Final Exam Questions

Please post questions about the Final Exam here.

1. I don't quite understand how humic substances are formed. From lecture note, it says "they are formed within the soil by breaking down organic matter and synthesizing new ones". Which organisms are responsible for this process and how are new organic matter formed?

2. Which physical process will contribute to aggregate formation, freezing of thawing? and Why?

Thank you in advance!

1. Humic substances are indeed formed as a result of various synthesis processes carried by numerous soil organisms (e.g. bacterial, fungi, actynomicetes).

2. Freezing / thawing process brings soil particles closer together, allowing flocculation to take place, which in turn enhances aggregation

Hi, Maja, I was wondering when do phosphate leaching become a problem?

Pollution of underground water

1. Humic substances are indeed formed as a result of various synthesis processes carried by numerous soil organisms (e.g. bacterial, fungi, actynomicetes).

2. Freezing / thawing process brings soil particles closer together, allowing flocculation to take place, which in turn enhances aggregation

Thank you! In the 2008 Final, I had trouble identifying the soil order of the following soil:

LFH( 5-0 cm)

Ah (0-8 cm)

Cgj (9-40 cm)

(mixed firest under cold and humid climate, somewhat restricted drainage)

I can't decide if this is a gleysolic soil or regosolic soil. It does not have a B horizon, which makes me think it is a regosol. However, the cgj layer is greater than 10 cm, which is characteristic of gelysol.

This is a Regosol since there is no B horizon.

Hi, Maja. For this 2008 final question, is that because the Cg horizon also has a j, which means the Cg horizon is not obvious, so it's not gleysol ? Or is there some other reasons? Thanks!

you are correct, this is a Regosol since glaying soil formation process is only at its beginning, as indicated by "gj" in the Cgj

So when there are both regosolic and chernozem diagnose (e.g., has an Ap of more than 10cm, and doesn't has a B horizon) I should still recognize it as chernozem ?

I am not sure that I understand your question..... Ap is not a diagnostic horizon for any of the 10 soil orders

Soil without B horizon is Regosol

This question was used in one of the class discussions, and also in an old final exam. What soil properties enhance nitrate leaching in soil?

Thank you!

Hi Julia, Could you pls re-post your question since a big part of it got cut off and is not visible

Hi Maja,

Sorry about that I did not realize it was cut off. Hopefully this works!

I was just wondering what soil properties would enhance the loss of nitrate via leaching from a soil?

Thank you!

OK< got it now. I already answered that question (on April 14) under "General course questions" stream

Hi Maja!

"List 4 soil properties that favor nitrate (NO3) leaching losses from the soil" is a question from a past final. What does it mean by soil properties? Like aggregation, denitrification etc?

yes, soil aggregation, denitrification are soil properties, but they are not examples of soil properties that are most directly related to nitrate leaching losses. High soil water content, dominance of large pores, coarse texture, high number of negative charges on soil particles are directly leading to high nitrate losses.

Could anyone tell me what these soils are? LF (5-0 cm)

Ah (0-8 cm) 
Ck1 (8-43 cm) 
Ck2 (43-100 cm) 

(mixed forest under cold and humid climate, good natural drainage) and... L (8-6 cm) 4

F (6-0 cm) 
C1 (0-3 cm) 
C2 (3-18 cm) 
C3 (18-65 cm) 
(tundra under extremely cold and humid climate, good natural drainage)

Thank you!

Both of those soils are Regosols, because they do not have B horizon since soil formation is limited in a cold climate.

Hi Maja,

Is absence of B horizon alone enough to classify a soil as Regosol? If, say, there is no B horizon but A horizon is >10cm thick, or there is no B horizon and A horizon is thin but C horizon is gleyed, would these soils still be classified as Regosol?

Thank you

What is hydraulic equilibrium?

This is a soil condition under which total water potentials throughout that soil are the same. Hence, there is no water movement (since there is no difference in water total potentials)at the hydraulic equilibrium

Hi Maja, am am wondering if the final exam will be cumulative of the entire course, or if it will just mainly focus on the material we learned after the midterm? In other words, should I review all the material prior to the midterm? I am sure this was talked about in class during the last week, however I missed the last week of class. Thanks!!!

The final will focus on the material covered after the midterm exam, but since the nature of the soil science material is such that numerous concepts are connected, you cannot completely overlook the material covered in the 1st half of the course. For example, question from our discussion no.3 re. the Goose soil is an excellent example of a question where numerous concepts will need to be summarized.

Hey Maja, for explaining why a Bn, Bf, Bt, horizon exists could we say that they were illuviated by substances from the A horizon and that they were enriched by the C horizon. If you had parent material with a high clay content couldn't that lead you to having a Bt horizon? I am just making sure that we should incorporate both the C and A when describing the factors that created the B. Thank you!

Some materials (clay, salts, organic matter) are brought into the B horizons from either A and/or C horizons. But then there are also numerous soil formation processes that occur in the B horizon itself. Some examples of those processes are gleying (Bg), shrinking/swelling of clay (Bss and Bv), weathering of minerals (Bf), freezing/thawing (By)...

Hi Maja, I was wondering about question 2 on the second practice final. The question provides a graph showing CEC of Humus and of Smectite changing with pH. I understand that the organic colloid will be entirely pH-dependent because of their carboxyl group, so CEC will change linearly with increasing pH. I also understand that smectite is similar to, or contains, montmorillonite and will have a high proportion of isomorphic substitutions which are pH-independent (CEC is constant up until pH 6.0)... however, I do not understand what is going on after the pH 6.0 mark for smectite. Why does CEC increase here? Thanks in advance!

Smectite (which is a group of phyllosilicates that includes mineral montmorillonite) predominantly has a constant (not pH-dependent) type of charge. However, OH groups on the broken edges & corners of smectite minerals will end up with the pH-dependent charges when reacting with either H ions (in acidic soil solution) or OH ions (in alkaline soil solution). Hence, the slight increase in CEC on smectite with the increasing pH.

By the way, that graph was taken out of your textbook and more details can be found there

Hi Maja, could you please explain what do C and N represent for in C/N ratio?

C = carbon, N= nitrogen Importance of the C/N ratio in soil organic matter is related to its susceptibility to decomposition. Organic matter with C/N < 25/1 is easily decomposed by soil organisms and as a result of that decomposition available N is released and it becomes available to plants

Hi Maja, I am feeling a bit unclear about determining soil pH from CEC and BS (question 7c on practice final #2). Could you explain the relationship? I thought that there would be a high amount of base cations on the colloid surfaces at low pH, because the base cations could not be in their ionic form in acidic conditions... but then I read somewhere that for a soil like the one in this example (loamy texture, base saturation 87.5, CEC 32) that the pH would be near neutral. Thank you!

Soil that has high base saturation (BS) has to have alkaline pH, since its exchange complex (or diffuse double layer)is dominated by the base-forming cations.

Cation exchange capacity (CEC) on its own does not tell us anything about the type of cations, it only gives us the total number of cations that that soil can adsorb. Hence, from the CEC itself (and without any other info) we cannot make any assumption about BS.

I am not sure what exactly was the question 7c (don't have time to go and look for it now) but I believe that it asked something along the line to choose one of 4 pH ranges based on the various types of info given in that question. Based on the high BS in that soil the most likely pH range would be around 7.5 to 8.5

Hi Maja, could you please tell me what is the distinction between soil association and soil complex? Thank you very much.

pls see lecture notes from March 28th on Soil Mapping

Yes Maja, I read through the lecture notes and I also asked several friends. However, none of them can clearly demonstrate what should we write when we are required to write the distinction between two terminologies. For example, if I write the distinction between soil association and soil complex, I would write "Soil association only represents one group of taxonomic soil, which is simple for people to predict the utility of that soil. Soil complex consists of several groups of taxonomic soil which intermix together, so soil complex is complicated for people to manage." Does it sound right? I always feel that I am repeating the definitions of those two terminologies. Thus, with all respects, could you please provide a sample distinction about soil association and soil complex for us? (By the way, all of my friends in APBI200 got -3 for all the sub-questions of the first question of the midterm. That's the reason why we really want to read a standard distinction.)

Thank you very much!

Here are the definitions (copied from the lecture notes) Soil association. A mapping unit in which a group of taxonomic soil units occur together in a regular pattern in about the same relative proportions throughout a geographic region (comparable to plant associations in many ways).

Soil complex. A mapping unit used in detailed soil surveys where two or more defined taxonomic units are so intimately intermixed geographically that it is undesirable or impractical (because of the map scale being used) to separate them.

Distinction: Soil complex represents a more intimate mixing of smaller areas of individual taxonomic units than that described under association. Also, these small areas of individual taxonomic units are not present in the same relative proportions within the soil complex.

Got it! Thank you!

Hi Maja, I'm slightly confused about question 5 on the 2005 final exam: List and briefly explain inputs and losses of soil phosphorus and sulfur.

I got that soil phosphorus will be lost through plant removal, erosion of phosphorus-carrying soil particles, and phosphorus dissolved in surface runoff water. However, I'm not too entirely sure what the inputs are.

Also, I looked all around online and in the textbook and was only able to find the following for inputs and outputs of soil sulfur, are these correct? Inputs: Use of highly concentrated fertilizers, mineralization of organic matter in release of plant-available sulfur Losses: Intensive cropping systems that increase crop yields causing more removal of sulfur from the field

Thanks for your help, Sophia

The most efficient way to answer this question would be to go to the lecture notes from March 14 and to check P & S cycles that are included there. All the losses are indicated by the orange lines, while all the inputs are indicated by blue lines.

If those slides are too small, you can also access them at our SoilWeb200 online tool (http://soilweb200.landfood.ubc.ca/). P cycle is available at http://soilweb200.landfood.ubc.ca/soil-biology/nutrient-cycles/2-phosphorus-p/, while S cycle is available at http://soilweb200.landfood.ubc.ca/soil-biology/nutrient-cycles/3-sulphur-s/

Hi Maja,

While studying the discussion question, im wondering how specific should we be when providing examples for the 4 soil formation process?

For example, as losses, is "calcium carbonate can be removed in the soil profile" specific enough, or should we be more specific: " Calcium carbonate in a soil layer dissolves in water that is percolating through the soil. The water flows deeper into groundwater and eventually enters a stream" ?

Thanks!

"calcium carbonate can be removed in the soil profile" is not finished or complete. Removal of salts (carbonates) can occur in several ways. Hence, your 2nd version is much better.

On the practice finals, specifically Question 3 from the 3rd Practice Final, it asks: "Discuss environmental and soil conditions under which following soil horizons can be present:". Can you please specify what qualifies as an environmental factor compared to what qualifies as a soil conditions?

Examples of environmental factors are: wet/dry climate, hot/cold climate, steep slope, flat topography, forest vs. grassland ecosystem, etc.

Examples of soil conditions are: poor / good drainage, saturated/unsaturated soil conditions, anaerobic/aerobic soil conditions, acidic / alkaline soil reaction, saline / non-saline soil, etc.

You shouldn't worry about would you lists something under environmental factors or soil conditions, as long as all relevant things are included into the answer for each horizon. We had this type of question at our discussion no.3

Hi Maja, on the 2004 final, the question 5b wants us to identify the soil order. (b) Ah (0-13 cm), Ae1 (13-25 cm), Ae2 (25-36 cm), Bt (36-66 cm), Btj (66-86 cm), Ck (86+ cm). (deciduous forest under humid to sub-humid climate; good natural drainage) I am hesitating between chernozemic order and luvisolic order. Based on the lab manual, chernozemic order should be identified before luvisolic order. However, the lab also said that the leaching process of luvisol is more intensive that chernozem, and that soil sample has a really huge Ae horizon. Could you please tell me which one is correct? Thank you very much!

That's my last question.

^Hey DepengR8198uan I am almost certain that soil is a luvisol, you wouldn't find a chernozem in the environment that this soil is referenced in. Chernozems are more in continental grassland environments.

Thanks Gwendolyn, I also think luvisol should be the correct one.

I second what Gwen has said. This soil is a Luvisol (diagnostic horizon = Bt, which is 30 cm thick). And as Gwen has said, you cannot have a Chernozem in a non-grassland ecosystem, in a region with a sub-humid climate.

You have to be careful re. making decisions about a soil being a Chernozem, since many soil types will have an Ah horizon. But not every Ah is the Chernozemic Ah (diagnostic horizon. The Chernozemic Ah horizon, need to have several specific properties (e.g., C/<17/1, base satauration >80%, Ca2+ as the dominant ion on the exchange complex)

Based on that, I guess the question will provide more information about Ah horizon for us, so we can identify that soil order. Thank you very much, Maja!

yes, that is correct - if a soil is Chernozem those additional details about the Ah horzion would be provided

Hi Maja,

Will a soil with no B horizon always be classified as Regosol? For example soil with Ap=10cm, Cg1=20cm, Cg2=30cm is not Chernozemic for sure, but could it be Gleyesol?

Thank you

Soil without B horizon is a Regosol

Hi, I am wondering what is the distinction between C factor and P factor for soil erosion? Is it correct to say that C factor only considers the influence of vegetative cover and related management such as tillage on erosion, whereas P factor incorporates various combinations of management practices?

C factor (in the Universal Soil Loss Equation) refers to all management practices (tillage, fertilization, addition of organic amendments, etc) that are needed to grow a particular crop on the site P factor refers to the conservation practices (terraces, contour cropping, etc) that are done to protect the soil from erosion