i was knocking up some potting mix the other day and realise I frequently add a bit of bone meal, or fish blood and bone, which of course I have to buy, but I reckon you could easily make that up by simply upping the proportion of manure. If you do buy stuff it is worth watching the prices. The last lot of bonemeal I bought was in 750 gr boxes from the cheap shop. I had seen a 20 kilo 'Bargin' tub advertised as though it was the bargain of the month in my local garden center at what worked out about 5p a kilo dearer. Ten little boxes has kept me going for ages.