Quiz 2

Question 1

The process involved in the conversion of N in organic materials to gaseous forms proceeds in which order:

1. Denitrification → mineralisation → nitrification
2. Nitrification → denitrification → mineralisation
3. Mineralisation → N-fixation → nitrification
4. Mineralisation → nitrification → denitrification

Answer 1

4. Mineralisation → nitrification → denitrification

Question 2

Ammonium is less readily transported through the soil than nitrate because:

1. Nitrification is largely an aerobic process
2. Ammonium is positively charged and nitrate is negatively charged, so nitrate is more mobile
3. Fungal hyphae are not able to take up nitrate from the soil
4. The negative charge of nitrate means that it more readily adheres to organic matter and clay particles

Answer 2

2. Ammonium is positively charged and nitrate is negatively charged, so nitrate is more mobile

Question 3

Choose the correct statement:

1. Nitrification only occurs under anaerobic conditions
2. Denitrification is an aerobic process that requires a carbon source for energy
3. Nitrous oxide is a potent greenhouse gas with a greater global warming potential (on a per molecule basis) than CO₂
4. The greenhouse gas NO_3⁻ is produced via the process of nitrification

Answer 3

3. Nitrous oxide is a potent greenhouse gas with a greater global warming potential (on a per molecule basis) than CO₂

Question 4

Choose the correct statement

1. Nitrogen only enters the soil in the solution phase (i.e. via surface run-off and/or leaching)
2. Nitrogen only enters waterways when bound to soil particles (i.e. via erosion)
3. Whereas nitrate typically enters waterways in solution, ammonium does so when bound to soil particles.
4. Precipitation of nitrate is required for it to enter the soil in the solution phase.

Answer 4

3. Whereas nitrate typically enters waterways in solution, ammonium does so when bound to soil particles.

Question 5

Choose the incorrect statement:

1. Plants can help reduce the amount of soil nutrient lost via leaching
2. Mycorrhizal fungi can help reduce the amount of soil nutrient lost via leaching
3. Ammonification is the conversion of ammonium to nitrate
4. Reducing N movement from soil into water bodies requires an integrated approach to land management

Answer 5

3. Ammonification is the conversion of ammonium to nitrate

Question 6

Choose the incorrect statement regarding water erosion in sodic soils:

1. Sodic soils are susceptible to gully erosion and gullies produced are often U-shaped
2. Gully erosion tends to extend in an uphill direction in sodic soils
3. Sodic soils are susceptible to tunnel erosion
4. Sodic soils are not susceptible to erosion because their poor structure results in low rates of infiltration

Answer 6

4. Sodic soils are not susceptible to erosion because their poor structure results in low rates of infiltration

Question 7

A farmer has access to two sources of water:

1. river water which has low salinity and low sodicity, and
2. bore water which has high salinity and high sodicity.

If she were to use one source of water for irrigation to grow one crop and the other source of water to grow a subsequent crop, which of the following management sequences would be worst for soil structure:

1. Irrigation with river water for the first crop followed by irrigation with bore water for the second crop followed by addition of gypsum
2. Irrigation with bore water for the first crop followed by irrigation with river water for the second crop followed by the addition of gypsum
3. Irrigation with bore water for the first crop followed by addition of gypsum followed by irrigation with river water for the second crop
4. Addition of gypsum followed by irrigation with bore water for the first crop followed by irrigation with river water for the second crop

Answer 7

2. Irrigation with bore water for the first crop followed by irrigation with river water for the second crop followed by the addition of gypsum

Question 8

Choose the correct statement:

1. Addition of gypsum to saline sodic bore water would decrease SAR
2. Addition of gypsum to saline sodic bore water would not affect its salinity
3. Addition of rain water to saline sodic bore water would increase its SAR
4. Addition of rain water to saline sodic bore water would not affect its SAR

Answer 8

1. Addition of gypsum to saline sodic bore water would decrease SAR

Question 9

Choose the correct statement

1. The “C-factor” of the USLE equation can be decreased by establishing well-maintained contour banks on gently sloping land
2. The “C-factor” of the USLE equation is usually lower for pasture soils than for cropping soils but well managed cropping soils can have a lower C-factor than poorly managed pasture soils
3. By definition, the “C-factor” of the USLE equation is equal to 0 for bare soils cultivated in the same direction as the slope
4. The “C-factor” of the USLE equation relates to erosivity

Answer 9

2. The “C-factor” of the USLE equation is usually lower for pasture soils than for cropping soils but well managed cropping soils can have a lower C-factor than poorly managed pasture soils

Question 10

Choose the correct statement

1. Erosivity of rainfall is generally higher in tropical regions than in temperate regions
2. Erosivity of rainfall is higher when rainfall is spread uniformly throughout the year
3. Erosivity of rainfall decreases with increasing raindrop size
4. Erosivity of rainfall can be decreased by maintaining continuous plant cover

Answer 10

1. Erosivity of rainfall is generally higher in tropical regions than in temperate regions

Question 11

Based on the following table:

1. What is the SAR of the bore water and river water?
2. What is the SAR of a 1:1 mixture of the bore water and river water?
3. What is the ESP of the soil?
4. Would irrigation of the soil with either river water or bore water result in degradation of soil structure? Explain.
5. Briefly describe one recommendation you’d make with regards to management of this soil. Explain.

Answer 11

1. SAR = [Na⁺]/√([Ca²⁺]+[Mg²⁺])

• SAR_bore = 50/√(20+5) = 10
• SAR_river = 6/√(6+3) = 2

2. SAR = (([Na⁺_bore] + [Na⁺_river])/2) / √((([Ca²⁺_bore] + [Ca²⁺_river])/2) + (([Mg²⁺_bore] + [Mg²⁺_river])/2)

• Basically, add the cation concentrations of the two waters together, divide by 2, then calculate the SAR.
• SAR = ((50+6)/2)/√(((20+6)/2) + ((3+5)/2))
• SAR = 28/√(13+4)
• SAR = 28/√17
• SAR = 6.8

3. ESP = [Na⁺]_ex/CEC*100

• ESP = 2/10*100
• ESP = 20%

4.

• ESP > 15, ∴ strongly sodic
• Irrigation with either bore or river water is likely to result in dispersion. Electrolyte concentration is insufficient to overcome the system’s desire to equilibrate the high cation concentration between particles (diffuse double layer forces). River water will disperse soil more than bore water (higher SAR).

5. Apply gypsum to displace the sodium cations from the cation exchange sites and replace them with calcium cations that are held more intimately to the CEC (CEComplex), which will result in reduced diffuse double-layer forces and a smaller diffuse layer, meaning less dispersion.