Now is the time of year for cleaning out the woodstove and spreading the ashes on the garden, a chore that both prepares the stove for summer vacation and sweetens the soil for the coming growing season. Those without access to wood ash usually make the trek to the hardware store for a bag of lime, which also sweetens the soil, though without the benefit of the added potassium in wood ash.
Why do we need to keep adding ashes or lime to our soils, year after year? Weren’t plants growing just fine hereabouts back before the days of lime and fertilizer?
Yes and no.
In relatively wet and humid parts of the globe, a description that includes Vermont and New Hampshire, soils tend to be acidic for the simple reason that calcium, the key ingredient in lime and ashes that adjusts pH, is a relatively soluble element that washes away over time. The underlying bedrock may be rich in calcium, but the constant action of rainwater and snowmelt washes the calcium into our rivers (and eventually into the oceans, where it becomes a key ingredient in shells, marine life, and coral reefs.)
Our native vegetation, therefore, tends to flourish in (or at least tolerate) somewhat acidic conditions. But over the past century, the emissions we’ve been adding to the atmosphere, from the combustion of coal, oil, and natural gas, have led to acid rain, which may be 100 times more acidic than natural rainfall. This increased acidity accelerates the natural leaching of calcium, which has caused our soils to acidify as calcium is removed faster than it can be replaced from the bedrock.
The situation is troublesome for our native vegetation but is acute in the garden, where many of our favored plants are not native to the Northeast and were never accustomed to growing under acidic conditions in the first place. Tomatoes and asparagus - two heroes of the New England garden - come immediately to mind.
Many gardeners, seeing their tomatoes and asparagus suffering, apply fertilizer to try to rejuvenate their plants. But if the soil pH is too low, the fertilizer has no effect because the acidity has locked up the nutrition in a form that the plants can’t use. The fertilizer runs off the garden, unused, a waste of money and source of pollution to ponds and brooks. Meanwhile the plants starve amidst plenty.
So a key spring task for every home gardener (and lawn grower, too) is to insure that soil pH is correct before fertilizing and planting. There are fancy test kits available for the job, or else state laboratories that will analyze your soil for a nominal fee. But there’s an even easier way to at least see if you have a problem: indicator plants.
Is there moss growing in your garden, lawn, or orchard? Reach for the lime. Mosses grow beautifully in acidic soils and are a certain sign that your vegetables, grass, or apples are suffering. Look in the thin patches in the lawn or around the edges of the garden to make sure this low pH indicator isn’t staring you in the face.
A second indicator, found more often in the orchard or pasture or rougher sections of the lawn, is wild strawberry, which looks just like cultivated strawberry only with much smaller leaves and fruit. Strawberries are very competitive on acidic soils and indicate that most other garden plants will not do well nearby.
Cultivated strawberries, of course, are also a desirable garden species. Serious gardeners will sometimes establish a lime-free section of the garden for growing strawberries and two other acid-loving plants: potatoes and carrots.
Are your potatoes routinely covered with scab? This could indicate that the pH is too high, not necessarily for tomatoes and asparagus, but certainly for spuds. Another indicator is carrots that are more woody than sweet. Many gardeners recall the carrots from their first year’s garden as far tastier than anything they’ve managed to grow subsequently. It’s not just that memories sweeten with age; it’s that lime is bad for carrots.
A final thought on sweet soils: they’re called “sweet” because that’s how they taste. Back before the days of fancy soil tests, farmers and gardeners would put a pinch of soil in their mouths to gauge the acidity. Soils that tasted sour required lime; soils that tasted sweet were ready to go. I’ve always aspired to impressing people with this skill, but lacking the years of careful practice and calibration, I’ve never been able to produce meaningful data. The only response I’ve been able to muster, standing in front of my expectant friends with a thin ribbon of mud running down my chin, is “tastes kind of gritty.”