We made up an experimental mixture. There are various recipes, but some use expensive commercial ingredients or things that are unsuitable for an organic farm.
Our recipe was equal parts sieved compost, worm castings, river sand, finely shredded paper, and commercial potting mix.
This worked pretty well. Our compost wasn’t of great quality – I suspect not enough water when it was cooking. The finely shredded paper I can get in bulk for free, and it replaces commercial fibrous matter. The potting mix really needed to be sieved too, so we might try doing that next time, or we might omit that as there can be some dubious substances in that stuff.
We made some big blocks and sowed some golden beetroot, roma tomatoes (for use in the greenhouse), carrots, leeks (4 to a block), kale and nasturtiums. I love the leaves and flowers of nasturtiums in salads, and they are a useful groundcover to keep weeds out.
We also made up some tiny blocks with borage, heartsease (another edible flower), onion and red mustard.
Both types of blocks fit nicely into commercial plastic seed trays, which can be reused forever.
I plan to water them with a fine spray gun. I couldn’t find it, so I just used the spray on the watering can, which was far too rough. But the blocks stood up well. I’ll report on how they progress, and we’ll make more trays with different soil mixtures in the coming days.