CREP Corner
By
Lee Ashford
10 tons per acre is a lot of soil erosion… or is it? I’ve seen this figure thrown about quite a lot over the years. Recently I read that an erosion rate of 10 tons per acre wouldn’t even be visible to the naked eye, so I decided to do the math, and figure out just what 10 tons per acre really means. I was surprised by the answer.
First, there are 2000 pounds in a ton, so 10 tons is 20,000 pounds. There are also 43,560 square feet in an acre, so that amounts to 20,000 pounds per 43,560 square feet. Dividing 20,000 by 43,560, we get 0.4591368 pounds per square foot. Multiply that by 16, and we get 7.3462 ounces per square foot; call it 7-1/3 ounces. So now we know that an erosion rate of 10 tons per acre amounts to 7-1/3 ounces of soil per square foot. However, that still doesn’t really tell us anything, does it?
Various sources agree that 3,000 pounds per cubic yard is a good “average” weight for generic soil, so let’s use that figure. We want to calculate what portion of a cubic yard 7-1/3 ounces amounts to. First, let’s multiply 3,000 by 16, to find there are 48,000 ounces in a cubic yard of soil. That’s not much help; we want to find out how deep 7-1/3 ounces of soil on a square foot will be, so let’s break down our cubic yard to cubic feet. A cubic yard is 3 feet by 3 feet by 3 feet; that equals 27 cubic feet (3 X 3 X 3 = 27). So let’s divide 48,000 ounces by 27, to find there are 1,777.77 ounces in a cubic foot of soil. Still not much help, is it? Well, let’s break it down to cubic inches!
A cubic foot has 12 inches per side, which equates to 1,728 cubic inches (12 X 12 X 12 = 1,728). Now we’re getting somewhere; if we divide 1,777.77 by 1,728, we discover that 1 cubic inch of soil weighs 1.028802 ounces! Since we have 7.3462 ounces spread out over a square foot, we can divide 7.3462 by 1.028802 to learn that each square foot of ground has lost to erosion a total of 7.1405382 cubic inches of soil.
Since a square foot contains 144 square inches (12 X 12 = 144), we need to spread our 7.1405382 cubic inches over 144 square inches to find out what 10 tons per acre actually amounts to. 7.1405382 cubic inches spread over 144 square inches equals 0.049587 inches (7.1405382 / 144 = 0.049587). In layman’s terms, that is a hair under 1/20 of an inch (0.05 = 1/20). Here is one case where going metric may be easier to understand; there are 25.4 millimeters in an inch, so 1/20 of an inch is just a hair over one millimeter.
So, an erosion rate of 10 tons per acre works out to just a hair more than one millimeter deep! Every time the wind or rain or irrigation runoff removes a millimeter of your topsoil, you’ve just lost another 10 tons of topsoil per acre!
Kind of makes you stop and think, doesn’t it?
A conservation buffer along your drain ditches, rivers,
streams, or other tributaries can help keep that topsoil on your farm, where
it will do the most good. The Conservation Reserve Enhancement Program
(CREP) is one USDA program available through the Farm Service Agency, to
help producers develop functioning buffers. If you think you might be
interested, give me a call in the
(By the way: my old high school math teacher would be proud; she always wanted me to show my work!)