Soil for your square foot garden.

Soil for your square foot garden is a mixture of:

8 cubic feet of compost
4 cubic feet washed sand
4 cubic feet coarse vermiculite
2 litres of wood ash
0.5 litres gardener's lime or 1 litre ground limestone

Try to include some good topsoil, mainly for the sake of the clay content. You can also use sphagnum peat moss, coconut coir, perlite, though they're inert and don't contain any nutrients. Sawdust is difficult stuff in garden soil, best avoid it -- especially sawdust from pressure-treated timber, which has poisons in it.