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Soil Properties And Runoff

• By Dr. Larry Oldham,
Mississippi State University •

soils

Photo by Lynda Richardson

Soils are the environmental regulators of rain: When it falls, soil properties determine if it goes into the soil (infiltration) or across the surface (runoff).

In the runoff water, there may be sediment and whatever is attached to it, nutrients and other elements dissolved in the water, and biological entities such as bacteria.

Exactly what ends up in runoff is a complex interaction of factors. Of the nutrients, phosphorus is one that probably has had the greatest attention in recent decades. However, blanket installation of conservation practices has not reduced P loss in many watersheds.

Work has shown that in many situations, relatively small areas of a watershed contribute more potential pollutants to the runoff. Why? Here are some things to consider.

Soil Texture

Soil texture — the relative amounts of clay, sand and silt present in a soil — affects infiltration. Water goes into sandier soils faster than clay soils. The more clay (and more organic matter) in a soil leads to more water-holding capacity and more filtration ability. More water-holding capacity of a soil leads to a longer lag time before runoff may start from a rain.

Restrictive Layers

Restrictive layers in soils lower water- storage capacity and can redirect subsurface water in the soil. Such layers may be natural or human caused.

Fragipans are dense, naturally occurring zones typically about 20 inches below the surface. They severely restrict water flow and root growth and are widespread in the Mississippi hill section. Man-made soil compaction results from traffic or some tillage operations.

Slope

An intuitive factor is the slope within a field. How the slope influences runoff potential is determined by its length and location in the landscape.

There must be hydrological connectivity to receiving waters for runoff containing potential pollutants to become an issue.
Movement of sediment within a landscape without entering surface water is another point.

Vegetation

Vegetation affects splash and interrill erosion (erosion that occurs when raindrops hit the soil) by slowing rain intensity.
The roots help limit rill erosion (slow movement of water along small channels typically on bare land) and gully erosion (fast-moving water in deeper channels).

Critical Source Areas

Soils have a variety of influences on runoff from soils. Therefore, some areas are more vulnerable to contribute pollutant loads to runoff water. It is more effective to address these “critical source areas” with management practices to minimize pollution risk.

One such tool is the phosphorus index risk assessment tool of nutrient management planning used in Mississippi primarily in livestock-producing areas. This tool uses site-specific information regarding soil test P levels, slope, available nutrient sources, predicted erosion and distance to water to guide fertility management recommendations.

The 4R nutrient stewardship principles also address these issues. They include using the right rate of (based on reputable soil testing results) the right source of fertilizer at the right time in the right place. This reduces nutrients not used by plants and potentially lost to the environment.

Not all farms, fields or sites within fields have the same capacity to contribute potential pollutants to runoff. Get to know to your critical source areas and manage accordingly for profit and stewardship.

Conservation Webinar, Sept. 22, noon CDT

Beth Baker and Gray Turnage team up for the September Conservation Webinar to discuss the benefits and management challenges of wetland conservation.

Baker is an assistant Extension professor at Mississippi State University with experience in areas of biology, biogeochemistry and Extension education. Turnage is research associate and invasive species expert with the Northern Gulf Institute and Geosystems Research Institute. Gray has 11 years of research experience in aquatic/wetland ecosystems and has worked as a private wetland consultant in Mississippi.

When you register at https://bit.ly/32hzcRZ you will receive a link to join the webinar via a zoom meeting link. Be sure to save the link to join.