Frequently I try to illustrate the carbon sequestration benefits of a fairly modest increase in topsoil organic matter (OM). Increasing 1% OM in 1,000 tons of topsoil enhances that parcel’s ability to retain moisture by 16,000 gallons of water/acre as well as increasing its residual carbon by 11,600 pounds/acre.
For simplicity’s sake, an acre is presumed to contain two million pounds (or 1,000 tons) of six-inch-thick topsoil. This means that the pain of moisture extremes – both drought and flood – will be intensified by soil loss. Trying to put a face on numbers like these is challenging. Toward that end, however, I got some help from Dale Ronfeldt, co-owner of Soil Solutions LLC.
On their company website (soilsolutions.net/save-the-topsoil), Ronfeldt wrote, “At the rest area (on I-80) near Casey, Iowa, they have an interesting display in front of the building about Iowa agriculture and the amount of soil loss that has occurred. According to this display, the average depth of topsoil in the year 1850 across Iowa was 14 inches. In 1900 it had decreased to 11.5 inches. After another 50 years it had dropped to 8.5 inches and in the 50 years from 1950 to 2000 it had decreased another three inches to 5.5 inches.”
If we line graph those values, we can sensibly conjecture that Iowa’s average topsoil depth has dropped down to about four inches since 2000. Sadly, much of the Mississippi Basin isn’t far behind Iowa in terms of soil loss. Lost soil is accompanied by carbon entering the atmosphere as greenhouse gases. It can be shown that the well-being of food security – as well as civilization itself – is very dependent on keeping carbon in soil and out of air.
Let’s elaborate on the Mississippi Basin. It gathers and drains 41% of the 48 contiguous United States. The basin covers more than 1,245,000 square miles, includes all or parts of 31 states and two Canadian provinces and roughly resembles a funnel which has its spout at the Gulf of Mexico. During summer 2022, the “trunk” of the Mississippi south of St. Louis suffered from seriously low water, and the associated transportation headaches like beached barges.
At that time, many folks in the Northeast tended to mentally relegate those problems to that distant venue. However, a small part of New York, a major part of Pennsylvania and most of Ohio contribute to the basin’s water resources (or lack thereof in the case of widespread drought). Here’s the geography supporting these last statements:
The Allegheny River is a principle tributary of the Ohio River and is located in the eastern U.S. It merges with the Monongahela River to form the Ohio River at Point State Park in downtown Pittsburgh, PA. The Allegheny River is by volume the main headstream of the Ohio River. The Allegheny starts in Potter Co., PA, then weaves its way north into New York, then back into the Quaker State. The Ohio River becomes a tributary of the Mississippi River directly south of Cairo, IL. At St. Louis, the Mississippi River welcomes the Missouri River which has just drained most of America’s plains regions and some of the Northwest.
With three main tributaries having formed the “Mighty Mississipp,” the likelihood of all three sub-basins experiencing the same moisture extreme simultaneously is statistically small. But in 2022 that’s what happened due to far-reaching droughts. The opposite happened in 1927 with the flood named for that year. The severity of ’27 was greatly intensified by widespread strip of deforestation practiced by largely unbridled logging enterprises.
Let’s compare how much moisture stress resilience has been depleted in the Corn Belt over 170 years, using 1850 as a base. We’ll examine the two most prominent Corn Belt states – Iowa and Illinois – in our example. Total U.S. corn acreage in 2020 was approximately 96 million, and total U.S. soybean acreage was approximately 76 million. For Iowa, corn and soybean acreage were 14 million and 10 million, respectively. For Illinois, corn and soybean acreage were 10.9 million and 10.5 million, respectively. Total corn and soybean acreage for both states is 45.4 million, which is a little over a quarter of the nation’s total corn and soybean acreage (approximately 172 million).
Let’s presume that all those 45.4 million acres in those two states averaged a topsoil depth of 14 inches in 1850. Let’s also presume that soil OM percentage has remained constant over the years. We know that at our base point, not only were Iowa topsoils deeper but they also had higher OMs with much more diverse cropping. But, again for simplicity sake, let’s presume that Iowa’s and Illinois’ soils averaged 5% OM over the 170-year period being evaluated.
In these two states, with a total of 45.4 million acres of non-fibrous root crops, we’ll calculate how much the total pounds of OM/acre has dropped in 170 years. With topsoil depth starting at 14 inches in 1850, and ending up at four inches in 2020, there’s been a loss of 10 inches of topsoil or 1.67 normal depth topsoil layers. This then means that per acre we have lost 3.34 million pounds of topsoil (1.67 x 2 million). At a presumed average of 5% OM, that acre’s water holding capacity has been reduced by 133,600 gallons (5% x 16,000 gal / % O.M. lost x 1.67 topsoil depths = 133,600 gallons).
Multiply this last product by 45.4 million acres (the land presently committed to corn/soy in Iowa and Illinois). We then see that the reservoir benefit on this acreage in these two states has been slashed by 6.06 trillion gallons – about 10% less than the 2.5 trillion gallons claimed by Seneca and Cayuga lakes.
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