Course talk:APBI200/Archive/2015-16WT2

Atmospheric deposition through precipitation occurs in both the N and S cycles (NOx and SO2). In the sulfur cycle, the input "through rainfall" is the same as through precipitation, and may be intercepted by plants and absorbed.

Hi APBI 200 students, Do you have a favorite question that you would like to see on the APBI 200 final exam? If yes, post it here and it might end up on the final exam this year. We will take your suggestions in consideration Cheers, Maja & Sandra

Q2 from problem set 3: Describe two activities, beneficial for soil productivity, carried out by each of the following groups of soil organisms - fungi, - earthworms, - protozoa.

questions about identify soil order through identifying diagnostic horizon, horizon sequences and horizon thickness.

roles of soil microflora in contributing to macronutrient availability to plants

Question 1 from both Discussion 2 and Discussion 3.

I would like to vote for any question on nitrification, ammonification and their role in the nitrogen cycle! ;)

Is this correct?: inputs: -fertilizers -sulfate on soil minerals -sulfate on clays -animal/human waste losses: -runoff -erosion -leaching -volatilization

organic matter and plant residues supply sulfur. so does the atmosphere, through dry deposition and acid rain. since S is a macronutrient and a constituent of some amino acids (proteins) there is loss through plant removal as well.

Thanks

Are there any other inputs of soil phosphorus other than fertilizers, primary and secondary minerals, human and animal residues and weathering? What about for losses, any other than runoff, erosion and leaching?

One more loss would be plant removal (harvesting).

Thanks

remember Phosphorus fixation too! it doesn't have to be eroded away to be considered a loss. it can stay in the soil profile but be unavailable to plants.

Thanks

How does freeze-thaw action "y" differ from shrink-swelling "v" in terms of the influence they have on soil characteristics and development?

they both have very similar results, which is mostly a vertical mixing of soil materials within the profile. finer particles will be translocated to lower horizons. we had a good slide that explained argillipedoturbation in vertisols and wikipedia has a decent explanation of cryoturbation. they both contribute to increased weathering (breakdown of rock and mineral particles, more surface area) through mixing under pressure. sandra mentioned that at depth there is no room for volume change so freezing and swelling will cause grinding, sliding and compression, etc.

Does the Ah (/Ap) horizon need to be > 10 cm thick in order for the soil to be classified as Chernozem ? As the table in the lab manual doesn't specify, but the slides say it must.

E.g., would this soil be a Chernozem or a Brunisol?

Ah (0-9 cm)
Bm (9-22 cm)
Ck1 (22-40)
Ck2 (40+ cm)

Yes, it has to be thicker than 10 cm and they also have to specify characteristics such as base saturation higher than 80% for it to be a chernozemic Ah and therefore a Chernozem, because that is not specified, the soil in this example is a Brunisol.

Cool, thanks for the answer

In the past final no 4, question 5 about the Delta soil, I think its a Gleysol, but I am a little stuck with the advantage and disadvantages for plant growth...I would say that because of water accumulation and little oxygen that it is only suitable for plants that grow in swamps...or rice plants... am i on the right track or am i completely off? :(

Thanks!

You are on the right track. This Delta soil is a real challenge for local farmers because of its properties. Pls note that the soil has been drained, since without drainage most sites in Delta are not suitable for agricultural production. Gleying is still the most dominant soil formation process in this soil even after drainage, but this soil is not now suffering from the lack of oxygen. Other properties are challenging.

There are multiple questions in the past exams where a soil will have an Ah layer thicker than 10 cm which should classify it as a Chernozemic soil. There is also a present thick Bt horizon as well, which should classify it as Luvisolic. In the answering key for soil orders in the lab manual, it indicates that a soil should be classified as chernozemic before luvisolic, but according to the problem, this site is within a forest (versus grasslands where chernozems should be present). It seems like the proper answer should be Luvisol in this case but I am confused as to whether the soil order classification key on page 79 of the lab manual is something which should be followed considering it causes some contradiction in this question.

You have to be careful about making a decision about soil order only based on the presence of the Ah horizon that is thicker than 10 cm. Many soil types will have such Ah horizon, but not every Ah horizon thicker than 10 cm is the Chernozemic Ah. The Chernozemic Ah horizon needs to fulfill ALL of the following criteria: to be thicker than 10 cm, to be present in a soil located in a semi-arid to arid climatic zone in a (natural) grassland ecosystem, to have C/N <17, to have Ca2+ ion as the dominant ion on exchange complex, and to have base saturation >80%.

1. Does anyone have a rough age for the different horizons of the ubc podzols? I'll assume the agriculture a horizon is around or less than 100 years old.

2. Does the glacial till parent material come from glacier movement originating from the north shore mountains or further east?

3. How is the C layer of these soils formed? is basal till, weathering and deposition of bedrock by glacier movement an adequate explanation?

4. Why does rainfall not significantly decrease the pH of these soils? excess rainfall leaches base cations from the soil, increasing the percent of Al3+ and H+, as well as rain being naturally slightly acidic. the pH we got was a consistent 6.5 which is neutral. What is the natural pH of the parent material? is it relatively neutral?

both soil sites were relatively high in organic matter, which is a strong buffer against changes in pH. CEC has that curious ability to retain cations against the percolating effects of water, but gladly releases them to plants. UBC soils are still relatively young (10,000) so a healthy supply of bases are still being released through weathering. and what's being lost is also being continually put back in through leaf litter, crop residues and management practices.

bedrock is not part of the regolith so it's of no consequence for soil formation. transported parent material is everything on top of the bedrock.

glacial till could have been transported on ice sheets over a kilometre high. my guess would be that materials came from here, there and everywhere, and without geological analysis we can't really know origin with much accuracy, but i'm not sure. an awesome question though! what's the age of the mountains vis a vis the glaciation and what would the ice sheet/mountain standoff have looked like???

The site where UBC Farm is located was cleared back in 1960s.

1. FA has higher acidity than HA, but one slide says CEC is low at low pH due to undissociated form of COOH. so why does FA has higher CEC?

2. between 4-6 pH, R-COOH=R-COO negative + H positive. it means it will dissociate in low pH, but why CEC is low at low pH

3."# of carboxyl groups determine the behavior of humic substances" HA has more COOH, why does it has lower CEC ? I am totally lost

I see the confusion. Slide #15 from that lecture says that "between pH 4 and 6, 85-90% of carboxylic groups are dissociated" which would mean an INCREASE in CEC, but slide #17 says "at low pH carboxylic groups on humic substances are undissociated", which would be a decrease in CEC…as far as I understand.

1. the "acidic character" of FA is different from soil pH. FA is more acidic because it does have more COOH groups, and can therefore contribute more H+ ions to the soil solution under the right conditions. pH will then determine how the functional groups behave.

3. Fulvic acid has more COOH groups (per unit mass) than Humic acid, which means it will have a higher CEC. And behavior of humic substances is determined by: 1) # of carboxyl groups AND 2) particle size. FA is much smaller with less molecular weight, so it has a higher SSA and overall it is more reactive and mobile in the soil.

Hope that helps for now. I just wanted to put in some thoughts, since it's Saturday and I know what it's like to have burning questions.

thank you for your help!

But i am still confused by the "acidity" of FA. if this is not equal with pH value. What does it imply? High acidity means more H+ in the soil?

One more other question about Ammonium fixation: is Ammonium fixation the same with isomorphic substitution ?

you're right. fulvic acid does have a low pH. an acid is just a compound or ion that contributes to H+ activity in the soil. isomorphic substitution occurs during the weathering of rocks and minerals, when primary minerals are altered or transformed into secondary minerals (ie. 2:1 phyllosilicate clays). remember the octahedral and tetrahedral sheets of montmorillonite for example. Mg can substitute for Al INSIDE the crystal structure, leading to a net negative PERMANENT charge on the colloid. ammonium fixation is when NH4 gets stuck BETWEEN two layers of vermiculite or illite (fine-grained mica) because it is just the right size. this also happens to K because of similar size, remember lyotropic series: Al3 > Ca2 > Mg2 > K=NH4 > Na. in this state the nutrient are non-exchangeable and only slowly available to plants, so it represents a decrease in available N. hope that helps.

Hi there, For the FA and HA, I am thinking that the FA is more acidic than HA since FA has more H+, and relatively it should has higher CEC. Since there will be more coo- that attract other cations such as fe3+ or al3+ and so on, FA "SOLUTION" (pure FA) will have higher CEC Refer back to lecture 17 slide 16, it says "CEC IS HIGH AT HIGH PH AND LOW AT LOW PH". Is it because coo- is organic anion that can lead to flocculation? (Since FA is so soluble that large amount of H+ being released with COO- leftover). Correct me if I am not thinking it in a right way. Thanks

Dr. Krzic and Dr. Brown,

On the final exam, are we allowed to list our answers or do they have to be in full sentences?

Thank you

The format does not matter, as long as your answer is correct and complete

What is a typical range for base saturation in soils? What base saturation percentage is high enough to predict an above average pH?

We did not discuss this in our course, since (as every other topic mentioned in this Introduction to Soil Science course) there is so much more behind the concepts covered. There is some disagreement among soil scientists about the value of using "optimum" or typical base saturation ranges. One school of thought holds that it is very important that the soil contain a specific base saturation, or ratio of saturations for each of the major cation nutrients (Ca, Mg, K). Practitioners of this approach will make recommendations designed to adjust the soil to specific saturation levels. The opposing view is that there can be a wide range of saturation for each of these major cations, with no significant benefit to having particular saturation levels or ratio of saturation levels. The evidence suggests that the primary need is for an adequate amount of each nutrient, regardless of the resulting percent saturation, and that the desired saturation range can be quite broad. To the degree that there is an "ideal" percent saturation range or ratio of cation nutrients, it would be affected by several other factors such as plant species, the intended use of the plants, the nature of the soil itself, etc.

1. Under what circumstances can phosphate leaching become a problem? is that when there is no Al/Fe/ca ions present in the soil solution?

2. Why soil animals like nematodes are restricted to the uppermost soil layers like algae do. Does nematode also need sunlight?

3. What soil and environmental conditons contributed to the forming of Ahb? is the erosion make Ah buried?

4.Can both First and second minerals contribute to the sources of P and S?

1. Under what circumstances can phosphate leaching become a problem? 1st review the P cycle. Consider why P leaching is seldom a problem (i.e. what happens to PO4 in soils)? Hint consider why available PO4 in soil solution is low. 2nd under what circumstances can phosphate leaching be an issue? Hints: consider inputs and pH.

2. Nematodes (threadworms) are micro-fauna (<0.2 mm). There are 2 major "types" of nematodes - what are they? To answer your question, consider their food sources and their role(s) in soils.

3. Ahb horizon. 1st give definitions for h and b (and consider the processes their associated processes). 2nd think about the environmental conditions which may commonly lead to the formation of an Ahb horizon. Note that erosion implies the removal of soil (not deposition) but you are on the right track.

4. Sources of P and S. Review the P and S cycles AND slide 16 of lecture 25 (soil as a source of plant nutrients). Sulfide minerals, phosphate minerals and SOM are important sources.

For the Brunisolic order, is the depth of the diagnostic Bm horizon greater than 5cm? And for Gleysol, a Bg greater than 10cm?

(For these two the depths are only given for the other potential diagnostic horizons but not for the main diagnostic horizon)

Thanks!

For the Brunisolic soil order the diagnostic horizons are a Bm, Bfj or Btj > 5 cm thick or Bf < 10 cm thick. Consider that Brunisols have undergone only slight development from parent material.

For Gleysol, the diagnostic horizon is a Bg (or Cg > 10 cm thick within 50 cm of the soil surface). Consider the conditions which lead to a gleyed horizon (e.g. Bg).

Why does nitrification take place in the soil when ammonium is already in plant available form? Is it because not all of the ammonium is taken up by plants so some of the ammonium in solution happens to get oxidation by autotrophs?

Both ammonium and nitrate serve the same purpose to plants and soil organisms right? They both sources of N just in different forms.

Also, what exactly does fixation and reduction mean? Since different fixation processes seem to render nutrients both plant available and unavailable.

Thanks!

Ammonification is done by heterotrophic microorganisms and convert the amines/a.a. released by bacteria and fungi decomposition to ammonium, which then can be converted to nitrites and nitrates, absorbed by plants, used by microorganisms, adsorb to clay minerals, or released to atmosphere. Nitrification is after ammonification and converts ammonium to nitrate. It has two steps: first, ammonium to nitrite. Then nitrite to nitrate.

Yes, both ammonium (NH+) and nitrate (NO3-) ions are available to plants.

We have discussed three types of fixation processes during the discussion session #3. In this course we mentioned N2 biological fixation (that converts N2 from atmospheric air to plant available forms through activity of N-fixing organisms), NH4+ fixation (which represents trapping of NH4+ ions in the inter layer space of certain phylosillicate minerals, reducing availability of this form of N to plants) and phosphate fixation (a chemical reaction with either Fe/Al or Ca ions which leads to formation of insoluble phosphate compounds that are not available to plants). Hence, the outcomes of these 3 types of "fixation" are different

Do we need to know the equations for ammonification and nitrification?

You need to know to describe those reactions (or any other chemical reaction mentioned in our lectures). Just knowing to write chemical reactions without knowing to describe what is going on is not what we are after in this course.

do we need a calculator for the exam?

No, i dont think so. Maja said today we dont need a calculator for the final.

what are examples of erosion? Is it just like water: rill, sheet and gully, and wind? Also, what are the effects other than the transformation of soil into sediment?

Soil erosion can be carried by water & wind. Types of water erosion are - sheet, rill and gully. Erosion removes soil material from one site & deposits it at another where it can burry plants and result in pollution due to higher conc on chemicals (fertilizers, pesticides) that are also transported and deposited by erosion. Additional effects of erosion are negative impacts on soil health (dust) and machinery

what exactly is the difference between sediment and soil?

Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by wind, water, or ice, and/or by the force of gravity acting on the particles.

And by now you should know what is soil (see definitions from lecture notes)

and do we need to know the equations for erodibility?

No

Because ammonium is a weak acid and it is converted to nitrate through nitrification, I am assuming soil pH will increase since there will be less of the acid present. Is this right? If not, what is the correct answer?

Nitrification, which is oxidation of ammonium to nitrite (NO2) and then oxidation of nitrite to nitrates (NO3), releases H ions into soil solution, which in turn lowers soil pH.