How Much Topsoil Do We Need For Agriculture, How Much Is Left, & Will We Run Out?

When it comes to natural resources, it might be more common to question when resources like water and fossil fuels might run out.

But, it might be less common to think about topsoil as a scarce resource.

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, how much is left, erosion vs replenishment (formation) rates, and whether we might run out in the future.

 

Summary – How Much Topsoil We Need, How Much Is Left, & Will We Run Out? 

What Is Topsoil?

Topsoil is usually the top layer of soil (but can sometimes be covered by a layer of humus), and it is made of mineral from the parent material underneath, as well as broken down organic matter. 

The soils.org resource has a good cross section diagram showing the different layers of soil, including topsoil.

 

How Much Topsoil We Need

Topsoil that meets certain criteria (such as being fertile soil) is required to grow crops – especially if the topsoil is to be productive and high yielding

The depth of topsoil is usually an important part of the criteria, because this is where most of the nutrients might be found for agricultural production

Different sources have different indications of what adequate topsoil depth is

Some sources say that the minimum amount of topsoil required for agricultural production is 10 to 30 cms (anything below 10 cms is shallow topsoil), and others say that the top 1 to 2 metres of topsoil is important for some farmers (as some plants can put their roots down three to four feet)

 

Soil Health & Soil Fertility

We don’t discuss this criteria for topsoil in the guide below, but in addition to soil depth, soil health and soil fertility are important considerations for the productivity of the soil

Having said this, synthetic fertilizers and other additives (like organic matter) can be added to soil material to increase productivity and yield too 

 

How Much Topsoil We Have Left

Average depth of topsoil is usually the way topsoil left is expressed.

It’s worth noting that average topsoil depth can different between regions (States and Provinces) within a country, but also across an individual plot of land.

In the US, some sources indicate an average of 8 inches of topsoil is left (but this differs between States).

In Canada, average topsoil depth varies anywhere between 10 to 25 cms depending on the area.

 

How Much Topsoil Has Been Lost Worldwide, Over How Long?

When looking purely at topsoil loss, one estimate says half the world’s topsoil has been lost in the last 150 years.

When looking at the amount of cropland abandoned because of soil erosion/degradation, about a third of cropland might have been abandoned over the last 40 years.

Intensive farming is a major reason for loss of soil (but other reasons like wind and water weathering, just as two example, can play a part)

 

How Much Topsoil Has Been Lost In The United States, Over How Long?

Some of the estimates for the United States’ soil loss are:

Half the soil has been lost since colonial days

Average topsoil depth across America might have decreased from 18 inches since farming first started, to 8 inches today

Specific States have their own average topsoil loss amounts. In Iowa for example, in the 20th century, average topsoil depth went from 14-18 inches, to 6 to 8 inches

About a third of cropland has been lost to soil erosion over time

 

Soil Erosion vs Soil Formation Rates Worldwide

Soil erosion rates exceed soil formation rates by 10 to 20 fold on average across most continents over the last few decades.

 

Soil Erosion vs Soil Renewal Rates In The United States

Soil might be eroded 10x faster than it replenishes at the moment.

However, some sources indicate that water and wind erosion decreased almost 50% between 1982 and 2007

 

Soil Erosion vs Soil Renewal Rates In India

As of 2006, India was losing soil 30 to 40 times faster than it was replenishing

 

Soil Erosion vs Soil Renewal Rates In China

As of 2006, China was losing soil 30 to 40 times faster than it was replenishing

 

Soil Erosion vs Soil Renewal Rates In Australia

Soil erosion rates compared to formation rates vary from State to State.

Erosion rates are generally rated poor to good (without being very good), and formation rates are below the global average.

Soil protection measures have been implemented in some States.

 

When Will We Run Out Of Topsoil?

Some estimates say that the world has about 60 years of topsoil left

In the United States, about a third of topsoil is forecast to be lost over the next 12 years, but the population size is forecast to increase as well. 

In the US, another third of topsoil loss would put average topsoil depth at around 5.5 inches, or 13.97cms. This is close to the 10cms and below of topsoil depth that is classified as shallow topsoil

But, there’s various factors that can impact topsoil levels, and the pressure put on topsoil resource (such as food demand, intensity of farming, level of effort put into maintaining soil health, topsoil degradation in different parts of the world, and more)

Other variables to future topsoil loss might include future breakthroughs in soil technology and practices, how effective soil amendments and organic matter can be in increasing soil health, how effective importing soil can be, and so on.

One of the issues with topsoil degradation and loss is that not only is it difficult to see the depth of the soil with the naked eye, but it’s difficult to see other things like soil health, soil structure, soil chemical and biological composition, and so on. So, it’s something that needs intentional ongoing assessment and observation.

 

How Quickly Topsoil Replenishes

Topsoil formation and replenish can vary over the world, and is related to factors like the soil type, rock type, climate, environment and other geographical factors.

Global averages for soil formation range anywhere from 3cm per 1000 years, up to 11.4cms per 1000 years.

Having said that, soil formation rates vary country to country, and vn within different regions in a country.

 

Is Topsoil A Renewable Resource?

Under natural conditions, topsoil isn’t a renewable resource at current erosion and degradation rates

 

How Are We Currently Producing At Scale In Agriculture With Topsoil Being Eroded?

Intensive agricultural practices, modern agricultural technology, and the use of synthetic fertilizers that provide nutrients to the soil (along with the use of synthetic pesticides) help boost and maintain the productivity of modern agriculture.

Various sources though say that this approach to agriculture is not sustainable long term, and more focus needs to be put on maintaining topsoil levels and health.

 

What Is Agricultural Topsoil?

It’s usually the more fertile upper/top layer of soil (containing decomposed organic matter, nutrients & minerals) used for crops and food production, amongst other things

 

How Much Topsoil Do We Need For Agriculture?

The amount of topsoil we refer to here is the depth of topsoil.

The reason for this is as vro.agriculture.vic.gov.au describes – ‘The topsoil is the area where most of the nutrients accumulate, and therefore, the deeper it is, the better it is for agriculture’

Different sources have different estimates on the depth of topsoil is important.

The minimum amount of topsoil might be 10 to 30 cms of topsoil, and anything less than that considered as ‘shallow topsoil’.

Some plants though can put their roots down up to 2 metres, or 120cms, so, the top metre to two metres of topsoil might be important to some agricultural producers. 

 

The minimal soil depth for agricultural production is 150 millimeters (fewresources.org)

 

[In Australia], shallow topsoil [might be considered to be] less than 10cm, and deep topsoil [might be considered to be] more than 10cm (vro.agriculture.vic.gov.au)

 

From producer.com:

… 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?

The amount of topsoil we have left can be measured by looking at the average topsoil depth we used to have, and looking at the average topsoil depth now, and seeing how much topsoil has been lost.

In the US, average topsoil loss varies between States, but a national average might be around 8 inches.

In Canada, anywhere from 10 to 25cms of topsoil is left, depending on the area.

It’s worth noting that in any country, average topsoil depth will vary both between regions (States and provinces), but also across an individual plot of land.

 

United States

[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]

– motherearthnews.com

 

Canada

… 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.

– producer.com

 

How Much Topsoil Has Been Lost Worldwide, Over How Long?

When looking purely at topsoil loss, one estimate says half the world’s topsoil has been lost in the last 150 years.

When looking at the amount of cropland abandoned because of soil erosion/degradation, about a third of cropland might have been abandoned over the last 40 years.

 

Half of the topsoil on the planet has been lost in the last 150 years (worldwildlife.org)

 

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.

As a result of erosion over the past 40 years, 30 percent of the world’s arable land has become unproductive

– fewresources.org

 

… a third of the world’s soil is now moderately to highly degraded (bbc.com)

 

… fertile soil is being lost at the rate of 24bn tonnes a year (theguardian.com)

 

We are losing 30 soccer fields of soil every minute, mostly due to intensive farming (scientificamerican.com)

 

How Much Soil Has Been Lost In The United States, Over How Long?

Some of the estimates for the United States’ soil loss are:

– Half the soil has been lost since colonial days

– Average topsoil depth across America might have decreased from 18 inches to 8 inches today

– Specific States have their own average topsoil loss amounts. We list Iowa’s below

– About a third of cropland has been lost to soil erosion

 

So far, America’s farms have lost about half their soil organic matter since colonial days (scientificamerican.com)

 

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.

– motherearthnews.com

 

[America has] lost roughly half the topsoil since farming started, but the loss is [gradual enough that is can’t be seen]

The average soil loss rate [in the US] is 5.8 tons per acre per year [and] Most farmers … believe their erosion loss is a fraction of that amount.

[the lost yield resulting from this loss of soil is] about 15 bushels per acre per year of lost potential.

– farmprogress.com

 

The average topsoil depth in Iowa decreased from around 14-18 inches (35-45cm) at the start of the 20th Century to 6-8 inches (15-20cm) by its end (bbc.com)

 

The United States alone loses almost 3 tons of topsoil per acre per year (wikipedia.org)

 

How Much Soil Has Been Lost In The Philippines, Over How Long?

In the Philippines … it is estimated that soil erosion carries away a volume of soil equivalent to one metre deep over 200 000 hectares every year (fao.org)

 

Soil Erosion vs Formation Rates Worldwide

Worldwide, erosion rates exceed formation rates according to certain measures.

 

Global rates of soil erosion have been exceeding those of new soil formation by 10- and 20-fold on most continents of the world in the last few decades (sciencedirect.com)

 

Soil Erosion vs Formation Rates In The United States

Soil might be eroded 10x faster than it replenishes at the moment.

However, some sources indicate that water and wind erosion decreased almost 50% between 1982 and 2007

 

As of 2006 … The United States [was] losing soil 10 times faster … than the natural replenishment rate (fewresources.org)

 

In the United States alone, soil disappears 10 times faster than it is naturally replenished … at an estimated rate of nearly 1.7 billion tons of farmland alone per year (theweek.com)

 

Between 1982 and 2007, soil erosion on U.S. cropland decreased 43%. 

[Specifically, water/sheet and rill erosion, and wind erosion decreased]

– nrcs.usda.gov

 

Soil Erosion vs Formation Rates In China

As of 2006 … China [was] losing soil 30 to 40 times faster … than the natural replenishment rate (fewresources.org)

 

Soil Erosion vs Formation Rates In India

As of 2006 … India [was] losing soil 30 to 40 times faster … than the natural replenishment rate (fewresources.org)

 

Soil Erosion vs Formation Rates In Australia

Soil erosion by water exceeds formation rates in Australia, whilst wind erosion rates vary.

Generally, Australia’s soil formation rates are below the global average, and soil erosion rates are somewhere between poor to good, without being very good.

Soil erosion rates have decreased, and soil protection measures have increased in some States.

 

Soil formation rates in some parts of Australia are below global averages

Current rates of soil erosion by water across much of Australia now exceed soil formation rates [whilst] Wind erosion rates can differ with annual climatic variation

Generally, soil erosion rates across Australia are rated as poor to good, on a scale from very poor to very good

Soil erosion rates and soil protection measures have improved in Victoria and South Australia over the last few decades

– soe.environment.gov.au

 

As can be seen from the data above, it’s possible for a country or region in the world to decrease their soil erosion rates via soil conservation measures, but still be losing soil at a greater rate than the soil replacement rate.

 

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 depth

– Soil erosion/loss rates

– Soil replenishment/reformation rates

… a projection can be made forward based on these key indicators.

But, other factors like population growth, food consumption and demand, a change in lifestyles, plus more, can change the results of projections.

Sustainable farming practices such as planting cover crops, installing crop drainage systems and reduced till farming might be a few examples of ways to address better soil health. They can also change projections going forward.

bbc.com also mentions that these things might help with topsoil soil and overall soil health – perennial crops (that can last up to 4 to 5 years without tilling or replanting, cover crops and intercropping (which can help the soil hold together, and via photosynthesis keep soil systems alive), beneficial microbes that can feed on the nutrients and minerals deposited by phosphorus fertilizers for example, eco engineering degraded soils via projects like Biosphere 2 in Arizona in the US, and more.

bbc.com also mentions that in Iowa, prairie strips, and cover crops have delivered a noticeable difference to soil health, and reduced certain types of soil erosion.

 

Using the US as an example:

What depth of topsoil we need as an estimate for agricultural production – 10cms is classified as shallow topsoil. So, we’d at least need more than that 

Current average soil levels – with … intensive agricultural practices, [American topsoil has been eroded] to around eight inches (about 203mm) [as an average topsoil depth]

Soil erosion/loss rates – At [the current rate] another third [will be lost] in the next dozen or so years, while the population almost doubles (motherearthnews.com). One third of 8 is between 2.5 to 3 inches, which would mean the US would have around 5.5 inches left after the next 12 years or so. Converting to cms, that’s 13.97 cms. That is getting close to the 10cm mark we identified above in this guide as being shallow topsoil

Soil replenishment/reformation rates – 3 to 11.4cms every 1000 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 getting very close to unsustainable even over the next 12 years, and something more might need to be done to conserve top soil.

 

On a worldwide scale:

On current trends, the world has about 60 years of topsoil left (wikipedia.org)

… if current rates of degradation continue all of the world’s top soil could be gone within 60 years (scientificamerican.com)

 

How Does Fertile Topsoil Form Naturally?

The evolution of a fertile soil, either from parent rock material or from a degraded system, is mediated by a complex set of tightly interwoven hydrological, geochemical, geomorphic and biological processes (bbc.com)

 

How Quickly Does Topsoil Naturally Form/Replenish? (On Average)

Topsoil formation and replenish can vary over the world, and is related to factors like the soil type, rock type, climate, environment and other geographical factors.

Global averages for soil formation range anywhere from 3cm per 1000 years, up to 11.4cms per 1000 years.

Having said that, soil formation rates vary country to country, and vn within different regions in a country.

 

The current high erosion rates throughout the world are of great concern because of the slow rate of topsoil renewal;

… it takes approximately 500 years for 2.5 cm layer of fertile topsoil to form under agricultural conditions

– globalagriculture.org

 

Generating three centimeters of top soil takes 1,000 years (scientificamerican.com)

 

Under the best case scenario, soil can only rebuild at a rate of 0.24 tons per acre per year. (farmprogress.com)

 

[Parts of Australia are examples of where soil formation fall below the average renewal rate:]

Soil formation from weathering rock is slower [than dune sands in moist environments], and varies with the environment and rock type

An average of about 10 millimetres per 1000 years is typical in New South Wales, increasing to about 75 millimetres per 1000 years in Arnhem Land, Northern Territory.

These soil formation rates are low compared with the estimated global average of 114 millimetres per 1000 years

– soe.environment.gov.au

 

Is Agricultural Topsoil A Renewable Resource?

Firstly, What Is A Renewable Resource?

A renewable resource is a natural resource which will replenish to replace the portion depleted by usage and consumption, either through natural reproduction or other recurring processes in a finite amount of time in a human time scale (wikipedia.org)

 

So, Is Agricultural Topsoil Renewable?

Topsoil is natural, as it’s made of dirt and decomposed organic matter.

But, under natural conditions, agricultural topsoil isn’t a renewable resource because it takes up to thousands of years on average to replenish 3 or more cms of fertile topsoil (although formation rates can differ between countries)

 

Consider this from fewresources.org:

It takes approximately 500 years to replace 25 millimeters (1 inch) of topsoil lost to erosion.

The minimal soil depth for agricultural production is 150 millimeters [5.9 inches]

From this perspective, productive fertile soil is a nonrenewable, endangered ecosystem.

 

What About Commercial Topsoil & Other Forms Of Man Made Topsoil?

There’s a difference between commercial topsoil which undergoes modification, or has additives added to it by humans, and soil or topsoil that develops/forms naturally.

Some types of soil can be made more fertile, or have their soil quality and soil health improved with organic matter, fertilizer, and other additives.

Some sustainable soil practices may also increase soil fertility, health and quality too.

But, in a natural setting, topsoil won’t replenish and reform quickly by itself.

There is certainly a difference between adding organic matter and nutrients to soil, and the topsoil material replenishing naturally.

 

FAO.org outlines the difference between replacing nutrients and soil organic matter vs replacing the actual topsoil material itself:

The effects of water and wind erosion are largely irreversible

Although plant nutrients and soil organic matter may be replaced, to replace the actual loss of soil material would require taking the soil out of use for many thousands of years, an impractical course of action.

In other cases, land degradation is reversible: soils with reduced organic matter can be restored by additions of plant residues, degraded pastures may recover under improved range management.

… land can only be restored by taking it out of productive use for some years, as in reclamation forestry

 

If Topsoil Isn’t Renewable, How Are We Currently Producing Agricultural Products Like Food At Scale?

Industrial agriculture has made it possible to produce a lot of food, despite soil erosion/soil loss, and slow soil replenishment rates.

Industrial farming machinery, as well as agricultural chemicals like fertilizer and pesticides/herbicides have boosted yields in the last half a century.

But, yield for several key crops are starting to signs of decreasing annual yields.

 

Sources

1. http://vro.agriculture.vic.gov.au/dpi/vro/vrosite.nsf/pages/soil_health_land_class_pdf/$FILE/step_10.pdf

2. https://www.producer.com/2010/12/agriculture-rests-on-two-feet-of-soil/

3. https://www.motherearthnews.com/nature-and-environment/topsoil-loss-zmaz80mjzraw 

4. https://blogs.scientificamerican.com/observations/to-feed-the-world-sustainably-repair-the-soil/ 

5. https://www.fewresources.org/soil-science-and-society-were-running-out-of-dirt.html 

6. https://www.worldwildlife.org/threats/soil-erosion-and-degradation 

7. https://theweek.com/articles/554677/america-running-soil 

8. https://en.wikipedia.org/wiki/Topsoil

9. http://www.fao.org/3/v4360e/V4360E03.htm

10. https://study.com/academy/answer/is-topsoil-a-renewable-resource.html  

11. https://en.wikipedia.org/wiki/Renewable_resource 

12. https://soe.environment.gov.au/theme/land/topic/2016/soil-formation-and-erosion

13. https://www.nrcs.usda.gov/wps/portal/nrcs/detail/national/soils/?cid=stelprdb1041887 

14. http://kidspressmagazine.com/science-for-kids/misc/misc/soil-horizons.html

15. https://www.farmprogress.com/soil-health/economics-soil-loss 

16. https://www.scientificamerican.com/article/only-60-years-of-farming-left-if-soil-degradation-continues/ 

17. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/land-degradation

18. https://www.sciencedirect.com/science/article/pii/S0143622814002793

19. https://www.theguardian.com/environment/2017/sep/12/third-of-earths-soil-acutely-degraded-due-to-agriculture-study

20. http://www.fao.org/3/v9909e/v9909e02.htm

21. https://www.bbc.com/future/bespoke/follow-the-food/why-soil-is-disappearing-from-farms/

22. https://www.soils.org/files/sssa/iys/november-soils-overview.pdf

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