I am sure that there are many soil types in any given area, from humus rich deposits to clay dust deposited by wind, and sand left by wind or water. Local topsoil, apart from it's variable composition may also behave very differently as small volumes in pots from the way it behaves as part of the ground, and plants which might have roots feeding through a very large volume of soil in the ground will need a lot of extra food in small pots. Seedlings in pots will need a constant supply of moisture (usually without water logging) which is a good reason for adding humus to a potting mix, (and sand/grit to prevent water logging), whereas in the ground their roots would go deeper. topsoil and loam are very good ingredients for most compost mixes but are usually best modified before used for sowing in or out of pots. Peter (UK) As in my case where I use a couple of tons of compost some years, boiling water is not always a practical method of sterilisation. On Wed, Jul 11, 2012 at 2:30 AM, Alberto Castillo <ezeizabotgard@hotmail.com > wrote: > > In your case, David, the best possible mix would be local gritty soil from > you area, previously sterilized with boiling water. > > After all., we are all mixing different ingredients trying to recreate > your well drained local soils for our South African seeds and bulbs. > >