The soil I use here for all my operations is the Pro-Mix BX. It is a soilless soil that is very fluffy and wets quickly. It has no fertilizer at all, however it has a hint of mycorhizal, an agent that promotes nutrient root intake. Unfortunately Pro-Mix comes only in large bales, 3.8 Cu Ft.(60 lbs) which are too large to a homeowner to buy.
I called Scott (makers of Miracle Gro) and they suggested a couple of products that come in small packages (2 quarts) which I looked up on Garden Web. Nobody seems to like those products, they don't wet easily and once wetted they feel like mud. Scotts does not have anything else that does not have fertilizer.
Farad, another company that makes soils also has an organic soil but it comes in 8 quart packages, too big for a homeowner to buy.
Please look around for soil that has no fertilizer in it, I have had bad experiences planting cuttings in soils that have fertilizer. the cuttings make a quick start, and die within a week or two.
You may have trouble finding the soil you need, I would suggest making calls rather than going to outlets but at the end, if you can't find any soil, I have broken down the Pro-Mix bales into small packages with enough soil for 3 grow bags. The shipping is more expensive than the soil itself. I am not making any money out of this, it is more trouble than it is worth. Just send me $4.00 via Paypal ( send to fgomez@hybridpoplars.com) and be sure to include your address because I will use the PayPal notification slip, to cut off your address and attach it to the envelope which will be returned to you immediately by return mail.
Whichever soil you use, be sure not to water the trees more than once a week. It is very easy to drown the trees. Sorry, I can't ship this soil outside the US.
.