Why You Should Use Soil Blockers Instead of Plastic Seed Trays

What if we could grow seedlings without adding to those piles of plastic waste and, while doing so, produce significantly healthier and more vigorous seedlings? Sounds jolly good doesn’t it – and there are ways to do it!

Sowing usually begins with plastic plug trays, seed trays, or pots. But the problem with them is that they crinkle and crack, and within a few seasons are good for one thing only: throwing out! You can buy versions that are far more durable and will last for many years, but they are more expensive. So let’s look at more sustainable alternatives…

Homemade Biodegradable Pots

I like my sturdy plastic plug trays, but I’m trying to reduce how much plastic is in my life generally, because plastics get everywhere. In fact, did you know that a recent study found that the average human brain now contains around 7 grams (a quarter of an ounce) of microplastic, which is about a teaspoonful. That can’t be good!

If you want to ditch plastic once and for all, there is an ingenious gadget to help you do that – but first, let’s look at biodegradable options, which reuse paper and cardboard items you’ll almost certainly have lying about at home.

Newspaper Pots

First up are newspaper pots, which are very satisfying to make. Don’t worry about the inks in newspaper – they’re almost certainly soy-based and perfectly safe.

• First find a glass or jar that’s about the same size you want your pots to be. This will be your mold.
• Cut newspaper into strips about an inch (3cm) taller than your mold.
• Fold a flap along the strip then unfold.
• Roll the flap side of the strip against the jar, facing inwards along the open end of the jar.
• Flip the jar upside down and then fold the rest of the newspaper inwards to create the base.
• Push down and pinch the edges to firm them up, then remove the mold.
• Sit your newspaper pot on its base, then fold down the flap to firm up the rim of the pot so it all holds together.

Once your newspaper pots are ready they can be filled with potting mix for sowing into. These are great for sowing bigger seeds like squashes, and different-sized jars can be used as molds to create larger or smaller pots.

Toilet paper tubes are fabulous for starting crops with deeper roots like corn, peas or beans - just fill with potting mix and sow. You can always cut down taller tubes to make squatter pots.

Cardboard egg trays are another great option for smaller seeds or seedlings that have a more compact root system. The great things about all of these biodegradable containers is that, when it’s time for your seedlings to go in the ground, you can just plant the whole thing. The newspaper or cardboard will quickly rot away and let roots push out into the surrounding soil.

For any pot made of cardboard or paper, take care to water just enough to keep the potting mix moist but not sodden, or the pot may weaken or turn moldy. Group them together in some sort of tray with drainage holes in the bottom so they don’t sag and topple over.

Cut Out Plastic When Sowing – Use Soil Blockers!

Any biodegradable option is a fantastic alternative to cut waste, but what if you could do away with the container altogether and use nothing more than the potting mix itself?

Soil blocking gadgets make this easy. Push the soil blocker down over potting mix, and then simply ease it out again by pushing down on the handle. Each cell contains little inserts which, rather handily, create depressions to sow into in the top of the soil block. These are easily removed and swapped out for different-sized inserts, depending on what you’re growing and how deeply the seeds need to be sown.

But won’t the potting mix just fall apart? I hear you ask. You’d think so wouldn’t you! But that’s where the right potting mix recipe comes into its own.

We need the potting mix to be ‘sticky’ enough to hold together until the roots can grow to the edges of the block or it’s time to plant. The secret ingredient that’ll help to hold everything together is coconut fiber or coir, which has lots of little fibers that, a bit like the straw used in Medieval wattle and daub buildings, will help to bind it all together. You can buy coconut fiber as a block, which can be rehydrated into something that looks like fibrous potting soil.

For my soil blocker recipe I mix two parts coconut fiber mixed with one part screened garden compost. This will add welcome bulk and nutrients to the mix. If you don’t have garden compost, just use any all-purpose potting mix. I also add one part garden soil for added weight and structure, then one part perlite for drainage and, to finish, a handful of volcanic rock dust (azomite or greensand will also work) to help keep the seedlings going a bit longer til they’re big enough to plant. Mix it all together.

Please don’t worry if you don’t have some of these ingredients to hand – there are as many different soil blocker recipes as there are recipes for chicken soup or macaroni cheese! – and some gardeners have great success with little more than a quality, all-purpose potting mix.

Your potting mix will just fall apart if it’s too dry, so we need one final ingredient to stick everything together: water. Trickle it in gradually, in stages, to avoid over-wetting the mix. It should be wetter than if you were just moistening the mix for filling a plug tray – what we’re looking for is a mud pie-like consistency, or something a bit like wet cement. If you pick up a handful it should hold together like a snowball, and if you squeeze it a little water should come out, but it shouldn’t be absolutely sopping.

How to Use Soil Blockers

Dip the soil blocker tool in water to moisten the sides. This will help the potting mix to slip out more easily. Place the soil blocker over your potting mix and push down quite hard. Don’t be afraid to pack it in, because we need it to hold tightly together. (This is where a flat-bottomed mixing tub helps because it makes it easier to fill the blocker in one go and get a good, even fill.) Scrape any excess off the bottom to leave a neat, flush finish, then press the spring-loaded handle to push out the blocks.

Cover your seeds with more potting mix. If the potting mix is still very moist you won’t need to water straight away but when you do it’s a good idea to use a mister or pump-action hand sprayer to avoid disturbing the surface too much.

Another option is to set your blocks on a tray with holes in the bottom to make it easy to put them into a container of water to soak. Once they’re moistened through, lift up the tray and blocks so they can drain off. This avoids all risk of disturbing the surface of the blocks.

Allow the surface to dry off between waterings to reduce the risk of algae or mold growing on the surface.

Block Busters!

As well as saving on plastic, the great joy of soil blockers is that they’re really good for the seedlings themselves. Because there are no walls, the roots get access to more air, which encourages a healthier root system and stronger top growth. When the roots reach the edge of their block they are naturally ‘air pruned’, meaning the roots don’t grow out any further, avoiding root-bound seedlings and creating an altogether sturdier little plant.

And, of course, by being able to easily see when the roots have grown out to the sides it’s a lot easier to know when the time has come for the seedling to be either potted on into larger blocks or containers, or transplanted out into its final growing position.

Seedlings don’t need to be pricked out at all – they’re simply planted or potted on once they reach the right size, so there’s no risk of transplant shock. It all makes for a seamless transition from nursery environment to final growing position. I love the self-reliance soil blockers offer – no need for seed trays, so no plastic waste, just seeds and potting mix!