Summer grilling can result in lots of leftover charcoal ashes. Mostly inert when dry, wetting charcoal ash activates its high alkalinity, so it is a good choice for use in the garden as a liming agent. In addition to dozens of essential plant nutrients, charcoal is especially rich in potassium. Charcoal ash is powerful and fast-acting, however, so use it only in areas that will benefit from a reduction in soil acidity. Briquette remnants and ash should never be used in the garden, however; these factory-produced fire bricks contain harmful accelerants and chemicals, and leftovers should be disposed of in the trash.
Insert the probe of the hand-held pH meter into the soil where you plan to use charcoal in order to measure the soil’s acidity. Most plants perform best in a pH range between 6.5 and 7.0, so if your pH is already in this range, only incorporate charcoal if you plan on growing plants that strongly prefer alkaline soils.
Measure the length and width of the area to be amended. Multiply the two figures to arrive at the square footage. If the area is not square, make a rough estimate.
Determine the rate of application for the charcoal. Use no more than 20 pounds per 1,000 feet, which equates to roughly a 5-gallon bucketful of ashes. A 100-square-foot garden will require only 5 pounds of ashes to raise the pH by about half a point -- for example, from 5.5 to 6.0.
Sprinkle the ashes evenly over the surface of the amendment area. A used coffee can is a handy way to distribute ashes, as a standard 16-ounce can holds roughly a pound of amendment.
Rake the ashes to spread them while also breaking up any clumps or thick deposits. Uniform application is essential to avoiding alkaline hot spots once the ash is incorporated into the soil.
Incorporate the ashes into the top several inches of soil, using a shovel to turn over dirt one section at a time. Break up soil clods with the shovel as you go to work the ash into the soil, then smooth out the entire area with the rake.
Water the entire amended area thoroughly, making sure the soil is soaked through at least an inch or two below the surface.
Measure the pH of the amended area again three to six months after application to determine whether another round of ashes is needed.