At this time of the year when we are using our fireplaces frequently, we generate a lot of wood ash.
The pH of wood ash varies but only slightly, depending on the type of wood that we are burning. The pH scale, which indicates the extent of acidity or alkalinity in soil, goes from 0.0 (which is the most acid) to 14 (which is the most alkaline).
The pH scale is a negative logarithmic scale so the lower case “p” in pH is the mathematical symbol for negative logarithm. The upper case “H” indicates the concentration of hydrogen ions.
It is a complicated scale, but fortunately gardeners only really need to grasp that 7 is the neutral number on the scale. Above 7 the soil is alkaline and below 7 the soil is acid. The 6-7 point range is the best gardening soil, because the organisms that make the nutrients available to our plants work best in that 6-7 point range.
British gardeners talk about chalky soil, which means a high pH with limestone causing high alkalinity. So if your soil has a high pH, never dispose of wood ash in your beds, and if your soil is acidic, use less than 20 pounds of ash per 100 square feet in your garden annually.