The Right Soil for Seed Starting - a Basic Guide to Success

Do you want to miss out on the learning curve and speed through to seed starting success? Using the right soil for seed starting is a great place to start! Just to avoid confusion, we’ll distinguish between starting seeds in the ground as opposed to starting seed in containers, and we’ll cover both these topics in one quick, easy-to-grasp article.

And in case you were wondering, garden soil is a world away from what you’d use in containers. The reason is quite simple. Natural soil isn’t something that occurs in isolation. It’s part of a system and it has structure. Take it out of its larger environment, and it loses all that. Garden soil becomes the worst possible medium for seed germination when used in containers – even though it may be just fine if left where it belongs.

Having said that, we’ll touch on seedbed preparation for open ground sowing before taking a closer look at container seed-growing soils.

Preparing Open Ground Seed Beds for Success

No matter what type of soil you have, the first basic you need for open ground seed sowing is compost. It improves water holding capacity in sandy soils, and it improves drainage in clay soils. As a rule of thumb, you’re looking at about one third compost to two thirds soil worked in to about a spade’s depth.

Work in some compost

You can achieve this easily with trenching. Simply dig a trench, transferring the soil you dig out to the far end of the bed. Now, dig a trench right next to your first one, shovelling and mixing in one spadeful of compost per every three to four spades of soil. Keep going with your trenches till you reach the end of the bed and then use the soil you removed from the first trench to fill your last one.

This isn’t the only method. Some gardeners like deeper trenching, but as a basic preparation method, this one works. Your soil surface will now be slightly raised above ground level, but that’s fine too.

Pick out the chunkiest bits

Now it’s time to think like a seed. How fine should the soil around your seeds be? The bigger the seeds are, the more easily they’ll overcome obstacles like stones or chunks of organic matter. Lot of horticulturists, both amateur and professional, refer to “tilth” which is basically how chunky the soil is.

For finer seeds, you will want a finer tilth, but it doesn’t have to be fine all the way down into the soil. All you need is a hospitable zone for your little seeds to get started. As the plants get stronger, they’ll be way less likely to get a fit of the sulks, and possibly die, if they encounter a minor obstacle.

I usually just pick out stones and crumble up any chunks close to the surface when I sow fine seeds. Remember, seeds shouldn’t be planted too deeply, so I find that fine soil just where the seeds come to rest and to cover them, is ample.

Once-off interventions for problem soils

Very stony soil can be the absolute pits. While you wouldn’t ordinarily have to sieve garden soil, you may have to do so if you have a lot of stones or rubble. I’ve seen soils that are literally more stones than earth.

It’s a devil to work at first, but once you have it roughly sieved, your soil will stay that way. In desperate cases, you might want to skip that altogether and build raised beds that you fill with potting mix or a combination of topsoil and compost. This also works well if you have super-heavy soil that’s more clay than anything else. Compost does work here, but you have to keep applying it just about every time you plant.

We could say a lot more about open ground soil prep for seed beds since there are many ways to approach it, but feel free to do some reading if you want alternatives to this basic guide.

The Right Soil for Starting Seeds in Containers

There are plenty of reasons why you might decide to start seeds in containers. If your seeds are very expensive, if they’re very small and could easily wash away with garden watering, or if you want to start them early and then move them to the garden, containers are the best solution.

To succeed, you need the right mixture. We shouldn’t even call it “soil” because for best results, you aren’t going to use soil at all. Instead, you’ll choose a soilless medium.

Commercial seed starting mix plus

There’s quite a complex science to making a good potted seedling mix. Doing it from scratch is the hard way, so I prefer to use a commercial seed starting mix. However, although Australia is known for the high standard of its container growing mixtures, I do like to add a few bits and pieces to make it even better.

The result you’re looking for is something light and porous. It should be fine but with some smaller chunks or fibers to give it structure, and it should hold water while allowing drainage. Even with our good commercial seed mixtures, I do find that a little coconut coir helps to make commercial mediums lighter, fluffier, and more porous – you can get it at any good garden centre or hardware store, usually in compressed bricks which expand when soaked.

I also like to add a little vermiculite. If you don’t know what that is, it’s the flakes of shiny stuff you’ll often see in potting soils. It holds a lot of water, so be sparing with it!

Making seed starting mix from scratch

There’s a lot more trial and error involved in making a seed starting mix from scratch. As we’ve already pointed out, you don’t want to let ordinary garden soil anywhere near it! You also want to exclude pathogens (germs) that could make your growing seedlings susceptible to damping off.

Damping off is what happens when your seedlings germinate, but then rot off at the root-stem junction. They topple over and die and the disease spreads like wildfire. Even with the best seed mixtures, you could experience damping off, but a disease-free mixture will reduce the chances of this disaster happening to you.

A lot of home seed starting mix recipes include compost – but our caveat is that this should be pasteurized (germ-free) compost. Big companies usually do this by heating it using expensive equipment. You can try emulating their results by heating the compost in your oven or microwave at home. Needless to say, you can only do small volumes at a time, which is why I start with the seed raising mix – it just saves so much time and effort!

For a basic mix which you can experiment with and adjust at will, try this seed starting mix recipe:

  • 4 parts pasteurized, sieved compost
  • 1 part horticultural vermiculite
  • 1 part perlite (or pasteurized coarse river sand)
  • 2 parts coco peat

Plant nutrition in container seed mixtures

While the media we’ve discussed so far are great for germination, they do lack a certain something – and that’s nutrition in the form of dissolved ions your plants can absorb.

Adding nutrients to a seed mix can be problematic. Instant release fertilizers can burn your tender seedlings as they germinate. Polymer-coated ones can solve the problem, but they’re super-expensive. Organic fertilizers, including compost teas, could contain pathogens.

Once germination has begun, nutrition from the soil solution (water with dissolved nutrients) is all-important to ensure that your plants will achieve their genetic potential once grown. My personal preference is to add soluble fertilizers to the water. If you prefer something organic, you can be reasonably sure that commercial products will be pathogen-free.

A good dose of liquid feed once a week should be enough for most seedlings. Of course, there are a lot of variables in this, so you may have to keep an eye on things and adjust the products you use, or the frequency with which you use them.

Slow-growing seedlings with a yellowish look are hungry. Very soft, blue-green ones could be overfed. With a little experimentation, you should be able to find the right balance.

Happy Growing With Seeds of Plenty

Whatever the methods you use, we’re hoping that this bit of extra information will help you to experience the joys of growing plants with Seeds of Plenty. If you happen on some bright ideas you’d like to share, feel free to write to us or post your recipes on our social media pages. We’d love to hear from you! After all, we are a community and communities love to share.

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