Module M4 „ Soils“: Examples of questions to prepare the exam
These questions are illustrative and will give you an idea about the questions you can expect during
the exam. This list of questions is not complete and do not expect to get exactly the same questions
during the exam! Use the information we provided during the lectures and the literature we
recommended to prepare for the exam and answer these questions.
1. What are the three main groups of soil functions? Explain one soil function in more detail! (4
points)
2. Give three ecosystem functions of soils and explain one in more detail! (4 points)
3. Explain the “clorpt” formula and how man is related to that! (6 points)
4. Explain the catena principle! (3 points)
5. What kind of different soils can be expected on a sandy and acidic bedrock in comparison to a
calcareous bedrock? Give two different soil forming processes and two soil types (one for each
of the two types of bedrock)! (4 points)
6. Give an example of a soil chronosequence (at least 3 soil types) developed on sandstone in a
humide climate! (3 points)
7. Sketch the general structure of a soil profile (from the topsoil to the subsoil) including the three
main soil horizons specified by one soil forming process, i.e. the principle depth gradient of soil
horizons of one soil type! (6 points)
8. Give five global challenges in soil sciences and explain one in more detail! (6 points)
9. Describe the principle structure of a three-sheet silicate and explain the main differences
between a three-sheet and two-sheet silicate! (3 points)
10. Explain the process of isomorphic substitution! What does it mean for the charge of the
minerals? (3 points)
11. Give three properties of clay minerals! (3 points)
12. Which properties of the silicates determine their weatherability? (3 points)
13. Give the most important oxides / hydroxides in soils (3 points)
14. Give the two most important chemical reactions of weathering and the three main principles! (5
points)
15. What is the chemical reason that olivine can be used to sequester atmospheric CO2? (1 point)
16. Describe the chemical weathering (dissolution) of lime stone! Why does increasing CO2
concentration in the atmosphere contribute to increasing chemical weathering of lime stone? (3
points)
17. The weatherability of biotite, calcite, quartz, potassium feldspar (orthoclase) and olivine differs
largely. Please provide the order of stability against weathering (starting with the least stable
mineral) and explain the reasons for that! (5 points)
18. Why does biotite (K(Fe1.5Mg1.5)(Si3Al)O10(OH)2) weather much faster than muscovite
(KAl2(Si3Al)O10(OH)2) although both phyllosilicates have a similar structure (2:1 minerals)? Give
two reasons! (2 points)
19. Why do many clay minerals have a permanent negative charge – explain the reasons for that! (2
points)
20. Explain what is meant by the point of zero charge (PZC) and how it is possible that some soils
have a point of zero charge while others do not. (5 points)
21. Indicate in the figure below the point (pH value) of zero charge of the different minerals! (2
points)
Figure 1: Charge (C) of different minerals (positive or negative charge per m2) in dependence on pH
22. Which minerals adsorb Mg2+ at a pH of 4.5 and which minerals adsorb NO3- at a pH of 7? (2
points)
23. What is the difference between potential and effective cation exchange capacity? (1 point)
24. What are the reasons for the variable charge in soils and give at least two carriers of variable
charges in soils (3 points)
25. Explain how the charge and the hydration effects determine the selectivity of cation exchange!
(2 points)
26. What is meant by the cation exchange capacity of soils and how is the base saturation of a soil
defined? (2 points)
27. Explain why Al3+ and Fe3+ are acid cations? (1 point)
28. Describe the structure of the electric double layer in soils, give two factors controlling the size of
the electric double layer and explain the relationship between the electric double layer and
dispersion of clay! (6 points)
29. What are the most important two differences between an inner-sphere and an outer-sphere
complex? (2 points)
30. What are the differences between the Langmuir and the Freundlich model describing adsorption
processes in soils? (2 points)
31. Please provide four H+ sources that contribute to soil acidification. Describe for two of these
sources the formation of H+ in more detail by giving the appropriate chemical reaction. (6
points)
32. What will happen with the base saturation of soils during soil acidification and why? (2 points)
33. Why does harvesting of biomass (e.g., deforestation) contribute to soil acidification? (2 points)
34. Which buffer processes dominate in (i) young soils with a neutral pH and without any
management and in (ii) an acidic forest soil (pH=4). Which soil properties determine the buffer
capacities in both soils? Describe the most important soil chemical changes by H+ buffering in
these two soils! (6 points)
35. Which buffer processes do you know and which changes in soil properties are related to these
buffer processes (10 points)? At which pH value in the soil do the four main buffer systems
work? (4 points)
36. Where do most of the electrons involved in redox reactions in soils come from?
37. Besides oxygen, other electron acceptors are present in soils. In which order will the following
ions / molecules be reduced if the redox potential decreases continuously (SO42-, Fe3+, O2, Mn4+,
NO3-)?
38. What are the main controls of O2 availability in soils? (2 points)
39. What are the main reasons for soil salinization? (3 points) and what parameters can be used to
determine the degree of salinization? (3 points)
40. How can the different solubility of various salts be used to determine the source (reason) for
salinization? (2 points)
41. Give the main textural soil classes! Which textural soil class can hold the least water against
gravity and why? (5 points)
42. Why can soils with the same pore volume have a different water holding capacity? (2 points)
43. Use the textural triangle to determine the soil textural class based on measured particle size
distribution (e.g. 15% clay, 15% fine silt, 20% medium silt, 10% coarse silt, 10 fine sand, 20%
medium sand, 10% coarse sand)
44. What is the main principle of the analysis of particle size distribution by sedimentation? (1 point)
45. Soil texture is one of the most important soil properties. Give four different soil properties
affected by soil texture and assess the change of two soil properties if texture change from sand
to clay! (6 points)
46. Explain the terms bulk density and porosity and give the necessary information to calculate bulk
density and soil porosity? (6 points)
47. Give the four main pore size classes, their equivalent diameters and their functions (12 points)
48. Which textural class contains most medium pores? (1 point)
49. Why is it necessary to disperse the soil prior soil texture analysis? (1 point)
50. Indicate a typical soil textural class for each of the given bulk density: 0.3, 1.0, 1.5 g cm-3! (3
points)
51. Describe the difference between soil texture and soil structure? (2 points)
52. Why is soil structure important for plant growth? (5 points)
53. Define soil water potential and give the different soil water potentials you know! (6 points)
54. What is the difference between saturated and unsaturated water flow in soil? What are the
main drivers and which pores are involved? (4 points)
55. Sketch a typical water retention curve of a sandy soil vs. a clay soil! (xy diagram: soil water
content / suction)! Indicate field capacity and the permanent wilting point at the y axis!
56. What is the main difference between autotroph and heterotroph organisms and give two
examples for each group of organism! (4 points)
57. What is the main difference between phototroph and chemotroph organisms and give two
examples for each group of organisms! (4 points)
58. The contents of organic carbon follow distinct patterns during soil development with depth.
Describe and sketch this general trend! (2 points)
59. The contents of organic carbon follow distinct patterns during soil development with time.
Describe and sketch this general trend! (2 points)
60. There are important exceptions from these general trends with depth and time related to
distinct soil forming processes and anthropogenic disturbances, respectively. Describe the effect
of at least one soil forming process and one anthropogenic disturbance on these general trends!
(4 points)
61. What are the main sources of soil organic matter? Give at least four different groups of
compounds! (4 points)
62. The stabilization of organic matter against microbial decay and therefore organic matter
accumulation in soils is mainly controlled by three different mechanisms. Provide these three
mechanisms and describe the key principle of each of them! (6 points)
63. Discuss the reasons for the strong binding of organic matter (e.g., organic acids) to Fe oxides.
Why is the binding particularly strong at a soil pH of 5? Take the point of zero charge of the Fe
oxides (ca. 9) and the pKa value of many organic acids (ca. 5) into account. (3 points)
64. What are the differences between a mull and a moder humus? Describe three main different
properties and give the most important reason for the observed differences! (4 points)
65. What are typical C/N ratios in a mor and a mull humus? (2 points)
66. What does a first order kinetic decomposition of organic matter mean in terms of changes in the
rate of decomposition over time? (3 points)
67. Explain, in terms of the balance between gains and losses, why cultivated soils generally contain
much lower levels of organic carbon than similar soils under natural vegetation. (2 points)
68. Calculate the amount of carbon (kg) stored in 0 - 25 cm of 1 ha arable soil with an average
content of organic carbon of 1.5%! Assume a bulk density of 1.35 kg dm-3.
69. What is the main difference in the composition of soil air in comparison to “normal”
atmospheric air and what is the main reason for this difference? (2 points)
70. Please explain the differences between autotrophic and heterotrophic organisms and
phototrophic and chemotrophic organisms! (4 points)
71. What positive effects of mycorrhiza fungi in soils do you know? (2 points)
72. Explain main functions of the soil macro-fauna? Why earth worms are particularly important for
soil functioning? (5 points)
73. What are the reasons for the high diversity of microorganisms in soils? (2 points)
74. Explain main functions of soil microorganisms? (4 points)
75. What is mineralization of organic matter? (2 points)
76. Which environmental factors are decisive for microorganisms in soils? (6 points)
77. Give the most important forms of plant available nitrogen in soils! Which of the main processes
of the N cycle are closely related to the formation of available N forms in soils? (4 points)
78. Which form of nitrogen in soils is particularly prone to leaching and why? (3 points)
79. What is the main reason that stocks of nitrate are much larger than stocks of ammonium in
most soils? (1 point)
80. Describe the conditions favoring the volatilization of ammonia from soils! (2 points)
81. Describe the conditions favoring losses of N from the soil in gaseous forms by denitrification! (4
points)
82. Why is soil texture (particle size distribution) an important property for controlling
denitrification (microbial reduction of nitrate) in soils?
83. Why is the leaching of phosphate from soils with a pH value of about 7.5 and below 4.5 very
low? (2 points)
84. Why is soil aeration (availability of O2) important for the leaching of nitrate from soils? Explain
the relationship between soil texture (grain size distribution) and soil aeration! (3 points)
85. What are the reasons for the ambivalent consequences of denitrification for the environment,
i.e. for water and air? (2 points)
86. Why is leaching a much larger problem for nitrate than for phosphate? (2 points)
87. Which measures can prevent or decrease soil erosion by water?
88. Give at least two examples for each of the main groups of soil formation (transformation,
translocation, turbation). Give at least one typical soil horizon and one typical soil type to each
of these three main groups of soil formation!
89. Explain the steps that lead to clay eluviation, resulting in the formation of a luvisol on lime-rich
parent material! (7 points)
90. Which diagnostic horizons (WRB) are characteristic for well-developed podzols? Give the names
and at least one characteristic property! (3 points)
91. Explain the four important steps of the soil forming process podsolization! (4 points)
92. Which site conditions favor podsolization? (4 points)
93. What are the main differences between a Stagnosol and a Gleysol? Which diagnostic properties
can be used in the field to distinguish both soil types (6 points)
94. What is the difference between soil type and soil texture? (2 points)