Wood Ash Calculator: how much ash your garden can take
Fireplace ash is a liming material with potassium, phosphorus, and no nitrogen at all. Used at the right rate on acidic soil it is a free amendment. Used heavily, or on soil that is already neutral, it pushes pH past the range most vegetables like and ties up nutrients. University of New Hampshire Extension puts the ceiling at about 20 pounds of ash per 1,000 square feet a year, which is one full 5-gallon bucket. Enter your bed size and your latest soil test pH to get pounds, buckets, and cups for your own garden. The calculator skips beds at pH 7.0 or higher, around blueberries, azaleas, rhododendrons, and hollies, and where potatoes grow.
Free wood ash calculator. Enter bed area and soil test pH and get pounds, 5-gallon buckets, and cups of ash for the year, capped at 20 lb per 1,000 square feet. Clear skip verdict at pH 7.0 or higher, and around blueberries, azaleas, rhododendrons, hollies, and potatoes. Sourced from university extension.
The rules
- Cap it at 20 lb per 1,000 sq ft a year, about one 5-gallon bucket (UNH, Iowa State).
- Skip it if your soil test shows pH 7.0 or higher (Iowa State, Purdue).
- Skip it around acid-loving plants and potato beds (UNH, Iowa State, Wisconsin, Purdue).
- Never mix ash with urea, ammonium nitrate, or ammonium sulfate. The mix releases ammonia gas (UNH, Iowa State).
- Apply in fall, work it into the top 2 to 4 inches, and do not add lime in the same year without a test.
Worked examples
| Bed size | Full rate | Half rate near pH 7 |
|---|---|---|
| 50 sq ft | 1 lb, about 4 cups | 0.5 lb, about 2 cups |
| 100 sq ft | 2 lb, about 8 cups | 1 lb, about 4 cups |
| 250 sq ft | 5 lb, about 20 cups | 2.5 lb, about 10 cups |
| 1,000 sq ft | 20 lb, one 5-gallon bucket | 10 lb, half a bucket |
A 5-gallon bucket holds 80 cups, so a cup of ash runs about a quarter pound. Weigh a batch if you want to be exact, because ash density varies.
Why the pH matters most
Ash raises pH the way limestone does, and it does it faster because it is highly water soluble (UNH). Its calcium carbonate equivalent is 25% to 59%, against 90% to 95% for lime, so it takes two to four times as much ash to match a lime application. Above pH 7.0, Purdue notes that phosphorus, iron, boron, manganese, copper, zinc, and potassium become chemically tied to the soil and less available to plants. That is why the tool stops rather than scaling down.
Guides
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Sources
- University of New Hampshire Extension, Managing Wood Ashes
- University of New Hampshire Extension, Guide to Using Wood Ash as an Agricultural Soil Amendment
- Iowa State University Extension, Using Wood Ashes in the Home Garden
- Wisconsin Horticulture, Using Wood Ash in the Home Garden
- Purdue University Consumer Horticulture, Wood Ash in the Garden
- University of Saskatchewan, Using Wood Ash in the Garden