Exam #1 ­ Key

(6) 1. How does a soil survey "map unit" differ from that of a "soil series" description?

A map unit defines the contents of the area inside a polygon on a soil map. It describes the dominant soil series that comprise the polygon and their approximate percentages along with information on soil inclusions within the polygon. The map unit description contains information on "genesis", "hydrologic properties" and on the "use and management" of the specific map unit.

In contrast, a soil series description defines the individual and allowable range of characteristics (e.g., chemical, physical and morphological properties) for a pedon to meet the requirement of the specific soil series. It contains detailed information on soil characteristics, such as the type and depth of horizons, soil color, texture, structure, roots, etc.

(2) 2. Which soil map unit provides the most detailed description of a map polygon? List the specific name given to this type of soil map unit; no explanation is necessary.

Consociation - a map unit in which greater than 75% of the unit is made up of soils named within the map unit name.

(2) 3. Does an Order 1 or Order 5 soil survey provide the greatest level of detail concerning the distribution of soils on the landscape?

Order 1 is the most detailed soil survey; map scale <1:12,000

Order 5 contains the lowest level of detail; map scale >1:250,000

(4) 4. What is the primary purpose of using geostatistical methods in soil survey investigations?

Geostatistics are used to understand, quantitatively, the spatial distribution (or variation) of soils and soil properties on the landscape. It can be used to i) select independent sampling points, ii) describe spatial variation of soil properties quantitatively, iii) predict values for points not sampled, and iv) help devise better soil sampling strategies.

(6) 5. List three purposes for which land evaluation systems such as the USDA-Land Capability Classification, Storie Index Rating, and LESA might be used?

i) how to use land (zoning, land-use planning)

ii) how much to tax land

iii) determine sustainable yields

iv) targeting for price support & conservation practices

v) targeting for land-retirement programs (taking highly erosive land out of ag production)

(6) 6. List three soil features that would be defined as a "root restricting" layer in soils.

i) cemented horizons (e.g, duripan=silica, petrocalcic=carbonates, placic/orstein=iron)

ii) bedrock

iii) clay pan (abrupt textural change)

iv) compacted soil layers (traffic pan)

v) extremely gravelly, cobbly or stony layers

vi) anoxic ground water

(6) 7. What is the rationale (i.e., reason) for using potential evapotranspiration for irrigated agricultural and actual evapotranspiration for dry land agricultural management in the USDA-Land Capability Classification?

Potential evapotranspiration is used for irrigated agriculture because it provides a measure of the availability of solar energy for sustaining the growth of plants. Given the availability of irrigation water, it is assumed that solar energy is the only climate limitation to plant growth. For example, a cold climate (Arctic or mountains) will have low potential evapotranspiration while a warm climate (tropics) will have high potential ET.

Actual evapotranspiration is used for dry land agriculture because it provides a combined measure of the solar energy and water availability. When temperature is not limiting, the actual ET value provides a direct measure of how much water is available for the growth of a crop.

(6) 8. What are two specific soil morphological features that indicate imperfect drainage (the absence of well-drained conditions) in a soil profile? How would you recognize them in a soil profile and what hydrologic conditions lead to their formation?

Mottles are spots or splotches of soil material having a different color from the soil matrix (red/orange=iron; black=manganese). Mottles are found in zones with a fluctuating water table that experiences alternating oxidation and reduction.

Gleying is characterized by shades of gray/blue/green color that have neutral hues and chromas of one or less in unmottled soils. The color is due to the removal of ferrous iron from the soil in the drainage waters. Gleying occurs is soils that are dominantly under reducing (anaerobic) conditions (i.e., permanently reduced).

(6) 9. Soil profile permeability is evaluated by different criteria in the USDA-Land Capability Classification compared to the Storie Timber Index. What are the different criteria used in the two systems and which one do you think provides a better measure (more accurate) of permeability?

Soil permeability in the LCC is determined by the texture of the least permeable horizon. In contrast, the Storie Timber Index uses the soil profile group (degree of Bt horizon development) as its criteria for soil profile permeability.

I would suggest that the LCC method using the texture of the least permeable horizon would generally provide a better measure of permeability. First, soil texture can be quantified relatively easily and accurately in the field. Second, there are soils such as Vertisols that have clay texture classes throughout the profile, yet have no development of a Bt (argillic horizon). These Vertisols would have a Storie soil profile group of 1 indicating good permeability, yet in reality the clay textures would lead to a very slow soil profile permeability.

(6) 10. What feature(s) of the USDA-Land Capability Classification rating system make it more appropriate as a national land rating system as compared to the Storie Agricultural Rating system. Explain briefly.

The Storie Agricultural Rating system assumes that irrigation water is available and that climate is not a limiting factor. Thus, it will not work well in cold regions or in regions where water is limiting plant growth. The LCC index takes into account the important climate factors of potential ET (irrigated) or actual ET (dry land) and the length of the frost-free season. It further takes into account the flooding hazard which is not considered in Storie.

(4) 11. To be considered prime farmland in the state of California, what two ratings are considered and what are the specific requirements for each rating? (The topic of one of your papers in discussion section)

LCC of Class I and II and

Storie Agricultural Ratings of 80 to 100

(6) 12. Two soils have identical Storie Agricultural ratings of 50%. The rating for Soil #1 is reduced due to a low value for factor A while the rating for Soil #2 is reduced due to a low value for factor X. If you wanted to use the soil for agricultural crops, which of the two soils would you buy? Why?

Soil #2 having the reduction due to the X factor would most likely be the best choice. Factors reducing the X factor can be relatively easily fixed at a reasonable cost through land reclamation strategies. In contrast, a soil that has its Storie rating reduced due to a Factor A problem will be very difficult (and expensive) to fix by management practices. This would involve changing the subsurface horizons (argillic horizon or indurated horizon) or dealing with shallow bedrock.

(4) 13. Height-age relationships are commonly used to evaluate the potential of land for forestry production. Under what circumstances would using the Storie Timber Index be more appropriate or necessary for evaluating the potential of land for forestry?

Evaluating forest land productivity using the height-age relationship requires the existence of a healthy forest. If a forest is affected by disease or pathogens, the height-age relationship will not accurately predict the productivity of the site. Perhaps more importantly, clearcutting or forest fires could destroy the forest canopy making it impossible to use the height-age relationships. Thus, the only option if this occurs is to base potential forest productivity on soil/climatic properties. Selective logging also makes the height-age relationship impractical since the best trees (dominants and co-dominants) have been removed leaving only the slow growing trees behind.

(6) 14. What are the two basic components of the LESA system? What are the general types of variables (factors) used for each of the two components?

Land evaluation - soil quality/productivity/potential factors such as LCC and Storie index ratings

Site assessment - socioeconomic factors such as proximity to other agricultural lands, viability of farming in the area, zoning for the site and adjacent sites, alternative uses for the site, urban conflict, urban infrastructure, comprehensive development plans for the area.