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.
This is why it’s so important to spread these amendments evenly—if you miss a spot, that spot won’t get any benefit from the Saturday afternoon you spent tending your lawn. The stuff won’t sort of spread itself around or even itself out. You can see this trait as co-operative (Here? Okay.), as stubborn (I’m not moving.), or as passive aggressive (You said to be here. Fine. I’m here.) but there it is: it stays where it’s put.
In a garden, this chemical immobility doesn’t cause much of a problem; you just incorporate the amendment into the soil, so it'll be in the root zone where plants can use them. But somehow “incorporation”—or to speak plainly, digging—doesn’t work so well with a lawn.
Ashes, in contrast, dissolve quickly and easily, so it’s not necessary to spread them absolutely evenly, and plants can quickly get at and make use of the calcium and other nutrients they contain.
Ashes also contain a wider range of essential nutrients than do either gypsum or lime. Dolomitic lime does contain magnesium, another of the three secondary nutrients, and some gypsum (mined from dolomite) can also contain magnesium. But wood ashes also contain potassium and phosphorus, and a number of micro-nutrients such as boron, copper, manganese, sodium, zinc, and iron, most of which are absent from lime or present in such small amounts that they can’t supply plant needs. Even in wood ashes, though, these elements are present in only trace amounts, which is one reason why heavy metals poisoning is essentially a non-issue unless you pile ashes on your lawn or garden to a depth of several feet.
Being an even-handed, honest promoter, I am honor-bound to acknowledge that wood-ashes are not for every lawn. They do raise pH—so if you’ve already got alkaline soil, ashes aren’t a great idea, especially on grass, which grows best in slightly acidic soil. It kills me to acknowledge that dandelions prefer a higher pH soil than grass, so raising pH will encourage this pernicious weed. It kills me all over again to admit that ashes, unlike lime, also contain a fair amount of potassium (more if they come from hardwood), which is needed by dandelions but not grass. If you’ve already got a significant dandelion problem—or if you think you do, which may amount to the same thing—it’s probably best to steer clear of wood ash on your lawn.
Just the logistics of dealing with ashes raises some hurdles. For one thing, as everyone knows, they’re so fine they’ll blow about with the slightest excuse or the slightest wind. This is not only inconvenient, it’s potentially harmful. The strong alkalinity of ashes (they measure 9 to 13 on the 14 point pH scale) only comes into play when they’re wet. You can run your (dry) hands through them safely, though this isn’t recommended. But once in contact with water, they become caustic, which isn’t surprising when you realize that you’re producing lye. (Pioneers made lye by straining water through ashes; the other major ingredient for a simple, if skin-wearing soap, was grease. Altogether an unappealing proposition.) Since the inside of your nasal passages (not to mention your lungs) is wet, ashes you breathe produce minute amounts of lye on any wet tissues they encounter. Breathing them is therefore even less recommended than touching them.
One last warning: ashes can change pH so quickly that it’s a mistake to use them within two weeks of planting anything. At least one source recommends spreading them in the fall, but this doesn’t make sense to me, since many of their nutrients would just wash away with rains and snows before plants got to use them. This is the downside of the fact that they are relatively “mobile,” unlike lime and gypsum: they will leach. For this reason, don’t add them to an open compost pile; they’ll lose most of their nutrient value long before you spread the compost.
So ashes are great—in the right place and at the right time. I’m feeling a bit like those ubiquitous medical ads on US television these days: Take X! It’s great! (Little skit with lovely young woman looking happy about taking X and testifying to how it changed her life.) Then: serious voice, speaking very fast, informing you that “Side effects are generally mild, and include indigestion, headache, migraine, an irresistible urge to beat your head against a wall, tongue cancer, and in rare cases asphixiation.” (Return to shot of lovely young woman; mature voice urges you to ask YOUR doctor about X.)
Aside from the fact that they encourage North America’s least-favorite weed (I don’t know about England, much less France or India), they’re impossible to deal with, they’ll eat away the inside of your nose, and they’re useless on alkaline soils, ashes are terrific. Ask your doctor about wood ashes today!