When wood or similar organic materials are heated at high temperatures in the absence of air, you get charcoal instead of ashes. Charcoal is a stable, highly porous material containing many of the nutrients of the organic matter from which it was made. When added to the soil, it increases the capacity for retention of plant nutrients and beneficial soil microbes by reducing the leaching of those essentials into ground water. Charcoal's low density also lightens heavy clay soils--promoting root growth and improving drainage and aeration. Additionally, charcoal acts similarly to lime as a pH buffer for acidic soils.
Pulverize the charcoal until pieces are about the size of BBs (about 1/8 inch in diameter) for best results. (Place charcoal in a sack and beat with a baseball bat, scrap of lumber or similar object to reduce them in size.) Large lumps lack sufficient surface area to be ideally effective, and a fine powder is messy, so aim for something in between.
Spread charcoal evenly over the surface of the soil. There is no set amount to use--plants have been grown successfully in varying amounts from 1 to 100 percent charcoal to soil mixes with no official consensus on the ideal charcoal-to-soil ratio. One soil scientist, contributing to Terra Preta, recommends a two-thirds compost/soil mix to one-third charcoal ratio.
Turn the charcoal into the soil. Hand cultivation with a rake and shovel is easiest for light soils in small gardens or yard. Heavier cultivating equipment such as a rototiller or garden tractor may be more practical for large areas. Concentrate the charcoal in the upper 2 or 3 inches of soil so that bacteria working in conjunction with surface moisture will more quickly take advantage of charcoal's porous nature to become established.