5

Soil Science 100

Final Exam 2 ID____________________

Friday 17 December 1999 Lab Section___________________

Value 150 points

Answer all questions in the space provided.

1. (15) Heterotrophic bacteria utilize preformed carbon sources for energy. How does their use of these carbon sources affect soil pH, composition of the soil atmosphere, and availability of nitrogen to plants? For complete credit, you must answer each part of this question by indicting how each component changes (e.g. up down, more less) and the mechanism or process that produces the change.

The pH is decreased as carbon in organic matter is oxidized to carbon dioxide.

Respiration depletes oxygen and increases carbon dioxide in the soil atmosphere.

Assimilation of carbon requires the mineralization and assimilation of some nitrogen. As the organisms die, the nitrogen is released, thus increasing the N supply to plants.

2. (10) Name each of the five factors of soil formation enunciated by Jenny (exclude the human factor).

Parent material, organisms (biota), topography, climate and time.

3. (20) Draw a diagram showing five components of the nitrogen cycle. For each component in the diagram use arrows to show transfers among the components. Label each arrow with the name of the process and the form of nitrogen.

4. (10) What are two differences in the reclamation of saline soil and sodic soil? What are the similarities in reclamation of these two problems?

The two main differences are the need for acid or acid producing compounds to lower the pH and cations other than sodium to replace sodium on the exchange complex when reclaiming sodic soils compared to those that are saline. Similarites are that good drainage (artificial or natural) and sufficient water for leaching are required by both reclamation processes.

5. (10) Write the Darcy equation for saturated flow in soils and define each of the variables in the equation.

Q = K*A*DH/DX or dY/dX

Q= flow rate, K= hydraulic conductivity or constant representing ease of flow, DH/DX or dY/dx the driving force, change in head or potential per length.

6. (15) Explain the results of the fertilizer experiment that produced the following graph. Your explanation must include statements about each of the four parts of the curve.

Region A fertilizer amount is too low to stimulate significant response from the crop.

Region B, yield increases rapidly as the fertilizer amount increases because the deficiency is being reduced.

Region C, yield plateau as the fertilizer is in sufficient quantity for meeting the plants genetic potential

Region D, Excess fertilizer leads to a toxicity which causes yield to decline.

7. (10) List one advantage and one disadvantage of the following kinds of diagnostic tests for diagnosing plant nutrient deficiencies: Soil tests, leaf tissue tests, field plot trials, greenhouse pot trials, plant deficiency symptoms.

Test	Advantage	Disadvantage
Soil test	quick, easy ?, relatively inexpensive, quantitative	sometimes expensive, timing is important
Leaf tissue test	quick, easy, relatively inexpensive, quantitative	sometimes expensive, timing is important (too late to respond)
Field plot trials	quantitative	long term, slow, expensive, timing
Greenhouse pot trials	quantitative, controlled	slow, expensive, timing
Deficiency symptoms	direct, cheap	timing, qualitative, difficult to interpret

8. (30) Listed below are five geomorphic positions you would find in an east to west transect from the Sierra Nevada to the Pacific Coast. For each position, complete the table. You are not expected to know the exact value of these variables, but you should be able to fill in each box so that the relative value of variables is consistent. If you think that a horizon does not exist at a location, fill in the space with the word missing or absent.

Position	A horizon color	A horizon pH	A horizon thickness (cm)	Bt horizon thickness (cm)	Bt horizon color	Bt horizon pH
Mid-elevation upland on granitic bedrock	Pale, light color, gray	Acidic	Thin to moderately thick	Thick	Red	acidic
Old alluvial fan on granitic alluvium	Brown	Acidic	Thin to moderately thick	Thick	Red	acidic
Basin rim	Almost any color	High	Almost any	Absent to thick	Grey or dark, maybe yellow	High
Low elevation upland on sedimentary bedrock	Brown	Neutral to slightly acid	Thin to moderately thick	Absent to thin	Light red to brown	Neutral to slightly acid
Serpentinite	Dark, reddish-black	Neutral to basic	Moderately thick to thick	Moderately thick	Dark red	Neutral to basic

9. (15) List three differences between inorganic fertilizer and organic fertilizer materials. For each, explain how the difference would influence your use of the material in your home garden.

Organic	Inorganic
Low analysis	High analysis
High variability	Low variability
Need lots	Need less
Cheep (sometimes)	Expensive (sometimes)
More difficult to apply	Easier to apply

10. (15) Complete the following table. For each condition,what surface horizon pH (acid, neutral or alkaline) do you expect to find? How (e.g. what process) does the condition contribute to the soil pH?

NOTE: Each condition must be considered separately. These are not conditions found for any single soil or location.

Condition	pH	Process
The soil forms under mixed deciduous forest	Neutral to slightly acidic	Accumulation of base-rich litter, decomposition leading to release of protons, balanced by nutrient cycling of bases.
Precipitation commonly exceeds evapotranspiration by a factor of 3 or 4 on limestone parent material.	Acid	Leaching, removing bases and substituting protons for bases on the exchange complex
Soil is flooded continuously during the summer.	Alkaline/neutral	Reduction consumes protons raising the soil pH
Soils are heavily fertilized with animal manure.	Acid	Mineralization of N releases protons. Also organic acids generated from organic matter additions lowers pH.
Free water (a water table) is found at 1 meter below the surface in a semi-arid climate zone.	Alkaline	Evaporation leaves behind salts and sodium raising the pH.