Potting mix

Someone asked me in a comment about my seed raising mix, so it’s a good opportunity to do a whole post about it. The mix has changed a little, now that I am working in a small, retrosuburbia garden. The old garden was on a ridge, inland, and rural. It had poor soil original soil too, but in a different way. That soil was shaley and heavy. This garden is suburban and coastal. The original soil here is sandy, mineral poor, and leaches nutrients at an alarming rate.

It has also changed with the availability of ingredients. The old recipe featured cow pats and creek gravel, neither of which is easily available here. But here I have chook made compost and bulk worm castings, and also crusher dust, neither of which was easily available there. But the concepts remain the same. Permaculture is like that – not a recipe but a system to be applied differently to every site.

The key concepts:

• To germinate seeds, you need a fine tilth, good drainage, no weed seeds, and consistant moisture. This is much easier to manage in a little box of seed germinating mix in a nursery or seed raising area than out in the garden at large. Digging up a bed, tilling it, keeping it watered just wastes water, destroys soil structure and invites weeds.
  
  For your little box of seed germinating mix, you want a potting mix that is fine but has good drainage and good moisture retention. Conventional potting mixes use perlite, coir and peat moss, but I don’t want to fill my garden up with that, and none of those have good fertility. You can make your own using any kind of coarse sand or gravel for drainage, and any kind of fine tilth organic matter, like mature compost, (seived if it is still a bit rough) for water holding.
  
• Planting seedlings out into the garden as advanced seedlings, up to 15 cm (6 inches) tall, is a major labour and resource saving strategy. It allows you to concentrate water and attention on the nursery – once plants are big enough to be planted out into the garden they are much more robust and don’t need much attention beyond harvesting. It allows you to place seedlings individually, creating guilds and minimising competition and pest damage. It allows you to choose the strongest, healthiest seedlings, and sequence plantings so as to avoid gluts. And, most importantly in a small space, it allows you to avoid using precious garden space, for months, on tiny seedlings that only need a fraction of their adult space.
  
  To grow advanced seedlings, you want a potting mix that has drainage and water holding, but also the major nutrients – nitrogen (N), phosphorus (P) and potassium (K) – in the right kind of ratio, and trace elements and minerals, also in the right kind of ratio, and a neutral pH to allow the plants to absorb them. Better quality conventional potting mixes have added soluble fertiliser (which I don’t want in my garden either), plus possibly things like blood and bone. But you can make your own by adding something like worm castings for NPK and trace elements, and rock dust for minerals to the basic seed raising mix above.

The ratios

Much like pastry or bread dough, I go by the feel. Which I know is not much good to you!

You are looking for a mixture that is loose and crumbly. If you water a potful, it absorbs that water rather than letting it flow through, but if you squeeze a handful, you can’t squeeze out water. You can make it into a ball in your hand but as soon as you try to put it down it crumbles.

As a rough guide, mine is something like two parts mature compost (sieved if it is a bit rough), one part cracker dust that is more fine gravel than dust, and one part worm castings without worms. I use the same mix for seed germinating and seedling raising. In my rural garden, the seed germinating mix was half creek sand (fine gravel) and half mowed old dry cow pats. The seedling raising mix was two parts compost, one part worm castings, and one part creek sand for drainage.

There are three kinds of seeds that need slightly different treatment.

• Fine seeds of little plants like carrots and beets, that don’t like being transplanted, can be sown thinly in a pot, then planted out as a set. (There’s a pot of carrots like this in the video).
• Large, fast growing reliable germinators like zucchini, squash, beans and corn can be planted individually, or in pairs in larger pots – I like to use 10 or 15 cm pots for them. They will be ready to plant out within a fortnight or so. (There’s a zucchini like this in the video too).
• The rest – lettuces, cabbages, broccoli, capsicums, eggplants – I sow thinly in a pot or a polystyrene box and transplant to individual pots at the two leaf stage. At this stage they suffer no transplant shock. I can choose the number I want to plant out (plus a few extra to account for losses) and throw or give away the rest. They will live on the potting bench for weeks or even, some of them, months before being planted out, and in a nice rich potting mix, they will thrive. And the pot of potting mix they spent their childhood and adolescence in will go on to enrich the garden soil for the next harvest.