Charcoal works as a highly effective slow-release fertilizer. The type of charcoal used, however, is important. Natural charcoal formed from biomasses such as wood will give the best results, with commercial charcoal briquettes, often containing chemical additives, being much less beneficial. This type of natural charcoal, purchased from natural-alternative outlets or made by slow-burning wood, will release vital nutrients into the soil, aiding the growth of your plants.
Put on gloves, a safety mask and clothes that you don't mind dirtying. Breathing charcoal powder can be detrimental to your lungs, and crushing the charcoal will make a mess.
Break up the pieces of charcoal with a hammer. Use a rolling pin to further crush the broken pieces, grinding it to powder. Don't worry about fully pulverizing all of the charcoal --- a mixture of small lumps and powder is ideal.
Spread the crushed charcoal over the soil you wish to fertilize to a depth of ¼ inch. Rake the ground to mix the charcoal in, working carefully if you've already planted your vegetables or flowers.