I've pointed out in previous Resilient Ag Landscapes columns that we live in a society of "quick fixes," but those quick fixes don't always apply to working with the land. A good case in point is building topsoil. In an earlier column, I pointed out that it only takes a few extreme rainfall or wind events to remove an inch of topsoil, but it takes at least 100 years to rebuild it.
"I have found that a lot of people think new soil is created overnight. And that's not the case," says Keith Glewen, University of Nebraska Extension educator.
Then there's organic matter — one of the key components of soil health and soil productivity. We've discussed different perspectives on defining soil health and soil productivity, with organic matter being a primary driver for both. It's one of the first things producers ask when adopting no-till or cover crops: How long will it take to see an improvement? The answer often lies in organic matter.
Factors affecting organic matter
A number of variables affect organic matter. As Neil Dominy, a soil scientist with the Natural Resources Conservation Service in Nebraska, notes, the potential to increase organic matter all depends on the environment and the soil type.
"The climate you're in is going to dictate some potential," Dominy says. "In western Nebraska with 16 inches of rain and in eastern Nebraska with 35 inches, there's quite a bit of difference in the potential to grow biomass above ground and root biomass below. If it's a cool and wet climate, you don't burn as much organic matter as you would in a hot and dry climate."
However, the position on the landscape also plays a role. "If you're comparing a side hill to a bottom ground piece, the potential for organic matter increase on a side hill or eroded positon may be greater," assuming erosion is under control, Dominy says. "If you're talking about a 17% slope vs. a 2% slope, I would inherently think the summit or low slope has more stable organic matter. There's less opportunity for erosion. If you're continually eroding your soil, you're not going to increase your organic matter."
There's also higher potential to build organic matter in the top 1 to 3 inches in the topsoil, because of a high density of root production at this depth and cycling of aboveground crop residues.
Of course, organic matter can't increase until the producer stops erosion first.
That amount of soil organic matter that's already in the soil is another factor. "In some situations, you can't afford to improve organic matter, because it's very near the steady state or above the steady state," says Charles Wortmann, Nebraska Extension soils specialist.
The organic matter steady state is reached when the organic matter reaches equilibrium — the amount of organic matter gained equals the amount lost. The soil organic matter steady state depends on management, location, soil properties and landscape.
Root biomass plays the biggest role in maintaining and increasing organic matter, and Wortmann notes root production can be increased in several ways. "The best way is to improve root production may be by a change in rotation, such as adding oats or winter wheat," Wortmann says. "In cases of high erosion and marginal profit potential from corn and soybean production, we should be thinking about perennials in the rotation and even permanent perennials in some cases."
An example is outlined in a recently published NebGuide, "Soil Management for Increased Soil Organic Matter." In silty clay loams soils in eastern Nebraska, where the steady state is at 1.5% organic matter on eroded soil, organic matter doubled in the top 8 inches over the next 60 years, by adding perennial grass as a harvested crop in an annual crop rotation.
While building soil organic matter is a long-term investment, depending on location, some producers have noted similar increases in a shorter amount of time.
Dan Gillespie, Nebraska NRCS no-till specialist who farms near Battle Creek, has seen a significant increase in his 25 years of no-tilling. He also has 15 years of organic matter testing from 1991 to 2017.
"When I introduce continuous cover crops into corn-soybean rotation, I'm averaging 0.15% to 0.2% increase per year. In five to seven years, I'd say you can build a percent organic matter," Gillespie says. "I'm doing this on irrigated silty clay loams; it's the predominant highly erodible soil in northeast Nebraska. I have been doing that on just a cereal rye cover crop."
Cover crops work well for mitigating erosion in any soil type. However, Gillespie notes, some soils are easier to increase organic matter in than others. For example, in sandier soils, it can be hard to increase organic matter. However, that doesn't mean there isn't a benefit to no-till and cover cropping on these soils.
"I believe cover crops give the most bang for the buck in coarse-textured soils. because the dead and drying cover crop root systems act somewhat like organic matter in the soil profile," he says. "In sandy soils, it's hard to build organic matter, but if you continuously cover crop it, you provide the root structure that can soak up water, slow down water movement through the profile and hold on to nitrates before they can get past the root zone."
Since the sod was first overturned over 150 years ago when the first Homestead Act was enacted, Nebraska has lost roughly 7 inches of topsoil per acre, according to NRCS calculations. Meanwhile, it takes at least 100 years to build an inch of topsoil — but it can take as many as 500 years. We live in a fast-paced society where quick fixes are often sought, but building topsoil and soil organic matter take long-term solutions. But before you can build, you've got to stop erosion and prevent the loss of organic matter.