Carl Sandburg probably wasn't thinking about his garden when he said that the past is a bucket of ashes. Yet many gardeners today wonder if the future is a bucket of ashes; that is, if the ashes they remove from their stoves in the winter might not benefit their next summer's crops.
The answer is an absolute yes. You really can turn yesterday's trees into tomorrow's tomatoes. Few problems arise by using wood ashes in the garden, and coal ashes can also be applied with caution.
The main value of wood ashes lies in their potassium and calcium carbonate (lime) contents. Wood ashes are about 45 percent calcium carbonate, with hardwood ash being about one-third higher in lime than softwood ash. You would have to apply about twice the weight of wood ashes as lime to meet a particular lime requirement.
Since an average application of limestone in the Northeast is about 50 pounds per 1,000 square feet, you could safely apply 50 to 100 pounds of wood ashes per 1,000 square feet of garden during the winter.
If you remove about 5 pounds of ashes from your stove each week, you could make 20 trips to a 1,000-square-foot garden without worrying about raising the pH of the soil too much. Gardeners who apply wood ashes to their gardens year after year should have the soil pH checked regularly, however.
A friend of mine thought he was applying only moderate amounts of ashes to his garden, but after two winters he had raised the soil pH to 8.0. A pH of 6.5 to 6.8 is considered best for most vegetables, and anything above that value becomes too alkaline to support healthy plant growth. Also, wood ashes should not be applied to soil around acid-loving plants, such as azaleas, rhododendrons , blueberries, and pin oaks.
The ash from chemically treated lumber should not be used in the garden or anywhere else around the home. No chemically treated wood should be burned in the home since toxic fumes may be released during the burning.
Wood ashes contain no nitrogen, but they do supply 1 to 2 percent phosphate and 4 to 10 percent potash. Thus, 100 pounds of wood ashes would supply about 2 pounds of phosphate and 4 to 10 pounds of potash per 1,000 square feet.
Since a high recommendation for potash in a garden is 4 pounds per 1,000 square feet, 100 pounds of wood ashes could meet all the needs of your soil for this element. Soil tests should be done regularly to make sure your soil is not becoming too high in potassium, since too much potassium can interfere with the uptake of other essential elements, such as calcium and magnesium.
Recommendations for phosphate range from 1 to 7 pounds per 1,000 square feet. Thus, little danger of oversupplying phosphorus from wood ashes exists.
Coal ashes can present problems of salt and boron toxicity to plants, however , and the coarser particles of coal can make a soil too gravelly. To use coal ashes, first sift out anything larger than sand-size particles (greater than 2 millimeters). Then apply the fine particles in a layer about one-third of an inch deep over the garden.
Such an application can probably be repeated for three or four years before potentially toxic levels of salt or boron are reached. These applications will supply most of the potassium, calcium, magnesium, and sulfur needed by the crops.
Other elements, such as nitrogen and phosphorus, will have to be supplied by other fertilizers.
Most coal ash has a pH of 6.0 to 7.0, so it should not be used around acid-loving plants. You should also avoid adding much or any ash to boron-sensitive and salt-sensitive crops such as fruit, garden beans, celery, radishes, cucumbers, squash, peas, and Jerusalem artichokes.
Plants that can tolerate boron and salt are asparagus, beets, spinach, broad beans, turnips, cabbage, broccoli, cauliflower, Brussels sprouts, and rutabaga.
If areas of leaves begin to die or appear scorched, especially along the leaf margins, toxic levels of boron may have been reached. Toxic levels will be reduced by weathering and leaching within three years if no more boron is added to the soil.