Wood Ashes and Soil pH

pH scale

pH scale

During the winter, we use our wood-burning fireplace a fair amount of time. We therefore have a significant amount of wood ash. I have heard conflicting opinions on using wood ash as a soil amendment. Since I have a large area that I grow my vegetables in, I would like your opinion as to whether or not I should continue rototilling wood ash into the ground.
>Marshall Maydeck, Arleta
Every sort of material rototilled or otherwise incorporated into the ground as soil amendment needs to be evaluated for its chemical and physical properties. The combination of the two will determine how the material in question will affect overall soil fertility.
The chemistry of anything added to the soil depends on its mineral content.
Minerals may enhance soil fertility by providing essential elements for plant growth or detract from soil fertility if they contain harmful elements.
Another important chemical property of soil amendments is pH, a measure of acidity or alkalinity, with 7.0 being neutral. The ideal pH for the growth of most plants is around 6.5. Acidic soil amendments, such as peat moss, may have a pH as low as 3.5.
Because of the high soil pH in our area, some local gardeners advocate dusting the ground with gypsum — which forms sulfuric acid in contact with water — several times a year to keep their soil pH leaning in a more acidic direction.
This brings us to wood ash. Wood ash is high in potassium and is applied to soil to increase the presence of this element. In Southern California, however, potassium is usually not lacking in the soil. In addition, wood ash contains abundant calcium, an element that elevates soil pH, a negative in our area since native soil pH is usually too high in any case.
Wood ash, when heavily applied, also increases soil salinity that is detrimental to plant growth. On the plus side, wood ash is prized for its desiccating attribute, which makes it repellent to snails, slugs and soft-bodied insects.
Thus, lightly applied in Southern California gardens, wood ash would be useful in pest control but not for increasing soil fertility.
If you do apply wood ash, do so carefully around the base of each plant, since its salts will burn flowers and foliage upon contact. Avoid mulching azaleas, which prefer a distinctly acidic pH, with wood ash. You should also handle wood ash with gloves and a dust mask because of the highly potent alkali compounds it contains.
The physical properties of materials such as peat moss, perlite, nitrolyzed redwood shavings, straw and compost increase soil fertility by making the soil spongier. Any type of problem soil, whether hard and poorly drained or overly sandy, will benefit from the addition of these materials.

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