Test soil before applying ash to lawn, garden

By David Sprague

OSU Extension master gardener volunteer

With cool temperatures, it’s the time of year people are using their fireplaces and wood stoves on a regular basis. Few things are as nice as a good fire on a cold evening, but what to do with the ash when the fire is gone? Some people have asked the OSU Extension Clinic if they can spread their wood ash on their lawns or gardens. The answer is yes, but with precautions.

Wood ash is considered a “low grade” fertilizer. The amount of nutrients in it varies, depending on the species of wood burned, but in general it has small amounts of potassium, phosphorus and traces of other micro nutrients. If ash were a commercial fertilizer, its numbers would be about 0-1-3 (N-P-K).

Ash’s biggest impact is its effect on soil pH. It is alkaline and will raise the pH of your soil. Soil pH is a measure of the soil’s acidity or alkalinity and is designated by a number on the pH scale ranging from 0 to 14. Numbers less than 7 indicate acidity while numbers greater than 7 indicate alkalinity. For best uptake of nutrients, most plants need a slightly acidic soil. Plants such as turf grass, flowers, vegetables, ornamental shrubs and fruits do best with a soil pH of 6.1 to 6.9. Some plants, such as blueberries, need very acid soil with a pH of 4.5 to 5.5.

Applying small amounts of wood ash to most soils will probably not adversely affect your lawn or garden, and ash does help replenish some nutrients. But because wood ash increases soil pH, adding large amounts can do more harm than good. The best thing to do before applying wood ash, or anything else to your soil, is to have your soil tested. That will tell you its pH and whether it needs to be raised or lowered, along with other valuable information. The Extension Office in Mahoning County offers laboratory soil testing for a nominal fee.

Also, never use ash from burned trash, cardboard, coal, pressure-treated lumber or painted or stained wood, which can contain potentially harmful substances. Avoid using ash around acid-loving plants such as azaleas, rhododendrons and blueberries. Do not apply ash at time of seeding; it contains too many salts for seedlings, affecting plant health. Do not use ash with fertilizers containing nitrogen, as the ash will cause the nitrogen to be lost before it can be used by the plants; wait about a month to fertilize.