To understand soil, read weeds

Last year, a section of our lawn was neglected, and it turned into a flower garden. Amid the tall, unmowed grass appeared bright spots of orange and yellow hawkweed, white daisies and blue violets. It was pretty, like the little flowery meads once planted in medieval cloisters to bring the outside world in. But it had been designed as a band of green to set off the real flower garden behind it.

This year, we set out to remove some of the nongrass species that had taken hold, but things were worse than we had thought. One part had almost no grass at all — just mats of sheep sorrel, hawkweed, plantain and dock.

Looking to see which weeds have moved into an area is useful because they often can tell you why they are there. Yes, I know, it’s because we didn’t weed, but also because any wild plant will choose to grow in the conditions that suit it best.

As we surveyed the lawn as a whole, it was clear the soil was quite acidic. The weedy interlopers grow in soil with a low pH, especially the sorrel and dock. (Other common acid-loving weeds include mulleins, buttercups and wild strawberries.) There were even some mossy patches — a dead giveaway. We had limed the soil when it was sown, but it had become more acidic over time. In his 1946 book “Weeds and What They Tell Us,” Ehrenfried Pfeiffer emphasizes that weeds are often an indication of changes in the soil rather than its original composition.

In the area where the grass had utterly surrendered, I went to work with a digging fork to pry the weeds loose, but the ground was unexpectedly hard. Although most of the yard has sandy soil, there are patches of clay here and there, turned up in the course of leveling the garden. This was one of them. No wonder the plantain loved it — that weed is often found in hardened, trodden-on paths.

Simply liming the soil in most of the lawn would improve it (in addition to more consistent mowing), but this bad area needed to be tilled and given lots of organic matter to aerate it and add fertility. In went a few wheelbarrow loads of weed-free manure and some peat moss, too, its acidity countered with extra garden lime.

It made me look around at the rest of the property to see what the weeds had to say. A large pile of blue marine clay we had stockpiled while digging our pond is covered with coltsfoot every spring, a pretty yellow flower that favors moist clay soil. Stinging nettles love our wet areas, and jewelweed thrives near the pond in part shade. White oxeye daisies pop up in every sunny spot where the soil is totally unimproved, showing how forgiving they are of poor land. Goldenrod is almost ubiquitous, but that’s only because there seems to be a goldenrod species for every condition — wet or dry, rich or poor.

The most consistent weed growth is always in the vegetable gardens, where most of our attention is focused. Cool-weather chickweed is always followed by the same warm-weather species: lamb’s-quarters, pigweed, prickly lettuce, galinsoga, purslane — all the ones that adore the loose, crumbly, nitrogen-rich soil we bestow on the edible crops that sustain us.

“Congratulations,” these invaders announce. “You have given your tomatoes and spinach a wonderful place in which to grow. Now weed it.”