If you can, get some potting mix, and mix in some high phosphorous bat or seabird guano and some potash. I think ash from any fire will do, it just has to have stayed dry since burning because the potassium washes out very easily so you can't use stuff that got rained on for example.
This is all just a guess as I've never used guano before so I don't know if it's immediately available, and the stuff about potash I just read from some gardening article.. I'm just trying to think of an organic way you can give them some kind of PK booster which would help them really fill those buds out a lot more than they will if you just put sand on top.