Updated: June 25, 2019
Soil is a great thing to have under our feet; we walk all over it and it doesn’t complain. It’s made up of a variety of fine particulate, depending on the particular geographic region where the soil is found. Common ingredients in soil include decaying organic matter like fallen leaves and lawn clippings, and inorganic matter like clay, sand, silt and gravel. The best soils in the world are rich in nutrients, allow roots to penetrate easily while still anchoring the plant and drain quickly, retaining just the right amount of water to keep plants hydrated. But that’s just the beginning. Soil also contains an ecosystem most people know nothing about. Life forms like bacteria and fungus are common soil inhabitants along with protozoans and other single-celled organisms. Bigger creatures like insects, worms and arthropods roam the pores in the soil, tunneling and mixing the layers together while the enormous mammals and reptiles create part-time dens below the soil’s surface. Soil is alive, and for that very reason it should never cross the threshold of your greenhouse.
In a greenhouse, we forget soil and use growing mediums. Even if you live in a region where soil is ideal, there’s a huge risk that you’ll bring diseases and pests into your greenhouse if you dig up the backyard and pack it inside. Once you’ve got a major problem in your greenhouse, trust me, it’ll be easier to torch the place than get the disaster under control—so let’s not let that happen, OK? Unlike soil, mediums are mixed to order—you can literally create any sort of conditions you can dream up, though most plants respond well to a light medium with moderate moisture retention. Many greenhouse hobbyists start with pre-packaged growing mixes, and there’s not a thing in the world wrong with that. The bags of seed starting mixes, potting mixes and so on contain detailed information about the composition of the contents, including a nutrient breakdown and often, the pH of the material. At some point most growers will try their hand at mixing their own mediums to save money and create what they believe to be the ideal conditions for their plants. Below, I’ve made a list of the most common ingredients in greenhouse mixes and added a little about what they do. Armed with this knowledge, you’ll be prepared to go out there and mix it up.
Common ingredients in greenhouse mediums
Peat – Peat comes in many forms, but for our purposes, we’re talking about plain old, shredded peat moss. It’s a brown color and is used extensively in gardening. Wetting peat can be a trick, but once it’s wet, it holds moisture well and is light enough for even delicate plant roots to push through as they grow. Peat is a lovely product, but controversial due to commercial over-harvesting from peat bogs.
Coconut coir – This is the same stuff they make hanging basket liners from, but with a much finer particle size. Coconut coir is highly absorbent, very light and easy to wet. Unlike peat, which is extracted from bogs, coconut coir is a by-product of the coconut industry—it’s just another form of garbage that gardeners love. Coconut coir is my favorite!
Sterilized compost – Compost, once sterilized, is a good way to feed your organically grown plants. It also makes nice mulch. Compost should be crumbly and heavier than peat or coir—it’s a great addition for plants that need to be anchored, since it usually increases the weight of your mix. Compost can be tricky business since the composition varies widely.
Bark – Bark is sometimes used in specialty mixes, or alone, for plants that require very large air pockets in the soil. Many houseplants, including orchids and Christmas cactus, dwell in trees in their natural habitats and have very little soil covering their roots. For these epiphytes, a bark-based medium is ideal, but you’ll have to water them frequently since bark isn’t very good at retaining moisture.
Agricultural by-products – Agricultural by-products like cotton seed hulls and rice hulls are available regionally. While they can add nutrients, aerate mediums and have some water holding capacity, these products must be used with great care, as many can add toxic levels of micronutrients to your medium as they break down.
Perlite – This lightweight substance is created by heating igneous rock to temperatures exceeding 1000 degrees Fahrenheit. This creates air pockets inside the particles, but they’re not porous, so it’s best used for increasing the drainage in heavy mixes.
Vermiculite – Similar to perlite, vermiculite is made by superheating a mineral called mica. Unlike perlite, vermiculite is porous and excellent at retaining water and nutrients. Check your vermiculite carefully, though, because non-U.S. sources may be considerably more alkaline than the typically neutral vermiculite produced in the United States.
Sand – Although it doesn’t add much to the mix, sand is a great way to add weight to your mix as long as you don’t overdo it. Fine sand can destroy those valuable little air pockets, so look for a coarse sand if you want to include it. If you have particularly top-heavy plants or problems with blow-over, sand can be your best friend.
Styrofoam – There are still some gardeners out there who advocate the use of Styrofoam in mixes because it’s cheap filler, but don’t be tempted by their beautiful words. It can increase drainage, that’s true, but at a heavy cost—Styrofoam is both difficult to sterilize and recycle. If you’re thinking about using Styrofoam, consider perlite instead. The environment will thank you.
The most basic mixes are made using half organic and half inorganic mediums. Combinations like peat and vermiculite or coconut coir and perlite are common for use with mature plants, but younger plants and seedlings may need a much lighter mix. For these delicate youngsters, try a formula made of 70 percent peat or coconut coir and 30 percent vermiculite. The peat or coir will keep the mixture very light, and the vermiculite will open some pores in the medium without giving up the absorption your little seedlings need.
Read more about soil in The importance of greenhouse sanitation and sterilizing soil>>