Blocking Up – experimenting with peat-free soil blocks

I can’t quite remember where I first came across the idea of soil blocks for seed sowing, but they’re a rather neat idea. Compacted cubes of growing medium are used instead of pots or modules, freestanding seed trays with narrow air gaps between the blocks.

Big claims are made about the benefits of soil blocks for raising young plants in terms of reduced root disturbance and how ‘air-pruning’ of the roots prevents root binding of seedlings, all of which may well be true – I haven’t really got the horticultural background to say! But actually the thing that really attracted me to soil blocks was practicality.

Sowing seeds into soil blocks

I really don’t like module trays – they’re a pain to wash and re-use, if nothing else, they tend to degrade badly and crack up so that they only last two or three seasons, and are expensive and enormously wasteful if you’re using them only once. While it makes sense  to start certain seeds directly into 3″ pots, that takes up an awful lot of window sill space – something that’s definitely at a premium in my house at this time of year. And as we’re aiming to be completely peat free in the new garden, those little peat jiffy pots are right out (and the cost adds up quickly). A useful compromise has been home-made newspaper pots, but there’s quite a lot of effort involved in making these and I don’t always have time (also, my paper potter tool seems to have gone astray in the house move)! I’d more or less settled to using plain seed trays and pricking out seedlings at a small size, but that’s quite a lot of work and it can be rather traumatising for the seedlings, and has to be done at just the right time when inevitably I have too much else to do. So, no perfect solution.

Half trays of soil blocks in heated propagatorSoil blocks combine the convenience of the plain seed tray with the ‘modularisation’ provided by plastic module trays or individual paper pots – so could they be the best of both worlds?

Despite a real lack of cash just now, I decided to dig deep and find the twenty pounds for a four-block two inch cube soil block maker, which I bought from GreenGardener.co.uk. Only after diving in feet first with the purchase did I stop to consider the peat free problem; almost everything I read suggested that a peat-based seed compost was essential for making functional soil blocks. So had I just wasted my money?

Unwilling to compromise on our principles, I set about mining websites and gardening forums in search of a peat free recipe for soil blocks. None was quite right but some themes kept recurring. A base of peat free compost (fine textured seed compost ideally), something to help drainage (sand, perlite and vermiculite were variously mentioned) and loam or topsoil to help bind the mixture.

I’ve mentioned that money is a bit tight; going shopping for extra growing media is not on the cards, but I have a reasonable supply of New Horizon peat free organic multi-purpose compost and a rather soggy old bag of vermiculite (I generally use the New Horizon compost mixed 3:1 with vermiculite as my seed mix). We also have an almost inexhaustible supply of molehills, as the efforts of Mr Mole to create a ‘des-res’ for Mrs Mole are in full swing on our paddock!

So with a ‘make-do and mend’ attitude I did a few experiments and have come up with the following mix that seems, so far, to be working well for me. It will be the basis of my soil blocks for this season, while I get a handle on how things go germination-wise, and consists of –

  • Dry soil block mixture4 parts peat free multipurpose compost (you could sieve this to take out the biggest bits – but I’m fundamentally lazy and don’t own a garden sieve)
  • 1 part vermiculite, and
  • 1 part molehill (you should ideally sterilise this to get rid of weed seeds, I suppose, though it seems like faff) or substitute with bought topsoil (being careful it’s not been mixed with peat compost, as it often is!)

Wetted soil block mixMix the components thoroughly and then add water, a little at a time, until you reach a consistency that holds well together when you take a handful and give it a good squeeze. I find this is at the stage that you can just squeeze a tiny bit of water back out of the mix. Any wetter and the blocks slide straight out of the block maker – drier and the blocks tend to crumble. If you find you’ve over-wet the mix just add a bit more compost to dry it out again. A little trial and experimentation and you should get a good idea what you’re aiming for.

Filling the soil block makerTo make the blocks, level the wet compost mix about two and a half inches deep and as even as possible. Push the block maker down firmly into the compost until it hits the bottom, then push down on the handle without moving the press, to squeeze down on the blocks. You may squeeze a out a little water (if you’re seeing a lot, your mix is too wet). Then, release the handle and carefully pick up the block maker, tilting it on it’s side to reduce the risk of blocks falling out.

Making up soil blocks in seed trayPosition it where you want your blocks to go, and then press down on the handle again pulling the block maker up at the same time. If it’s all worked perfectly, you will have four even, neat little cubes each with a dimple in the middle. I find I can fit three rows of four blocks (12 blocks in total) into a half sized seed tray or seven rows of blocks (28 in all) in a full sized tray.

Half seed trays of soil blocksNow you’re ready to sow your seeds. Pop one seed into each dimple (or more than one if they’re small seeds and you plan to thin any extra seedlings) and cover loosely with a pinch of dry compost or vermiculite – or leave the seed uncovered if light is required for good germination. Then place the seed tray in your desired spot. I currently have two half trays of soil blocks in my heated propagator with tomato seeds in, and two full trays of salad leaves and brassicas in the unheated greenhouse.

Completed tray of soil blocksThe soil blocks are supposed to contain enough water to allow the seeds to germinate without further watering being required, but this depends on preventing excessive evaporation.

Covered seed trays in greenhouseI wrap the seed trays in the greenhouse with cling film (wetting the edge of the seed tray like a pie dish gives a good seal) and those in the heated propagator are under a closed lid. But if they do get a bit dry, don’t panic – mine held together fine when I watered them with a normal watering can and medium rose, just be gentle! Obviously you can’t use normal plant markers easily if you’re using cling film, so I label my trays using masking tape stuck to the seed tray. Keep your eyes open for signs of germination and remove the cling film before the seedlings reach it.

I’m waiting to see how things germinate in them now, and how they grow on, with no small degree of excitement!

Are you experimenting with soil blocks this year? How are you getting along?

Read more from the Country Skills blog >>

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