An easy way I recharge potting soil, whether or not anything is growing in it at the time, is with urine. Dilute it four or five parts water to one part urine if plants are still in the pots....it's a wonderful pick-me-up, or add it full strength to potting mix not currently growing anything and let it mellow a while before planting in it.
Wood ashes can be mixed in too. The trick is estimating quantities....you could do it by the numbers, looking up nutrient contents and so on, or just guess. I would add no more than half a cup of ash to a five-gallon bucket of mix, and a half gallon of urine undiluted. Wet the mix down some and let it sit a few days. If there's any odor to it at that point you've got too much urine....dilute with more mix.....
Fresh urine from a healthy person carries minimal disease germs. Just don't let it sit around in containers too long. If you do, send it through a prolonged or hot compost process, as one would humanure.....