Course talk:APBI200/Archive/2014-15WT2

1) Solenetzic soils have columnar structure when dry and massive when wet (because the sodium deflocculates the aggregates)? Right? What other relationships between soil orders and structures should I take note of?

2) Chernozemic soils have an Ah thicker than 10cm, have high base saturation and the main cation is Ca. But what about the Ap? Can this be in place of the Ah and still be a chernozem?

3) I am confused about pH and CEC. Does CEC increase when acidity increases (pH lowers)? How does this relate to dissociation, flocculation and dispersion?

- I think that CEC increases as pH increases. But I have a note next to losses in the sulphur cycle - it says that there is less loss of soluble sulphur in acidic conditions as it gets held to the colloid surface by adsorption. So is this indicating that there is a higher CEC in acidic soils?? - And then how does organic matter change pH and CEC?

4) Which soil organisms names should I know in more detail instead of just group (fungi, algae, bacteria, protozoa)? For example: Nitrobacter, rhizobium and nitrosomonas.

Soil organisms:

Carla, you should know the major groups (as per the class notes and text) as you indicate above, but within fungi and bacteria, as they include a wide range of organisms with different functions it would be useful to know important subgroups e.g. heterotrophic bacteria, nitrifying bacteria (your examples of nitrobacter and nitrosomas), N-fixing bacteria (your example of rhizobium with legumes), cyano-bacteria; similarly for fungi - heterotrophic fungi (decomposers), and mycorrhyzal fungi.

Thank you

CEC:

You are correct that CEC increases as pH increases (pH dependent charge), due to the deprotonation (loss of H⁺) of function groups (e.g. R-COOH + OH^- ---> R-COO^- + H₂O).

Soil Organic Matter (SOM), pH and CEC:

SOM has an important influence on CEC and pH as SOM contains a large number of functional groups (e.g. R-COOH, R-OH etc.) and deprotonation of these functional groups will 1) increase the net negative charge of the soil colloids (e.g. R-COO^-), and 2) increase H in the soil solution (buffer capacity).

Additionally, decomposition of SOM by soil organisms releases CO₂, which combines with water to form carbonic acid (lowering soil pH)

Sulfur:

Many sulfate compounds are quite soluble, so similar to nitrate, sulfate may be leached from the soil in humid regions. However, most soils have some AEC. Recall that AEC is greater at lower pH due to the protonation of hydroxyl (OH) groups. Consider the diagram in your text in the section of Chapter 12 on sulfur retention and exchange, the figure showing the effect of lowering pH on an Al-oxide. As pH decreases AlO^- --> AlOH --> AlOH⁺ giving the soil colloid net +ve change and AEC; thus SO₄ is attracted to the colloid

Soil orders:

The lab manual and soil web (http://soilweb.landfood.ubc.ca/classification/) provides a good overview of the characteristics of Soil Orders in the Canadian Classification system (supplementing our class notes). 1st I suggest focusing on the diagnostic horizons / diagnostic characteristics of the Soil Orders.

Columnar structure in the B horizon of Solonetzic soils is indicative of Na salts (as you indicate). Additionally in Luvisolic soils (Bt), the enrichment in clay may result in a blocky structure in the B horizon. What about Chernozems, developed under grasslands, would you expect an influence on soil structure? hint: earthworms. How about vertisols? consider wedge shaped soil peds - how are they formed.

Theoretically any soil order can have an Ap horizon.

I'm still confused on the losses and sources of phosphorus, sulfur, and potassium. can someone help me with this?

Sophia, have a look at the nutrient cycles in the class notes. You will notice that the arrows are colour coded. Red = losses, blue = inputs, black = transformations.

Phosphorous: Additions (sources):

- Fertilizers (Excess amounts must be applied to over come the phosphorus fixing capacity) - Decaying organic matter - Deposition from eroded rocks by wind and water (dust & precipitation)

Losses: - Plant removal - Uptake by plants via mycorryzal networks - Leaching (But rarely happens because inorganic (plant available) phosphorus gets fixed. By Ca in alkaline soils and by Fe and Al in acidic soils. It gets fixed into an insoluble and immobile form. - Erosion of phosphorous carrying particles

Sulfur:

Additions (sources): - Decaying organic matter - Minerals - Coal burns into the atmosphere then is added to the soil by acid rain or by direct absorption from the atmosphere by plants

Losses: - Leaching of sulfates in solution - Volitilzation of S(2-) to sulfur gasses - Erosion - Plant Removal

Potassium:

Additions (sources): - Fertilizers - Decaying organic matter - Minerals

Losses: - Leaching (translocation down through profile and exit into ground water) - Runoff (to rivers from irrigation or rain) - Erosion

Hope this helps! :-)

Hi,

sorry...more questions...

What soil properties enhance sulfate (SO42-) leaching losses? Explain your answer.

-Soil that is high in sand has poor water retention, but good drainage which are more susceptible for sulfate leaching. -soil in high pH values cannot hold onto SO42- because high pH means more OH- in the soil solution. OH- then pulls H+ from the edge of the phyllosilicate..which eventually carries negatvie charges, so it's repelling SO42-...)

That's all I can think of..please help.

Thank you!

Good question! You are definitely on the right track with your answers, but there is more to add. Think 1st, under what conditions there will be lots of leaching of any ion? Key factors include: a) Abundance of water in the soil, b) Ions predominantly present in the soil solution, b) Abundant source of a specific ion c) Soil mainly has large pores that are filled with water

... now for each of those key factors, think about soil (or climate for factor "a") properties that would contribute to it. For example, soil will end up having predominantly large pores, when it is very coarse textured (ie dominated by sand) and with no aggregate formation.

Instead of me completing this answer, I'll leave it for you (and your colleagues, who are hopefully reading this) to complete the answer for the sulfate ions

So, the following enhances sulphate leaching:

- Abundance of water in the soil due to high precipitation and moderate water retention to dissolve the ions into soil solution within the macro pore spaces . But also having moderate to good drainage so the dissolved ions in solution can permeate through macro pores and exit the soil profile.

- Coarse textured soils that have macro pores will allow the soil water to percolate down the profile. In addition poor aggregation from lack of biotic activity or high sodium levels will contribute to a granular structure that favours macro pores.

- High abundance of sulphate ions in soil profile to begin with. Added by decomposition of organic matter, acid rain from atmospheric sulphur or the chemical weathering of minerals.

- High pH levels will increase sulphate leaching as there is less adsorption by colloids.

I'm unsure how you want us to answer the questions "Discuss environmental and soil conditions under which following

soil horizons are most commonly present". Do you simply want us to just define the horizon, or actually go into depth? For example, can you answer the question for say, a Bv horizon?

Well you can discuss the basics of each horizon - like LFH horizon occurs in forests with litter fall, A is a horizon that typically has illuviation of material building up in the B horizon and C horizons are most similar to the parent material.

You can then discuss specifics within the horizons like Ae with a Bf beneath it shows that the environmental conditions are high rainfall that has resulted in the iron washing out of the A horizon and accumulating in the B horizon. In terms of soil conditions you could discuss that there needs to be iron present in the first place - perhaps from the parent material and that the texture of the soil would need to have a relatively even distribution of pore sizes in order to allow for the translocation to occur (if dominated by clay the profile would not allow for movement of cations as they would be held within the micropores).

For the Bv, that is one of the diagnostic horizons for a vertisol - which are dominated by smectite clays. So you should think of environmental examples that would create a high proportion of clay. Precipitation? Primary minerals? Parent material? Weathering? Think about what specific environmental/soil conditions that would cause the agrilipedoturbation (hint: wetting and drying).

Hope this helps.

We were told we wouldn't have to do any calculations on the exam, so would calculating the CEC be included or removed? For example, a question like this:

7. A surface soil horizon of an uncultivated site has a loam texture with clay content of 20%. The clay-size fraction is dominated by montmorillonite clay mineral. Organic matter content of this horizon is 6%. 5 (a) Assuming that the CEC of montmorillonite is 100 cmolc/kg and organic matter has CEC of 200 cmolc/kg determine the net negative charge (cmolc/kg) of this soil. (b) The concentrations of exchangeable cations in this surface are: Ca2+ = 23.0 cmolc/kg Mg2+ = 4.4 cmolc/kg K+

= 0.5 cmolc/kg 

Na+

= 0.1 cmolc/kg 

Calculate the percentage base saturation (BS) of this soil horizon

Sophia, You do not require a calculator for the final exam. There are no calculation questions (like the one on the midterm). You should however understand the principal behind the example you give above - that CEC of clay (or organic matter) does not equate to the CEC of the soil. If you have a soil with high clay content (e.g. 40%) what are the implications for CEC (of the soil)? If you have a soil with 15% organic matter, what are the implications for CEC (of the soil)

Hello!

I please need a hint. In sample exams the question about soil algae and nematodes comes up repeatedly. Why they occur near the soil surface and why they are smaller than their aboveground and aquatic counterparts. and why this is not the case for bacteria. My apologies, but I just do not get it, neither can I find any notes of mine on this nor valid information in the textbook. This might be very stupid and only my fault but its fact… Thank you very much for any valuable suggestion.

Best regards

hey..you are not alone. I am wondering about the same thing too!

I was part of the group 4 of discussion #2 with the questions... Why are the soil algae usually restricted to the uppermost soil layers? Soil algae are usually smaller than their aquatic counterparts? Explain why this is the case. Explain why this is not true for soil bacteria?

Our answer was:

Soil algae are restricted to the uppermost soil layers because:

• Soil algae require photosynthesis therefore require sunlight (hence being at uppermost layers)

Soil algae smaller than aquatic algae because:

Space:

• -there is more open space in water
• -free floating: can be widespread

Resource availability:

• -maximum sunlight in water
• -water availability in aquatic environment

Why this is not the case for soil bacteria: [Pls note that Maja edited this part of the answer]

Soil bacteria are unicellular organisms, therefore...

• -restrictive space in soils is not an issue for them

Thank you Tanya!

Julia, 1 hint when answering this type of question 1st) define the organism If you understand what algae are, e.g. autotrophs, live in water, have chlorophyll and perform photosynthesis, then you can deduce some of their environmental conditions.

Thank you Sandra! Thank you Tanya!

These are great explanations for algae and bacteria, however why are nematodes found near the surface and why are they smaller than their aquatic counterparts?

I assume that this is because nematodes feed on organic material, helping with decomposition, and most organic material is found on the top layers of soil.

Generally there is more pore space at or close to the soil surface, and (generally) pores became smaller with increasing depth. Nematodes (which are not unicellular organisms like bacteria) require pore space to move around. Hence, abundance of organic matter & presence of larger pores at or close to the soil surface enhance larger number of nematodes at soil surface relative to the deeper depths.

As for why are soil nematodes smaller than nematodes that live in aquatic systems, the explanation is the same as for algae.

If you look at Final exam 2004, Question 4: Soil algae and soil animals (e.g. nematodes) are usually............. I believe nematodes use the same answer as I wrote above...

I understand why phosphate leaching from the soil is seldom a problem but can't seem to find when it would become a problem. Any hints?

OK, here is a hint: Phosphate leaching does not commonly occur because of the phosphate fixation. Phosphate fixation is caused by either Fe& Al (in acidic soils), Ca (in alkaline soils) or silicate clays (in neutral pH soils). So, can you think of a soil type (mentioned in this course) that does not have either Fe, Al, Ca, and silicate clay minerals. In that soil type, phosphate leaching can be an issue especially if phosphate fertilizers are applied in large quantities (which on its own can lead to phosphate leaching)

is the answer is none? is there any soil other than those 3 types?

thank you

no, there is a soil type (actually the whole soil order) in which phosphate leaching will happen, since that soil does not have silicate clay minerals (or any other mineral particles) nor Fe/Al or Ca ions........ Do not think about soil pH, but what causes phosphate fixation....

Regosolic order?..

Also, soil that has abundant negative charges, like montmorillonite will repel anion like phosphate, which will lead to phosphate leaching. Am I correct?...

Thank you.

Sorry, but no. It is not a Regosol. Regosol being a mineral soil could potentially have Fe/Al or Ca ions which all contribute to phosphate fixation

Hi,

I was looking at the soil series chart. I am wondering..how do I know the CEC value given on the chart is high or low? For example, 2014 sample exam question #5, the first layer (Ap) has CEC of 36, how do I know if this is consider high or low?

Thank you!

Here are some general ranges of CEC (in cmolc/kg soil) for various soil orders

• Luvisols: 10-30 cmolc/kg soil
• Podzols: 10-20 cmolc/kg soil
• Solonetz: 15-30 cmolc/kg soil
• Chernozem: 20-40 cmolc/kg soil
• Vertisol: 35-50 cmolc/kg soil
• Organic soils: 130-150 cmolc/kg soil

There values are dependent on pH of soil, since portion of their CEC is due to organic matter, which changes with soil pH

Hello! Here are few questions in the sample final, I not sure about the right answer! 1. Provide examples and briefly discuss effects of (accelerated) soil erosion (caused by water and wind).

2. What soil properties enhance sulfate (SO4 2-) leaching losses? Explain your answer.

3. What conditions favour nitrification?

4. How is nitrification useful to the organisms which carry it out?

5. Under what circumstances can P leaching become a problem? Briefly explain.

6. What soil conditions are best for plant growth?(sand% silt% clay% organic c% pH CEC exchangeable cations: Ca Mg Na K)

Thanks for helping!

Several of these questions are already answered in this Discussion Forum. Pls see responses from April 16 re. soil erosion question and sulfate leaching On Apr 17, I answered question re. nitrification and why organisms will carry it out, while on Apr 18, I answered question re. P leaching.

Q. What conditions favour nitrification? Nitrification is a biological oxidation of ammonium to nitrite and the oxidation of nitrite to nitrate. Nitrification is carried out by a bacteria Nitrosomonas sp. Hence, nitrification can only be carried out under aerobic conditions (ie in presence of oxygen) and when Nitrosomonas bacteria are present.

Q. What soil conditions are best for plant growth? -This is a really big question that summarizes numerous concepts covered in this course. Think about aeration, water movement and retention, nutrient retention and supply when developing the response to this question.

Hey,

I just had a quick question in regards to indicating diagnostic horizons from the previous exams. For example:

For each soil listed below, also indicate the diagnostic horizon(s). (a) Ah C (7-40 cm) Ahb (40-48 cm) C (48-80 cm) (located on a floodplain with a good natural drainage, in a sub-humid climate)

I understand how to classify the soil order in these examples but always get confused how would you indicate the diagnostic horizon, would you indicate it by each layer or just by the top layer of the soil? Thanks

Raza, you need to give the 1 diagnostic horizon e.g. in a Luvisol the diagnostic horizon is Bt

in your example what you put as the diagnostic horizon?

Hey,

I classifed this example as a Regesolic order and the diagnostic horizon is there is no B horizon present in this soil sample.

Would this be correct?

Yes, that is correct. This soil has no B horizon and it is a Regosol.

Additional info that would confirm this decision (ie that this is a Regolos) are: -presence of Ahb (ie buried horizon) at depth of the soil profile, and -location of this soil on a floodplain; hence, burial of the original Ah happened due to flooding event

I recall Dr. Brown saying in class that there would not be calculations on the final, is this still the case? Thanks.

Yes, this is correct. There will be no calculation question on the final exam

I am a little confused about how soil organic material related to PH. What I am thinking is that the decomposition of SOM will release CO2 which combines with H2O, so carbonic acid forms. Therefore pH decreases. Am I right?

Could you tell me more about the relations between CEC ,SOM and pH? Rex

Rex, you are correct that decomposition of SOM leads to the formation of carbonic acid, and characteristically lower pH. But also consider the impact of functional groups e.g. R-COOH and buffering capacity.

CEC is linked to pH in soils with significant SOM; consider pH dependent charge.

Hello again,

My next question is about the relationship with pH and CEC. I remember talking about their relationship in class, but also remember it being said that they are not directly correlated. How are they related or how do they effect each other?

Thanks!!

Soil pH has an effect on cation exchange capacity (CEC) of soil colloids that have pH-dependent type of charge (ie soil organic colloids adn sesquioxides). During yesterday's review session, I went over the graph from your textbook (I believe from the page 257) that relates soil pH and CEC. I cannot copy the graph here, but it is included into the final exam from 2005 posted on "Old exam" page.

The explanation of that graph is as follows:

Below pH 6 the charge for the clay mineral is negative and relatively constant. This charge is considered permanent, since it is formed as a result of the isomorphic substitution during mineral weathering. Permanent type of charge on montomorillonite (or any other phylosilicate) does not change under the influence of soil pH. Above pH 6 the charge of mineral colloid increases slightly because of ionization (deprotonation) of hydrogen from exposed hydroxyl groups at crystal edges. Note – these exposed hydroxyl functional groups are responsible for the formation of small portion of the pH-dependant charge on smectite (montmorillonite) clay minerals. In contrast to clay (i.e. montmorillonite mineral), essentially all charges on organic colloid are pH-dependant and as such are under the influence of pH of the surrounding soil solution. Consequently, as soil pH increases, ionization (deprotonation) of hydrogen from functional groups (e.g. carboxyl, phenolic hydroxyl, alcoholic hydroxyl) on organic colloid increases (i.e. number of negative charges on the surface of organic colloids increases) resulting in the increase of the cation exchange capacity.

If you need more explanation on the relationship between soil pH and CEC, pls refer to your textbook.

Hello,

Why is phosphate leaching from soil seldom a problem? Under what circumstances can it become a problem? Explain.

I am still confused about why/how phosphate is stuck in the soil in in soluble conditions.

Thanks!

For an ion (e.g. phosphate) to be leached out of a soil, the ion needs to be in soil solution. Very few phosphate ions will be in solution of any soil due to the process of phosphate fixation. Phosphate fixation transforms phosphate ions from soluble, mobile ionic form into an insoluble form of either iron-phosphate and aluminum-phosphate (in acidic soils) or calcium-phosphate (in alkaline soils).

I have a question about soil orders. can one group of the horizon sequences have two soil orders? ) For example:

Ah (0-13 cm) 
Ae1 (13-25 cm) 
Ae2 (25-36 cm) 
Bt (36-66 cm) 
Btj (66-86 cm) 
C (86+ cm) 
(Deciduous forest under humid to sub-humid climate; good natural drainage)

it satisfies two soil order(Luvisolic and brunisolic order). which one is the right order? Or it can have two soil orders?

thanks!

No, one sequence of horizons can ONLY belong to one order. The example you showed, can only be a Luvisol, since its Bt horizon is very well developed (i.e. thick) and the main soil formation process (i.e. accumulation of clay) is present in 50 cm of this soil profile. For this soil to be a Brunisol, it would not have a Bt horizon and its Btj horizon would need to be much thinner.

Hi, I am very confused by those 2 question: 1- how is nitrification useful to the organism which carry it out? first, i think about symbiotic relation between them and conversion of NH4 to NO3 2- how can nitrification influence soil pH sorry.. i may need some hints, cuz i have no idea right now thanks!!

Nitrification is a biological oxidation of ammonium to nitrite and the oxidation of nitrite to nitrate. The 1st step of nitrification is carried out by bacteria Nitrosomonas sp. (under aerobic conditions). Nitrosomonas is a chemo-autotroph which utilizes energy released during oxidation, and synthesize their organic matter from carbon dioxide. The second step of nitrification is carried out by a chemo-autotroph bacteria Nitrobacter for the same reason - it uses energy and C released during oxydation.

Nitrification acidifies the soil, since it yields nitric acid, with 2 hydrogen ions tending to be produced for every ammonium ion nitrified:

NH4+ + 2O2 = NO3- + 2H+ + H2O

and the fact that H+ ions are released in this reaction explains the acidification of the soil

Hi!

I have a question about the processes of both Potassium and Ammonium fixation. I am confused about their purpose in each cycle and how they influence the nutrient availability and overall cycle of each element.

Thanks!

Both K & NH4 fixation result in removal of available (ionic) forms of these 2 nutrients from the soil solution. K+ and NH4+ ions have exactly the same ionic radius and their radius also happens to match the interlayer spacing of some phyllosilicates (ie fine grained mica). So in soils with abundance of that type of phyllosilicate numerous K+ and NH4+ ions are trapped (or fixed) in unavailable form. The only way how these ions can become available is through weathering of that phyllosilicate mineral and this takes a long time

I think we didn't cover wind erosion much..I don't really know how to properly answer this question.

Provide examples and briefly discuss effects of (accelerated) soil erosion (caused by water and wind).

Thanks for the help!

We did not cover wind erosion and there will be no questions about it on the exams. Water erosion, on the other hand, has been covered in lectures and questions re. it might appear on the final exam.

To answer your question, you should 1st define accelerated erosion, followed by a brief explanation of general effects of erosion (ie removal of soil from one site & deposition of that soil or sediment on another site). Then you should list examples of water erosion (ie sheer, rill and gully erosion), with a brief explanation what are effects (or consequences) of each of them.

I have question regarding the difference between rill and gully erosion. Rill erosion is caused by slow movement of water along small channels on bare land with less vegetative cover. Gully erosion creates a deep channels that the surface runoff is further enhanced. The water movement is faster, creating a deeper channels.

Due to the slow movement of water in rill erosion, the soil particles are finer than gully erosion lands. Rills are channels small enough to be smoothed (rectified) by normal tillage. Gullies are larger channels that cannot be restored by normal tillage. is this correct?..

Thanks!!

Yes, this is all correct and this is an example of a complete answer. Well done!

Are we expected to be able to draw out each nutrient cycle? Or is it sufficient to know the sources and losses of each cycle?

No need to worry about memorizing the complete nutrient cycles. It is enough to know what are sources, losses and transformations of nutrients (N, P, K, S, Ca and Mg) within the soil. Also, you should be able to explain those transformations, not just to list them.

Perfect! Thank you Maja!