By Linda Blum
Piedmont Master Gardener
The soil beneath our feet is one of our most precious resources. Yet we are destroying this resource at rates faster than ever before. Soil erosion was a major factor in the decline of past civilizations, following deforestation around the globe. In the last 150 years, half the world’s topsoil has been lost due to erosion, disappearing at a rate of about half an inch per year from agricultural lands worldwide. Soil loss is far outpacing soil formation, posing a long-term threat to all the essential benefits that soils provide to us and the natural world.
By taking a few simple steps to protect the soil in our yards and gardens, we can all do our part to help solve this problem.
What Is Soil Erosion?
Soil erosion is the wearing away of the soil surface. Water is the dominant force causing soil erosion, but soils are also eroded by wind, ice and gravity. Soil erosion involves several different processes. Splash erosion occurs when droplets of rainwater fall on soil, dislodge the soil particles and splash them into the air. When the splashed soil particles are carried by water flowing across the soil surface, the erosion is called sheet erosion. As the water carrying the soil particles flows quickly downhill, it concentrates into small channels or rills, and more soil particles are lost in a process called rill erosion.
Rills are common, especially on bare land, and once formed can cause substantial soil loss. Gully erosion takes place when rills come together and the increased water velocity cuts deeper into the soil. While gullies or channels are large and easy to see, most erosion occurs as sheet and rill erosion and is less visible on the landscape. Look for soil deposited at the base of steep slopes, a classic indicator of soil erosion.
How Human Activities Increase Erosion
Some soil erosion takes place naturally without our help. It occurs slowly and is an important way to form new soil. Humans accelerate this process when we disturb the soil by harvesting lumber, grazing animals or clearing land. The result is unsustainable rates of soil loss. In the U.S., only half the erosion is from agricultural activities; the other half is a result of road and building construction, including homes.
Erosion’s Impacts on Plant Growth and Water Quality
Human-induced erosion is the single greatest threat to our soil and its ability to support plant growth. Erosion removes topsoil, decreasing the depth of soil available for development of plant roots; it reduces soil organic matter and degrades the ability of soil to retain water and provide nutrients to plants; and it destroys soil structure, the mix of pores and aggregates that help plants grow. Erosion also increases soil compaction, which decreases soil-water retention and makes it more difficult for plant roots to penetrate the soil. The eroded material is carried to streams, lakes, reservoirs and coastal waters and leads to water quality problems such as increased nutrient pollution, algal blooms, low oxygen and fish kills.
Conditions Influencing Erosion
Three major factors affect the rate of sheet and rill erosion: climate (especially rainfall intensity, frequency and duration), soil properties (topography and the ability of soil to absorb rainfall), and land management (vegetative cover and erosion control practices).
Rainfall intensity. Heavy downpours can cause severe erosion while gentle rain causes little erosion. The type of precipitation, which is influenced by temperature, also has an effect. No erosion occurs when it snows, when the ground is covered with snow or when the soil is frozen. Rainfall varies dramatically from place to place and from year to year due to the climate, and so does the potential for soil erosion.
- Soil Properties
Soil Topography. Steep slopes (33 to 50 percent) are especially susceptible to erosion. South-facing slopes absorb water and dry out more rapidly than north-facing slopes, and therefore are less susceptible to erosion.
Absorption of Rainfall. Soils that resist erosion are high in organic matter (which increases water absorption), high in most types of clays (which are slow to absorb water but once eroded are easily transported) or contain a high proportion of large sand grains. Soils most susceptible to erosion have high proportions of fine sands and silts, low organic matter content, and/or contain a certain kind of clay (known as shrink-swell clay) that erodes easily. Gravel and gravel-sand mixtures are least erodible.
- Land Management
Vegetative Cover. Vegetation shields the soil surface from raindrop impact by reducing splash erosion, and plants with extensive root systems hold the soil in place and slow water flow over the surface. Plants add organic matter to the soil, which increases the soil’s ability to absorb water. Plants also remove water from the soil between rainfalls through evapotranspiration, the process that draws water up through the plant’s conductive tissues and releases it into the atmosphere.
Some plants are better than others at controlling erosion. Undisturbed forests and dense grass provide the best protection. Forage cover crops (legumes and grasses) are effective erosion control because the plants grow as a dense cover. Crops grown in rows (beans, corn, tomatoes and potatoes, for example) provide little protection until the plants are sufficiently large to cover the soil.
Home Gardeners Can Help Solve the Problem
Some erosion occurs whenever water runs over the surface of the soil. However, there are two main ways home gardeners and land managers can slow erosion: 1) cover the soil and 2) slow the water flow and promote infiltration. The benefits of erosion control can result in improved plant growth in gardens and lawns, cleaner water and air, and higher property values. Try these strategies to reduce soil erosion:
Cover the Soil
For ornamental gardens, select perennial plants. With more extensive root systems than annuals, perennials are better for erosion prevention. Plant cover crops between rows of vegetables, fruit trees and perennials, both during the growing season and again during the off-season. Use no-till techniques, especially for row crops. Keep bare soil covered with straw or mulch. Inspect the mulch after each rainfall, at least weekly in the absence of rain, and replace it as necessary.
Slow the Flow and Promote Infiltration
Install rain barrels at downspouts to collect water from gutters to slow water flow from the property and to provide water for plants between rainfalls. Keep gutters free of debris and downspouts connected to maximize the amount of water collected. Plant a rain garden to allow water to collect and infiltrate slowly into the soil between rain events. Reduce impervious surfaces by replacing sidewalks and driveways with steppingstones or pervious materials. Construct pathways across, rather than parallel with, slopes.
Make It Your Business
Let’s all make it our business to reduce soil erosion in our gardens, lawns and woodlots.
Home gardeners are often looking for ways to have more productive gardens and ecologically resilient landscapes. Soil erosion control techniques will improve soil quality, support healthy plant growth, and reduce the need for fertilizer and for disease and insect pest controls. And, the environmental benefits will reach far beyond individual properties, resulting in improved water and air quality in the local water- and air-shed. For more information on soil erosion and soil health management, visit www.4thesoil.org/.
Fall is the best time to plant trees, shrubs and hardy perennials to increase vegetation cover in your landscape (www.plantmoreplants.com) and a great way to begin to celebrate World Soil Day on December 5th (www.fao.org/world-soil-day/en/ and #WorldSoilDay)
Dr. Linda K. Blum is a Research Associate Professor of Environmental Sciences at the University of Virginia. She earned her bachelor’s degree in forest soils and her master’s degree in forest soils microbiology at Michigan Technological University and completed her Ph.D. degree in agronomy at Cornell University.