It takes 500 years to thousands of years to create an inch of topsoil! Why? Soil is often derived from rock. First, the rock has to be broken into small pieces. This happens by physical weathering…..things like freezing and thawing in the colder climates or chemical weathering in the warmer climates. Then, once cracks form and early succession plants can take hold, they continue to break up the rock into smaller pieces by root action and start to add organic matter. During that time, chemical weathering continues, turning the rock pieces into the sand, silt and clay particles that make up our soil; and plant action continues to add organic matter while also weathering the rock to soil.
Thinking of how soil forms, we can say that soil is formed by 5 factors. Parent material - or what type of rock or unconsolidated material we start with; Vegetation - the action of roots and the addition of organic matter; climate - which controls the physical and chemical weathering processes; topography - which controls the stability of the soil in the landscape; and lastly TIME - as mentioned above.
So it can take an incredible amount of time to form topsoil. Luckily two things are in our favor that allows us to have so much topsoil in this country. First, not all soil forms from solid rock. The glaciers of the north crushed the soil and did much of the physical weathering for us. Many soils following glaciers are only about 10,000 to 20,000 years old. Other soils have been deposited by moving water or wind and are already physically weathered and can form topsoil much faster. Second, outside of glaciated areas we have had a very long time to form soil. For example, in the panhandle of Florida, the landscape has been exposed to continuous weathering for close to 1,000,000 years. So while it takes much time to form topsoil, we have had that time.
What is the value of topsoil?
Another interesting fact about topsoil that may be of interest is the dollar value of topsoil to society. The soil is a source of nutrient for plants - both natural ecosystems and those systems we use to grow food and fiber. Topsoil filters all of our water that shows up in deep wells or surface waters, such as streams. Topsoil decomposes wastes and can detoxify harmful chemicals and topsoil is a reservoir of organism diversity - billions of organisms can be found in a gram of topsoil. Based on $26 per ton of topsoil, a typical U.S. topsoil would have an acre value to society of about $28,000. That would make the topsoil in the U.S. to have a combined value of about $64.3 trillion dollars (rough estimate) or about 6 times the U.S. national debt.
Data from: http://soils.usda.gov/sqi/concepts/soil_organic_matter/som_value.html
(dollar values from 1997 were recalculated to reflect the value in 2010 dollars)
Regardless of the monetary value of topsoil, the value is priceless as it forms the base of civilization – providing us with food and clean water.
