"If I wanted to have a happy garden, I must ally myself with my soil; study and help it to the utmost, untiringly. Always, the soil must come first."
- Marion Cran, If I Were Beginning Again
Like many things, potting soil seems simple and self-explanatory. We're talking about dirt, right?
Why not just stick a shovel in the ground and get some dirt to pot your plants in? There are several reasons why actual soil is less than ideal as a planting medium for use in containers. Oddly enough, many commercial brands of potting "soil" contain little or no soil. Instead we find a mixture of organic and inorganic ingredients which each brand claims is ideal. For this reason, other (maybe more accurate) names for "potting soil" are "potting mix", "potting medium", or even "potting soil mix".
Let's mention a couple of things you don't want in your potting soil.
The soil in your yard or garden is likely to contain viable seeds that are patiently waiting for the right conditions to germinate. Whether our potting soil is going to be used for germinating garden seeds, or transplanting plants that are already growing, the right conditions are likely going to be provided. The weed seeds will sprout and compete with the plants we actually want to grow. Materials can be pasteurized to kill weed seeds and some pathogens. An oven, crockpot, roaster or solar cooker (summer sun on a black trash bag?) may be used. The key here is to heat the soil (or other media) to 160° F to 180° F (71° C to 82° C) for at least an hour. A link is provided below for techniques used by large scale operations for control of weed seeds and plant pathogens.
Material that will compact
That soil you dug up in your yard is likely to contain mostly clay, silt, sand and similar inorganic materials. For our purposes here the term "organic" means simply something which is, or once was, alive. Even very good quality garden soil still consists mostly of inorganic minerals. Ever notice how hard a lump of soil can become once it gets dried out? Bricks are fine for building houses out of, but not so great for growing plants in. So, am I saying not to use topsoil? No, just be careful how much you use. A general rule of thumb is no more than half and I recommend one third, or less. (I don't add any).
Am I saying that the only option is to buy a retail potting mix
? That is certainly an option and there are valid reasons for choosing it. If you choose to go that route see the link below for a current (2020) comparison between brands. I have been able to make my own potting soil and have been reasonably pleased with the result. I still have challenges keeping weed seeds out of both my potting medium and my garden. On the other hand, potting soil packaged for retail tends to be either expensive or poor quality. I use a lot of it! So I continue to work on methods of producing quality potting soil at home. The materials to do this are surprisingly inexpensive if you don't mind doing some work. In my case that work consists of collecting three types
of "free" (unless you count labor and time!) biomass.
First, I have put the word out that I'll pick up bagged leaves around town. They rake 'em and bag 'em, and I haul 'em. They become a source of biomass for my compost pile. They are shredded with the bagger mower, if I have time. Often I don't and they are just watered, piled and covered with a tarp. I actually came across a windfall when I first started making my own potting soil years ago. A neighbor had a pile of material that I drove by occasionally. I thought it was wood chips. I slowed down to rubberneck as I went by and the owner was mowing nearby. He stopped his riding mower and came over to ask what I was doing. Turns out, He had asked the city to dump leaves that had been collected around town and ended up with this huge pile after using as much as he wanted. They were already composted and sitting on a concrete slab. He offered to load them with his front-end loader and I hauled sixteen pickup loads to my place. That pile provided literally tons of compost for potting soil.
Another source is my own lawn clippings. My neighbors think I'm crazy. They're not wrong. But there is method in my madness. I have a perfectly good zero turn riding mower and I use it a lot. But it doesn't have a bagger to catch clippings. So, if I have time, I use my self-propelled push mower, which does have a bagger, on my acre of lawn to collect grass clippings. These are good biomass, but need careful handling to compost them. If not handled properly, they are green manure and will look, and smell, like it. The best way I've found to handle this material is to mix it thoroughly with some of the following one.
Sawdust and wood shavings. I have a neighbor who is a cabinet maker. He gladly lets me help clean his cabinet shop when the piles of shavings and sawdust get too deep for him to find his power tools. The sawdust and fresh grass clippings, when thoroughly mixed, make a perfect blend. Make sure the wood being sawed or shaved is not treated wood. There are also some local small sawmills that let people haul off sawdust for free. When I get ambitious enough, I'll haul some of that.
Manure. (OK, that makes four types). A man just a few blocks from me has horses. When it gets too deep in the stall, I show up with a pickup and a shovel. Beware of weed seeds in barnyard manure.
When the pile is ready, the composting worms show up. I actually purchased a couple of pounds of red wigglers several years ago but I now believe that was a waste of money. With regard to compost and red wigglers, "If you build it they will come", as momomom has already mentioned. Sorry if this sounds like a treatise on composting. If you want to make your own potting soil, you'll either have to learn to compost or buy compost. I wouldn't want to try to make potting soil without it. The exception to this is seed germinating soil or mix can be made without compost if the seedlings will be quickly moved to the garden or to a mix containing compost.
There are as many recipes for potting mix as there are people making it and this includes the potting soil you buy at the garden center. I'll give some proportions soon but keep in mind that they are approximations only. There are a couple of key qualities that make for a good potting soil and they are:
Simply put, a friable mix is one which will hold its shape when moist but not sodden. Pick up a fistful of potting soil and squeeze hard. Now open your fist and let the lump of soil rest on your open palm. Next, tap it sharply with a finger of your free hand. If it falls apart readily, it is friable.
The second key quality relates to the ability of our potting soil to absorb water. Keep in mind that a bone dry mix will be somewhat hydrophobic. This tendency relates to surface tension. It will try to repel water when first moistened. For this reason, I prefer to have the ingredients pre-moistened a bit when mixing.
I like to use either peat moss or coco coir in my potting mix. Either is fine and you can omit these if you have plenty of good compost. Using them helps to achieve the two key qualities just discussed more easily. Of the two, coco coir, a by-product of coconut processing (ground husks), is considered more environmentally friendly. I use coir when I can and peat when I can't. Coco coir is tough to source locally and expensive to ship. Whichever you choose will make up about half of the mix. The other half will be good compost. If you've chosen to add good topsoil use an equal part of sifted soil.
The other ingredients are optional. Vermiculite™ and Perlite™ may be added and I wouldn't use more than about 10% of these by volume. I like to add a handful or two of the following, if available: Bone meal (for phosphorus), Azomite™ (or other rock powder), and ground limestone (AKA Calcium carbonate). I buy these (except Azomite™, which I order online) at a local feed store. If you aren't trying to keep it organic (in the chemical-free sense of the word), you may want to add a sprinkle of timed release fertilizer such as Osmocote™. I no longer add this but it works well. So does worm compost. I sift everything used through a homemade sifter (1/4 inch mesh hardware cloth and scrap wood) that will lay across my garden cart. It not only gets large chunks out but also helps blend everything.
So that's it! If you want to make your own potting soil, you can, and the project can be fun. Like anything else, it gets easier with practice and your best indicator of success will be how your plants respond.
10 Best Potting Soils 2020
Soil Pasteurization (added February 9, 2020)