“Just a quick survey,” my teacher asked, “how long did it take you to complete the potting soil assignment?”

“Less than an hour?” A few hands went up

“1-2 hours?” Most of the class raised their hands.

“2-6 hours?” Fewer hands this time.

My hand, however, was still firmly down.

“How long did you spend?” she asked, looking right at me.

“Uh . . . maybe 12 hours?” 😬

Potting soil is a deeeeeep topic. And in my intro to farming class, I went down many a rabbit hole trying to find “the” answer.

--> I’ll save you the time: there is no one answer. Potting soil is as varied as bread. The result depends on who’s making it, and with what ingredients. There are lots - LOTS - of recipes.

But, like bread, the basics are simple, and anyone can make a decent batch. No kneading or rising required.

 

If you’d like to learn to make your own potting soil, you’ve come to the right place!

First things first.

There’s soil soil, and then there’s potting soil.

Soil soil is the dirt you dig up outside. Potting soil generally has no soil soil in it at all, and is instead a mixture of organic materials and sand.

For most potting projects, from seed-starting to houseplants, soil soil is a bad idea. It’s heavy. It’s usually full of weed seeds. It compacts easily, making it hard for baby roots to grow. And depending on its composition, soil soil usually either holds too much water or not enough.

Read on to learn what potting soil actually is, and how to make your own.

Good potting soil has 4 main jobs:

1. Water retention
2. Air flow
3. Nutrition
4. Microbial life

We’ll talk about each element in turn, and then go to some recipes for how to put them all together.

Water retention:

Water retention is the easiest element to understand. Plants need water, and potting mixes should stay moist for a while so your plants can drink, and you don’t have to water 3x per day!

Air flow:

Roots need air as much as they need water! It’s easy to drown your plants if soils get too soggy. That’s why you always need drainage holes in any potting container. And it’s why potting mixes need structure that allows for air flow.

Nutrition:

Plants need food as well as water, though not very much. Remember they make most of their food through photosynthesis. But they do need some main nutrients: Nitrogen, Potassium, & Phosphorous, plus lots of other trace minerals.

Microbial life:

The symbiotic relationships between plants and microbes - bacteria and fungi primarily - is a new area of study, and not completely understood. But it is clear that a thriving microbiome promotes healthy plant growth just as much, or even more than, strong fertilizers.

What ingredients provide the essential functions of potting soil?

 

Water & Air

Water retention and air flow are generally achieved by balancing ingredients that both hold water and air.

Traditionally, the main ingredient for water and air has been peat moss, but most responsible gardeners are moving away from it.

Peat is disastrous for the environment. The bogs where it’s mined are like miniature old-growth forests. They are not at all a renewable resource! If you’d like to read more about peat and its impacts, especially within the horticultural industry, here is a detailed look, written by Sara Venn, a nursery professional in the UK.

We at Orta are completely peat free.

There are LOTS of alternatives to peat. (Remember – all it does is hold water and air):

• Coconut coir. This is a material similar to peat in texture and color - it’s the finely ground byproduct fibers from the coconut industry which are extremely springy and water retentive. They also don’t compact easily, which means that air pores stay open unless it’s totally saturated. If you want to adapt a peat-based recipe, you can just substitute for coco coir, and leave out the lime. (Peat is somewhat acidic and needs lime to balance the pH. Coco coir has a neutral pH.) Coconut coir comes compressed in a brick. Soak it for an hour or so to expand it into a light, fluffy material.
• Vermiculite. This is a natural mineral that has been expanded like popcorn in an industrial furnace before it gets to you. It feels like styrofoam pellets, but it’s actually rocks! It holds both water and air, and is popular on its own as a seed starting medium.
• Perlite. Just like vermiculite, but a different mineral, and holds a bit less water. It’s a better choice for drier potting mixes (like for succulents) or for complimenting ingredients that hold water and can get soggy. *** Beware! *** I’ve found some perlite brands “helpfully” come impregnated with synthetic fertilizers. If you’re committed to organic gardening, check the label carefully to avoid artificial chemicals. Plain perlite is fine - it’s just rocks.
• Grit. This is the general term for sand / lava rock / ground pumice materials that increase drainage while holding some water (but not very much!).
• Wool. I’ve never used it, but in the UK, where wool is abundant, it’s a popular peat alternative
• Straw, cut up into little pieces. Again, I’ve never used it, but I know lots of gardeners who swear by it.
• Leaf mold. This is decomposed leaves. It’s crumbly, water retentive and a lovely texture for young plants. It tends to get water logged, so should be combined with perlite or other grit. (It looks a lot like compost, but has almost no nutrition.)

Nutrition

The nutrition part of potting soil generally comes from compost, worm castings, or well-rotted manure.

If you have homemade compost, that’s excellent! It’s a good source of nutrition, and it’s free. It’s a good idea to sieve the compost before adding it to your mix to remove the big chunks.

Bagged worm castings are expensive, but they are easy, reliable, and a great source of nutrition. You can create your own worm castings if you use a worm bin.

You can also buy commercial composts, soil amendments and composted manures at most garden centers. If you choose to use a soil amendment, a good ingredient list should include organic compost and various manures. Check to make sure it’s peat-free and isn’t mostly woodchips. Amendments vary wildly in quality.

Good compost also has the added benefit of being water retentive, ticking two of the functional boxes at once.

Finally, I often add a small scoop of organic fertilizer to my potting mixes, especially because they contain the trace minerals that may not be present in compost or worm castings.

Microbes

Microbial life is easy to come by. Just add a small amount of garden soil - about a tablespoonful for every 5 gallons of potting mix you make. The beneficial microbes naturally present in your soil will easily multiply in your potting mix, but the weed seeds won’t! You get the benefit of live soil without the problem of weeds.

It’s also becoming easier these days to buy mycorrhizal fungi, beneficial fungi that attach to the roots of plants, helping them absorb more water and nutrients. It comes as a powder that you can sprinkle into your potting soil in small amounts.

Putting it all together.

A basic all-purpose recipe for potting:

1 part expanded coconut coir
1 part perlite / grit
1 part compost
1 tbsp garden soil per 5 gallons of finished potting soil
Mycorrhizae according to package instructions
Organic fertilizer according to package instructions, err on the side of less.

For hungrier plants, increase the compost proportion.
For succulents / cacti, double the perlite.
For plants that like lots of water, increase the coco coir or decrease the perlite.

Here’s a video of me making my own water-retentive blend with worm castings instead of compost. (It was from before I could easily find mycorrhizae.)

For seed starting, the needs are a bit different. Seeds carry all their own nutrition for the first 2 sets of leaves, which means a seed-starting mix is low nutrition compared to potting mix for actively growing plants.

Also – seeds are often very small, with teeny baby roots. A very fine texture helps the baby roots grow and get anchored. Potting mix can have a texture like rolled oats, or even coleslaw, but seed starting mix should be about as fine as cornmeal (with a few bigger perlite chunks.)

General purpose seed-starting mix:

4 parts coco coir
3 parts perlite
1 part worm castings
1 tbsp garden soil per 5 gallons of finished potting soil
Mycorrhizae according to package instructions

Sourcing

• Garden centers sell compost, manures, perlite & vermiculite, organic fertilizers, plus whatever grit is local. It’s also very common to find bagged peat, which makes me sad - like buying printer paper made from old-growth redwoods. Mycorrhizal fungi and coco coir bricks are becoming more common at garden centers. You can also get coco coir in our store here.
• Most metro areas have at least one bulk soil and stone source. Primarily for the landscape trades, these places can be excellent for low prices on bulk materials like local grits (around here it’s lava rock), coconut coir and composts. They’re usually a lot less secretive than proprietary potting soil makers – they’ll just tell you where the compost comes from and what’s in it.
• Finally - if this all seems like too much work, we sell a peat-free seed starting blend here, that we ship without any plastic.