Spring - 1998 Name

Soil Science 118 - Exam #2

pts

(7) 1. a . World-wide, which causes the greatest loss of soil, water erosion or wind erosion?

Water erosion accounts for approximately 2/3 of the total soil erosion world-wide.

b. Describe the concept of "on-site" versus "off-site" effects of soil erosion and

provide a specific example of each for both water erosion and wind erosion.

On-site erosion involves the effects of the erosion to the soil ecosystem that is being eroded. This includes the loss of nutrient- and water-holding capacity and the loss of a rooting medium in some cases. Wind erosion may also have on-site effects due to abrasion of plants growing on the eroding soil.

Off-site effects are the consequences of soil erosion that occur at a location remote from the site of the actual erosion. With respect to water erosion, effects include deterioration of water quality in the form of sediment deposition, and nutrient, chemical and pathogen additions. Off-site wind erosion effects include detrimental deposition, air pollution, and cleaning and maintenance costs.

(4) 2. The water erosion process involves two distinct processes (or steps). What are the two steps, what are the mechanisms related to these two steps, and how is the velocity of moving water related to them?

Soil detachment - involves raindrop impact and overland flow detachment breaking soil aggregates into individual particles. (Kinetic energy = ½ mv²)

Soil transport - raindrop splash and overland flow transport move sediments down slope. The carrying capacity of running water is proportional to v⁵.

(5) 3. What soil properties determine the erodibility (K factor) of a given soil? Which soil property is most important?

Soil texture = silt + very fine sand content and fine through very coarse sand content

Organic matter concentration

Soil structure

Soil permeability

Texture is most important!

(7) 4. Which factors in the Universal Soil Loss equation are relatively fixed (not affected by management practices) and which factors can be changed by management practices? For those factors that can be changed, provide an example of a specific management practice that is commonly used to change the value of this factor.

R = fixed

K = relatively fixed, but can be changed somewhat by organic matter and permeability adjustment. Conservation tillage or mulching would improve.

L = can be adjusted; by terracing

S = difficult to adjust; but can be changed by bench terracing

C = easy to change through conservation tillage, cover cropping, mulching, etc.

P = easy to change through application of contour tillage operations

(2) 5. Who was the primary scientist responsible for the development of the Universal Soil Loss Equation?

Wischmeier

(4) 6. What is the conceptual meaning of the USLE's "R" factor? Describe the concept using words rather than equations.

The "R" factor represents the energy input that is a function of rainfall intensity and is related to the climate of the site. It accounts for both the amount of rainfall and the intensity of the rainfall.

(9) 7. Wind erosion processes are broken down into three categories or mechanisms. Name each of the three wind erosion processes and provide a brief explanation describing the conceptual mechanism involved in each.

Suspension - lifting of particles high into the air where they may be carried long distances

Surface creep - rolling or sliding of particles along the soil surface

Saltation - jumping or bouncing of particles along the soil surface. Collision of saltating particles with the soil surface often initiates both surface creep and suspension.

(2) 8. Of the three processes you described above (Question #7), which process is most important with respect to transport of wind-eroded material?

Saltation is the most important process!

(8) 9. Which factors in the Wind Erosion Equation are relatively fixed (not affected by management practices) and which factors can be changed by management practices? For those factors that can be changed, provide an example of a specific management practice that is commonly used to change the value of this factor.

I = soil erodibility; a function of soil texture and can not be easily changed

C = climate factor; can not be changed

K = surface roughness; can be changed by tillage operations

L = unprotected width of field; can be altered by planting wind breaks or strip cropping

V = vegetative cover; can be altered by cover crops or surface residues

(2) 10. What is the primary force responsible for mass movements? Gravity

(4) 11. Mass movements, in particular rotational slides, are commonly observed to reach a maximum frequency 5 to 10 years following clearcut harvesting of forests. What factor(s) are responsible for this increase in mass movements following harvest?

The soils become wetter following clearcutting because evapotranspiration is decreased. After about 5 years the rooting system from the previous trees weakens and it provides little strength to hold the soil mass together.

(3) 12. How does the soil water content affect mass movements? I'm looking for specific effects (mechanisms).

Water increases the weight of the soil

Decreases friction

Increases pore water pressure which forces particles apart

(7) 13a. Define soil quality.

The ability of the soil to sustain plant and animal productivity, maintain or enhance water and air quality (environmental quality), and support human/animal health and habitation.

b. List four critical management strategies for maintaining soil quality.

Prevent erosion, maintain good soil physical properties, maintain or enhance organic matter concentrations, control insects, diseases, and weeds without chemicals, maintain or enhance nutrient levels, and minimize accumulations of plant/soil organism limiting compounds.

(6) 14. Consider a soil at three contrasting moisture contents: i) air dry (<2% H₂O), ii) near field capacity, and iii) saturated. How will the water content affect the maximum compaction potential at each of these water contents? Which water content will have the greatest compaction?

Proctor test concept: the field capacity sample will have the greatest compaction potential.

At the lowest water content there is insufficient water to lubricate the particles and compaction is low. At saturation, the water content reduces compaction by pressing against particles and inhibiting rearrangement. At field capacity, the water content lubricates without reducing the rearrangement.