A seed starting mix recipe is not rocket science. It’s really pretty easy. But what is the recipe for the very best seed starting mix? Now that is a little more complicated.
Still, just about everybody agrees on two key points.
A seed starting mix should:
- Hold moisture
- Drain well
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Another area where everyone agrees is that a recipe should include at least some of the following ingredients:
Worm castings or screened compost are often mentioned as organic amendments. Limestone is frequently added to alter the pH and greensand to add potassium. Because this blog is about growing organic, I won’t discuss non-organic alternatives.
Also commercial seed starting mix products almost always include a “wetting agent. ” This helps the mix absorb and hold water more easily.
Winter harvest expert Eliot Coleman, who starts a lot of vegetables in hoop houses, uses this seed starting mix recipe:
- Three 8-quart buckets of peat
- Three 8-quart buckets of compost
- Two cups of organic fertilizer
- One 8-quart bucket of perlite
Another area of consensus is that even the best garden soil is NOT a good media for starting seeds. Garden soil creates two problems for seed starters:
- Dampening off
The most common problem indoor seed starters face is dampening off, and garden soil encourages it. Garden soil also drains badly and is difficult for young roots to penetrate because it tends to get very hard.
Peat and Coir
Peat moss and the more environmentally-friendly coir are popular because they retain moisture well. Peat is often harvested un-sustainably, ruining peat bogs in the process. Coir, on the other hand, is made from coconuts, a sustainable resource.
Vermiculite also aids in moisture retention and is light weight, making it easy for young roots to move it aside. But I don’t use vermiculite for two reasons:
- Not all, but some vermiculite has been known to include asbestos. No thank you!
- It makes me cough. I know this from working with it.
Perlite holds both water and air. It’s a great alternative to vermiculite.
Sand, which must be clean and sterile, promotes good drainage so that seeds and roots aren’t sitting in water for long periods of time.
Finally, soil amendments provide nutrients for the seedling once the seed’s food supply storehouse is exhausted.
The big question
In the past, many commercially-available starter mixes did not include nutrients. I guess the makers’ plan was to use liquid fertilizer until you repot or plant outside.
If you plan to keep your seedlings inside (under the lights) for a while without repotting, this could be a problem. This is especially true if the plants are heavy feeders.
Some experts suggest that liquid fertilizer is not enough. In other words, you must add soil amendments or chemical fertilizers for good results.
But the potential problem with this is nutrients may promote the development of problem causing bacteria. The big question is should you use seed starter mix that includes organic soil amendments, such as screened compost or worm castings?
My Seed Starting Mix Recipe
Last year, I bought seed starting mix and use it to make soil blocks. It just doesn’t pay to buy the ingredients in bulk. I use a soil blocker because soil blocks are the best, most inexpensive way to start seedlings.
Once well established, I transplanted the seedlings into larger pots with Fox Farm planting mix (not seed starting mix). Then I add Neptune’s Harvest liquid fertilizer at one quarter strength every two weeks.
One of the Brandywine tomato plants grown using this method was exceptionally productive. I put it in the hoop house early, and the tomatoes started ripening around the Fourth of July. It kept giving until early October, which is really exceptional in New Hampshire.
But I still would like to avoid repotting my seedlings if I can help it. I no longer use indoor lighting for my seeds and use my hoop house instead. While I don’t have to use electricity because the hoop house gets plenty of sun, the plants grow so big in the hoop house that I have to repot them several times.
This year I’m going to try successive plantings to try to avoid pot bound plants and having to repot over and over.
Related articles you might enjoy:
- Build an Indoor Seedling Starting Unit
- Transplant Seedlings at Proper Soil Temperature
- Getting Rid of Gnats on Seedlings
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