On any given Saturday, if you run into Mike and Philip, you‘re likely to find them covered in dirt. My friends recently dug into a new pursuit: gardening. At present, this means incubating delicate seedlings on windowsills while they haul bags of topsoil from garden store to garden plot, adding nutrients and substrate for their future vegetables.

In time for this spring (rather, summer, in my New England lexicon) flurry of activity, the Environmental Working Group released a report last week on soil erosion in the Iowa cornbelt. The news isn’t good.

With the folly that is corn-based ethanol and rising food prices worldwide, American corn (and soybean) growers have seen crop values spike. As a result, it’s become economical to plant more marginal cropland, which tends to lie on steeper slopes with higher erosion potential. So the amount of topsoil we lose each year (already at 2 billion tons nationally) is increasing.

This has both short and long term consequences: Immediately downstream, waterways are clouded by soil and polluted by fertilizer and pesticides. Back at the farm, erosion rates set the agricultural lifespan of the land.

There’s only so much that nutrient-booster shots can do, and once the soil has blown or washed away, the land is lost to farming. Indeed, according to Jared Diamond, soil erosion and consequent loss of soil fertility may have contributed to the collapse of ancient civilizations, like the Mayan Empire.

So it’s critical that rates of soil erosion don’t outstrip the natural rates of soil production (by weathering of rock and deposition from uphill and upstream sources). Measuring soil formation is no mean feat, but in a 2007 Proceedings of the National Academy of Sciences paper, David Montgomery calculated rates ranging from 0.01 to 0.1mm of soil per year, depending on how much geological action the local landscape was getting.

By comparison, erosion on Iowan farmland results in a loss of 1mm of soil per year. The USDA’s threshold values (the rates at which regulatory alarm bells are supposed to sound) are set at 0.4 to 1mm per year.

In other words, we’re eroding our topsoil at 10 to 100 times the rate it can be replenished, and our (supposedly scientific) farm authorities say that’s OK.

Still, we’re holding onto our soil more successfully than other countries. In parts of Asia and South America, erosion rates are double or triple ours. But erosion rates are half of ours in Europe, where the right set of (enforced) incentives has farmers planting cover crops (to hold soil in place after harvesting), edging fields with buffer zones (grassy strips that entrain soil before it washes into drainages) and practicing no-till farming.

“Zero-tillage” is an increasingly popular technique in which the soil isn’t plowed into ridges and furrows before planting. It requires less energy and fewer tractor hours and has the potential to eliminate erosion completely (and reduce surface runoff by 99 percent). But untilled fields give weeds a head start, so no-till is often coupled with heavy pesticide usage.

While we could implement an array of soil conservation practices (some more effective than others, of course), our social and economic motivations are failing us. In Iowa, most farmland is rented; a third of the landowners live outside the state. Many farms are huge amalgamations of land disconnected from the individual with a deeply personal stake in the soil’s health.

Since 1985, we’ve used economics to mimic that commitment: farmers are supposed to follow local soil conservation guidelines in order to receive subsidy payments. At first, this strategy successfully reduced erosion by 45 percent. But in the last decade, compliance has dropped under dwindling enforcement, and now the most critical conservation programs — the Environmental Quality Incentives Program and the Conservation Stewardship Program — face $110 million cuts under the just-passed rescue budget. (Of course, our subsidy system has always emphasized production over sustainability: in the last 15 years, only 12 percent of subsidy payments came from conservation funds.)

Today, though, it’s more important than ever to address agricultural sustainability. Increasingly violent weather patterns — and attendant erosion pulses — are predicted to accompany climate change. Our farm system is crippled by outdated subsidies and overreliance on intensive, fossil fuel-based inputs. Yet globally, we must still find a way to feed 3 billion new mouths by 2050.

It seems increasingly clear that the only way to address all these issues is by shrinking our agricultural operations back down toward human scale. Industrial agriculture feeds the world, but it also decouples us from our resource base. We may call the Plains states flat and monotonous, but a farmer intimately familiar with his land knows that there are drainages here and fertile flatlands there, that he should plant crops on this side and buffer strips on that one. A farmer intimately familiar with her land knows that the right thing for her pocket in today’s economy may not be the right thing for the land she plans to leave her children.

It’s our job, then, to help bring these values back into synchrony. It begins with a bag of topsoil, a shovel and Mike and Philip’s garden.

Holly welcomes reader comments — and fresh backs for the garden — at [email protected].