Topsoil is the uppermost layer of soil and is vitally important for plant growth and agriculture. However, extensive topsoil loss has occurred globally due to a variety of factors, posing a major threat to food security and the environment. Identifying the greatest causes of topsoil loss is critical in order to implement preventative measures.
What is topsoil?
Topsoil is the surface layer of soil composed of mineral particles, organic matter, fungi, bacteria and other organisms. It is the most fertile part of the soil and contains much of the essential nutrients, microorganisms and organic matter vital for plant growth and agriculture. The topsoil layer typically ranges from 2 to 10 inches (5-25cm) in depth.
Some key facts about topsoil:
– It takes approximately 500 years for 1 inch of topsoil to form under normal agricultural conditions. This means topsoil is an extremely slow renewable resource.
– Healthy topsoil contains around 5% organic matter by weight. Organic matter helps bind soil particles together and retains moisture and nutrients.
– Topsoil contains much higher concentrations of nutrients like nitrogen, phosphorus and potassium compared to the subsoil layers underneath it. Plant roots predominantly grow in the topsoil zone.
– Topsoil is home to billions of beneficial microorganisms per handful that play vital roles in the soil ecosystem. These include bacteria, fungi, protozoa, nematodes and arthropods.
– Topsoil acts as a buffer and filter for the groundwater below it. It can help remove impurities and prevent toxins from leaching into groundwater supplies.
In summary, topsoil is a thin but extremely important layer critical for agriculture, food production and the health of the overall ecosystem. Once lost due to erosion or exhaustion, topsoil is difficult if not impossible to regenerate within a human lifespan.
Why is topsoil loss a concern?
Topsoil loss threatens food production and security. With the expanding global population and changes in diet driving increased food demand, reduced soil fertility and crop yields risk exacerbating food shortages and malnutrition in susceptible regions.
Additionally, topsoil loss can:
– Reduce the ability of soil to absorb and retain water, increasing the risks of drought, flooding and desertification.
– Harm soil biodiversity and ecosystem functioning.
– Release carbon stored in the soil, contributing to climate change.
– Pollute waterways with sediment and contaminants.
– Necessitate increased use of fertilizers to maintain yields.
– Eventually lead to the inability to farm the land altogether if the subsoil layers become exposed.
With these wide-ranging impacts, extensive topsoil loss can threaten whole societies that depend on agriculture and the natural environment. Estimates suggest that approximately 40% of agricultural soils worldwide are already degraded, with erosion a leading cause of degradation.
What causes topsoil loss?
The main causes of topsoil loss include:
Erosion
Erosion is the detachment and movement of soil particles from wind, rain, or irrigation water. It is considered the primary cause of topsoil loss globally. Factors influencing erosion rates include:
– Rainfall intensity and wind speed
– Slope gradient – steeper slopes increase erosion
– Vegetative cover – bare soils are far more prone to erosion
– Soil structure – soils with poor aggregation erode more readily
– Farming practices – tilling, residue burning, monocropping
Water erosion occurs when rain or irrigation detaches soil and transports it downslope. The splash from raindrop impacts dislodges particles, while the shallow sheet flow generated transports the particles downslope. Rill and gully erosion are more concentrated forms of water erosion occurring in small channels.
Wind erosion is also a major problem, especially in flat, dry regions. Soil particles detached by wind action are transported horizontally offsite, sometimes over long distances.
Globally, water erosion accounts for 60% of total erosion, while wind erosion accounts for 36%. The remaining 4% is attributed to tillage erosion.
Unsustainable farming practices
Many modern industrial farming practices accelerate erosion and degrade topsoil over the long term:
– Tilling and plowing disturb soil structure and break down soil aggregates, reducing stability. This leaves soil loose and prone to erosion.
– Overgrazing compacts soils, reduces vegetation cover, and exposes soil to erosion.
– Monocropping and reduced fallow periods deplete organic matter and nutrients over successive seasons.
– Fertilizer and pesticide use can reduce beneficial soil organisms and organic matter.
– Burning crop residues after harvest removes protective cover and releases carbon.
Sustainable practices like no-till farming, crop rotations, cover crops, conservation tillage, and controlled grazing help protect topsoil. However, adoption of such practices remains limited on many industrial farms.
Deforestation and vegetation clearing
Removing natural vegetation exposes the soil to weathering and erosion. Forests or grasslands are converted to cropland or used for fuelwood, timber and development. Brazil, Indonesia and parts of Southeast Asia have experienced high rates of topsoil loss in deforested areas.
Construction and development
Construction activities like cut and fill operations, grading, excavations and soil compaction result in localized topsoil disturbance and erosion. Soil stabilization measures are often required to protect topsoil.
In urban areas, the coverage of pervious surfaces with impervious materials such as concrete and asphalt impedes rainfall infiltration and removes vegetation cover. This increases stormwater runoff velocity, transporting topsoil particles into drains and waterways.
Mining
Surface mining techniques like strip mining and mountaintop removal severely disrupt vegetation, topography and topsoil systems. Surfaces are scraped clean of topsoil to reach subsurface coal and mineral deposits, destroying existing soil profiles. Remediation to restore productive soils once mining ceases is extremely difficult.
Off-road vehicle use
The use of off-road vehicles like dirt bikes and four-wheel drivescompact soil and shred protective vegetation mats. This leads to accelerated wind and water erosion, especially in fragile environments like deserts. Concentrated recreational off-road vehicle use has caused extensive topsoil damage and erosion in localized areas. Strict regulation is necessary to prevent uncontrolled cross-country travel.
Measuring topsoil loss
There are several techniques used for measuring topsoil loss rates:
– **Runoff plots** – bounded plots of land are monitored for a period under natural or simulated rainfall. Sediment collected from runoff is measured to calculate actual soil loss.
– **Cs-137 analysis** – cesium-137 fallout isotopes strongly bind to fine particles within soil. Comparing Cs-137 inventories across a landscape allows estimation of erosion and deposition over decades.
– **Historical records** – old soil surveys, aerial photos and landuse records help estimate changes in soil depth over time.
– **Modeling** – complex computer models like the Revised Universal Soil Loss Equation (RUSLE) calculate theoretical soil loss based on climate, topography, soil type, land use and management practices.
Globally, average annual topsoil loss rates are estimated at ~15 tons/hectare but vary considerably based on region, climate and land use. Some key statistics include:
– North America – Average loss rate of 17 tons/hectare/year
– Australia – Average loss rate of 4 tons/hectare/year
– Asia – Average loss rate of 30 tons/hectare/year but over 100 tons/hectare/year in parts of China.
These represent average values across different land use types. Actual erosion rates on cropland are often much higher.
Which regions experience the greatest topsoil loss?
Some regions with very high rates of topsoil loss include:
Midwestern United States
The Midwestern corn belt has experienced severe topsoil loss since settlement by European farmers in the 19th century. Deep fertile prairie soils that took thousands of years to form have been depleted over just a few generations of intensive agriculture. Water and wind erosion are high under conventional tillage practices on sloping lands without cover crops. Erosion is typically highest in spring when massive fields are bare. No-till adoption has increased but more sustainable practices are needed to halt topsoil loss.
Average topsoil loss rates of up to 26 tons per hectare per year have been measured in the Midwest.
China
Massive erosion has occurred in China due to high population pressure on the land. By the 1990s, nearly 20% of China’s farmland was affected by erosion with average losses of over 100 tons/hectare/year. The Loess Plateau region of North Central China has been particularly impacted. Massive gully systems up to 500 feet wide and 100 feet deep have formed. China has made restoration of soil health a priority under initiatives like Grain for Green. Ongoing soil conservation efforts include terracing, no-till, cover crops and reforestation.
India
It is estimated that India loses over 5 billion metric tons of topsoil every year to erosion, much of it from agricultural lands. This equates to an average loss rate of 16 tons per hectare per year. However, rates exceeding 100 tons/hectare/year have been measured in some regions. Northern India faces significant erosion due to steeply sloping croplands. Wind erosion also impacts western states like Rajasthan.
Australia
The intensive cultivation of marginal lands in Australia combined with drought and sparse native vegetation cover has accelerated wind erosion and dust storms in regions like New South Wales, Queensland and South Australia. Overgrazing and introduction of invasive species have degraded fragile semi-arid ecosystems. Salinity has also reduced productivity as rising saline groundwater tables deposit salt onto the soil surface. Careful land management focused on soil health is now practiced more widely.
Sahel Region of Africa
Hot and dry conditions coupled with recurrent drought makes the Sahel prone to wind erosion. Desertification has encroached southward due to climate change and human pressures like intensive grazing, wood harvesting and inappropriate irrigation. Maintaining vegetative cover through practices like agroforestry is a priority for reducing erosion.
Southeast Asia
Deforestation has exposed soils to heavy monsoonal rains in Indonesia, Malaysia, Thailand, the Philippines and other nations. An estimated 2 billion metric tons of topsoil are lost every year in this region due to unsustainable logging, palm oil plantations and slash-and-burn clearing for farming. Stricter protections on remaining forests paired with erosion control on cleared lands is improving conditions.
Worldwide trends
The United Nations Food and Agriculture Organization provides global topsoil loss estimates:
| Land Use | Estimated annual topsoil loss (tons/hectare) |
| Virgin forest and natural land | 0.004 – 0.2 |
| Managed forest | 0.01 – 1 |
| Permanent pasture | 0.5 – 2 |
| Arable cropland | 6 – 70 |
Key observations:
– Topsoil loss under native vegetation is typically very low. Undisturbed forests, wetlands and grasslands retain soil well.
– Sustainably managed lands with permanent cover like pasture and managed forests have relatively low erosion rates.
– Conventional agricultural practices lead to soil loss up to 175 times higher than background rates.
As seen above, conversion to cropland often dramatically increases erosion far beyond natural baselines. However globally, cropland occupies only about 12% of land area. Grazing lands occupy 26%, forests 31% and the remainder is mixed use and barren land.
So while cropland has by far the highest erosion rates per unit area, the actual contribution to total global topsoil loss is estimated as:
– Grazing lands – 52%
– Croplands – 33%
– Forests – 15%
This demonstrates that the extensive global area of pasture is likely the single greatest land use contributor to total topsoil loss worldwide. While erosion rates are lower than cropland per hectare, the sheer global area results in pasture contributing the majority of total eroded topsoil. This points to the huge role that sustainable grazing management must play in reversing global topsoil loss.
Forestry, agriculture, mining and development interests must be balanced to protect remaining soil resources. Continuing population growth and food demand will place increasing pressure on vulnerable soils.
Projections and potential solutions
Under business-as-usual projections, topsoil loss is expected to continue increasing this century due to factors like:
– Expansion of agricultural lands, particularly in the developing world.
– Shortening fallow periods and continuous cropping.
– Increasing off-road vehicle recreational use in fragile environments.
– Additional pressure for biofuels, livestock fodder and timber.
Climate change may exacerbate soil loss by altering temperature and rainfall patterns. More extreme floods and droughts overload natural erosion resistance mechanisms.
However, global topsoil loss is not a foregone conclusion. Many proven solutions exist to reduce erosion and regenerate soil health, including:
**Croplands**
– No-till and conservation tillage
– Cover cropping and green manures
– Strip cropping, terraces and contour farming
– Improved irrigation management
– Integrated pest management
– Crop rotations and polycultures
**Grazing Lands**
– Holistic planned grazing
– Silvopasture and pasture cropping
– Riparian buffers
– Reincorporation of forage legumes
– Off-stream watering infrastructure
**General**
– Agroforestry and permaculture
– Rainwater harvesting earthworks
– Promotion of soil macrobiota
– Reduction of bare fallows
– Limited vehicular access in wild lands
Adoption of such regenerative land management practices globally offers hope for stabilization of remaining topsoil resources while supporting food production. However, action is urgently needed before damage becomes irreversible.
Conclusion
Topsoil is an extremely precious resource that is rapidly eroding worldwide. The greatest single contributor to global topsoil loss is likely grazing lands, which account for over half of total eroded soil due to their vast collective area. However, on a per hectare basis, annual soil loss under conventional agriculture is up to 175 times higher than natural rates.
Erosion, unsustainable farming practices, deforestation, mining and development all deplete topsoil. But solutions exist in the form of regenerative agriculture, holistic grazing, agroforestry and other sustainable land management practices. Widespread adoption of such techniques offers hope for feeding the world while conserving remaining topsoil for future generations. Timely action is essential as soil lost now may take thousands of years to regenerate.