We often hear about and ask when the world is going to run out of water and fossil fuels.
But, it’s less common to think about topsoil.
With topsoil being so important to food production and the growth and production of other resources, we’ve put together a guide on how much of we need, and whether we might run out.
Summary – How Much Topsoil We Need, How Much Is Left, & Will We Run Out?
How Much We Need
Shallow topsoil is considered to be less than 10cms, and deep topsoil more than 10 cms.
Some sources indicate that 15cms (150 millimetres) is the minimal soil depth for agricultural production (5.9 inches)
Some prairie farmers are interested in the top 20 to 30cms of top soil
When looking at plants in general, they put their roots down in the first three to four feet (up to roughly 120cms)
How Much We Have Left
On average, the US has lost one third to one half of their topsoil on arable land since colonial days.
On average, topsoil has decreased from 18 inches to 8 inches (rough estimates).
The actual amount lost though varies from State to State
On a worldwide scale, it’s estimated half of the world’s topsoil has been lost in the last 150 years
When Will We Run Out?
Some estimates say the US will start running out of topsoil in some regions in 12 years time
Some estimates say that that the world has about 60 years of topsoil left
Overall though, there’s several factors that contribute to and have the ability to change replenishment and erosion rates.
For example, there’s different causes of topsoil degradation in different parts of the world (like intensive agriculture, water and wind weathering, and so on), and there’s different things that can be done to protect or amend soil.
How Much Topsoil Do We Need For Agriculture?
The minimal soil depth for agricultural production is 150 millimeters
The topsoil is the area where most of the nutrients accumulate, and therefore, the deeper it is, the better it is for agriculture
[In Australia], shallow topsoil [might be considered to be] less than 10cm, and deep topsoil [might be considered to be] more than 10cm
… for prairie farmers, all they’re really interested in is the top couple of metres, especially the top 20 to 30 centimetres [of topsoil]
Our agriculture depends on the top few feet of the earth’s surface
Most plants put their roots down to three or four feet (90 to 120 cm) so it’s the top metre or two that’s important for farmers.
How Much Topsoil Do We Have Left?
[over time, US topsoil levels have been] eroded … to [an average of] around eight inches
[note though that soil depth and levels vary from state to state and region to region – they aren’t the same everywhere]
… the thickness of topsoil varies widely, not only between different geographical regions but also within fields.
Topsoil depth ranges from 10 to 15 cm in the drier areas of Saskatchewan to 20 to 25 cm in more moist areas such as southern Manitoba, southeastern Saskatchewan and central Alberta.
How Much Topsoil Are We Losing Via Land Degradation & Soil Erosion?
Half of the topsoil on the planet has been lost in the last 150 years.
During the past 40 years nearly one-third of the world’s cropland (1.5 billion hectares) has been abandoned because of soil erosion and degradation.
So far, America’s farms have lost about half their soil organic matter [on arable land] since colonial days [when people arrived there]
When … European ancestors arrived [in the US], topsoil averaged around 18 inches in depth. With … intensive agricultural practices, [it’s been eroded] to around eight inches
Each day … 30 hundred-acre farms are being lost down the river . . . 10,000 farms a year . . . 15 tons of topsoil a second . . . a yearly loss of one ton for each person on earth.
… America have lost about one-third of … arable land since people arrived there.
At [the current rate] another third [will be lost] in the next dozen or so years, while the population almost doubles.
Consider this statement about topsoil loss rates in the US:
In the United States alone, soil disappears 10 times faster than it is naturally replenished, according to the Cornell study, at an estimated rate of nearly 1.7 billion tons of farmland alone per year.
How Quickly Does Topsoil Form/Replenish?
Read about soil replenishment rates in this guide.
But, in general, an average rate might be approximately 500 years to replace 25 millimeters (1 inch) of topsoil lost to erosion.
Other estimates say 1000 years for 3 cms, or at a rate of 0.24 tons per acre per year.
But, it can depend on the soil type, rock type, climate, environment and part of the world.
Will The World Run Out Of Topsoil In The Future?
To give an estimate to this question, we have to consider:
What depth of topsoil we need as an estimate for agricultural production
Current soil levels
Soil erosion/loss rates
Soil replenishment/reformation rates
Using the US as an example:
What depth of topsoil we need as an estimate for agricultural production – 150mm
Current average soil levels – eight inches (about 203mm)
Soil erosion/loss rates – will lose a third of that in the next 12 years or so, which is about 2 to 3 inches. This puts soil levels at around 5 inches, or 127mm
Soil replenishment/reformation rates – 1 inch every 500 years
From this quick breakdown (which is a very rough estimate – and shouldn’t be taken as a precise forecast), current soil level loss is unsustainable even over the next 12 years, and something more needs to be done to conserve top soil.
Sustainable farming practices such as planting cover crops, installing crop drainage systems and reduced till farming are just a few examples.
On current trends, the world has about 60 years of topsoil left