Midterm Exam Questions
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
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?
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!
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?
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?
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.