Reduce Soil Erosion by Understanding the Factors that Cause it

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Unsustainable soil erosion

Our ability to maintain or improve the quality of Nova Scotia’s agricultural soils will determine our capacity to sustainably produce healthy crops into the future. There are several ways in which soil quality can decline over time and soil erosion is a major one. Erosion is the loss of soil through the action of running water, wind, tillage or ice. In our wet maritime climate, pounding rain and running water are the primary causes of soil erosion. When erosion is first visible in a field, it has already reached an unsustainable level.

Soil erosion not only represents an economic cost to farmers in terms of reduced productive capacity, it can also represent a cost to the wider environment when eroded soil particles carrying nutrients, pesticides and microorganisms find their way into natural systems including watercourses and wetlands.

Soil specialists since the 1930’s have attempted to better understand erosion by breaking it down into several component parts.  The Universal Soil Loss Equation (USLE) which has been improved upon and revised over the years is a model that predicts erosion as the result of six factors. This article will briefly look at each of these factors and discuss opportunities to implement management changes to reduce erosion risk.

Erosion = R K L S C P
where;
R = rainfall erosivity
K = soil erodibility
L = slope length
S = slope steepness
C = cover and management
P = erosion control practices


R (rainfall erosivity)
This factor takes into account the total rainfall amount, the intensity of the rainfall and the distribution of rainfall across the season. In general, higher intensity rain events have more erosive power than low intensity rain events. Big rains have larger droplets which have more energy to dislodge soil particles and a larger volume of water to carry dislodged soil particles away. Whereas fine rains have smaller raindrops which land gently on the soil and have the ability to be absorbed more slowly over time. We can’t control when the rain comes or how much falls, so we’ll have to settle for management practices to protect the soil from the rain.

K (soil erodibility) This factor takes into account the natural properties of a soil that affect its vulnerability to erosion. The two main components of erodibility are the texture of the soil (sand, silt and clay content) and the ability of the soil to absorb water. Soils containing silts and very fine sands are most prone to erosion where soils with higher clay or coarse sand contents are less prone. The structure of a soil, or arrangement of particles within the soil (sometimes measured as density) affects the amount of pore space a soil has, and this in turn affects whether water will be absorbed or run-off. As we know, water running over the surface of a soil has the capacity to erode. We can’t control the texture of our soils in any practical way but we can improve soil structure by keeping organic matter levels high and reducing compaction. Reduce soil erodibility by applying manure or compost, planting forages and deep-rooted cover crops and by reducing tillage and traffic.

L S (slope length and slope steepness) The slope length and the steepness of the slope play a role in erosion. Imagine rolling a tire down a hill. The length and steepness of the hill will determine how much speed the tire gains.  The erosive power of water is directly related to the speed at which it travels. Anything that can be done to slow the forward speed of water running down hill can reduce erosion.  Several practices that can slow water down will be discussed in the erosion-control practices section.

C (cover and management) Soil erosion can most readily be controlled by managing vegetation, plant residues, and tillage. Various crop types provide differing levels of protection from soil erosion with perennial forages providing the most stability, small grains providing moderate stability and row crops being most prone to erosion. Nova Scotia is particularly well suited to growing forages and the benefits to soil health are many, so attempt to incorporate forages into all rotations.

Cover crops can protect the soil after crop harvest when the soil would otherwise be exposed. Look for windows of opportunity where the soil is exposed and where a cover crop could be incorporated into the rotation. For example, some farms are broadcasting annual ryegrass into a young silage corn crop to ensure soil coverage over the winter where traditionally the soil would be exposed. If a cover crop hasn’t been established by November 1st, it is recommended that hay or straw mulching be implemented at a rate of 3.3 t/ha (1.5 t/ac), or on areas of the field prone to erosion, the mulch should be applied at a rate of 4.5 t/ha (2 t/ac). At these rates, the mulch will cover 85 to 100% of the soil surface respectively. It is also recommended that mulching be done across the slope to ensure that there are no wheel ruts up and down the slope that would allow water to collect and cause more severe erosion in the wheel tracks.

Reducing tillage to maximize the amount of residue left on the soil surface is also an important management strategy. The EFP office has a residue management kit that can be borrowed to measure the percentage soil cover.

P (erosion-control practices) There are several management practices that can be used to guide and slow the movement of water and thereby reduce erosion. Pages could be written about any of these practices, so if you are interested in additional information please refer to the resources listed below.

  • Contour tillage: is the practice of conducting field operations along the contour of a field. This has the benefit of preventing downslope pathways for water to gain speed formed by wheel ruts, furrows, etc.
  • Strip cropping: The practice of growing crops that require different management in strips along the contours of a field. For example, strips of a row crop that would be more susceptible to erosion, alternated with strips of grass forage.
  • Diversion terraces: Are shallow grassed ditches, with a berm on the downhill side which are constructed across the slope to intercept surface water runoff. Terraces decrease the slope length and remove large flows of water safely from the field.
  • Grassed waterways: For areas where persistent erosion occurs, permanently grassed waterways can be used. The waterways should be shallow channels approximately 6 m wide by 30 cm deep formed into a saucer shape following the natural depressions in a field.  They allow large volumes of runoff to flow downhill without causing erosion

Erosion is a complicated issue with many aspects contributing to the overall susceptibility of a given soil.  Some aspects like soil type and slope are outside of a farmers control. In today’s world where climactic variability appears to be increasing and where rainfall events with larger volumes and higher intensities may increase, it is more important than ever to protect our soils using management practices that we do have control over.

Resources

The environmental farm Plan has the following resources available to farmers:

http://sis.agr.gc.ca/cansis/publications/surveys/ns/index.html

 

Cory Roberts, Environmental Farm Plan Coordinator
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