The mixture on which seed is sown should be low in organic matter such as compost, dark-coloured humus or peat moss. Water should drain through the mixture very easily because a free-draining mix lowers the risk of disease.

Any one of the following is suitable:
  • medium-grade river sand, or
  • 1:1 mixture (by volume) of river sand and loamy soil, or
  • 2:1 mixture (by volume) of vermiculite and horticultural perlite but avoid builders' perlite as some is very limey.
Always use light free-draining soils such as sands, loamy sands and sandy loams. Avoid any soil which:
  • becomes sticky when wet;
  • stays water-logged and holds a pool of water on top;
  • dries to powdery dust; or
  • forms a hard surface crust.
Most native trees prefer soil with an acid to neutral pH value that is, a pH value between about 5 and 6.5. The CSIRO booklet listed below at 'Useful references' gives more information.

Mix the ingredients, then sieve them through a fine mesh one with holes of about 2 mm. A kitchen strainer will sieve small quantities, and a plasterer's sieve is useful for larger amounts.

Heat treatment of sowing mix
Sowing mix made from new perlite and vermiculite should not contain plant diseases or weed seeds. However, fertile soil teems with microscopic plants and animals, and while some assist seedlings, others hinder them and may even cause fatal diseases. Most topsoil also contains plenty of weed seeds. So a mix which contains soil or river sand may contain diseases and weed seeds, but such a the mix can be partially sterilised
by heat.

For small-scale treatment, moisten the mix then put it in a plastic oven bag and heat it in a kitchen oven. Aim to raise the temperature throughout the mix to 60 C for up to 30 minutes - this will kill many weed seed and disease-causing pathogens.

Heat treatment should not be overdone, so do not try to cook the mix. High temperatures and long heating times are unnecessary and undesirable, as they kill tiny soil organisms which aid plants. High temperatures may also harm soil structure and the soil chemistry.

Treated mix should be used as soon as possible, or stored in a closed container - this can prevent pathogens returning.