The Soil Series # 3
Having gone on at some length a while ago (twice!) about how wood ashes aren’t going to cause lead or mercury or cadmium poisoning if you use them in your garden, I am now prepared to tout them as the nearly-perfect lawn fertilizer. Since it seems that I’ve adopted this poor, misunderstood amendment as my own, I might as well do it thoroughly.
Grass needs calcium, which might be considered a non-renewable resource in a lawn: once the roots have used it up, it’s gone. So additions are necessary. The most common materials for such additions are gypsum, which contains about 22% calcium, and lime, at about 30%. The calcium content of ashes varies widely depending on type of wood, but even softwoods will produce ashes containing about 15% calcium, and hardwood ash may be as high as 50%.
A couple of things make ash a superior amendment, especially for lawns. For one thing, both gypsum and lime are quite insoluble. The term “immobile” seems an excellent metaphor for how they behave when applied to grass, but it’s also the technical term for a compound that doesn’t dissolve easily and therefore doesn’t move with water into and through soil. As a result, it is hard to get lime and gypsum into a plant: unless they’re snugged right up against the roots, they might as well be on Mars, for all the good they’ll do.
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