Making Seed Raising Mixture (Potting Compost) by Nev Sweeney

Editors Note – In England we might also call this potting compost! Nev has also found a substitute to peat that can be used! I recommend using this mixture in conjunction with biodegradable newspaper pots.

Ingredients, compost, peat moss, sand,Vermiculite or Perlite, warning

Nev holing a tray of seedling mixFor most of the past 20 years I have grown veggies in the back yard, and for most of that time I have bought seedlings of various types. This has left me with a world class collection of empty punnets and labels, but has no doubt over the years cost me a considerable amount of cash. Punnets of veggie seedlings now cost over $3 (Aud) each(£1.25 or $2.30 USD). There is also a point of ethics here, when one is striving for a measure of self sufficiency to go and buy seedlings seems to be working counter to the cause!

I therefore thought that this year I would not only raise my own seedlings (as I had dabbled with in previous years) but I would also make my own seed raising mixture from scratch, which was something new. I dragged out my old Farm Technology textbooks on plant propagation and did a bit of searching on the net. I wanted a seed raising mixture based on easily available and (at least partially) on home produced materials.

The Ingredients

Compost

After several reasonably successful goes at compost making I found that I had a good supply of this raw material – it smelled like rich soil but looked like a dark gluggy mass, obviously there was some treatment required here. Once I had dug it out and let it dry a bit, I passed it through a good old-fashioned garden sieve. This broke it up and turned it into friable, loose material that was easily mixed with other ingredients. The compost helps to retain moisture and provides nutrients.

more about compost….

Peat moss

Peat moss is a classic component of seed raising mixtures but digging up peat is not the best from an ecological point of view and due to its acid nature, requires lime to be added to the seed raising mix. So while on the look out for a substitute I found a compressed block of coconut fibre which when soaked in water gave up to 9 litres(16 pints)of peat like material. It was also non-acid and recycled a material which may otherwise be dumped, although it is imported. I have seen it sold as “Fast Peat” or horticultural coir. It is also gluggy when reconstituted but when passed through the sieve it becomes friable and easily mixed with the remainder of the ingredients. The coir keeps the mixture moist by retaining moisture.

Sand

adding the sandThe sand (according to the experts) should be coarse so that it does not crust over, keeping an open structure and it should be river rather than beach sand, which is contaminated with salt. If you have this available on your property good for you, but I had to buy my sand at $5(AUD) a bag(£2/$3.80usd) from our local produce merchant. The sand keeps the structure open and free draining and prevents the rest of the ingredients clagging together (a technical term).

Vermiculite or Perlite

adding perlite to the mixThese are two more expensive components that are expanded mica and volcanic rock respectively. They both assist in keeping the mixture aerated while retaining moisture. Neither are home producible but I have found that a better germination rate is obtained with perlite at least.

So now having found, manufactured, mined or bought your raw materials, it is time to mix them. Just before we get to that I want to let you know about one of the most mind bogglingly useful pieces of apparatus that I have ever found – the pet litter tray. These are a decent sized plastic tray that retails in the El Cheapo type merchandise shops for around $2 (83p or $1.50usd) , and they are Australian made! So far I have used them for:

  • Holding the compost while it dries out
  • Mixing the seed raising mixture in
  • A work platform while filling punnets with the seed raising mix
  • Setting up as a capillary bed (more later)
  • Washing second hand punnets in disinfectant before re-use
  • And if all else fails you can let your cat crap in them!

I have half a dozen of them in various colours and find a new use for them almost everyday.

mixing up the seed rasing mixtureSo now to mixing up your seed raising mix – I tried a number of different combinations but the one that seems to work best for me, time after time, for both flowers and veggies is as follows;

2 parts by volume Compost,

2 parts by volume horticultural coir,

1 part by volume sharp sand,

1 part by volume perlite

I use a round, approximately 500ml(1 pint) Chinese food container as the measure and the above mix makes enough so that it is still comfortably able to be mixed in my mixing tray and fills approximately 10 standard punnets. I just mix it through with my hands until it is homogenous, which takes about 5 minutes, and it is ready for use.


A Quiet Warning

I don’t know the technicalities, but some people have gotten sick with Legionella infections after working with commercial potting mixes, when they inhaled the dust. If you keep all your raw materials moist that should keep the dust down, but if you are still concerned it might be worth purchasing an Australian Standards approved dust mask to wear while doing this work.


You can now store your seed raising mix in a closed container (to keep it moist) or use it to fill your punnets or flats and plant your seeds into them. But if, like me, your punnets are all second hand, you should wash them out in a mild disinfectant solution like Dettol ® so that you do not transfer any bugs or greeblies (another technical term) between batches of seedlings.

Once I have planted my seeds into the punnets, I make a capillary bed by half filling one of my all-purpose cat litter trays with the coarse sand. The sand is then soaked with water until there is a small amount that remains on the surface of the sand. The punnets can be now placed on the wet sand and the moisture is drawn up into the punnets by capillary action, if the seed raising mix is moist to begin with. From this point on you can water into the sand, which prevents the seeds from being washed out by the force of the water and the sand acts as a store of moisture to keep the punnets moist on even very hot days.

I have used this approach for over three years now and last autumn I raised a decent size batch of edible flowers ( marigolds, carnations, poppies, dianthus etc) and, with the exception of chillies, (I think that it was a dud batch of seed or maybe a bit too old) I did not buy any seedlings for my veggy patch this Spring. As I write this there are more seeds germinating in the hot house to replace the ones growing in the patch as they go past their prime (tomatoes or cucumber) or are eaten (lettuce, Brassicas etc.). It was very rewarding from the financial view point but also because of the level of satisfaction gained from being that teeny bit more self sufficient, so try it yourself – it’s worth the effort.

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