundation to hold the plant upright.
The material surrounding the roots is called the growing medium. The two most common types of growing media used for containers are potting soils and hydroponic media, but there are plenty of alternatives, as you can see from the list below:
Basic potting soils are usually made as a mix of materials—common ingredients are actively decomposing plant material, stable plant material and mineral materials.
Decomposing plant matter will degrade into compost, which in turn can be used to grow more plants. A naturally-occurring renewable resource, it requires little processing. As long as this decomposition is taking place (it will continue even after the compost is mature and ready for use), the material is compost. Compost has a dark earthy smell and contains nutrients that are readily available for plants.
Eventually compost will completely decompose and settle into its stable form—which is known as humus. Humus is so stable that it can remain unchanged for hundreds, if not thousands, of years. However, since decomposition is complete humus contains little nutrient value, although it still provides for less compact soil density and general soil improvement.
Peat moss is the decayed remains of sphagnum moss, a stable material harvested from bogs. While peat moss is naturally-occurring, overharvesting has damaged or destroyed bogs in many areas—coir is a more sustainable alternative.
Perlite is made from puffed volcanic glass and is often added to potting soils to improve physical structure and aeration. (More on perlite later…)
Sand is often added to media to allow better drainage and provide structure. It also helps give weight to the mix and adds some density. It can also be used on its own in hydroponic systems. When sand is being used this way, it's considered sand culture, which is a variation of gravel culture where the sand, which is used primarily to anchor plants in the grow bed or tray, is a lot finer than the gravel grow medium option.
Pre-mixed and homemade
Commercial mixes often fortify their products with beneficial ingredients like nutrients and other additives, which can save all the time and labor involved for the grower in assembling ingredients and compiling additives to include.
A basic homemade potting soil can be made from a mix of two parts homemade compost, one part coir or humus and a half part sand or clean dirt. There is a wide variety of homemade potting mix recipes available—ranging from the simple to the complex—and each gardener tends to have their own version.
The term hydroponics refers to using a growing medium other than soil, which can offer a habitat for soil-laying insects. Just as there are different kinds of garden soils, there are several different hydroponic growing media available, each with its own set of characteristics.
In general, the primary function of hydroponic media is to supply a structure for the root system. The medium itself has little to no nutritional value of its own, but can help keep air, nutrients and moisture available to the plants.
Your ideal medium will depend on both your chosen hydroponic method and environmental conditions. Fast-drying media can be helpful in preventing overwatering in humid conditions, while slow-drying media can help mitigate the problems associated with low-humidity environments. There is no single best medium for all applications!
Coco fiber (coir)
Coco fiber is made from coconut husks and is a renewable byproduct of coconut processing. The natural fibers have a high cellulose content and are often woven to create mats or used as upholstery stuffing.
Coir is often sold in compressed blocks that are expanded before use. It is sometimes necessary to add additional calcium to nutrient solutions when using coir, as it can absorb available calcium applied in average amounts. Using coir is more like growing in soil than most other hydro media and may be a good choice for a confident soil gardener trying their hand at hydroponics for the first time.
Diatomite (diatomaceous earth or DE)
Freshwater diatomite is primarily silicon oxide from the fossilized remains of diatoms (unicellular plants). A porous, chalky, mined sedimentary rock that absorbs moisture well, it is often used to assist in insect control, as a cat litter or to clean up oil spills.
While commonly known as a non-chemical insecticide, it may also be used as a growing medium. Diatomite is very light and has a low density, which means that it is not suitable for flooding systems as it may float when dry. Most of the diatomite used in a container can be recovered and reused after harvest.
Lightweight expanded clay aggregate (LECA)
LECA is clay that has been heated to expand and solidify. It is extremely airy and well suited for systems that have overwatering issues. LECA may be available either as irregular chunks or rolled into balls—it does not compact with use and allows for open space when used in a container.
It is often used to fill netting in deep water culture or nutrient film systems, but it is not well suited for sprouting and may float if flooded. LECA is reusable after washing (with a diluted sterilizing solution) and rinsing.
There are hydroponic systems that use little to no media to hold the root structure. With a deep water culture system (DWC), for instance, the roots are allowed to dangle into an aerated nutrient solution, while in an aeroponic garden the nutrient solution is sprayed onto the roots. Neither arrangement requires much in the way of media: the roots dangle below the plant with little or no support.
Perlite is natural volcanic glass with internal moisture locked inside. When heated at high temperatures the glass becomes molten and the internal moisture vaporizes into steam, expanding the glass. Dry perlite can be a dust hazard, so care should be taken to wet and rinse new product.
Perlite is extremely airy and difficult to overwater, but—as a tradeoff—it does not retain water as well as some other options. It will wick nutrient solution, though, which makes it a good choice for passive systems. Since dry perlite will float until it becomes saturated, it is not well suited for ebb and flow systems.
Most perlite is sold in bags containing small pieces, which retain moisture longer than larger pieces. One benefit to using perlite is that as long as the harvested plants were healthy, it can be rinsed and reused. As an alternate to perlite there is a similar product named Growstone, which is made from recycled glass and a foaming additive.
PET1 is a common plastic used to make a huge variety of products, including soda bottles and polar fleece. This pH-neutral plastic can also be spun into a fiber for use as a sterile hydroponic medium and as it tends to be very dry it works well in situations where it will be watered often—such as an ebb and flow system.
While the medium itself does not break down much over time, it is not very reusable as the roots will permeate the material and embed themselves throughout.
Thermosetting and thermoplastic foams
Polystyrenes and other polymers can be formed into materials similar to florist’s foam. These foams tend to be very light and airy when dry and can hold several times their weight in water when saturated.
Rockwool, also called stonewool, is melted basalt spun like fiberglass or cotton candy. One of the most popular hydroponic medium choices, rockwool has been around since the late 19th century. It can be an irritant when dry and can cause small itchy cuts in the skin (similar to fiberglass).
Depending on the manufacturer, it may be slightly basic (high pH) when new, so it might have to be rinsed with a light acid such as lemon juice before use. It is less prone to overwatering than coir, but more so than perlite or LECA. It retains water well but it is not very reusable.
Vermiculite is heated at high temperatures and the moisture it contains expands to puff up—much like popcorn. Dry vermiculite is dusty and may be an irritant, so precautions should be taken to limit exposure. Vermiculite, which compacts and breaks down over repeated use, is very airy and more commonly used in conjunction with other media than by itself.
Gravel can be used as a hydroponic growing medium and in fact was one of the earliest types of hydroponic media. The gravel culture hydroponic method maximizes root aeration as well as nutrient delivery. The gravel can also be re-used over and over, is inexpensive, and can never become waterlogged. While it is the easiest medium to keep clean, it is extremely heavy to work with.
As you can see, there are many choices of media to select from—each with their own ideal set of application parameters. Depending on what type of garden you have, some will be better suited than others. Match your medium to your garden and to your needs—and both you and your plants will enjoy the benefits.