Editor’s Note: Guest author Dr. Linda K. Blum earned her BS degree in Forest Soils, her MS degree in Forest Soils Microbiology from Michigan Technological University, and completed her PhD in Agronomy at Cornell University in 1980. Her research focused on the interaction of nitrogen-fixing soil bacteria, pathogenic root-fungi, and protozoans in bean crops in developing countries. After joining the faculty at the University of Virginia in 1984, she continued to explore questions about plant-soil-microorganism interactions. Her passion for vegetable gardening is lifelong. Her first memory of gardening is planting tomatoes with her father when she was four years old. She became an Extension Master Gardener in spring 2021.
Please Note: The reference numbers contained in parentheses refer to the numbered citations at the end of the article.
The soil beneath our feet is one of humanity’s most precious resources. Yet, humans are destroying this resource at rates faster than ever before in human history (1). Soil erosion was a major factor in the decline of past civilizations following deforestation in the Middle East, Greece, Rome, Mesoamerica, Norse Greenland, Easter Island, and North Africa (2, 3). Currently, soil erosion is a more pressing problem than at any time in history as human populations continue to increase at dramatic rates and desertification of formerly productive land continues at unprecedented rates. In the last 150 years, half the topsoil on the globe has been lost due to erosion (4). Global average rates of soil erosion are 5 to 7 US tons per acre per year (5), the equivalent of losing about 0.5 inches of soil per year from the surface of the World’s agricultural lands. This rate of loss is more rapid than the rate of soil formation, and therefore, poses a long-term threat to soils and the services they provide (2).
Evidence of Erosion Throughout the Landscape
Soil erosion is the physical process that wears away the soil surface by the action of water, wind, ice, and gravity. Erosion by water is the dominant mechanism causing erosion. When rain drops fall on soil, the droplets dislodge soil particles and splash them into the air. Splash erosion makes the soil particles vulnerable to water flowing across the surface of the land. (6). When soil particles are carried by water flowing across the surface of the soil, the erosion is called sheet erosion. As the water carrying the soil particles flows downhill, it concentrates into small channels called rills where the water velocity increases, and more soil particles are eroded from the surface in a process called rill erosion.
Rills are common, especially on bare land (1). Once rills have formed, substantial amounts of erosion may occur (7). Rills can be further concentrated as water velocity increases and cuts deeper into the soil to form gullies. This is gully erosion. While gullies or channels are large (ordinary tillage cannot remove them), most erosion occurs as sheet and rill erosion (1). Rills and gullies or soil deposited at the base of steep slopes are classic indicators of soil erosion.
Human Activities Increase Erosion
Some soil erosion takes place naturally without human activity. This type of erosion, termed geologic erosion, occurs slowly and is an important geological and soil-forming process (2). It is accelerated erosion, which occurs when human activities disturb the soil by harvesting lumber, grazing animals, and clearing land to grow crops, construct roads, and erect buildings, that results in unsustainable rates of soil erosion. In the US, only half the erosion is from agricultural activities; the other half is a result of road and building construction, including homes. Any human activity that exposes the soil surface has the potential to accelerate the rate of erosion over the geological rate; thus, rates of accelerated erosion in the US are 10 to 1000 times that of geologic erosion (1).
Erosion Decreases Environmental Quality
There is no more destructive soil process than human-induced soil erosion (1). Soil erosion decreases the ability of soil to support plant growth. Erosion removes topsoil, decreasing the depth of soil available for development of plant roots, reduces soil organic matter, decreasing the ability of soil to retain water and provide nutrients to plants, and destroys soil structure, increasing soil compaction which decreases soil-water retention and makes it more difficult for plant roots to penetrate the soil. Impacts of soil erosion are not limited to soil loss. The eroded material, called sediment, is carried to waterbodies (streams, lakes, reservoirs, and coastal waters) and leads to water quality problems such as increase in nutrients, algal blooms, low oxygen, and fish kills (8).
Conditions Influencing Erosion
In 1978, scientists developed the Universal Soil Loss Equation (USLE) to quantify sheet and rill soil-erosion (9). The factors included in this equation are those that have the greatest impact on erosion and focus on three groups of conditions, including climate (especially rainfall characteristics), soil properties (topography and the ability of soil to absorb rainfall), and land management (vegetative cover and erosion control practices). Let’s examine these factors in a little more detail.
Climate: The driving force for sheet and rill erosion is the frequency, duration, and intensity of rainfall, especially the intensity of rainfall (1). Intensity is of great importance because intense rain has large raindrops which are more able to detach particles from the soil, causing rainwater and soil particles to flow across the soil surface. Gentle rain causes little erosion; heavy downpours can cause severe erosion. Temperature is important because it influences the type of precipitation. There is no erosion when it snows, when the ground is covered with snow, or when the soil is frozen (snow melt is another story!). Rainfall varies dramatically from place-to-place and from year-to-year and so does the potential for soil erosion.
Soil Properties: Soils are not equally susceptible to erosion. When soils are able to absorb rainfall rapidly, less water will be available to flow across the soil surface (6). When water stays on the soil surface, raindrops are more likely to splash and detach soil particles. Well-aggregated soils (soil particles held together primarily by fungi and organic matter) resist being broken apart by raindrops and have a higher capacity to absorb water than soils that are not well-aggregated. Soils with a sandy texture generally absorb water rapidly; for example, beach sand. Soils high in clay are slow to absorb water, but here in central Virginia the soils are high in iron and aluminum which form highly stable aggregates that resist erosive processes (6). Soils that resist erosion are high in organic matter (increases water absorption), high in most types of clays (but once eroded are easily transported), or contain a high proportion of large sand grains. Soils that are most susceptible to erosion have high proportions of fine sands and silts, low organic matter content, and/or shrink-swell clays (found throughout Virginia). Gravel and gravel-sand mixtures are least erodible.
Soil Topography: The two primary components of topography that influence soil erosion are the length and the steepness of the soil surface slope (1). The longer and steeper the slope, the more opportunity there is for water to detach soil particles. Steep slopes (33% – 50%) are especially susceptible to erosion. Another factor to consider is the orientation or aspect of the slope. South-facing slopes dry-out more rapidly than north-facing slopes, and therefore, have greater capacity to absorb water; however, slope steepness has the greatest impact on soil erosion.
Vegetative Cover: The presence and type of vegetation play a major role in influencing soil erosion (6). Vegetation shields the soil surface from raindrop impact, reducing splash erosion (10). Plant root systems hold soil particles in place and slow water flow over the soil surface (11). Plants add organic matter to the soil, which increases the soil’s capacity to absorb water (12, 13) and removes water from the soil between rainfalls through evapotranspiration (evaporation from plant surfaces and water moved through the plant conductive tissues to the atmosphere) (14).
Not all plants provide similar erosion control benefits (9, 15). Undisturbed forests and dense grass provide the best protection. Forage crops (legumes and grasses), which home gardeners may grow as a cover crop, are effective because the plants grow as a dense cover. Crops grown in rows (beans, corn, tomatoes, potatoes, etc.) provide little protection until the plants are sufficiently large to cover the soil (1).
Solving the Soil Erosion Problem
“Combating soil erosion is everybody’s business” (1). If water runs over the surface of the soil, erosion will occur. However, there are ways to slow erosion and derive the benefits of erosion control. Benefits may include improved plant growth in gardens and lawns, cleaner water and air, and higher property values.
In general, there are two steps that homeowners and land managers can use to limit erosion from their property: keep the soil covered and promote on-site infiltration (water absorption into the soil) to slow the rate of water flow.
- Keep the soil covered with vegetation: plant vegetation that forms a dense canopy to limit splash erosion and extensive root systems to hold the soil in place. Perennial plants have more extensive root systems than annuals and are better for erosion prevention. Plant cover crops during the off-season for vegetables. Take advantage of between growing-season cover-crops and use no-till approaches especially for row crops. Cover crops are a very effective way to reduce soil erosion between perennial plants and fruit trees while providing weed control.
- Keep the soil covered with straw or compost mulch: both are effective at reducing soil erosion (16, 17). When using mulch, it is important to inspect and replace after each rainfall. Even in the absence of rain, it is recommended that mulches be inspected at least weekly to be sure these materials are still in place.
- Promote infiltration and slow flow: Allow water to collect in rain gardens and infiltrate slowly into the soil between rain events (18).
- Promote infiltration and slow flow: Collect water from gutters by installing rain barrels at downspouts to slow water flow from the property and for watering plants during periods between rainfall. To maximize the amount of water collected, keep building gutters free of debris and downspouts connected (19).
- Promote infiltration and slow flow: Reduce impervious surfaces by replacing sidewalks and driveways with steppingstones or pervious materials. Construct pathways across, rather than parallel with, slopes (20).
Make Reducing Erosion Your Business
Maintaining productive gardens and ecologically resilient landscapes depends upon soil quality – the soil’s ability to provide the water, nutrients, and rooting-medium to support plant growth, nutrient cycling, and organic carbon storage. Conversely, soil quality depends upon vegetative cover. Soil erosion leads to soil degradation and the ability of the soil to store organic carbon and to provide water, nutrients, and physical support to plants. Where soil is disturbed and remains exposed after vegetation removal, soil erosion is enhanced, soils are degraded, and plant health is reduced. Vegetation with dense canopies and extensive root systems lessens soil erosion and promotes soil quality. Wise plant choices and management practices for home gardens and lawns can reduce soil erosion and promote soil quality; plants are more productive with lower fertilizer application needs, and disease- and pest-control requirements. The environmental impact of erosion control actions can have beneficial impacts that reach beyond individual properties and result in improved water and air quality in the local water- and air-shed. Let’s all make it our business to reduce soil erosion in our gardens, lawns, and woodlots.
Featured Photo: Gully erosion. Photo: USDA Natural Resources Conservation Service Oregon, CC BY-ND 2.0
Citations
1. Brady, N.C. and R.R. Weil. 2004. Elements of the Nature and Properties of Soils, 2nd Edition. Pearson Education, Inc., Upper Saddle River, NJ. p. 606.
2. Montgomery, D. R. 2007. Soil erosion and agricultural sustainability. PNAS 104(33):13268-13272.
3. Diamond, J. 2005. Collapse: How Societies Choose to Fail or Survive. Viking Press, New York City, NY. p. 592
4. www.worldwildlife.org/soil-erosion-and-degradationhttps://www.worldwildlife.org/threats/soil-erosion-and-degradation; accessed 7-19-2022
5. den Biggelaar, C., R. Lal, K. Wiebe, H. Eswaran, V. Breneman, and P. Reich. 2003. The global impact of soil erosion on productivity: II. Effects on crop yields and production over time. Advances in Agronomy 81:49-95.
6. Virginia Department of Conservation and Recreation Division of Soil and Water Conservation. 1992. Virginia Erosion and Sediment Control Handbook, 3rd Edition. Virginia Department of Environmental Quality, Richmond, VA. p. 834. https://assets.vbt.io/public/files/6975/VA_Resources_Construction/Virginia_DEQ_Erosion_and_Sediment_Control_Handbook.pdf; accessed 7-22-202
7. Gilley, J.E. 2005. Erosion: Water-Induced. In: Encyclopedia of the Soils in the Environment, (eds.) Hillel, D. and J.L. Hatfield. Elsevier Science Publishing, Amsterdam, The Netherlands. p. 2200.
8.https://static1.squarespace.com/static/5201a163e4b01f15d7f763c6/t/520af5b8e4b033742444e254/1376449976308/erosion_control_homeowner_guide.pdf; accessed 7-22-2022.
9. Wischmeier, W.J. and D.D. Smith. 1978. Predicting rainfall Erosion Loss – A Guide to Conservation Planning. Agricultural Handbook no. 537. Washington, D.C., USA
10. Bochet E., J.L. Rubio, and J. Poesen. 1998. Relative efficiency of three representative matorral species in reducing water erosion at the microscale in a semi-arid climate, Geomorphology 23: 139–150.
11. Ola, A., I. C. Dodd, and J. N. Quinton. 2015. Can we manipulate root system architecture to control soil erosion? SOIL 1:603–612. https://doi.org/10.5194/soil-1-603-2015; accessed 8-23-2022.
12. Wainwright J. 1996. Infiltration, runoff and erosion characteristics of agricultural land in extreme storm event, SE France. Catena 26:27–47.
13. Ziegler A.D. and T.W. Giambelluca. 1998. Influence of revegetation efforts on hydrologic response and erosion, Kaho’Olawe Island, Hawaii, Land Degradation and Development 9:189–206.
14. Zuazo, V.H.D. and C.R.R. Pleguezuelo. 2009. Soil-erosion and runoff prevention by plant covers: A review. In: Lichtfouse, E., Navarrete, M., Debaeke, P., Véronique, S., Alberola, C. (eds) Sustainable Agriculture. Springer, Dordrecht. https://doi.org/10.1007/978-90-481-2666-8_48
15. Schwab, G.O., D.D. Fangmeier, and W.J. Elliot. 1995. Soil and Water management Systems, 4th ed. Wiley. New York, NY, USA. 384p. ISBN: 978-0-471-10973-0
16.https://www.nrcs.usda.gov/Internet/FSE_PLANTMATERIALS/publications/mipmctn11904.pdf; accessed 8-23-2022
17. https://extension.uga.edu/publications/detail.html?number=B1200&title=Compost%20Utilization%20for%20Erosion%20Control); accessed 8-22-2022
18. ; accessed 8-20-2022.
19. https://canr.udel.edu/wp-content/uploads/sites/16/2018/03/12024201/Harvesting_Water.pdf; accessed 8-20-2022
20. https://canr.udel.edu/wp-content/uploads/sites/16/2018/03/12024201/Permeable_Impermeable_Surfaces.pdf; accessed 8-20-2022
