How To Make Your Own Seed Starting Mix

Have you ever wandered the aisles at your local gardening store and felt overwhelmed at all the different choices for dirt?

So many bags with colorful labels advertising for potting soil, compost, garden soil, topsoil, seed starting mix, coconut coir, peat moss, etc.

If you’re just starting out it can be really confusing to figure out what you actually need.

Plus, if you are interested in growing organically, or just limiting your use of chemical fertilizers and pesticides, how do you know what to get?

Generally to start your seeds indoors, you are looking for Seed Starting Mix. It can be different from potting soil that is used for containers of larger plants.

Seed starting mix needs:

to be light and fluffy, not heavy and full of clay or rocks,
to retain moisture so your seeds won’t dry out while they send out their first roots,
to have the right nutrition for the kinds of seeds you are planting, and
to be free of weed seeds and diseases to keep your seedlings healthy.

So, how do you figure out what can meet all of those needs? I’ve got you covered. Let’s go through the types of materials you’re looking for.

You can look for those items on the ingredients lists of the bags at your local garden center. Or you can make your own. I’ve got a recipe for how to make your own seed starting mix at the end.

Texture – Light and Fluffy, not Heavy

Seedlings are small when they first sprout. If the soil is too heavy or forms a hard crust on top, they won’t be able to break through. Or they will use up a lot of their energy just getting to the surface before they get a chance to see the sun to make more.

There are several types of materials that you can use in your mix to keep the texture light and fluffy. One is a volcanic mineral called pearlite. It is very light and doesn’t absorb water, so it helps with drainage. Pumice is another volcanic stone option, but it is a little heavier. Or there is vermiculite which is another naturally occurring mineral. Vermiculite is super light and it does absorb some water and nutrients.

You can also use sand in your mix as an alternative to the other minerals. It is a little heavier, but it doesn’t absorb water and can help with drainage. Plus it is easy to find and relatively cheap. Try to find playground sand rather than builders sand for the best drainage advantage.

Moisture – Not Too Much, Not Too Little

A lot of commercial soil mixes use peat moss as a significant percentage of the mix. It does a great job of retaining the right amount of moisture while still allowing enough air into the soil to prevent your seedlings from rotting. The main problem is peat moss is only found in peat bogs and digging it out destroys the bogs which can cause significant environmental damage because the bogs take thousands of years to recover.

A more eco-friendly alternative is coconut coir (the shredded up fibers of the coconut husk). It also retains water really well and is anti-fungal and sterile which can help keep your seedlings healthy. You usually find it in large blocks or small disks that rapidly expand when you add water.

If you don’t want to go the moss or coconut route, you can substitute composted leaves (aka leaf mould), bark, or wood shavings. Just make sure you get it from sources that weren’t treated with any chemicals.

Nutrition – Enough to Feed Your Seedlings

The classic form of nutrition in a seed starting mix is compost. There are lots of different kinds you can use. You can make your own from kitchen scraps and yard clippings. Or, you can start a worm bin or use composted manure from herbivores like rabbits, sheep, or cows. They also sell pure compost in the garden store, or some local governments make composted yard waste available for their residents.

The key is to understand what the nutrition needs are for the seeds you are starting. Having too much nutrition is less of a problem than having too little. But, having too much compost in the mix can make it a bit pricey if you don’t have access to a lot of it at a reasonable price.

Some seeds that want a lot of nutrition include tomatoes, peppers, potatoes, cabbage and most annual flowers. Seeds in a more medium range include onions, garlic, pumpkins, squash, and cucumbers. The low nutrition seeds are most herbs, lettuces, and salad greens.

If you’re planning on transplanting your seedlings into larger containers pretty soon after sprouting, then you don’t need too much nutrition in the seed starting mix. But if you are growing in containers or planning to keep your seedlings in the same trays for several weeks or months, then getting sufficient nutrition in the mix is key.

Sterile – Weed and Disease Free

You want to keep your seedlings as healthy as possible. If they have to compete with weeds or if the soil has any fungal or bacterial diseases, they could really struggle. Weeds can sometimes be difficult to tell from your sprouts and if you try to pull them out, it can disturb the delicate roots of your plant. So, starting with a mix that is free of weed seeds and diseases is key.

Luckily, most of the things I’ve listed already like pearlite, vermiculite, sand, peat moss or coconut coir are already pretty sterile. Where you’ve got to be careful is compost. If the composting process gets up to a high enough temperature (which naturally happens as the materials break down), then the weed seeds and diseases will usually be killed off. But if you are making your own compost, or getting it from a friend, some weed seeds might make it through. If you’re just starting out, you may want to purchase the compost you need to be safe.

The Recipe!

Here’s my favorite tomato seed starting mix recipe:

4 parts coconut coir or leaf mould
3 parts sand or pearlite
3 parts compost

I tend to start all my seeds in this kind of mix, but if you are starting seeds that can live with a lower level of nutrition, you can drop the compost to 2 parts and increase the coconut coir or leaf mould to 5 parts.

So, take this recipe and whip up some great seed starting mix and then get planting!

Need more help with your basic plant care? Check out Mission: Healthy Plants for a skill-building mini-course with everything you need to know about basic plant care in three quick modules. Enroll today!

Trying to decide where to start your seeds? I’ve laid out all the pros and cons of starting seeds indoors and out in this post. Check it out!