Topsoil is the uppermost layer of soil and is often the most fertile and productive soil in an ecosystem. While topsoil provides many benefits for plant growth, it also has some potential drawbacks that can affect agricultural productivity and environmental quality.
Erosion
One of the biggest risks associated with topsoil is erosion. Because topsoil is loose, granular, and located on the very surface, it can be easily eroded by wind and water. Excessive erosion can strip away the nutrient-rich upper layers of topsoil, reducing soil fertility and crop yields over time. There are several factors that can make topsoil erosion worse:
- Overgrazing – Animals remove protective plant cover
- Over-tilling – Destroys soil structure
- Lack of plant cover – Exposes soil to erosion
- Slopes – Increases water runoff velocity
Erosion removes topsoil faster than it can be naturally replenished in many areas, leading to a permanent loss of this valuable resource. Globally, erosion has damaged around 1/3 of the world’s arable land since the industrial revolution.
Runoff and Leaching
Nutrients in topsoil like nitrogen and phosphorus can be lost through leaching and surface runoff. Because topsoil is located at the surface, it can easily be washed away by excess rain or irrigation. Key nutrients then flow away from plant root zones and may end up in groundwater or surface waters, causing pollution. Some topsoil management practices that can worsen runoff and leaching include:
- Over-irrigation – More water than the soil can absorb
- Bare fallow – No plants to absorb nutrients
- Over-fertilization – Excess nutrients not utilized by crops
- Tilling sloped land – Promotes water runoff
Losses from leaching and runoff result in less available nutrients for crops. Farmers may then need to apply additional fertilizers to replenish nutrients, which further increases costs.
Compaction
Repeated traffic and heavy machinery can compact topsoil, which reduces pore space and increases bulk density. Compacted soils have higher mechanical impedance, which restricts root growth and limits the diffusion of air and water in soil. This can choke plant growth by limiting access to nutrients, water, and oxygen.
Tilling compacted soils provides only temporary relief. It destroys soil structure which leads to re-compaction. The best way to avoid compaction is to reduce traffic over fields and add organic matter to improve soil structure. Crop rotation with deep-rooted plants can also help break up compacted layers.
Loss of Organic Matter
Organic matter provides many essential benefits for soil health and crop growth. Soil organic matter:
- Stores nutrients and water
- Improves soil structure
- Increases cation exchange capacity
- Stores carbon
Regular tillage, erosion, leaching, and lack of additions can burn up soil organic matter over time. Conventional agriculture practices have caused many areas to lose 30-60% of their original topsoil organic matter content.
Cover crops, compost additions, reduced tillage, and mulches can help replenish soil organic matter. This improves moisture retention, nutrient availability, and stimulation of soil biological activity.
Loss of Biodiversity
Healthy topsoil is teeming with life. A single teaspoon of productive soil generally contains billions of microorganisms belonging to thousands of species. This includes bacteria, fungi, protozoa, nematodes, arthropods, and earthworms. But modern agriculture practices have reduced topsoil biodiversity in many areas through:
- Heavy pesticide/herbicide use
- Fertilizer runoff
- Monocropping
- Overgrazing
- Erosion
Higher microbial diversity improves topsoil resiliency and function. Adding organic matter, reducing chemical inputs, and using cover crops can restore topsoil biodiversity over time.
Poor Water Retention
Healthy topsoil has good porosity and structure that allows infiltration and retention of moisture. But poor soil management can degrade these properties, reducing water holding capacity. Practices that impair water retention include:
- Compaction
- Loss of organic matter
- Erosion
- Excessive tillage
As the water holding ability of topsoil declines, crops become more vulnerable to drought stress. Farmers may over-irrigate fields to compensate, which wastes water and can increase problems like erosion and nutrient leaching.
Acidification
Use of acid-forming nitrogen fertilizers and presence of acid rain can lower topsoil pH over time. Soil acidification generally reduces nutrient availability, increases solubility of toxic metals like aluminum, and impairs soil biology. Lime or ash applications can be used to increase pH if soils become too acidic. But prevention through reduced use of acid-forming nitrogen fertilizers is preferable.
Salinization
Irrigation water containing salts can leave residues in the soil as water evaporates, leading to a build-up of soluble salts over time. Excess salts make it harder for plants to take up water from the soil, causing crop losses. Salinization issues are especially common on arid lands with poor drainage. Improvement of drainage, reduction in irrigation frequency, and use of salt-tolerant crops can help manage saline soils.
Nutrient Deficiencies
Intensive cultivation without rebuilding soil fertility can deplete essential plant nutrients in topsoil like nitrogen, phosphorus, and potassium. Deficiencies reduce plant growth, crop quality, and yields. Routine soil testing helps monitor changes in fertility, and targeted fertilizer applications can replenish depleted nutrients. Using leguminous cover crops also increases available nitrogen.
However, over-fertilization to correct deficiencies can damage soil biology and contaminate waterways. Following integrated nutrient management plans tailored to specific soils and cropping systems optimizes topsoil fertility while minimizing environmental risks.
Toxicity
Build-up of toxic metals and metalloids in topsoil from industrial pollution, mining, and smelting can make soils unsafe for crop production in some areas. Metals like arsenic, cadmium, chromium, lead, and mercury are toxic to plants at high concentrations. Binding these contaminants reduces their solubility and uptake by plants. But remediation of severely contaminated topsoil is often extremely difficult and expensive.
Pesticides and hydrocarbon contamination from spills are other sources of topsoil toxicity. Careful management of chemical inputs and following all safety procedures during storage, transport, and application can help prevent accidental contamination.
Loss of Structure
Topsoil has granular structure produced by a mixture of sand, silt, and clay particles aggregated together with organic matter. This gives topsoil good aeration, drainage, and water retention. Excessive cultivation, erosion, low organic matter, and compaction degrade soil structure over time. Poor soil structure reduces root growth, nutrient availability, and water infiltration. Practices to improve aggregation include:
- Reduce tillage
- Add organic amendments
- Use cover crops
- Avoid over-grazing and machinery compaction
Surface Sealing and Crusting
Sealing and crusting occurs when the top layer of topsoil forms a relatively impermeable surface barrier. This is exacerbated by loss of organic matter, compaction, poor structure, and erosion of topsoil aggregates. Sealing prevents water infiltration and seedling emergence. Excess irrigation and rainfall can also trigger sealing and crusting. Strategies to reduce these issues include:
- Reduce erosion
- Add organic matter
- Improve soil structure
- Avoid over-irrigation
- Use cover crops
Respiration and Oxidation
Biological respiration and chemical oxidation within the topsoil layer continuously depletes organic matter and releases carbon dioxide. While inevitable, excessive loss of soil carbon degrades topsoil health. Practices that accelerate respiration and oxidation include:
- Over-tilling
- Low organic matter inputs
- Too much nitrogen fertilizer
- Warm, moist conditions
Improving soil aeration, reducing tillage, adding carbon inputs, and maintaining plant cover help replenish organic matter lost through oxidation.
Subsoil Compaction
Compaction damage can extend beyond the topsoil layer into the subsoil zone. This restricts root penetration into deeper subsoil with available water and nutrients. Plow pans are common examples of subsurface compaction. Deep compaction issues require specialized equipment like ripper-shanks to break up dense subsurface layers.
Waterlogging
Poor drainage can lead to waterlogged topsoil, especially in low-lying areas. Excess water displaces oxygen in soil pores required by plant roots and soil organisms. Prolonged saturation causes anaerobic conditions that alter soil chemistry and produce phytotoxic compounds. Drainage control through subsurface tile installation and drainage ditches helps prevent waterlogging damage to topsoil.
Freeze-Thaw Cycles
In temperate and boreal regions, repeated freezing and thawing of topsoil in winter can degrade soil structure. Ice lens formation can physically disrupt and pry apart soil aggregates. Maintaining plant cover and minimizing tillage in late fall helps protect topsoil from freeze-thaw damage.
Desertification
In drylands, excessive irrigation, overgrazing, erosion, and declining organic matter can trigger desertification. This irreversible degradation turns productive grasslands or croplands into desert conditions. Preventing loss of protective plant cover is key to halting desertification. Other desertification control measures include:
- Reduce tillage
- Improve irrigation practices
- Re-vegetate bare areas
- Control grazing
Urbanization
Expanding urban areas bury fertile topsoil under roads, buildings, and other infrastructure. Most covered topsoil becomes effectively lost for food production. Urban planning to protect croplands and regulations limiting construction on prime farmland can preserve topsoil.
Inadequate Organic Matter Inputs
Regular additions of organic materials like compost, cover crops, and animal manure are needed to maintain soil organic matter levels. Insufficient organic matter inputs lead to loss of fertility, poorer water retention, and reduced carbon storage. Diversified crop rotations with high biomass cover crops and incorporation of crop residues help return more organic matter to the soil.
Declining Soil Depth
Ongoing erosion, degradation, and insufficient topsoil formation can produce an overall decline in topsoil depth over time. For example, Iowa has lost an average of 6 to 8 inches of topsoil depth since settlement as a result of erosion. Shallower topsoil depth provides less rooting zone depth for crops. Preventing erosion and improving subsoil conditions using deep rooting cover crops helps maintain topsoil depth.
Heavy Metal Contamination
Topsoil can become contaminated by heavy metals through industrial pollution, incinerator emissions, mining, smelting, and excessive sewage sludge application. Excessive accumulation of heavy metals like cadmium, zinc, nickel, lead, mercury, and chromium can cause toxicity to plants. Preventing topsoil contamination is critical, as remediation options are very limited.
Conclusion
While topsoil is a valuable resource, it faces many threats from erosion, degradation, contamination, and depletion. Careful management through conservation tillage, cover cropping, integrated fertility management, controlled traffic, and addition of organic matter can help ensure the long-term sustainability and productivity of topsoil.
However, there are no quick fixes for degraded topsoil. Preventing damage and implementing protective practices before problems occur is critical. We all depend on finite topsoil resources to sustain food production and civilization. Being good stewards will preserve topsoil for future generations.
