A statement1 in JM Greer’s blog last month challenged everything I thought I knew about soil management in American cropland. At today’s rate of erosion, he wrote, the topsoil would be gone by 2075. Gone! The land might look like Providence Canyon7, where poor soil management in the 1820’s triggered runaway erosion that is still going on.
Providence Canyon, the "Little Grand Canyon" formed in southwest Georgia by manmade erosion in a tricky geological context. (permission requested)
"Gone by 2075" seemed an absurd projection for the well-tended midwestern topsoil. Driving there, you pass mile after mile of fields with furrows always running across the slope, patches of permanent grass where a gully might form, strips of grass and shrub along creeksides, sometimes bands of grass alternating with cultivated strips across steeply sloping fields. Many fields are no longer plowed or cultivated, but farmed on a no-till system.
You can’t stop all erosion, of course, unless in a rice paddy-type field. Most farmers seek a lesser goal4, formalized by the Soil Conservation Service in its early years: Keep erosion slow enough that it doesn’t damage the soil’s productivity.
How much erosion is acceptable? For the better soils of the Midwestern corn belt, about five tons8 per acre per year — in theory. The Soil Conservation Service, who are now the Natural Resources Conservation Service (NRCS), speak of the acceptable erosion rate as the Soil Loss Tolerance Rate, or sometimes the "T factor."
NRCS reported in 20075 that only 28% of US cropland was eroding faster than the acceptable rate "T" — not an ideal situation, but apparently much better than "gone by 2075." Yet the more detailed NRCS data don’t fully support this statement. And the T factor itself is a puzzling legacy.
NRCS publications don’t say how T was calculated, though it’s tabulated in the soil survey for every soil in every county of the US. The calculation took into account the natural rate of soil formation, but only as one factor among many. Judging from some discussions6 back in 1956, T was mainly designed to keep the topsoil deep enough for crop production, free of serious gullying, and capable of holding most of the nutrients applied to it. The soil formation rates considered may have been some early, highly optimistic estimates.
Current estimates suggest that soil forms at about half a ton per acre per year in some representative Iowa and Minnesota soils. Even the lowest T values are many times higher. Soil scientist Leonard Johnson6 reviewed the research behind the T factor in 1986. He concluded that erosion control based on T values "..should be considered as provisional or short-range."
A 2014 webinar9 at ISU by Prof. Rick Cruse concluded that using T as the criterion for soil erosion control was effectively "a depletion schedule" for topsoil.
From R. Cruse, Pritchard Lecture 2014.10
The long range goal Johnson, Cruse and many others recommend is to keep topsoil erosion slower than the rate at which it is being replaced by nature. If this were achieved, hills and ridges would level out slowly over geologic time, always blanketed by a continually renewed layer of topsoil where crops could be grown.
Conservation planning is difficult because the accepted method of estimating soil erosion in a given field, refined and extended since the 1930s, is still inadequate. New studies, in which actual runoff9 from test fields is collected and measured, show that actual soil erosion is 100-200% worse than the most sophisticated estimate. (One of the test fields is being farmed by the best no-till methods.)
With actual soil erosion this severe, we may be losing topsoil at 10 to 30 times the rate it is forming. That range echoes the 10x-40x2 estimate of an Australian soil scientist8, for the entire world’s cropland. He told Time magazine in 2012 that
"A rough calculation of current rates of soil degradation suggests we have about 60 years of topsoil left. Even the well-maintained farming land in Europe, which may look idyllic, is being lost at unsustainable rates."
Tolerable vs actual soil erosion rates in Europe. F.G.A. Verheijen, R.J.A. Jones, R.J. Rickson and C.J. Smith, Earth Science Reviews :24-38. From Prof. R. Cruse, Pritchard Lecture 201410
There are other soil management issues. No-till methods require herbicides, which have accumulated in groundwater to such an extent that many farm families rely on bottled drinking water. Herbicides can also kill grasses planted for erosion control.
Soil erosion estimates take into account a number of climate measurements, all of which may be changing as the earth warms. Revised values may be needed in the many tables of the soil survey. Some values reflect the frequency of heavy rainstorms, which may be increasing. A major storm can cause erosion close to the annual total in a single day.
Despite the questionable T-value guidance, NRCS remains an invaluable safety net, helping farmers to preserve and protect cropland. It assists in planning, financing and sustaining all types of erosion control structures. Among other services, it provides guidance for farmers making the transition from conventional to organic farming. All its work since 1933 has been helpful — just not as successful as we thought.
Holding erosion below the rate of soil formation isn’t impossible, but may require some unfamiliar and costly practices. Cover crops would help, as would the long-cycle crop rotations specified in every county’s soil survey. I’ll elaborate in another post, after I learn more.
Iowa cropland seen from the air after a rain. Light patches are subsoil exposed on hilltops and ridges. From Prof. R. Cruise, ISU webinar9.
1. J.M. Greer, "Dark Age America: A Bitter Legacy"
2. Prof John Crawford, Univ of Sydney "What If the World’s Soil Runs Out?". Time magazine, December 14, 2012.
3. Mike Duffy, retired economist, "Value of Soil Erosion to the Land Owner", ISU Ag Decision Maker,
4. NRCS, "State of the Land"
5. NRCS, "Soil erosion on cropland 2007"
6. Leonard C. Johnson, "Soil Loss Tolerance: Fact or Myth", Journal of Soil and Water Conservation, May-June 1987
7. Paul S. Sutter, What Gullies Mean: Georgia’s "Little Grand Canyon" and Southern Environmental History
8. Prof. Rick Cruse, "Soil Erosion – What will the future bring?",
9. Prof Rick Cruse, "Soil Erosion in Iowa: How much is really happening?" ISU webinar April 2014
10. Prof. Rick Cruse, "Is soil and water degradation invitable? Don’t bet your life on it.", Pritchard Lecture 2014