1. Unit 13: Soil Erosion
Chapter 11

2. Objectives How soil erosion affects your life
Magnitude of the soil erosion problem
Causes & methods of soil erosion
Controlling erosion

3. Introduction North America’s most devastating environmental disaster
Much topsoil lost resulting in greatly decreased production capability
Plugs channels & raises riverbeds, increasing flood risks
Geologic Soil Erosion – natural wearing away of land surface by water, wind, or ice
Not addressed here
We’ll address erosion related to human activity

4. The Soil Erosion Problem
Erosion problem two-fold
Lost productivity
Sediment pollution
Exposed B horizons, much less productive
9-18% corn yield reductions
17-24% SB yield reductions
Al toxicity risks much higher

5. The Soil Erosion Problem
Sediment
Major carrier of pesticides & fertilizer that pollutes the ecosystem where it settles
Also make lakes, rivers more shallow
Ex. Eastern Washington – in last century, 40% of topsoil has eroded
Much focus on stream bank erosion, erosion at construction sites (land left unprotected for extended time)

6. The Soil Erosion Problem
Minimization is the goal (elimination of erosion impossible)
Since 1935 $30 b spent to erosion control in U.S.

7. Nature of Water Erosion
Effects: destroys human-made structures; fill reservoirs, lakes, rivers; damages land
Mud, silt, sediment
Eroded material usually the richest part of the soil
Highest nutritive content
Most organic matter

8. Nature of Water Erosion
Causes of Water Erosion
Peds disintegrate w/ impact of rain drops
Soil aggregates separate
Particles can move ~5’ w/ splash erosion
Most destructive on bare soil
Rainfall then moves particles w/ water flow on soil surface
Scours channels in soil surface
Each subsequent rain adds to depth/width of these channels
Form: gullies, rills

9. Nature of Water Erosion
Causes: excessive tillage, burning crop residue, overgrazing, clear-cutting forests
Raindrop Splash Erosion
Not as detrimental in clay soils due to strong adhesion forces
More common in fine sands, silty soils
Clays more likely to crust over, decreasing aeration, slows infiltration
Water then must wash over soil surface

10. Nature of Water Erosion
Sheet & Channelized-Flow Erosion
Sheet erosion – water moves an entire layer of soil
Rills – water-cut channels in the soil
Can be erased by tillage
Gullies – large, deep channels caused by excessive movement of water
Can’t cross w/ equipment
Sheet & rill most common & problematic
Harder to detect

14. Erosion Tolerance
Erosion Tolerance Level (T) – maximum rate of annual soil loss that will permit crop productivity to be maintained indefinitely
Highly erodable land – reduce erosion to 5 t/ac/yr
Some soils – 1-2 t/ac/yr

15. Water Erosion Control Controlling Soil Detachment
Cropping/vegetative practices the keep soil surface covered as much as possible
Energy from rain drops dissipated/minimized by hitting residue first
Leaving residue cover:
Not plowing under stubble
Planting a cover crop

16. Water Erosion Control Controlling Soil Transport Slow eroding water
Reduce steepness of slopes (if possible)
Construct barriers or terraces
Reduces slope, appropriate for areas of high-intensity rainfall
Reduces velocity of runoff
Can be costly

17. Water Erosion Control Cultivate on the contour Strip crop
Plant at right angles to natural slope
Appropriate for slopes 2-8%
Strip crop
Plant on contour
Alternate row crops w/ close-growing crops
Not only slows erosion, but can filter what is eroded
Filter strips
Plant on low end of field to prevent chemicals/fertilizer from entering adjacent ditches/streams
IA study – 10’ filter strip reduced sediment load 70%, 30’ strip 85%

20. Water Erosion Control Cover Crops – grown during off-season
Legumes, cool-season grasses
Hairy vetch, clover – can also add N to soil
Good cover crops should:
Increase soil organic matter
Improve water infiltration
Reduce runoff
May harvest, or burn down w/ herbicide
In dry areas – use is limited due to risk of depleting soil water content

21. Water Erosion Control Erosion Control for Construction Sites
Erosion can far exceed any agricultural erosion
Greater effort needed to minimize erosion
Construction site barren soil can erode 1 t/ac over a few storms
Methods used to control erosion
Mulch
Soil binders
Logs
Blankets

22. Nature of Wind Erosion
Rates can be high on dry, bare, weak aggregate soils
Soil particles can be held in suspension and blown away
44% of erosion caused by wind
Most of our land: wind erosion risk small
Highest risk areas: TX, NM
1/3 of land surface – high risk for wind erosion

23. Nature of Wind Erosion Arid & semiarid soils at highest risk
Less vegetation, less cover
Less clay, less aggregation
Less soil moisture, lighter wt
Desertification – process of soil degradation by wind erosion, resulting in coarser soils, lessen ability to retain water

24. Wind-Erosion Control
Ridge-till – decreases ability of wind to blow away soil steadily across surface
Reduces wind velocity
Traps soil
2-4” ridges ideal
Reduce field length
Increase soil cover
Use windbreaks

25. Assignment