Course talk:APBI200/Archive/2013-14WT2

What depth of knowledge are we expected to know for soil organisms, similar to the information given to us in the lab manual or more depth information like that given to us in the textbook because the textbook has a much more detailed explanation of soil organisms and their roles.

Here you can post any general questions you have about the APBI 200 course.

Could you please explain what submergence potential is?

And is it the same thing as the hydrostatic potential that us mentioned in the textbook?

Submergence potential is a component of the total water potential and it is operational only in saturated soils. Submergence potential is formed in the same way as matric potential (operational in unsaturated soils) due to attraction between charged soil particles and polar water molecules and capilarity.

More on this topic can be found in your textbook in chapter 5

Hi, Maya. R the role in soil of mites and nematode just grazing on soil bacteria, and stimulate the nitrogen cycle? And does heterotrophic bacteria have any roles other than decompose organic matter? Thank you!

Nematodes (but not mites) feed on bacteria and due to their low N requirement nematodes release surplus of N into soil solution, which then becomes available to other organisms (including plants)

Mites perform other functions such as shredding & mixing of soil material, the also do a bit of burrowing. The can also enhance aggregate formation by ingesting mineral & organic soil particles.

Heterotrophic bacteria decompose organic matter and by doing that they help with the release of the available forms of N, S and P.

All soil organisms also add to the pool of soil organic matter once they die

Hi Maja, should we memorize the multiple steps of mineralization, I mean the chemical reactions of aminization, ammonification and nitrification?

you don't need to memorize the chemical reactions, but you should be able to describe them correctly

Question: I have a question about montmorillonite and kaolinite. Based on my notes, Kaolinite is more stable than montmorillonite. I feel confused about that concept. Could you please briefly explain it? Are they both phyllosilicates?

Answer: Yes, both montmorillonite & kaolinite are members of the phyllosilicate group. However, they have a very different crystalline structure, which also translated into different stability and susceptibility to isomorphic substitution. With 2:1 type of structure, montomorillonite is characterized with weaker bonds within its crystallyne structure and is more prone to isomorphic substitutions and consequently larged number of charges than kaolinite (wiht its 1:1 type of structure)

More on these 2 minerals can be found at http://soilweb200.landfood.ubc.ca/soil-components/1-mineral-components/#Types_of_Soil_Colloids as well as in your textbook (see p.240-244)

Hi Maja:

Im wondering how does a high pH in soil help enhacing the decomposition?

I wouldn't say that pH is ENHANCING decomposition of organic matter, but that pH AFFECTS decomposition of organic matter.

The effect of soil pH on decomposition of organic matter is indirect, since decomposes (i.e heterotrophic soil organisms) all have an optimum pH range in which they are most active.

Hi Maja,

I'm confused with what soil properties enhance nitrate leaching losses

This was one of the questions that was part of the discussion session no.2, so I'll only briefly answer it here.

Nitrate is an anion, hence as such it will mainly be in the soil solution (since as an anion it will be repelled by the negatively charged soil particles). So, think about what conditions lead to easy movement of soil solution....... keeping in mind that soil solution moves through the pore system....

Also to consider are sources of nitrates in soils (i.e., from where are nitrates released and how are they added to the soil).

Hi Maja, I was wondering what is the reason for soil algae and nematodes being smaller than the other kinds of nematodes and algea? Thank you

Soil species of algae & nematodes don't have as much space to grow as their aquatic relatives. Hence, soil species end up being smaller.

Maja, the sizes of the algae cells and the bacteria cells are almost the same. Why the room in soil is limited to algae? Is that because algae cells like to clump together?

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!

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!

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

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)

Hello Maja, I was wondering if you could explain the relationship between the pH and the CEC. Thank you.

Soil pH only has an effect on CEC of the soil colloids (organic matter, sesqui oxides) that have pH-dependent charge.

Here is an example how a sesquioxide may develop a pH-dependent charge: Al(OH)3 + H+ Al(OH)2+ + H2O

A clay-size particle of hydrous aluminum oxide has a positive charge under acid conditions (as indicated in the reaction above). Such a particle being a part of the soil matrix, rather than a cation in solution, contributes to the soil’s anion exchange capacity (AEC). At sufficiently high pH, sesquioxides may even contribute to cation exchange capacity, by acquiring extra hydroxyl groups: Al(OH)3 + OH- Al(OH)4-

However, most soils in which sesquioxides are abundant are naturally acidic, so they are contributors to AEC.

Please post questions about Lab Assignment 7 here.

Hey! I was wondering what could be some reasons I found the soil profile at the field pit location to have finer soil texture. I thought soil texture was really hard to change so was I wrong when I felt a finer texture? Thanks!

Our campus soils have quite coarse texture with very little clay in them. It is possible that organic matter (that accumulated in the A horizon of the cultivated soil pit) confused you to think that the texture is fine in that horizon. Organic matter tends to give that soapy fleeing, similar to silt.

Hey Maja, I am sorry that I forgot the relationship between soil pH and soil formation. My assumption is: Soil that has low pH can have relatively stable aggregates because the concentration of base cations (e.g. Na+) in acidic soil is really low, so the DDL is relatively thin, so the aggregates are more stable. Is that correct?

By the way, the reason why forest soil has low pH is that plant residues are decomposed by soil organisms, and that decomposition can cause the release of some materials which can form acid. Does it sound right? Because both forest pit and agricultural pit are in UBC farm, so the difference of precipitation is negligible. Based on that statement, what is the main factor that cause the difference of soil pH between forest soil and agricultural land soil?

Thank you very much!

In your 1st question, I am not sure what do you refer to by "soil formation". The process of aggregate stability that you described is correct, but aggregate stability is not a soil formation...

Soils formed under coniferous forests are acidic because their debris is acidic and when decomposed various organic acids are released. These acids then enhance mineral weathering (which results in release of Fe & Al ions, which in turn further acidify soils).

Soils at UBC campus under their natural setting tend to be acidic. Soils at the UBC Farm have been in the agricultural production for 50 yrs and since their natural acidity is not suitable for majority of agricultural crops liming is commonly done to raise their pH.

That is valuable! Thank you Maja

Can we have Ct and Af horizons?

Good question. There is no such thing as Ct horizon, since "t" is only associated with the B horizon (and no there is no At horizon either). The "t" implies enrichment (accumulation) of illuvial clay particles that have been moved from a horizon of eluviation, which then translates into a higher clay content in the illuviated (i.e. Bt) horizon than eluviated (Ae) horizon above it. Generally, in soils there isn't enough water (moving through its profile) to do the same translocation (movement) of clay particles from B to C horizon. Hence, there is no Ct

There is no Af horizon either. Accumulation of Al and Fe oxides (released due to weathering of secondary & primary minerals), as designated by "f", occurs in the B horizon.

Hi Maja, what is the designation of pure sandy horizon? I chose Ae to represent sandy horizon, but I am not sure.

There is no designation for a pure sandy horizon. Sand was included into the question (2a) to indicate that this is a mineral horizon. More hints about this question can be found in my answer to another question as shown below

Hi, Maja, can I say the horizon with both A and B horizon properties is AB for Q 2d? And for 2a, should I count it as LFH or Ah?

Re 2b - All so called transitional horizons have properties that integrate 2 horizons above and below it.

In question 2a it is specified that horizon is sandy; hence, that is a mineral horizon and as such it cannot be LFH since those are organic horizon. Ah horizon has a soil formation process (accumulation of organic matter occurring in it, as signified by the letter "h"). Consequently, Ah is not the correct answer for question 2a. You have to think of a mineral horizon in which there is no soil development or no soil formation process.......

Hey Maja, for 2 a) could E be a good answer? It is a mineral horizon used in the united states, and I don't think any soil development occurs on that horizon given that mostly quartz is left behind. Or should we only consider A, B, and C as the mineral horizons.

E (as used in the US soil classification system) is the same as Ae (in Canadian system). But regardless of the system, E (or Ae) is the horizon characterized by the soil process of eluviation (or leaching out of either clay particles and/or organic matter) indicating that soil development is taking place. Hence, E (or Ae) cannot be the answer for question 2a. The answer is among A, B and C horizons........

Should we know some characteristic of soil orders, or just know how to identify them?

Tobe able to identify soil orders, you need to know their key characteristics.

Here you can post questions about Problem Set #3.

Hi. I have done all of problem set 3 except for question 1. I am having trouble calculating the matric potential. Could you please help me do that for question 1A.

Thanks

I assume that you are referring to problem set no.2 (not no. 3), if yes then for question 1a, you need to know matric potential and gravitational potential. This is very similar to the example shown during lecture on Jan 24, 2014.

For question 2b), I assume we take the total CEC calculated from 2a) to find the percent Base Saturation in 2b), correct?

Yes, that is correct

For 5, what can happen to H2S,is this H2S oxidated to form H+ and sulfate?

Hi Maja,

I don't know how to approach question 4.

Re. question #5 - yes H+ and sulfate ions are form and then you need to elaborate the consequences of the formation of those 2 ions (ie what could happen to them in the soil)

Please post questions about Lab Assignment 5 here.

What determines the capability of a parent material to supply plant nutrients? Is it true that soil dominated by finer particles such as clay can supply more nutrients than soil dominated by coarse particles?

Two things: 1) Mineralogy (ie the composition of rocks that make the parent material); if primary minerals dominate then such parent material will have limited charge, and

2) Size of particles in the parent material (small-size particles such as clay, have a large specific surface area)

And yes, clay size particles will hold (and eventually supply) more nutrients (ions) to plants than inert, sand-size particles

Hi, Maja, for question 2, is it because those four factors need time to have an affect on the formation of soil, so parent material has the most effect on the soil profile formation during the early stage?

yes, that is correct

Please post questions about Lab Assignment 4 here.

HI Maja,

I am confused with question 4c of the lab assignment.

One of the benefits of adding gypsum (CaSO4 2H2O) is that Ca ion will replace Na ion on the exchange complex.

Hi, Maja, for question 4b, following the irrigation, because Na ions promote dispersion and colloids are far apart, so we just need to consider ESP to see which is most likely to disperse? Thanks!

yes, that is correct. Of all info provided in this question, only ESP gives you relevant info re. sodium

If Na is present in small amount in the exchange complex, is it still beneficial to replace Na with Ca? Or it is not necessary?

If a soil has a low conc. of sodium there is no need to replace it with calcium (or any other cation). The replacement would be beneficial, but it isn't necessary

Please post questions about the midterm exam here.

-Pls note that the the cut-off point for the material that will be included in the upcoming midterm exam is Feb 12 (inclusive). In other words, all topics up to soil organic matter will be on the midterm exam.

-Pls note that NO formula sheets or cheat sheets are allowed at the APBI 200 midterm exam.

-Review session will be held on Monday Feb 24 (9-11 am) in Scarfe Building (2125 Main Mall), room 100

Hello, what type of calculator is permitted on the exam? Is a graphing calculator ok?

Any kind of calculator is accepitbe, including graphing ones

According to the lecture notes, soil water content decreases when matric potential becomes more negative. I am not sure why this is the case. As matric potential decreases(becomes more negative), soil holds water more strongly. Wht would soil content decrease if the water is held more strongly? Thank you in advance.

hello Maja.

1.For the Q4 of 2008 mid term, could you please explain why those two ( sand and massive clay) soils differ in terms of water rentention?
2.why are the soil colliods the most reactive part of soil matrix? Is that because of they have permanent charges and there are many exchangeable ions bond to them? 

thank you .

1) Massive clay only has small (micro) pores, while sand only has large (macro) pores. Consequently, large pores in sand louse water very quickly (as indicated by a prominent drop in its water retention curve)with the deceasing matric potential. On the other hand, small proes of the massive clay, hold tightly on the water molecules and there is very little change in water content with a decreasing matric potential (as shown by its almost flat curve)

2)Soil colloids are small and as such they have large specific surface area and in addition they usually have lots of charges on their surfaces.

Is the midterm taking place in class?

Yes, midterm is during our regular lecture time (11:00 -11:50 am) in FSC 1005

Hi Maja, will you provide answers of previous exams for us?

Yes I will during the review session on Monday Feb 24th (9-11 am, Scarfe 100). However, no written answers will be posted at the UBC Wiki

Questions: 1. Can I say the exchangeable ions are just ions located in DDL? Does the DDL include the soil solution ? 2. Does the AEV correspond to the matric potential at field capacity? 3. In the question 6 of 2006 old exam, I got the porosity is 0.363, but how can I see if it's good for plant growth?

Answers: 1. Yes, exchangeable ions are those present in the DDL. The DDL does NOT include ions in the soil solution. However, ions in the DDL are in constantly going into reactions of ion exchange with ions in soil solution, until the equilibrium between DDL and soil solution is reached.

2. AEV (air entry value) does NOT correspond to the matric potential at field capacity. AEV corresponds to matric potential at which water leaves the largest pores in your soil, and that condition is wetter than the field capacity.

3. Generally, mineral soils have porosity between 0.3 and 0.6 cm3/cm3 (or 30-60%) [see your lecture notes for lecture no.3]. The lower part of this range is an indication that a soil has been compacted, while the higher part of this range is an indicator of lack of compaction and good soil conditions for plant growth.

Hi Maja, based on my notes, Kaolinite is more stable than montmorillonite. I feel confused about that concept. Could you please briefly explain it? Thank you.

Both montmorillonite & kaolinite are members of the phyllosilicate group. However, they have a very different crystalline structure, which also translated into different stability and susceptibility to isomorphic substitution. With 2:1 type of structure, montomorillonite is characterized with weaker bonds within its crystallyne structure and is more prone to isomorphic substitutions and consequently larger number of charges than kaolinite (wiht its 1:1 type of structure)

More on these 2 minerals can be found at http://soilweb200.landfood.ubc.ca/soil-components/1-mineral-components/#Types_of_Soil_Colloids as well as in your textbook (see p.240-244)

Hi, Maja, in 2008 midterm , how to identify the soil structure, I can only tell that the first one is sand and second one might be clay since it has higher water retentive ability, for structure, should I answer something like spheroid, blocky? Thank you!

To have such a flat water retention curve, as shown in the 2008 midterm, soil #2 could only be a massive (i.e. structureless) clay. Any other clay textured soil with any formation of stable aggregates would not have such a flat water retention curve. Such shape of the water retention curve is indicative that the soil is only having small pores, and that can only be the case in a massive clay.

Hi Maja, is there a chance that you could post some of the answers for the practice midterms online for those that have class during the review session on Monday?? Or could we maybe email you our specific questions?

In class we only covered a few slides from the lecture on "effects of cattle grazing on soils in coniferous forests". Are we responsible for knowing all the material covered in this lecture? Thank you!

You should not worry about that lecture.

I am not going to post any answers for past midterms. If you have any questions pls post them here and I'll answer them

which chapters in the textbook should we review for the midterm exam?

Pls refer to your course syllabus. Chapters are indicated there

Hi Maja, I am a little confused about Question 4 b on Practice midterm #1. We are given this information:

pore volume = 0.55 m3 air entry value of soil water tension = 0.005 m volumetric water content = 0.19 m3 field capacity = 0.28 m3/m3 permanent wilting point = 0.11 m3/m3

-->We are then asked to find the soil air volume in m3... I understand that Vf= Va+Vw, and I tried to find Vw using Volumetric water content= Vw/Vt ... but we are not given Vt. Is there another way of finding Vw that I am missing? Or of calculating Vt with the given information?

Hi Maja, I was wondering what are considered ideal conditions for anion exchange to occur. The only thing I was able to come up with was a lower ph, therefore a more acidic soil.

Low (i.e. acidic) pH is part of the answer. The other part is related to the type of the colloids, since not all soil colloids will change it charge with change in pH of the soil solution. Only organic matter and Fe/Al oxides/hydroxides change their pH with a change in pH of soil solution. Elaboration of how that change occurs is also needed as part of the answer for this question (pls see lecture notes from Feb 12)

Soil air volume (Va) can be calculated as Va = Vf – Vw Hence, you don't need Vt. And both Vf and Vw are given in the question

For question 2b on midterm 2006, what is it asking for when it asks for what the type of charge is called?

Answer for 2B is permanent or constant charge

Re Kyla Sheehan's question about question 4b) exam 2005, only Volumetric water content is given ( and according to Soil Web Volumetric Water Content is the ratio of water volume (Vw) to total soil volume (Vt), (i.e. Vw/Vt). So Volume of water is not given as far as I can tell. I'm not sure how to find Soil air volume with the information given.

Hello, I was just wondering about the relationship between thermal admittance and heat flux, when you have high admittance do you typically see HIGH or LOW heat fluctuations in soil temp?

Soils with high thermal admittance (e.g., mineral soils) experience limited (or low) surface temperature fluctuations

Based on lecture, H and Al ions that contribute to residual acidity are bound on soil particles. Is it possible to exchange those ions? In other words, can they undergo ion exchange reactions?

H & Al ions that contribute to residual acidity are strongly bonded to the soil particles. There are numerous ways how this strong bonding (much stronger than electrostatic attraction) occurs, but the only one that has been mentioned in this course is bonding in the inetrlayer space of the phyllosilicates. Hence, for those strongly bonded H & AL ions to be exchanged, phyllosilicate will need to weather 1st and then these ions could be exchanged.

Okay im really really confused. You said that the midterm cut of is up to feb 12 but you said mid term will also include soil organic which was taught on feb 14 and will be taught on feb 24 Im really really confused. For example, i have to know what Chelate means? since it is about soil organic matter

As I said before, topics up to (but NOT including) soil organic matter will be on the midterm exam.

According to the lecture notes, soil water content decreases when matric potential becomes more negative. I am not sure why this is the case. As matric potential decreases(becomes more negative), soil holds water more strongly. Wht would soil content decrease if the water is held more strongly? Thank you!

Water retention curve (which illustrates relationship between water content and matric potential) indicates that as matric potential decreases, the remaining water left in the soil is held more tightly by the soil matrix (or by soil solids).

You always need to know what is impacting what. In the case of the water retention curve, matric potential (plotted on x-axis, as independent variable) is impacting water content (potted on the y-axis as dependent variable). Hence, by applying lower and lower matric potential (as we did in our water retention lab) one is able to pull out more and more water. At the same time, water that is left in the soil is held more strongly.

1. Why would physical and chemical processes such as wetting/drying and freezing/thawing contribute to aggregate formation?

2. Would field capacity be lower in a sandy soil than in a clay soil?

3. Midterm 2008 question 5: Can we assume the density of the organic matter is 1.3g/cm^3?

1. Wetting/drying and freezing/thawing are physical (not chemical) processes that enhance aggregate formation. They do so, since they (namely thawing and wetting) bring individual particles closer together, allowing them to bond better (which leads to flocculation as the 1st per-requisite for aggregate formation).

2. Clay soil with its higher porosity (than sandy soil), will have the higher field capacity.

3. There was a typo in question #5 in 2008 midterm, hence I replaced it with another example

Low matric potential of a soil sample represents low water content inside the sample,and it also represents strong adsorption and capillarity between soil particles and water. Is that right?

Yes, that is correct

Please post questions about Lab Assignment 6 here.

Can anyone tell me what dominant pore size is?

You have to calculate the size of dominant pores (or the most common size of pores in your sample) using the capillary raise equation and tension that corresponds to air intrusion value (AIV). As it is explained in the lab manual on p.33 "The AIV is the tension corresponding to air intrusion into the pore size dominating the porosity"

Here you can post questions about Problem Set #2.

Question 1 part b asks for the water potential gradient between the two depths. I got a negative number for this and I am not sure if this is correct. Since water always moves from high potential to low potential, is the potential gradient supposed to be positive all the time? Thank you!

There is nothing wrong with total water potential being negative. It

For the 3rd question, is it asking us to find the gravimetric water content and volumetric water content of the moist soil or the oven dried soil?

For gravimetric water content you need to know mass of oven dry soil Volumetric water content you need to know volume of water & total volume of soil

Q.For question 3c, do I need to calculate the bulk density or just estimate to 2.65? Since in 3b, the soil is unsaturated, so the density maybe different.

A. You should assume particle density of 2.65 g/cm3 (and this is particle density, not bulk density). Particle density is NOT affected by the water content.

Q. re. question 4b, I don't really know the soil condition of BC forest, is that just because there is too much rain, so the Al ions are washed away, so it's less acidic? Or anything else?

A. Pls refer back to the lecture slide (shown on Friday Feb 7)showing the most common ranges of pH values in soils in humid & arid regions. I went over this explanation in the class and there is similar explanation in the textbook from where that slide was taken. Hint - coastal BC has a humid climate and Al is not as easily washed out of the soil as some other ions (e.g. Ca, and Mg).

where can i get the other data for lab 3? i thought that the other data are posted on the web.... i got the data for fine sand but i dont have the data for medium sand...

Water retention data for both samples used in lab no.3 are posted in the appendix A of your lab manual (see p.92)

Here you can post questions about Lab Assignment #2.

Can anyone tell me what the dried soil sample's mass for the bulk density calculations was? I may have missed it during the explanation

It might be too late, but just in case here it is: 437.3 g

Here you can post questions about Problem Set #1.

Can you help me with question #2?