Questions and Answers (Q&A) --- Biotic System

Ecosystem

1. What is net primary production ? Is it related to energy transfer ?

Soil

1. What causes an unstratified soil be formed in high latitudes?
2. What are bases and humus ?
3. What is the relationship between cations and soil colloids ? Are they important to plant growth ?
4. Is Aridisol reddish or light in colour ? Is it light because of little humus; but reddish because of accumulation of sesquioxides ?
5. What are pedalfers ? Do they mean soils with Fe₂O₃ and Al₂O₃ ? Which types of soil can be treated as pedalfers ?
6. Is it correct to say that both humus and bases make soil dark in colour ?
7. What is the difference between eluviation, leaching, and infiltration ?
8. Can anyone distinguish black peat and hard pan ?
9. Do I need to remember names of chemical elements and compounds to answer questions on soil ?

1. What is the difference between morphological adaptation and physiological adaptation ?
2. Can you give an explanation of sunken stomata ?
3. Transpiration causes plants to lose water, why is there still transpiration ?
4. Can anyone define osmosis ? Is it harmful to plant growth ?

Ecosystem

A: Yes, it is related to energy transfer in a biotic community. (Gross) Primary Production refers to the amount of biomass generated at the producer level. Net Primary Production is the amount after subtracting what is consumed by the producers themselves in metabolism or respiration. You may refer to "primary productivity of major terrestrial biome" for further details.

(answer provided by cm kwok)

A: The unstratified soil in high latitudes is closely related to the freeze and thaw cycle as conditioned by the polar climate. There is a seasonal expansion and contraction of the soil volume which leads to a vertical mixing that prevents soil layers to be clearly defined.

(answer provided by cm kwok)

A: Bases are any chemical compounds that exhibit a pH value above 7. Humus is an organic substance resulted from the decomposition of plant litter. Because of frequent presence of humic and other organic acids, humus uses to have a pH value below 7.

(answer provided by cm kwok)

A: Soil colloids hold the cations (positive ions) in soil solution by ionic attractions onto their surfaces. As a result, their presence affects the cation-holding capacity of a soil. (There is a formal term for the amount of cations that could be held by soil colloids : cation-exchange capacity (CEC)).

Many cations are nutrients to further plant growth. Thus, the role of soil colloids, the clay-humus in particular, is crucial from an agricultural viewpoint. For without the cation-holding capacity as rendered by these colloidal particles, fertilisers applied to the soil surface would be rapidly washed away and wasted. Hence sandy soils with a low clay-humus content have a low CEC and a correspondingly low fertility.

(answer provided by cm kwok)

Q: Is the colour of Aridisol reddish or light ? It is light because of little humus; but reddish because of accumulation of sesquioxides ?

A: A number of substances, including humus and various minerals, may impart colors to soil.

(a) The presence of humus may give the soil a darkish color, especially at the upper horizons. The lack of vegetation in an arid environment may mean less litter and subsequently scant humus, thus freeing the soil from the darkish color of this substance. Also, most Aridisols are saline and rich in salts, such as NaCl and/or MgCl₂ which are white in color. Hence, a light color is not uncommon with Aridisols.

(b) A reddish color is often related to the presence of iron compounds (in ferric or oxidized state, i.e. Fe³⁺ ) in nature. It is not just iron oxides/sesquioxides but also other iron (ferric) compounds that could impart a reddish color to an Aridisol in localities rich in such compounds. Al₂O₃ (aluminium oxides/sesquioxides), on the other hand, is white in color.

(answer provided by cm kwok)

A. Pedalfers refer to soils that experience heavy leaching activities and in which the clay particles are broken down by these activities. Oxisol is one of the examples; in the absence of humic acids, Fe₂O₃ and Al₂O₃ remain insoluble and accumulate in soil. However, this may not be the case for other pedalfers. It should not be generalised that pedalfers are just soils with Fe₂O₃ and Al₂O₃.

(answer provided by cm kwok)

A. Not so for bases. Many base minerals or compounds in soil do not possess a darkish color.

(answer provided by cm kwok)

A: "Infiltration" refers to the movement of water on soil surface into the soil layers below. Both "leaching" and "eluviation" are translocation processes that involve movements of soil matters from one part of the soil to another. For "leaching", the movement of material is in solution, i.e., the material translocated is soluble. For "eluviation", the transport is in suspension, i.e., the material translocated is insoluble.

There is also the closely associated process of "illuviation". It refers to the precipitation of leached materials and/or deposition of eluviated ones. All these processes are significant in the differentiation of soil horizons as follows :

1. those subject to no translocation, only in situ weathering
2. horizon of loss (eluviation and/or significant leaching)
3. horizon of gain (illuviation)

(answer provided by cm kwok)

A: "Peat" refers to the dark, barely decomposed organic matter in the upper soil horizon. (see also apdx 1). "Hard pan" refers to the concretion of minerals in soil (see also apdx 2)

<<< apdx 1 >>>

It's likely that you may have come across various types of organic matter decay, apart from "peat". They are :

1. "mull"
   with neutral or alkaline soil conditions, complete breakdown of organic matter occurs. A multitude of soil animals mix the material evenly into the upper part of soil. This type of organic matter distribution is called "mull".
2. "moder"
   if soils are more acid, decay is slower because there are fewer micro-organisms at work. As a result, a thin layer of litter remains clearly visible on the surface with a layer of partly decayed material beneath it. This arrangement is called "moder".
3. "mor"
   under extremely acid conditions (as under coniferous forest), there are so few soil animals that a thick layer of black, greasy, partly-decayed organic matter lies below the surface litter. This arrangement is called "mor".
4. "peat"
   under conditions of extreme waterlogging, most micro-organisms cannot get sufficient air to survive; plant tissue is little decomposed, instead accumulating on the surface to form "peat"

<<< apdx 2 >>>

Hard pan is an accumulation and hardening of leached material that precipitated out and/or eluviated material that deposited. The formation is closely related to the processes of capillary rise and evaporation, and is favored by the fluctuating ground water level. It reflects a deficit in moisture regime at times.

(answer provided by cm kwok)

A: Chemical compounds are those imparting the characteristics of a soil. It is hard for a clear elucidation of soil features not touching on the diagnostic compounds involved.

As affected by a rigid arts/sciences class streaming, most students of AL geog may lack a chemistry knowledge to help deal with the formulae and reactions. Yet, the AL geog curriculum is not to test you on chemistry knowledge but rather if you can understand the interplay of environmental factors in favouring different sorts of natural formation and landscapes.

The scope of questions examiner can test you on soil is not wide. They cannot go into minute details such as a micro physical or chemical aspect. You stand a good chance in scoring high marks for a soil question if you study your material well. After all, neither do most of your fellow students or teachers possess a strong chemistry background.

For the chemical names and formulae, my personal preference is to use the formulae instead of their corresponding long English names. The former are more convenient to use, easier to remember, and there's a lower chance in committing spelling errors. Questions on soils in Paper I often come with a profile diagram listing out the chemical formulae. So, you need to know these formulae. Also, you will find it neat and quick to annotate your own sketch profiles with chemical formulae in elucidating textual answers.

The chemical formulae or names to remember for exam will be confined in particular to those diagnostic chemicals involved in the formation of Oxisols and Aridisols. You need to refer to your study notes or material for them.

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It will be helpful if you can differentiate the following :

• compound, element, and ion;
  • instead of "iron and aluminium sesquioxides", you can say "compounds such as Fe₂O₃ and Al₂O₃" to avoid remember the long spelling of chemical names. If you are sure the compounds concerned are oxides, you can then say "oxides such as ....".
  • "soluble elements such as magnesium, ... are leached away"; you can write "soluble elements such as Mg ..." likewise.
  • instead of cations (+ve) and anions (-ve), just use positive ions and negative ions. Most positive ions are the soluble bases, such as Na⁺ , Ca²⁺, Mg²⁺. These are important plant nutrients. As you may not be familiar with the ionic valency or chemical periodicity, you may have a difficult time in remembering those superscript plus or minus signs. Try saying "ions of Na, Ca, Mg ...." In this way, you can avoid specifying whether it is a plus or minus; how many number of plus or minus, etc.
  • If you mix up these 3 chemical terms, in the worst case, write this : "chemicals such as Fe₂CO₃ or Na or Mg²⁺". "Chemicals" is a loose term that may represent a compound, an element, or an ion.
• acid vs base (alkaline)
  this has something to do with pH value : <7 --- acidic; 7 --- neutral; >7 --- alkaline; water has a pH value of 7;
• soluble vs insoluble
  being soluble, so easily leached away by downward flowing water; such as most bases as needed by plants for futher growth;
• anaerobic vs aerobic
  anaerobic means oxygen (O₂) lacking as in a waterlogging situation while vice versa for aerobic;
• oxidation vs reduction
  Oxidation is favored in aerobic situation while reduction in anaerobic one. The Fe₂O₃ present in Oxisols is a product of oxidation; so it is less likely to be found in a waterlogging (anaerobic) environment.

(answer provided by cm kwok)

A: Morphological adaptation refers to adaptative features related to "growth form" while physiological to those on "growth behaviour".

1. Morphological :
   Let's take desert vegetation as an illustration. Some plants may have needle-shaped leaves. The reduced leaf surface area helps cutting down water loss through evapo-transpiration. This adaptation is related to the growth form / morphology. Examples of this category also include waxy leaf surfaces, succulent stems, and deep tap root or a dense shallow root network, etc.
2. Physiological :
   Still on the issue of cutting down water loss through evapo-transpiration. Other plants may choose to turn their leaves edgewise to direct solar insolation or some will have their stomata (pores on leaves where water vapour transpires) closed intermittently. These adaptations are related to growth behaviours or physiology.
3. Synecological :
   There is still a third category of adaptative feature related to the (plant) community --- synecological. In a desert environment where moisture is a limiting factor, plants of the same community may be found widely spaced apart. This may help reduce intra-specific competition. The root system of an individual plant may have a greater area in collecting moisture with this spatial arrangement.

(answer provided by cm kwok)

A: Stomata are small openings found on the underside of leaves that are connected to vascular plant tissues. Water from the plant, in vapour form, is lost through these openings to the outer atmosphere (the process of transpiration).

Sunken stomata refer to those stomata sunk into the leaves, thus allowing small pits of outer air be formed in-between these openings and the wider atmosphere. These pits are kept moist by vapour transpired through the openings.

Under an arid climate, such structures help moderate the humidity gradient between the stomata and outer atmosphere. As transpiration is conditioned by atmospheric humidity, the moderated gradient helps slow down transpiration, thus cutting down water loss through this process.

(answer provided by cm kwok)

A: If a process is justified by the functions it performs, then transpiration is there for

• the transpirational pull it sets up which aids translocation of water and nutrients from soil into roots and then to various parts of the plant,
• the cooling effect it brings upon which helps keep the plant from becoming overheated.

(answer provided by cm kwok)

Q: Can anyone define osmosis ? Is it harmful to plant growth ?

A: Osmosis is the movement of water molecules through a semi-permeable membrane from a solution of lower solute concentration (or higher water concentration) to one of higher solute concentration (or lower water concentration). The semi-permeable membrane is selectively permeable. Because of its ultra-microscopic pores, it allows very small molecules, e.g. water, to pass through whereas large molecules not.

In the uptake of soil water by plant root, the cell sap of root hairs, which contain accumulated salts and sugar, is often a more concentrated solution than soil water. By osmosis, water moves from soil to root cells. It is, in fact, a very important process to plant growth.

However, in a saline soil where the soil solution is more concentrated than the cell sap, water will be drawn out from root cells by osmosis. This is a crucial reason why saline soils are harmful to most plant growth.

(answer provided by cm kwok)