Last summer I had a grand weed clearance of docks, thistles and ragwort in a new section of paddock for the pigs and also the remnant branches and trunks etc., of the fir trees that had been cut down. I just made a huge solid bonfire which was about 14 feet high and 20 feet across, waited until all the greenery had dried out and was brown then set fire to it. It burned for about 10 days, I kept turning and shovelling any unburnt bits back into the centre and it ended in a small heap of pure ash about 2 feet high and 3 feet across. After it had cooled completely, I just spread it around a bit and let the pigs in the new area. They briefly investigated the ash, spread it around a bit more and trod it in the ground, after a couple of rainfalls there was little to see. Since then I have broadcast grass seed over the area and it's now grassing over nicely - albeit with the odd rooting hole here and there.
Basically, the ash from vegetation and trees is simply potash which is good for the soil, the important thing is to turn it all into ash by paying regular attention to the bonfire - don't leave any partially or unburnt bits lying around - especially anything that in it's 'green' state could be poisonus.