Growing Media

James Semper
Jan 11, 2017
6 min read

There are various types of growing media and amendments. Buying soil can be expensive and the wrong kind could be devastating on your plants. Hopefully after reading this guide, we can help you with your soil purchases and show you how to make your own blend.

Note: This is definitely not everything. There are various substrates I failed to mention. These are the ones I work with the most.

Inorganic Growing Media

Perlite - A volcanic glass that is heated until it pops like popcorn. This is the white "Styrofoam-like" particle found in most potting mixes. Perlite is good at facilitating water absorption into roots. However, perlite is quite porous and is good for adding air into a growing media. Adding perlite to your growing media will decrease weight per pot and will reduce the chance of root rot. Perlite does wonders to resist compaction as well. Some growers use perlite on a misting bench to root cuttings.

Vermiculite - Vermiculite is a mineral that is mined and processed to form a puffy sponge-like material. Vermiculite helps loosen soil and gives room for roots to grow. Unlike perlite, it is significantly better at absorption of liquids. For this reason, it is even used to clean up chemical spills. Add vermiculite to potting mix for plants that love water.

Sand - Sand is quite common. Sand increases drainage and gives plenty of pore space for roots to grow. This large pore size leads to the smallest capillary action out of the different soils. Sand is a mostly inert substance made of the same mineral that makes up most of the molecules in glass. Being inert and having a very small ability to move around cations, sand isn't the best for nutrition. If you grow in pure sand, you will need to fertilize and water more frequently. Sand also has the disadvantage of being heavy.

Silt - Silt is a mineral that is smaller in size than sand. For this reason, it has much more capillary action so it holds water better which leads to less aeration than sand. Silt has a higher ability to move around cations leading to better nutrition exchange to roots grown in it. Silt is often seen as a happy middle between sand and clay. Silt also has the disadvantage of being heavy.

Clay - Clay minerals are the smallest particles found in soil. Clay is so small that it has an enormous amount of surface area per mass leading to the greatest capillary action compared to sand and silt. This allows for clay to hold more water. Unfortunately, the small pores do not allow for adequate root growth and can lead to root rot due to lack of air. Clay does have a much better ability to exchange cations with the water in the soil. Clay also has the disadvantage of being heavy. Clay pellets are sometimes used for aquaponics or hydroponics.

Loam - A loam, whether a sandy loam, clayey loam, or silty loam, contains properties of sand, silt, and clay. This is the most ideal blend for growing crops. There is good water retention but plenty of pore space for air and roots. This also tends to be a good blend for releasing the cations needed for healthy growth. Some loams are better than others. For example, if growing watermelon you want a soil like a sandy loam. This is because too much water retention leads to rot. Loams are still quite heavy.

Gravel - Gravel is even larger than sand. This allows for more air space but at the expense of being heavier. Gravel is poor for growing almost everything but can be ideal for hydroponics or aquaponics.

Styrofoam - Little Styrofoam beads can be used in soil to keep soil from packing and to allow for more air space. Since Styrofoam isn't readily broken down, this isn't a favorite for those who want soil they can easily dispose of.

Air and Water - Technically these are both types of growing media. Combined, they make up about 50% of soil/soil-less media by volume. And then there's aeroponics, aquaponics, and hydroponics which don't always use solid media. And, of course, we have the oddball plants which prefer to grow in water or in dry environments.

Organic Growing Media

Sphagnum Peat Moss - Sphagnum peat moss is the most used soil-less media. It is on the acidic side so it works well for plants that prefer acidic soils. But it works just fine with most plants. Sphagnum peat moss can hold about 20 times its weight in water. However, when dry, it repels water. For this reason, most potting "soils" add a wetting agent. With a little bit of patience, this isn't an issue. Sphagnum peat moss isn't the richest in nutrition but will release a few nutrients as it decays. Your primary source of nutrition will be from other components of your media and fertilizer.

Sphagnum Moss - Unlike peat moss, this isn't formed in a bog. Sphagnum is the dried plant. This is good for layering plants but like with any uncomposted material, assume you need to add nitrogen if you plant directly into it.

Compost - Compost is a whole topic on its own. It comes in many different varieties. First off, compost you buy in the store is typically good to go. You can check ratings before you buy to help confirm or deny this. Many composts you buy are mostly sand but also contain various trace nutrients and are void of seeds. This is a plus when you are making your own soils and don't want weeds. Some composts are made from manure and can be extremely rich in some nutrients. And of course you can make your own. Make sure you have a good ratio of carbon to nitrogen and it should heat up, killing the weeds present. Additionally, stirring the compost frequently adds the air it needs for the bacteria to process their food. This can be a wonderful amendment to any garden or your potting mix. Pesticides and disease can be a problem though so beware what you put in the mix.

Spanish Moss - Similar to sphagnum moss in properties but isn't acidic so pathogens can be more of an issue. If you collect you own, you will have to sterilize it. Additionally, Spanish moss works quite well as a mulch.

Humus - Humus is the dark layer on many soils. This is essentially compost that has built up over the years. It is high in organic material and hasn't fully mineralized. It is a wonderful source of nutrition and like any organic matter, it is great for exchanging cations and loves to retain water.

Mulch - Technically this belongs under both organic and inorganic media. Mulches are designed to be placed on the surface of a soil to retain water, shade the ground, slowly release nutrients, etc. They can be made up of leaf matter, bark, hay, newspaper, rocks, plastic, rubber, etc. Organic mulches can sap a lot of nitrogen out of soils while they decompose. This is fine so long as they aren't tilled under. Orchids are a type of plant that are typically grown on bark.

Coir - Coir is the fiber from coconut husks. Just like with the mosses, the leveled of decomposition will determine how much nitrogen this uses up. That being said, it is still a great media and is often mixed with sphagnum peat moss. Some people choose to use coir in place of sphagnum since coir doesn't require huge mines that can pose issues on the environment.

Worm Castings - Worm castings are the excrement of worms. This can be a really good soil amendment as it has been broken down a step further by worms. Castings are essentially a form of humus. Castings are one of the end products of vermiculture.

So what do you buy?

Personally, I like to buy large bricks of Sphagnum peat moss and mix it with a cheap bag of compost and some perlite. This allows for the water retention of peat moss, nutrients of compost, and porosity of perlite. While mixing, I add water. This is a must when using dry peat moss since it will repel water until wet. I am also experimenting with humus and homemade compost at the moment and this will probably be added to my future recipes to cut costs even more.

This decision is more on yourself. You can learn a lot by experimenting and mixing. One thing to be aware of is buzzwords like "all-natural" or "organic" as this don't usually mean much as far as effectiveness goes. This is a great way of spending more money for less product, but this isn't always the case.

Some people like to buy brand name things like MiracleGro Sphagnum Peat Moss as opposed to generic peat moss. MiracleGro and others are great in the fact that they typically already contain fertilizer and a wetting agent. However, I just add my own fertilizer and I do not need the wetting agent. Little things like this can save a lot of money for the big time gardeners. For infrequent gardeners, I advise buying these pre-made potting mixes so you have less soil laying around. However, I encourage you to create your own soil, experiment a little, take a few cuttings, and share the love with neighbors and friends. I personally have potted well over 150 small plants and gave them away with less than $8 worth of soil I made myself. It can be quite rewarding to all involved.

Best wishes to all of you. Happy experimenting. Any questions, additions, or just general comments, feel free to add them below.