We are losing our topsoil at an unacceptable rate, and very few people are talking about it. Crop production cannot be sustainable until we stop soil erosion. Iowa produces more corn than any other state, but the average annual soil loss from Iowa’s cropland is 5.3 tons/acre. Under ideal conditions, soil can be generated from parent material at rates up to 0.25 ton/acre/year. This means we are losing soil at a rate of 20-30 times faster than soil formation. This is not sustainable nor acceptable. On average in the Corn Belt, we have lost about one-half of our topsoil in 150 years of farming, and some indications are that the problem is accelerating.
So why are we losing top soil? Increasing crop yields over time due to improved genetics and production technology, but this may be masking the problem of soil loss and associated loss of yield potential. Rick Cruse at the Iowa State Daily Erosion Project estimates that soil erosion costs the average Iowa Corn farmer 15 bushels/acre in lost yield potential or about $1B in yield each year. Recent publications by NOAA and other climatologists note that recent precipitation patterns are changing. Since 2012, we have seen more variability in the amounts, frequency and timing of large rainfall events than at any time since we began keeping records. The state of Minnesota reported a one-in-five-hundred-year frequency rain storm event in three of the last five years and some of the largest rain events in Iowa and Illinois in 2016 occurred in the fall and winter months.
Energy from rainfall disrupts and disturbs soil particles causing soil to detach and move through three distinct types of erosion. Sheet erosion is the overall movement of a uniform layer of soil like tearing off a page of paper from a tablet each time rains. Rill erosion is the formation of very small channels or gullies in the soil where flow begins to concentrate and accelerate the detachment of more soil particles. Gully erosion is the formation of channels that are larger and deeper than what we consider rills (i.e. more than a few inches). Typically farmers think of gullies as those channels too big to drive across, whereas evidence of rill erosion can be erased with tillage.
Each erosion event removes some of the most valuable topsoil; that surface layer is the most valuable topsoil out there as it usually contains the most nutrients and is most biologically active compared to soil deeper in the profile. Once topsoil is lost, the yield potential of that field is lost forever. A few farmers have attempted to capture topsoil before it leaves the field and transport and re-deposit it back up the hill, but this is expensive and there is limited evidence that this will pay off in higher yields. It’s far more cost-effective to understand the process of erosion and take steps to keep soil in place by doing less tillage, building soil health, and consider using cover crops to maintain soil cover throughout the year.
Most farmers simply don’t know how much soil they are losing to erosion. Farmers and their advisors need to recognize erosion as a major threat to the sustainability of our farming operations. We are investing in some new tools and technologies to help understand soil loss and what can be done to reduce it. Farmers will find a way to save their topsoil, and we look forward to working with them to understand and take steps to protect soil resources.