how to mix your own potting soil & seeding 101.

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When I’m seeding I like to mix up my own potting soil. For the volume of starts that I plant, it’s WAY cheaper, and I can control the amounts of everything I put into the mix; the plant’s needs will vary depending on its stage of growth. Take peat moss, for example: I like to use a little more for seed starting, and a little less for potting up. That’s important because with seed starting, you want the soil to be nice & airy; its essential for the water to be able flow THROUGH the cell, and not get stuck in pockets of dense moist soil, potentially drowning or rotting the seed. But after the seed has taken root and the plant is established, you’re better off having equal amounts of compost & peat in your mix, so the plant can start to absorb the N-P-K found in the compost component of the soil, which it needs to grow leaves and a strong & sturdy root system.

Ok, wait. Let’s back up a bit.

There are many factors that effect germination in your seeds including moisture, air, temperature, light, and seed depth. By mixing your own potting soil, you can help control some of those factors, mainly moisture and air. The seed is a living organism that is held in a suspended state of dormancy, and is dehydrated (it usually contains about 10-15% water). Absorbing more water from the soil activates the seed & allows it to metabolize its stored food reserves. The embryo begins to swell, and the softened outer layer of the seed then ruptures, and voila! Germination. However, if your soil is too wet or dense, you have {accidentally!} created an anaerobic condition where the seed wont be able to germinate. And that’s why good airflow is important.

There are 5 ingredients in this homemade potting mix, and they each serve a purpose:

• Compost: retains water in the soil, and provides nutrients to the plants.
• Peat Moss: holds water and air, and doesn’t decompose quickly.
• Sand: adds space between compost particles, providing airflow.
• Perlite (a light-weight material made from volcanic rock): Increases air space, and improves water drainage.
• Vermiculite (mica {a mineral, I think!} that has been heated and expanded to become light-weight): creates air space and holds water. Also contains calcium & magnesium.

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I usually mix this in an old bath tub we have, but it’s pouring rain, and I don’t feel like getting soaked… so today, I am mixing it in a wheelbarrow, inside our greenhouse…

Here’s the recipe:
½ wheelbarrow full of compost
½ wheelbarrow full of peat (more if doing seed starting mix)
a shovel full of sand
a heaping handful each of perlite & vermiculite

You could use a shovel, but I like to get my hands in there to mix it all up; it helps to break up the larger compost clumps…

After mixing, simply fill your seed starting trays, & tamp down the soil a bit (not too firmly though!). Then plant your seed in each cell of the tray. A general rule of thumb is to plant your seed at a depth of twice the size of the seed. For example, if your seed is a ½ inch long, go down 1 inch. Next, cover the seeds with a bit more soil, tamp down again, and give your trays a good soak with the watering can.

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If your seeds need a warm temperature to germinate (onions & peppers, for example), you can place them on a heated seeding mat or on top of your refrigerator. And always set them up in a well-lit place: in the greenhouse, a south-facing window, or under grow lights.

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Lastly, don’t forget to label everything… VERY important! Things get moved around all the time, and you THINK you’ll remember, but you never do. I also like to add some other info on my labels, like: the date started, estimated germination time, approximate transplanting time, and any other info that specific tray might need (extra watering or fertilizing, for example). Then you can manage your starts at a glance.

And that’s it!

Happy seeding!

p.s. For those of you who don’t know, I’m also a contributing writer for Agrarian Organics. This post can also be seen on AO: the GROW Blog.

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