Are You Mining Your Soil?

Are You Mining Your Soil or Managing Sustainably?

By Brian Dougherty, Understanding Ag Consultant

In my Fertilizer Fallacy article, I made the case that conventional fertility recommendations often drain farmer bank accounts without a reliable yield response to show for it. However, a common concern with reducing nutrient applications is a fear of “mining” your soil if you do not apply fertilizer to compensate for the nutrients removed via harvest.

It’s a fair concern on the surface. Nobody wants to be accused of being a bad steward. But if we’re going to use the word “mining,” we need to use it honestly. Because the truth is that most farms are already a mining operation. The question isn’t whether you’re mining — it’s what you’re mining and how fast. Let’s walk through it.

Mining Truth #1: Erosion is the Original Mine

If your soil is moving downhill, downstream, or downwind, you are mining your soil. Full stop. It doesn’t matter how much DAP and potash you spread. You cannot compensate for erosion by buying fertilizer, because erosion doesn’t just take nutrients. It takes the soil itself, along with the structure, the carbon, the biology, and the water-holding capacity that was built over centuries. 

Soil erosion is a natural process. It occurs slowly over time at a rate of roughly 1,350 lb/acre/yr mainly during extreme wind and rain events. The rate is slow and roughly matches the natural rate of soil formation because nature tries to cover the soil with living plants.  Today however, the average rate of erosion across U.S. cropland runs about three times the natural rate, and on rolling ground in hillier areas it can be far worse than that. The Iowa Daily Erosion Project routinely shows hillslopes losing 5 to 30+ tons per acre in a single year.

At a modest 5 ton/acre erosion rate, you lose roughly 3 inches of topsoil in 100 years. This is the T value or “tolerable” erosion rate on our home farm in NE Iowa according to NRCS standards. At first glance, that may not seem like much to worry about. But if your great-grandfather started with a foot of topsoil, and each generation lost a little more, your great-grandchildren could be left farming subsoil because the topsoil is gone. 

For a deeper dive into the actual costs of that erosion, you can read my previous blog series on the topic. In this article, I want to focus on the soil mine that you should be concerned about.

Mining Truth #2: Carbon Loss is the Real Threat

Most soil fertility conversations focus on N, P, and K. But the nutrient that actually limits productivity in degraded soil isn’t any of those. It’s carbon.

Carbon is the currency of biological function. It feeds the microbes that mineralize nitrogen, solubilize phosphorus, mine potassium out of mineral particles, and hold the whole system together with the glues that build soil aggregates. Without aggregates, you don’t have a functioning water cycle. Without a functioning water cycle, you don’t have functioning biology. Without functioning biology, you don’t have nutrient cycling — and you’re back on the fertilizer treadmill.

Every day you leave the soil bare you are mining carbon. Here’s why:

• Bare soil doesn’t capture sunlight energy. A 200 bushel/acre corn crop pulls roughly 30,000 lb of CO₂ out of the air per acre over a relatively short growing season. That is the power of biology. Bare ground pulls zero. Sunlight on bare soil bakes the biology and bleeds CO2 back into the air with nothing to show for it. 
• Bare soil oxidizes. Sun, rain, and wind on uncovered ground accelerates the breakdown of existing organic matter. Microbes consume the carbon they have available to them and then go dormant or die — taking their nutrient-cycling work with them.
• Bare soil erodes. Erosion strips the carbon-rich topsoil from the landscape.

Leaving the ground bare for months out of the year is the functional equivalent of running a strip mine on your own farm. You wouldn’t lease your back forty to a coal company. But every fall, when the combine pulls out and the ground sits naked through the winter and into late spring, that’s exactly what’s happening — just with a slower truck and no royalty check. Throw in fall tillage and you make the problem even worse. 

Mining Truth #3: The Midwest is a Huge Mine

Let’s put some numbers on it, because this is where the scale of what we’ve done becomes hard to ignore. The consensus among soil scientists is that we’ve lost, on average, roughly half of our original topsoil and soil organic matter percentage is roughly half what it was pre-settlement.  Prairie and oak savanna soils across the Upper Midwest typically carried 5–7% organic matter (OM) in a deep, dark A horizon that often ran 12–18 inches before grading into the B horizon. Today, those same soils — under 150-ish years of tillage and cropping — typically test 2–3.5% organic matter, with a much shallower A horizon thanks to erosion. 

A 6” slice of soil over one acre weighs around 1.8 million lb, and organic matter is roughly half carbon. Using an average pre-settlement topsoil depth of 15” at 6% OM, that topsoil would weigh 4.5 million lbs and contain 135,000 lb of carbon. If we are now at 7.5” of topsoil with 3% OM, that works out to roughly 2.25 million pounds of soil and 33,750 lb of carbon per acre in the topsoil horizon. That’s a loss of 2.25 million lbs (1,125 tons) of soil and over 100,000 lb of carbon per acre. 

Now scale that up. Recent research estimates that the U.S. Midwest has lost about 57.6 Billion TONS of topsoil in the last 160 years. If we assume that topsoil was originally 6% OM, that’s 1.73 billion tons of carbon that has relocated elsewhere. Nationally the numbers scale up from there. That’s a heck of a big mine. The pit just happens to be shallow and spread across the country. This mine didn’t pay royalties to the landowner. The soil just left, taking the productivity, the water-holding capacity, and nutrient-cycling capability of those soils with it.

Mining Truth #4: Higher Yields and Fertility Inputs Don’t Make You Sustainable

This is the part of the conversation that tends to make people uncomfortable, so I’ll just say it plainly.

If your yields are climbing every decade but your organic matter is declining or your erosion rate is greater than natural background erosion – your farm is not sustainable, and you are not “leaving it better than you found it”. You are riding a yield curve that’s being propped up by genetics, fossil-fuel inputs, purchased fertility, and mining of carbon that nature laid down for free.

This is the lie at the center of modern agriculture: that yield is the scoreboard. It isn’t. Yield is one output of a complex system, and it can be increased significantly for decades while the underlying system falls out from underneath it. The actual removal of nutrients from the farm in grain, forage, or animal products is a minor concern relative to what most soils and parent materials contain. The real mining concern is topsoil and carbon. Ask your input supplier how their products or services will help you keep those in your field. If they don’t, start asking serious questions about what exactly you are getting for your money.

The true metrics for determining if you are leaving the soil better than you found it are these: Is aggregation improving? Is organic matter trending up? Is water infiltration improving? Is the biological community more diverse and more active? Is the system requiring fewer purchased inputs over time rather than more? Are your children really going to inherit soil that is better than what you inherited, rather than just a higher yielding higher cost system?

What’s the Alternative?

The good news is that you don’t have to choose between “mining the soil” or buying fertilizer. That’s a false choice the input industry would like you to keep believing. Digging nutrients out of a big hole in the ground and burning finite fossil fuels to ship it hundreds to thousands of miles to your farm never has been and never will be sustainable. The alternative is to “mine” the right vault — the one that’s already on your farm – and work on ways of recycling nutrients back to the farm locally over time so they are replaced.  

At current crop removal rates there are hundreds to thousands of years’ worth of nutrients already sitting under your feet in most soils. The catch is that these nutrients are locked in mineral and organic forms, and only biology has the combination to the vault. Mycorrhizal fungi, P-solubilizing bacteria, free-living N fixers, protozoa, nematodes, and the rest of the soil food web are the ones who do the actual mining work — for free, every day, as long as you don’t kill them off. Conventional farming systems will run short of topsoil and organic matter long before they run short of nutrients.  

The even better news is that if you farm in a way that improves biological activity, you can start turning subsoil into topsoil to compensate for what has been lost. That means the path forward isn’t more inputs. It’s the one we emphasize over and over: know your context, minimize disturbance, maximize plant diversity, keep living roots in the ground as much of the year as possible, keep the soil armored, and integrate livestock where you can. The 6 Principles of Soil Health work everywhere because they reduce erosion, build carbon, and feed biology.

When you evaluate any product, practice, or recommendation, run it through that filter: does this support or undermine soil biology? If a fertility input is killing the very organisms that do your nutrient cycling for free and hold your soil in place, you’re paying twice — once for the product, and again in lost soil function. If you are concerned about your kids paying a third time in lost future productivity, reach out to a consultant at Understanding Ag.