Just to add to your pro-mix details.....I use the pro-mix BX which costs around $35 a bale. It has 107 litres if I recall, but it is compressed, so it will give you more than that.
I find the peat in the BX makes the PH drop like hell after a few months in the same bucket, but the plants still get all they need at a ph level of 6.2-6.5 for some reason.
Dolomite lime (1tablespoon per gallon of soil) has helped stablize my PH later on though.
Anyway, I use it, and think its well worth the $. Thats my
on pro-mix BX.
