Course talk:APBI200/Archive/2016-17WT2

Big thank you to all APBI 200 students from all of us on the APBI 200 teaching team!
Yours, Akhit, Kiran, Zineb, Jason, Emma, Sean, Elana, Sandra & Maja
[APBI 200 Teaching team celebrating the end of the term  ;-) ]

Looking good everyone! Maja, it looks like you're celebrating with a salad and water....boring! 😁😉 Just teasing. Im actually just wondering if I can see my final? I want to see if I got that pH buffering question right...haha so funny, I know. Not sure why, but I would like to see it. Thanks for everything, Sarah.

Sarah and all, You are welcome to come and see your final exams. Just send me an e-mail so that we can schedule a meeting

Thank you so much to Dr. Sandra and Dr. Maja for an incredible class. I learned a ton and I appreciate the time, energy, and effort you both have put into our success. You are both awesome!

Thank you Brian! We really appreciate it Maja & Sandra

Thank you Maja and Sandra, under your teaching it is impossible not to be inspired and motivated. You guys are my role model and the best teachers I have ever seen. My dear classmates you are really lucky to have such great teachers!

Coming from an exchange student.

Thank you very much Richard! Despite all the ups and downs and all the hard work that goes into it, it's our pleasure to teach this particular course. Maja & Sandra

I don't understand how would the percentage of base saturation affect soil properties/behaviours? Will high b.s lead to high pH?

In the lab when pH was measured in water and then with 0.01 M CaCl2, why was the exchangeable acidity lower than the active? Is it because of the buffering capacity of the soil?

Thanks.

Sarah, try not to mix concepts

Buffer capacity is the ability of a soil to resist a change in pH. See lecture 15, slides 11 & 12.

Exchangeable acidity (measured in CaCl₂) is lower than active acidity (measured in water). In the lab when you add H₂O, you are measuring the H+ and Al3+ ions in soil solution. When you add CaCl₂, the Ca replaces H⁺ and Al³⁺ on the exchange complex (by mass action); i.e. H⁺ and Al³⁺ previously on the exchange complex are "driven" into soil solution, so the pH goes down (i.e. more acidic). See slides 9 & 10.

Thank you! For days I have been searching for that missing piece that wasn't allowing me to put everything together in my mind.... for some reason that little tidbit, "driven", did it for me. I'm so happy! Yah! So silly.

Hi,I have two questiones about thermal admittance and thermal diffusivity. -why soil with low thermal admittance subject to extreme surface temperature fluctuation? -why does high thermal diffusivity result in large and rapid subsurface temperature response to surface temperature change?

The answers to these 2 questions were covered in detail during the lecture #12, so I'll only briefly answer the 1st question here and you should try to use that explanation for the 2nd question.
Assume that there are 2 soil types (organic and mineral soil) adjacent to each other, receiving the same amount of solar energy, and having the same water content. Solar energy absorbed at the surface is converted to heat energy. In the organic soil, as surface temperature rises, a temperature gradient develops, causing some (but slow) heat conduction downward (due to low thermal conductivity). The heat energy tends to remain near the surface of this organic soil. The increased heat energy content makes soil surface temperature rise very high, because the low thermal capacity (Cv) means that even a little increase in soil heat content will cause a large increase temperature. In contrast, mineral soil (having high thermal conductivity) will conduct heat rapidly away from the surface and (heaving high thermal capacity Cv) would need a great increase of heat content in the surface if it were to develop a high surface temperature.

Thanks!!

Hi Maja/Sandra,

I was wondering if you could elaborate on the second part of this question. All i can gather from that lecture and slides is that higher thermal diffusivity means the more rapid thermal heat change at depth. If we drain said soil it would warm rapidly..... I find the lecture posted does not give enough background or depth to this. Its just scratching the surface.....why does high thermal diffusivity result in large and rapid subsurface temperature response to surface temperature change? a short to the point answer would suffice. Thank you

Steven, thermal diffusivity (slides 21-22 of lecture #12) is driven in part by thermal conductivity (which increases exponentially in mineral soils with a small amount of soil moisture - see slide 15). Consider 2 soils (1 mineral and 1 organic) both of which are subject to a 1 degree increase in soil temperature - the mineral soil will rapidly conduct heat downwards due to it's high thermal conductivity. While it takes more energy to warm a "wet" mineral soil, once the temperature increases, that heat will be conducted down.

Perfect thanks Sandra!!!

Two soil samples with similar textures are placed in close contact. one has matric potential of -1m, the other -0.01m. Which soil likely has the higher water content? In which direction will water move and why.

I am thinking that the second sample with a matric potential of -0.01 has higher water content and I know the water will move in the direction from wet to dry. But I dont know how to explain myself. help

At saturation, matric potential = 0 As the soil dries, large pores will empty 1st, water is held within small pores (by capillary) and on thin water films around soil colloids (by adhesion). The drier the soil, the smaller matric potential (i.e. larger –ve); the tighter the soil water is held (higher tension), the lower freedom of movement. Therefore: the soil with the higher matric potential (smaller –ve) in this example – 0,01 m will have the higher water content.

Thank you Maja that helps a ton!

I'm wondering why nitrogen content decreases from humin to fulvic acid, wouldn't there be more nh4+ on the functional groups of fulvic acid?

Bn - Presence of a high % of Na ions
Cs - Detectable soluble salts (NaCl, Na2SO4).
Csa - Secondary enrichment of salts more soluble than Ca and Mg (generally Na).

Ck - Presence of carbonates (CaCO3).
Bca - Secondary carbonate enrichment from horizons above, horizon > 10 cm thick.

I am wondering what the differences between these suffixes are, exactly. It seems from the above descriptions that n, s and k refer to the simple presence of the salts or carbonates, while sa and ca refer to "secondary enrichment", but I'm not sure what that means. Also not sure what the difference between n and s are - maybe just that n is used for B and s is used for C and A horizons?

In this video between 5:30 and 6:30 he covers some of these suffixes and says that n "signifies salt in the soil", s "stands for salts" and Csa is "concentrated salt ... indicates an actual layer of salt accumulated in the profile".

Thanks!

Mitchell, see the lab manual for details, BUT I recommend you focus on learning the diagnostic horizons (e.g. Bn, Bt etc.)

For the lab manual:

n = presence of a high % of Na ions. It is used with B alone (Bn), or B and t (Bnt). It leads to distinctive prismatic or columnar structure.

s = accumulation of soluble salts, e.g. Cs

sa = secondary enrichment of salts more soluble than Ca and Mg carbonates (generally Na). The concentration of salts exceeds that in the un-enriched parent material. You may see a Csa horizon for example.

So since Solonetzic soils have a Bn horizon, meaning a high percentage of exchangeable sodium, does this mean this horizon will be sodic?

Yes

Lecture notes on SOM mentions various factors influencing decomposition time, including pH and water content. How does pH affect SOM decomposition, is it slower in acidic conditions? And does increased water content (when well drained and unsaturated) increase decomposition?

Acidic soil inhibits microbial activity, and hence slows down organic matter decomposition. Increasing water content reduces decomposition as there will be less oxygen available for the aerobic microbes. Most of the microbes that decompose organic matter are aerobic; and anaerobic decomposition is much slower than aerobic one. Having said that, aerobic microbial decomposition requires some moisture: the ideal soil water contents for the decomposition is around the soil's field capacity.

Through biological fixation, N2 is converted to NH3. After that, do plants just absorb the NH3 as their nutrients? Or the NH3 will convert to NH4+ through ammonia volatilization then plants uptake them? I am confused about which forms of nitrogen are the inorganic forms (or plant available forms)?

Ammonia (NH3) is not available to plants, but ammonium (NH4+) ion is. To get from ammonia (NH3) to ammonium (NH4+), ammonia (NH3) would need to be transformed (in the 2nd step of aminization process) to NH4+ ion. After NH4+ ion is formed (by aminization) it can either be: (i) taken up by plants or (ii) taken up by soil organisms (eg bacteria), or (iii) adsorbed on exchange complex (ie by negatively charged soil particles), or (iv) transformed back to ammonia (NH3).
Inorganic (or mineral) forms of N are NH4+ and NO3-, while organic forms are various organic compounds that contain N.

In my notes, aminization is the process of protein transforming to R-NH2, and ammonia volatilization is the process of transformation between NH3 and NH4+. But you just mentioned that NH3 transform to NH4+ through aminization. I am a bit confused about these processes. Also, NH3 is not inorganic forms of N?

Ammonia volatilization is the conversion of NH4 to NH3 gas, which is lost to the atmosphere. So it holds that NH3 becomes NH4 via aminization.

Be careful gentlemen. See lecture #25, slide #5 the steps in N mineralization: aminization, ammonification and nitrification. The gaseous loss of NH₃⁺ is volatilization - lecture #26, slide #13.

Another question is one of the bacteria's roles is to decompose organic matter by participation in nitrification process. I don't understand how decomposition of SOM and nitrification are related? Thanks!

or (v) the Ammonium can undergo nitrification to become Nitrate which is the other plant-available form of N

YuhaoZhou, regarding bacteria and nitrification, my understanding is that various types of bacteria play different roles in nitrification:

1) Certain types of decomposing bacteria (and actinomycetes) decompose detritus which contains organic N, storing the N in their bodies (pretty sure as NH2¹). This is aminization.
2) Next, the decomposing bacteria are eaten by protozoa and nematodes, which have lower N requirements than most bacteria, and they excrete the excess N as ammonia (inorganic N), as explained here.
3) Next, this ammonia reacts with H+ or H2O to form NH4 (ammonium). This took me a lot of digging to figure out, I hope it's right.
4) Finally, other types of bacteria (and actinomycetes), called nitrifying bacteria, convert this ammonium to nitrite, then nitrate.

¹Somewhere along the way the NH2 becomes NH3, and according to this page that process is part of ammonification (so is step 3). I'm not sure whether it's within the decomposing bacteria, or within the protozoa and nematodes after they eat the bacteria.

See lecture #25 slide 5 and 6. What we are asking you to understand is the major roles of soil organisms. With respect to bacteria, Mitchell is correct that they are a diverse group of organisms and play a range of roles including decomposition of SOM.

If you are interested in knowing more, you may wish to consider APBI 342 / FRST 310 Soil Biology next year.

Thx! Your answer is so clear!

I'm wondering where can we get the answer for the sample exam question?

They are not available George. If you have specific questions (not already asked), you can post a question or a response for us to review

In the lab manual it says that glacial till can be a mixture from fine clay to coarse fragments, but glacio-fluvial has no silt or clay. Why can't fine material carried by the glacier also be reworked by meltwater? Also, how are the fragments deposited by glacio-fluvial?

Yehuda, if glacial till is reworked by water the fines (silt and clay) will be "washed" downstream. Because glaciers carry so much material, there is not enough energy in the water to move all the material. Thus glacio-fluvial deposits are sub-rounded, and coarse.

How does nitrification influence soil pH?

Thanks! :)

Through the first step of nitrification (NH4+ to NO2-), hydrogen ions are released, hence decrease the soil pH.

Interestingly, I think during ammonification (stage of mineralization before nitrification), NH3 reacts with H2O to form NH4 and release OH- (increasing pH). So I wonder how these two processes balance out, or what the net effect is on pH.

That would require determination of a H⁺ budget

I was wondering under what conditions will phosphate leaching out of soil? Why is this not common? Thank you!

Natalie, see April 16 post "Phosphate leaching"

Can we think of the concept of exchangeable/active/residual acidity similar to non-exchangeable potassium/exchangeable potassium/Soil Solution potassium?

I.E. Residual acidity is H+/Al3+ trapped between clay minerals. Non-exchangeable potassium is K+ between clay minerals. Once the H+ and Al3+ ions become unbound, they are part of the exchangeable acidity which can be exchanged into the soil solution, just as exchangeable potassium is held near clay colloids, but can exchange into the soil solution? Kind of the same concept?

Furthermore, I'm not sure I fully understand how potassium increases drought tolerance. Is it just the fact that it is a solute in the soil solution that decreases osmotic potential, meaning water will tend to flow towards it.

Lucas, I would not mix concepts - although in this example both are related to exchange reactions.

1) active acidity - due to H⁺ and Al³⁺ ions in the soil solution; Exchangeable acidity - associated with H⁺ and Al³⁺ ions that are easily exchanged by other cations in the soil solution; and Reserve acidity - associated with H⁺ and Al³⁺ ions that are bound (non-exchangeable) on soil particles.

2) K cycle - the main sources of K are: minerals such as micas and K-feldspar (weathering releases K); SOM; and potash fertilizer (KCl a soluble salt). There is a gradation between exchangeable and "fixed" K. K⁺ electrostatically attracted to negative charges on soil colloids (e.g. clay or SOM) is exchangeable. The non-exchangeable pool contains less available K+ ions that are trapped in between the structure of the clays and are released slowly by weathering. The fixed or mineral pool contains K held within the mineral and so is very slowly by the weathering.

Regarding the role of K in drought tolerance it is my understanding that K plays a role in guard cell regulation and thus in the opening and closing of stomata.

Thanks Sandra!

Hello, according to the lab manual the Regosolic soil order is the only order w/o a B horizon (I could've had misinterpret their meaning though), while on the Canadian system of soil classification stated that Regosolic occurs when all requirements of other soil order has not been met.

So, on the final exam, there are soil order identification questions (given a list of soil horizon, their depths and some extra info, determine soil order and diagnostic horizon). On those questions, is it safe to say that a soil is Regosolic if there are no B horizon present, or should we consider other options if a requirement not involving B horizon is present. (example being that a soil have no B horizon, but has over 10cm of Ah/Ap horizon, so is the soil Chernozemic, or is it Regosolic). (another example being that a soil again has no B horizon, but has a Cz (permafrost layer) within 1m or the soil surface, so is the soil Cryosolic, or Regosolic)

Furthermore, what diagnostic horizon can we identify for Regosolic order? (should we just say that it lacks a B horizon / that it failed to meet any requirement of other soil order. or there is a specific diagnostic horizon even for Regosolic?)

Angelo Be clear on the diagnostic horizons. e.g. Chernozemic Ah > 10 cm BUT also has between 1-17% organic C, C:N ratio < 17:1, base saturation > 80% and Ca2+ is the dominant cation. Be careful not to jump to conclusion that a soil is Chernozem just because the Ah is > 10 cm. Use the other clues given in the question re. the site description.

In this class we focus on soil order. We did give you one example in the lecture notes (lecture #31) of Great Group for Chernozems (Brown Chernozem, Dark Brown Chernozem, Black Chernozem). Thus, combinations such as a Regosolic Cryosol exist (e.g. your example of a soil lacking a B horizon but with permafrost within 1 m of the soil surface). Note Organic soils also do not have a B horizon. However, you are correct that the diagnostic feature of a Regosol is the lack of B horizon. Again, use the information provided in the description portion of the exam (e.g. located on a floodplain such as the example question 7a) on the sample final exam posted in the wiki).

Page 84 of the Lab Manual is really helpful for this. Basically you go through the steps in order, so this tells you which diagnostic horizons override the others. If it has a chernozemic Ah horizon but also has no B horizon, it's a chernozem rather than a regosol. A regosol can't have any of the other diagnostic horizons.

Good suggestion Mitchell!

Thanks for the answers :D, im assuming regosol happens when all else fails

indeed.... but it is still a bit better than just a rock ;-)

I understand that the advantage for soil with high CEC is high CEC leads to high nutrient retention. Will there be some disadvantages for high CEC?

yes there is.... soils with a high CEC will also attract and retain various heavy metals and organic ions that are toxic for plants.... remember, all ions on the exchange complex can be replaced (through ion exchange reactions) by other ions in soil solution

Yes, in the three types of humid substances, I remember there is one type (Is it humin?) that tends to attract the heavy metal ions than the others, and it is not usually absorbed by plants?

In the lecture notes it says that flocculation of clay particles is caused by electrostatic, van der waals, and hydrogen bonds. What is creating these forces and why don't negatively charged clay particles repel each other? I'm imagining it that water molecules and/or cat-ions adsorbed to a clay particles attract another clay particle. Is this true? Is this the same process for flocculation of humic substances?

Yudel, things are way more complicated in real-life soils than what we have covered in this course. And yes of course, there are numerous ions and water molecules attracted to the charged surfaces of soil particles and all those ions and water molecules act as bridges between interacting solid particles. But that story is left for an upper-level soil science courses.
Hence, in a simplified example covered in this course, re. inter-particle attraction we wanted you to understand that (a) solid particles are charged and (b) they interact with each other through those inter-particle forces.

Are secondary clay sized minerals formed from the weathering of primary minerals, or are they formed to begin with under different conditions such a lower temperature or pressure?

Some secondary mineral are created as a result of partial change of crystalline structure of some other mineral (either primary or secondary). Example of that is weathering through hydration of hematite to limonite (see lecture notes from Jan 9). In addition, some other secondary mineral are created as a result of complete destruction of crystalline structure of another mineral that releases various ions into soil solution. New (secondary) minerals could be created in soil from those ions