AGR3LS Exam Revision

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  1. What property contributes to the substantial difference in the ability of soil colloids, in comparison to the silt and sand fraction, to hold nutrients?
    Clay and organic colloids have a substantial capacity to hold nutrients due to the much greater surface area. Clay has more than 1000 times the surface area of sand.
  2. How are plant nutrients held in the soil?
    Adsorbed, by electrostatic attraction to the negatively charged surfaces. This enables an exchange process to occur between the soil solution, and the colloid surface. If nutrients are too strongly adsorbed, such as when P is strongly sorbed through specific adsorption, then exchange does not readily occur.
  3. Describe the process where plant roots may acquire nutrients from the soil solution, and how do we measure the capacity of soil to hold nutrients?
    Nutrients become available in the soil solution through ion exchange processes. Cation exchange is measured using a leaching solution of a salt solution of a cation, such as ammonium chloride. Cation exchange is many times greater than anion exchange.
  4. Name the sources that supply nutrients to the soil?
    SOM, atmosphere, primary and secondary minerals.
  5. What are exchangeable bases and which one of these is not essential for plant requirements?
    Ca, Mg, K are plant nutrients, while Na is a non-essential plant nutrient.
  6. Describe the process of N supply to plants. What form of N do plants mostly acquire from the soil solution?
    Mineralization of organic matter is the major source of N to plants. However, the fixation of N by legumes is another major source, especially in some low nutrient natural ecosystems. Most plants will acquire their N as nitrate, although in some acidic soils plants have adapted to take up ammonium.
  7. Of course some cations in the soil solution are not essential for plant growth. These can become increasingly soluble at low pH, and hence toxic to plant growth. What are these, and at what pH do these become more soluble?
    Below pH the concentration in the soil solution of Al increases to a level that becomes toxic for plants. The other metal cation is Mn, which can also increase to toxic levels in acidic soils.
  8. What property is easily measured in the laboratory and has a profound effect on many soil physical properties? What is this measurement? Hint: you will need to collect an intact core of soil.
    Soil porosity can be measured/estimated by the measurement of soil bulk density.
  9. What sort of pores will be water-filled at Field Capacity?
    Meso- and micro-pores.
  10. What pores are able to hold water most tightly against gravitational force. What is the pore size, and the matric potential of this water?
    Micropores hold water that has a MP of -1500 kPa (Wilting point). This water is considered unavailable to plants. The micropores would be 100 nm.
  11. Usually loam soil is considered to have a ‘well balanced’ distribution of pore sizes. Thus loams usually have good infiltration, and water holding capacity. However, what can be added to a loam soil to aid water retention?
    Organic matter can improve the water-holding capacity.
  12. What component of the soil may strongly affect water-stable aggregates? Especially with respect to slaking.
    Organic matter
  13. Describe the changes that occur to the distribution of pore sizes when well-structured clay is compacted? What are the changes in plant available water?
    Compaction largely reduces the macroporosity, thus the infiltration/drainage will be affected. However, there is also the possibility that there some compaction of mesopores, and hence a slight reduction in the plant available water.
  14. Hydraulic potential is the sum of which two components? This is important for predicting water movement. At what conditions is water likely to move?
    Hydraulic potential is the sum of matric and gravitational potential. Water will move from a high (such as a saturated soil) to low potential (dry soil).
  15. Soil structure can be measured in many ways, but one of the most useful is to study the soil profile. What important property does a well-structured soil (visible aggregates) have in comparison to a poorly-structured soil? What does this affect in the soil?
    Well-structured soil will have a much higher porosity than soil with few large aggregates. This facilitates good aeration, water movement, biological activity and root growth.
  16. What do clusters consist of?
    Clay and organic material, including bacterial and fungal cells.
  17. What promotes stable macroaggregates? What management technique can enhance this process?
    The presence of roots and fungal hyphae can be promoted through the use of no tillage.
  18. What are sesquioxides and in what soils are they most abundant?
    These are amorphous iron oxides, which are particularly abundant in highly weathered soils.
  19. When is goethite most likely to form?
    Although these are both forms of Fe(III) oxides, the hydrated form, Goethite is predominant in moister and cooler environments.
  20. Soils that consist of a high proportion of the clay mineral Illite are considered to have an adequate level of K. Where are these K+ ions located?
    Illite contains a inter layer of free K+ ions. Weathering of the mineral releases these ions into the soil solution.
  21. Why does Montmorillonite swell and Kaolinite not?
    As montmorillonite is weakly bonded between the crystal layers, water can enter between, and enlarge the space between these layers. In comparison, Kaolinite is strongly bonded by hydrogen bonding.
  22. Define salinity and sodicity?
    Saline soils are defined when the EC> 4 dS/m. Sodic soils have ESP>6%.
  23. What physical constraints are imposed in sodic soils? How can we alleviate these constraints?
    Poor soil structure, which can lead to reduced infiltration, aeration and prone to waterlogging. Although gypsum can be useful for these soil structure, the use of organic matter can be more beneficial in the long term.
  24. The electrical conductivity of a soil is measured in the extract of:
    a)  1 M ammonium chloride in water
    b)  0.1 M HCl
    c)  deionised (distilled) water
    d)  0.01 M calcium chloride in water
  25. Which of the following statement is NOT true of slaking?
    a)  Slaking occurs on rapid wetting
    b)  Slaking is caused by double layer swelling
    c)  Wetting under vacuum prevents slaking
    d)  Dry aggregates are more prone to slaking than are wet ones
  26. Which of the following is not a component potential of the total soil water potential?
    a)  Osmotic potential
    b)  Matric potential
    c)  Gravitational potential
    d)  Electric potential
  27. Which two soil physical properties are considered by the least-limiting water range?
    a)  Bulk density and porosity
    b)  Porosity and soil strength
    c)  Soil temperature and porosity
    d)  Aeration and soil strength
  28. Micropores are most important for:
    a)  Storage of soil water.
    b)  Drainage of soil water.
    c)  Aeration of wet soil.
    d)  Determining the air-dry moisture content.
  29. Which of the following is true of a soil drying below field capacity?
    a)  Water moves from zones of high suction to zones of low suction
    b)  The soil may be dried by evaporation and transpiration
    c)  All the micropores are air-filled
    d)  The rate of soil water movement is faster than at saturation
  30. Which of the following makes water move from soil B to soil A?
    a)  A is a clayey soil; B is a sandy soil (Both soils have the same Ψt)
    b)  B contains a higher level of salt than does A (A and B are the same type of soil and have the same θv)
    c)  A has Ψt = -100 kPa; B has Ψt = -20 kPa
    d)  A is a sandy soil; B is a clayey soil (Both soils have the same θv)
  31. To generate a water retention curve of a soil, which of the following method can be used?
    a)  Neutron moisture probe
    b)  Pressure plate
    c)  Tensiometer
    d)  Time domain reflectometry (TDR)
  32. Waterlogging of soil increases the availability of which of the following nutrients?
    a)  Manganese (Mn)
    b)  Nitrogen (N)
    c)  Calcium (Ca)
    d)  Potassium (K)
  33. Which of the following fertilizers is preferred for a paddy (waterlogged) soil to minimize nitrogen loss? (All the fertilizers are applied at the same rate of N)
    a)  Calcium nitrate [Ca(NO3)2]
    b)  Urea [(NH2)2CO]
    c)  Ammonium nitrate (NH4NO3)
    d)  Ammonium sulphate [(NH4)2SO4]
  34. Which of the following is (are) greenhouse gasses contributed to the atmosphere by soils?
    a)  Nitrous oxides
    b)  Carbon dioxide
    c)  Methane
    d)  All above
  35. 0.01M CaCl2 is recommended for the measurement of soil pH because:
    a)  0.01M CaCl2 is more susceptible to any current seasonal variation
    b)  after the 30 minutes standing, the Ca2+ present has coagulated the soil suspension giving a clearer supernatent necessary for measuring soil pH
    c)  0.01M CaCl2 is well buffered compared with water
    d)  0.01M CaCl2 has similar ionic strength to the soil solution
  36. Which of the following fertilizers acidifies soil least when applied to the soil? (All the fertilizers are applied at the same rate of N)
    a)  Ammonium nitrate (NH4NO3)
    b)  Ammonium sulphate [(NH4)2SO4]
    c)  Calcium nitrate [Ca(NO3)2]
    d)  Urea [(NH2)2CO]
  37. Exchangeable cations in the soil may be determined by analysing the filtrate obtained after leaching the soil with
    a)  Aqueous ethanol
    b)  Ammonium chloride solution
    c)  Calcium and potassium nitrates in solution
    d)  Pure water
  38. Permanent negative charge in the soil results from
    a)  unfilled positions in clay crystals
    b)  substitution of ions with others of higher positive charge in clay crystals
    c)  clay crystal edges upon reaction with water
    d)  substitution of ions with others of lower positive charge in clay crystals
  39. To completely exchange 30 cmolc/kg of Na+ in soil, we need:
    a)  10 cmol/kg of Al3+
    b)  20 cmol/kg of Ca2+
    c)  30 mmol/kg of K+
    d)  30 cmol/kg of Fe3+
  40. For a soil with a CEC of 20 cmol(+)/kg, what is the maximum number of moles of Ca2+ that could be adsorbed by 1 kg soil?
    a)  0.1 mol
    b)  0.2 mol
    c)  1 mol
    d)  2 mol
  41. For a soil with a CEC of 200 cmol(+)/kg, what is the maximum number of moles of Ca2+ that could be adsorbed by 1 kg soil?
    a)  0.2 mol
    b)  0.5 mol
    c)  1 mol
    d)  5 mo
  42. Which of the following has the most availability of potassium (K) to plants?
    a)  K on exchangeable sites
    b)  K in primary minerals
    c)  K in soil solution
    d)  K in interlayer of clay minerals
  43. Which of the following has the least availability of potassium (K) to plants?
    a)  K on exchangeable sites
    b)  K in primary minerals
    c)  K in soil solution
    d)  K in interlayer of clay minerals
  44. Which of the following micronutrients is most likely to be deficient in acid soils?
    a)  Iron (Fe)
    b)  Manganese (Mn)
    c)  Zinc (Zn)
    d)  Molybdenum (Mo)
  45. Plants that are efficient in iron (Fe) uptake in a calcareous soil are known to employ which of the following methods?
    a)  Lower rhizosphere pH
    b)  Produce reducing agents that reduce Fe3+ to Fe2+ in the rhizosphere
    c)  Produce organic compounds (phytosiderophores) that can form stable chelates with iron
    d)  All of the above
  46. Which of the following statements is correct?
    a)  Divalent manganese is oxidized by the addition of lime to forms that are not toxic to plants
    b)  Elemental bioavailability of both Mn and Mo decreases with an increase in soil pH
    c)  After extraction, manganese is most commonly measured by flame emission spectroscopy
    d)  The rule of thumb is that liming soil to pH 5.5 – 6.0 will overcome manganese toxicity
  47. Which of the following crops would you recommend the farmer to grow if his paddock is contaminated with cadmium?
    a)  Carrot
    b)  Grazing pasture
    c)  Wheat
    d)  All above
  48. Which of the following statements is correct?
    a)  Plants improve phosphorus uptake by increasing the length and density of root hairs
    b)  Release of organic acids by plant roots increases phosphorus availability in soils
    c)  Rhizosphere acidification increases phosphorus uptake by plants grown in alkaline soils
    d)  All above
  49. Soil compaction can adversely affect plant growth in several ways. Which of the following is least likely to be a consequence of soil compaction?
    a)  Poor soil aeration
    b)  High soil strength
    c)  P deficiency
    d)  High soil temperature
  50. From your understanding of the behaviour of ions in the soil solution, which of the following would you expect to be the most likely to contaminate groundwater?
    a)  K+
    b)  H2PO4-
    c)  NO3-
    d)  Ca2+
  51. Which of the following statements is NOT correct?
    a)  Reduction of sulfur is mediated by bacteria Desulfovibrio spp.
    b)  Oxidation of sulfur is mediated by bacteria Thiobacillus spp.
    c)  Net mobilization will occur and plant available S will be released during decomposition of organic materials with the C:S ratio > 400:1
    d)  Organic sulfur can be mineralized by high temperatures
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AGR3LS Exam Revision
AGR3LS Exam Revision
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