Course talk:APBI200

Hello!

I just finished the final, and I am slightly worried about the marking of the fill-in-the-blank questions. In some of my other courses, there have been issues with getting fill-in-the-blank questions wrong due to Canvas only accepting one particular way of writing the answer, even if someone else wrote a correct answer in a different way.

Will the fill-in-the-blank questions be marked manually or automatically, and if they are automatic, will we lose marks for formatting an answer differently (ex. 'vertisol' vs. 'Vertisol' vs. 'vertisolic' vs. 'Vertisolic')?

Blaise, pls do not worry - we will mark this question manually and will NOT subtract marks for incorrect spelling

Thank you!

I lost a bunch of marks on Q5 on midterm 2 (the management question). I'd appreciate any feedback for how I could have answered this better (or, if you got full marks on this question, I wouldn't mind seeing your answer for comparison.) My answer from the exam is below.

---

a) L1: Has the best nutrient retention (highest CEC=35) and second best supply (percent org mattter=7)

L2: Has the second best nutrient retention (CEC=33), but the best supply (percent org matter=10)

L3: Has by far the worst supply (org matter=2) and the worst retention (CEC=15)

b) L1: Will have the highest water retention (since percent clay=50, and also good aggregation holds water in micropores)

L2: Will have the second highest water retention (percent clay=30, and also good aggregation holds water in micropores)

L3: will have the lowest water retention (percent sand=75, and no micropores in aggregates to retain water)

c) L1: should have the second best drainage. Although it's a clayey soil, the fact that it's well-aggregated means that it should drain efficiently via macropores. In addition, marine parent material has a higher inflitration rate than alluvial parent material.

L2: Should have the worst drainage (assuming the assumptions about L3 hold). Although the fact that it's well aggregated means that it should drain well via macropores, its alluvial parent material won't drain as well as L1's marine parent material.)

L3: should have the highest drainage, assuming the parent material is loose ablation till instead of basal till, and the soil has not been severely compacted.

d) I would recommend L1 as the ideal site for a grassy picnic area, given that it seems to be the best compromise between the above factors. I would choose it instead of L2 given that it's likely to have better drainage, an important factor when considering a high traffic area that's likely to suffer from drainage issues due to compaction.

In addition to Maja's comments: Please be aware that there were different versions of the Data Interpretation Question randomly assigned to different students; thus the answers above are not application to other student's exams. Maja's comments below however are generic and useful.

Spencer,
In the part of the answer re. nutrient retention you did not mention differences in clay among the 3 soils, but based your very brief explanation of differences in CEC just on organic matter. And, those differences in organic matter contents among the 3 soils needed to be explained (you just stated what they are). The explanation should have included statements that both organic matter & clay particles cary lots of charges on their surfaces and are consequently responsible for nutrient retention.
Parts of answer on water retention & drainage were much better (ie complete). But the final selection of the most suitable location (or soil) was not correct. The best soil was #2 since it had the most balanced combination of nutrient and water retention properties for the city park.

So, here is an important advice to all of you who are reading this (and I sincerely hope that you are reading this) - in these so-called explanation type of questions, pls make sure that you provide both the reason for your observation and explanation.

I'm not sure how to differentiate between the two, if a question asking us to compare the two comes in the final?

Primary particles are sand, silt and clay
Primary minerals are those minerals that have a very stable crystalline structure which does not weather easily and in which there are very limited isomorphic substitutions and consequently very limited number of charges. Examples of primary minerals are quartz, feldspars, olivine (see lecture from Jan 17, slide #9)

Edit: I just found the thread with answers to this question but I can't figure out how to delete this post sorry!!

Olivia, see the discussion below "Practice exam #3 - QUESTION 2" recall that you need to login using your CWL to see all posts You are on the right track: as indicated below by Blaise "On soils containing iron sulphide, the iron sulphide will oxidize to produce iron oxides and sulphuric acid." So think about ways to reduce O₂ in these soils

I received a Canvas notification today that the Final exam due date has been changed to 8:30 am. Does that mean that we need to start the exam at 7:00 in the morning? I have been assuming (from past notifications and the UBC wiki page) that the exam starts on 8:30 and ends at 9:50.

Please clarify so that I can start and complete the exam on time.

Eakin, you are correct. The final exam starts at 8:30 a.m (Vancouver time) and is 1 hour 20 minutes long. The notification in canvas has been corrected.

Hello! I have a few questions about P and K nutrient cycles. 1) Is the reason phosphates are so low in soil because they are easily fixed? 2) In the potassium cycle, are primary minerals (once the crystalline structure has been weathered) considered an input since they make up the majority of K+ released into soil solution? Or are fertilizers and residues the only two inputs since those have the blue arrows on potassium cycle slide? 3) Is fixation considered a loss even thought the K+ is still technically in the soil? 4) Could you please explain the arrow pointing from residues to soil solution that says "leaching" on the K cycle slide? I always associated leaching with the loss of a nutrient, so I'm confused how that arrow applies here.

P cycle
You are partially right Olivia. Have a look at lecture #25 - you will find: 1) details on P fixation (i.e., different ways that phosphates are fixed in soils) and 2) additional reasons why phosphate ions in soil solution are low.

K cycle
In all the nutrient cycle diagrams in your class notes, inputs are show by blue lines, losses by red lines and transformations by black lines. Have a look at the K cycle diagram in lecture #25/#26 and see what you think.

Losses from the soil imply that a nutrient or compound is lost from the soil (e.g. via erosion, leaching or gaseous losses). In the case of K fixation, K is not longer available to plants - but it is not lost from the soil profile.
The arrow from K in soil solution labelled leaching indicates that K⁺ in solution can be lost via leaching. Note that cations held on exchanges sites are not readily leached.

Thanks for your reply! I had another look at lecture 25. Would the other main reason for lack of phosphate ions in soil solution be that minerals containing P have pretty low solubility and therefore little phosphorus can be released from the crystalline structure into soil solution? I also had a question about the nitrogen cycle diagram in lecture 26: Could you please explain why the arrow labelled fixation that points from fertilizers/industrial fixation to nitrates is blue? My understanding of any kind of fixation was that it makes nutrients unavailable to plants by adsorption to colloids etc, so wouldn't it be a loss and therefore colored in red? How is this kind of fixation considered an input?

P cycle
Olivia, in your first post you identified 1 reason: P fixation; here you identify a 2nd reason - minerals containing P have v. low solubility. Both reasons contribute.

N cycle
Industrial N fixation on the diagram refers to the Haber-Bosch process by which atmospheric N₂ is converted to NH₃; this is how synthetic fertilizers are made. You could replace Fertilizer / Industrial Fertilizer with synthetic or chemical fertilizers (and they are an input to the soil)

The term fixation: does it always indicate that nutrients are made unavailable to plants? Hint think about inputs to the N cycle?

From looking at the N cycle again I would say fixation doesn't always render plant nutrients unavailable. I can see there is another input arrow for photochemical fixation (NHO3) that goes directly into the nitrate pool. My understanding is that atmospheric N compounds are fixed into NHO3 and when they enter the soil solution, they dissociate, releasing a proton and forming NO3- which is plant available. Is that correct? In more general terms, is any kind of N fixation that's labelled as an input described as the conversion of atmospheric N to an ionic, reactive form of N such as nitrite, carried out by soil organisms?

What Sandra was hinting to is the process of ammonium (NH4+) fixation that traps (or fixes) ammonium ion in the inter-layer spacing of certain phyllosilicate minerals. Through that process of ammonium fixation, ammonium ions from soil solution (where they are available to plants) are made unavailable to plants.

Hello!

I was wondering how potassium lowers osmotic potential? Also, how would this improve drought tolerance? Is this because a lower osmotic potential means the water cannot move as freely and therefore cannot move down and become ground water or be absorbed by plants as quickly?

Thanks very much!

Gretchen, potassium lowers osmotic potential in plants and this is something that we did not cover in our course.

Thank you!

All questions are posted at the "Lecture notes" page here in wiki. Please post here your discussion, answers, and/or requests for clarification re. the QUESTION #2

a)These soils contain iron sulfides, which when dried react with oxygen and form sulfuric acid. Release of this sulfuric acid from the soil can in turn weather other constituents (like clay minerals containing Fe/Al) in the soil and release iron and aluminum (amongst other things). Via reactions with these compounds many excess H+ ions are released, lowering soil acidity.

b)Lime the soil, or do something to contribute to it’s buffering capacity?

I am particularly bad at explaining this stuff…..but I get it! I think….

b) Could you also add organic matter or a clay with high SSA like montmorillonite that would increase the CEC, creating a bigger buffering capacity. Also not sure how you would do it but could you increase the base forming cations (such as Ca, maybe that's what lime does)?

Bennett, increasing the buffering capacity would have minimal impact on the extreme acidity produced by the oxidation of Fe-sulfides (e.g., acid mine drainage). See Blaise's suggestion below.

a) On soils containing iron sulphide, the iron sulphide will oxidize to produce iron oxides and sulphuric acid. Sulphuric acid is one of the most acidic acids in existence, and easily releases its H+ cations into the soil solution, which will drastically lower the soil pH.

b) To reduce H+ release from these soils, you could keep it at saturation for part of the year. This would create anaerobic conditions that cause reductive conditions to dominate that would prevent FeS2 from oxidizing and releasing H+ ions. Alternatively, you could add large amounts of basic compounds like lime to soils to absorb the release H+ ions and offset increases in acidity.

Blaise has a good option, keeping these soils saturated.... preventing oxidation and the release of H⁺ ions in the first place.
As indicated here as well, lime will help nuetralize acidic conditions, but large amounts required.

2.) a.) FeS2 disassociates in solution forming Fe(OH)3 and SO4- releasing H+ ions. b.) The Addition of Na and Mg ions can help slow the release of H+ by forming salts.

a) The reason for that kind of parent material end up with having very low pH is iron sulfide would have the chemical reaction (oxidation). When the soil is unsaturated or drained it reacts with oxygen and releases hydrogen to lower the pH.

b) First: keep the soil saturated Second: add basic ion (C, Mg, K, Na)

Oxidation of pyrite (or iron sulfide) produces soil pH of 3 or 3.5. Hence, to bring such low pH to something that most plants can tolerate (around at least pH 5) would require large quantities of lime (or any other material that would contain Ca and/or Mg) and frequent applications of that material would be needed. Pls note that we do not want to add material containing sodium because of sodium's negative effect of aggregate formation. Sodium is a bad news for soils ;-)

a) Low pH indicates a high concentration of hydrogen ions. The reaction of pyrite with water and oxygen releases just such a preponderance of hydrogen: 2FeS2 + 9O2 + 4H2O -> 8H^+ + 4SO4^2- + 2Fe(OH)3

b) The acidity is a product of the dissolution of pyrite in water. Therefore, programs to limit the amount of water in the soil may reduce the acidity. This could be done by improving drainage, potentially by adding sand or larger size-fraction particles to the soil. Alternately, alkaline compounds such as lime could be added to the soil to raise the pH.

Spencer, please see Maja's explanation about S oxidation above, and refer to lecture #24.So when answering part b) limiting the amount of water would not reduce the oxidation of the sulfides (and thus would not reduce the acidity). In fact, drainage would make the problem worse (not better) - see comments above e.g. from Blaise.

All questions are posted at the "Lecture notes" page here in wiki. Please post here your discussion, answers, and/or requests for clarification re. the QUESTION #6

a) Since there is Ah>10cm, bs > 80% where Ca is the dominant cation, has a massive texture lower in the soil, and maybe its a grassland, I said it belonged to Chernozemic soil order. It also has a Bg layer, so i guess it could also be a Gleyzolic but it doesnt state that the water table fluctuates, so I think Chernozemic is the best fit.

Bennett, consider what is dominant gleying or the Chernozemic Ah?

I'm confused why we can conclusively say that this soil is of the Gleysolic order - it has a Chernozemic A horizon and is in a grassland, and also fits the properties of a Gleysol. Is it possible for it to be both? Why does one take precedence over the other?

Thanks!

A) The Goose soil belongs to the Gleysolic order. Its diagnostic horizon is the Bg (also the ABg and BCg horizons) which indicate a fluctuating water table and fluctuating oxidation/reduction conditions.

B) the granular and subgranular structure in the Ah and ABg horizons would allow room for root expansion and good aeration with the presence of more macropores. However, the massive structure due to high clay percentage in lower horizons would be an obstacle for root growth, as well as prevent drainage contributing to a lack of aeration. The fluctuation of the water table would mean that plants would occasionally be in completely saturated conditions which would restrict their access to oxygen and limit growth. The pH throughout the soil profile is mostly lightly acidic which is a condition that many plants can thrive in. The high CEC, particularly in the upper horizons would indicate that the soil has a good ability to store plant-available nutrients which would benefit their growth, however, it may also indicate that the soil is prone to leaching.

a)Gleysolic order, Bg diagnostic horizon

b)Well, the Goose soil has very low sodium content throughout its layers. The presence of a semi-thick Ah layer would be good for some plants, but this environment seems like it would largely make growth difficult for plants. To start, we have multiplegleyed horizons, implying spells of intense anaerobic conditions. Nd All layers below the Ah have a high percentage of clay and therefore micro pores. The small pore size and massive structure will make water extremely difficult for plant roots to access, as is borne out in our data for Goose.

That's all for me! Look forward to corrections and other peoples' responses :)

a) Gleysolic order. Diagnostic horizon: Bg, BCg, Cgk. b) Some advantages of this soil order is that it has a high CEC, which means it can store large amounts of nutrients. It also has a pH that lies within the 6-7 range for optimal plant growth. It also has a 5 cm thick LFH, indicating that there is a good source of organic residues providing nutrients into the soil.

   Some disadvantages of this soil order is its extremely high clay content. This will create very poor drainage conditions in the soil, leading to poor aeration that will inhibit plant growth. Also, the lower horizons have a low Ca:Mg ratio, which is bad for plant growth. The structure of the lower horizons are massive, which also causes extremely poor aeration, and is hard for plant roots to penetrate.

Generally, the top 20 cm of this soil are suitable for plant growth, but below that is extremely unfavourable.

Good start. When answering this type of question (in general) consider clearly separating your advantages from your disadvantages (e.g. two bullet lists)

a) Goose soil belongs to Gleysolic Order. The diagnostic horizon is Bg.

b)advantages: because silt and clay accounted for the large proportion of soil texture, Goose soil would have high water retention. Then organic matter would form soil aggregates to retain nutrients. High CEC which give the soil high buffering capacity, if there is acid rain the soil would not much affect. Soil pH is neutral.

disadvantages: soil contains small percentages of sand and high percentages of clay would cause low infiltration, poor drained.

I am not sure about how does Ca, Mg, K, Na influence plants grow. They provide nutrients, but high Ca would cause phosphorus deficiency in soil. While, high Ca exchangeable cations can exchange more nutrient? Also, if the roots are abundant in A horizon does it means the soil has a low decomposition rate? I checked the note of soil structure where can i find more details.

Yiming, consider how soil properties, such as texture and structure change with depth (as well as root abundance).
With respect to P fixation, consider pH (not just Ca).
Ca, Mg, K are all essential plant nutrients. Na is only an issue if levels are high (i.e., sodic soil)
High root abundance does not suggest slow decomposition, but good plant growing conditions.

a. Goose soil is a gleysol, as diagnosed by the Bg horizon, and the fact that gleying is a dominant process in the soil from 15-55+ cm. b. . i. Advantages: good supply of organic matter, close to neutral pH, high concentrations of base-forming cations, structure above 25cm conducive to plant growth. ii. Disadvantages: poor drainage (as indicated by the fact that it is a gleysol, and the fact that it has massive structure), unfavourable ratios of Ca:Mg:K:Na, poor texture for plant growth, poor structure for plant growth below 25cm.

Yes, this is a Glesyol... and yes its diagnostic horizon is Bg (and gleying is also evident in ABg, BCg, Cgk)

Advantages:
Everything Spencer that you listed is correct, but for the full mark the explanation would need to be included too. For example, consider elaboration of the following :

• -what are implications of good organic matter content? In other words, what other soil properties are impacted by high organic matter?
• -what are implications of neutral pH?
• -why is structure above 25cm in this soil conductive to plant growth? In what way?

Disadvantages:
Once again, what is listed is all correct, but elaboration/explanation is needed.
What is missing is mention of the fine texture, including a very low sand content. Then elaboration would be needed to explain what are consequences for other soil properties of such type of texture

Hello, I was wondering what the dominant soil forming process in regasolic soil? Since it's diagnostic horizon is lack of a B horizon, I'm not sure how to know this dominant soil forming process since Maja explained in her most recent video that dominant soil forming processes are determined based off the lowercase letter in the diagnostic horizon. I was also wondering if we should know what areas regasolic soils are found? I couldn't find anything specific in the lab manual. Once last more general question: would accumulation of certain things in soil such as clay or Na+ be considered a translocation in terms of the 4 soil formation processes? Thank you!

There is NO dominant soil forming process in Regosols. These are soils at the very beginning of soil formation, and whatever soil forming processes do occur in them, they are just starting. Consequently, there is no B horizon in them and even if A or O or LFH horizons are present they are very thin; while C horizon is often labeled just as 'C' or with that modifier 'j' (eg Cgj).
Accumulation of clay or Na+ can be considered as examples of translocation soil forming processes, as long at they came to that horizon from another horizon.

Ok thank you! If accumulation of certain things don't come from a different horizon, would that instead be considered an example of addition soil forming processes?

Yes, that is correct

Hello,

My apologies if this has already been answered somewhere else. I was wondering whether we're supposed to list all possible diagnostic horizons or only those that pertain to the soil order that we've identified when answering "What are the diagnostic horizon(s)?" styled questions.

Thanks for your time, Lukas

Lukas, IF you are referring to the questions where you are given a profile; be specific to that profile. For example, in the tips for soil classification which Maja uploaded (under lectures) the 2nd example is:
LFH (4-0 cm)
Ah (0-10 cm)
Bg1 (11-23 cm)
Bg2 (23-58 cm)
Cg (58-103 cm)
As indicated, Bg1, Bg2 and Cg are all diagnostic (thus all should be listed)

But be clear that you should only list diagnostic horizons for that specific soil profile and its soil order. Using the 1st example Maja provides in her tips: there is only 1 diagnostic horizon (Bt) in that example
see https://www.dropbox.com/s/cqw4wf5s5dggigu/Untitled%20Project.mp4?dl=0

Question: Was just confused about the carbon content in humic acid. On lecture 8, there is picture that says that humic acid has a higher carbon content compared to fulvic acid but on the next slide it says that humic acid has a lower number of carboxyl groups per unit mass than fulvic acid. I am a little confused about this. The important concequence of this is that humic acids will be more difficult to decompose by soil microorganisms, and that release of plant available forms of nutrients (eg N, S, P) will be slower.

Answer: In organic compounds, C can be present in some (but not all) functional groups (eg COOH group), but C is also present in open chains and cyclical rings. Hence, statement that humic acids have higher C content than fulvic acid is in agreement that humic acid molecules are larger and of larger molecular weight (and also more complex) than molecules of fulvic acids.
In addition, one needs to keep in mind that humic acid has lower number of COOH groups (and other functionsl groups) per unit mass.

Hello!

How is the dominant soil formation process of each of the 10 soil orders determined? For example, the Podzolic soil order has a diagnostic horizon of podzolic B. This includes Bh, Bf, and Bhf. If we are to determine the dominant soil formation process from these diagnostic horizons, would the dominant soil formation process be enrichment with organic matter or enrichment with Fe and Al oxides linking to an addition?

Thanks very much!

yes, that is correct - in Podzols there could be 2 dominant soil formation processes - accumulation of Fe/Al oxides/hidroxides (indicated as Bf) and accumulation of organic matter in B horizon (indicated by Bh). Some (but not all) Podzols will have either both of those processes (indicated by Bhf) or just one of those 2 processes.

In general, assigning the specific dominant soil forming processes to each of 10 soil order is based on the current, overall understanding of soil formation (genesis).
However, if you are asking how is an individual soil scientist or land manager deciding what is a dominant soil forming process in a specific soil, than the answer is - based on field observation and description and (if necessary) lab soil analysis.

Hello!

In lecture 24, nutrient cycles N and S, on page 17 there is a small picture illustrating the different reserves of nutrients. I was wondering, for each of these forms, if I am correct in thinking that the arrow going towards the form (e.g. minerals precipitate) is saying that that is how nutrients become stored in the mineral?

Thanks very much!

Close but not quite exactly "stored in the mineral" .... That particular arrow that you mention is indicating the pathway of mineral (or ionic) form of an element becoming plant unavailable through a process of chemical precipitation (ie formation of a solid crust.
Without going into deeper explanation here, what would be important to remember from that particular slide is that for each nutrient there are processes that make it available to plants, and those that make them unavailable to plants.

Thank you!

On the SOM summary, it says that non-humic substance is a primary component and humic substances are secondary components. What does it mean to be a secondary component? Are humic substances created from the further breakdown of non-humic substances?

It also says that "at pH 4-6, 85-90% of carboxyl groups are dissociated". Does the dissociation further lowers the pH? Kinda like a positive feedback loop?

Thank you so much!

Non-humic substances are primary components such as carbohydrates, proteins, lipids, lignin. Primary in this context means that these components are derived from the organic residues/litter.
Humic substances are decomposition products, i.e., secondary components which have been microbially transformed. As indicated in the slide "SOM components - a summary" posted with lecture #8, micro-organisms through decomposition processes synthesize new organic compounds (i.e., secondary components)

Lecture #19 further discusses SOM, in particular pH dependent charge and the dissociation of functional groups. Note that the dissociation of functional groups occurs over a range of pH. As you indicate, carboxyl groups (R-COOH) are largely dissociated over the range of pH common in soils.
You are correct that the dissociation of OH groups contributes to the acidic nature of organic soils (e.g. wetlands), however the relationship with pH is complex. The important thing to remember is that as the pH increases (i.e., more OH^- in soil solution), the greater the dissociation of R-COOH groups, increasing the net negative charge and thus increasing the CEC. Consider:
R-COOH + OH^- goes to R-COO^- + H₂O

Hi I was just wondering if the practice final on canvas is an accurate representation of the weighting of types of questions and length of the final? On Canvas there was only 8 questions and only 2 of them were short answer. For the real final I assume there will be more than 2 short answer questions. I believe I saw somewhere on wiki that said that the final would be 3/4 short answer? Is that correct? Thanks!

The practice exam posted in Canvas is supposed to give you an idea about the format of the final exam (ie to show you the type of questions that you will have on the final exam). Pls note that the practice exam is much shorter than the final exam will be.
About half of the points on the final exam (46.7% to be exact) will come from the short answer type of questions.

Hello,

I'm curious if there is a standard range for what are considered to be low, moderate, and high CEC values. If I were to guess, I'd wager (approximately) the following. Does this seem reasonable?

Very low : < 5 Low: 5-10 Moderately low: 10 plus-15 Moderate: 15 plus-25 High:25 plus-50 Very high: 50-300 plus

There are ranges of CEC that are considered low, moderate, and high but those ranges are affected by numerous soil properties and there is not one universal set of CEC ranges that will be applicable for all soil types. We stayed away from this explanation on purpose, since it goes beyond the scope of this course

Does a dominant soil forming process for Regosols include the beginning of loss of soluble salts, along with poor soil development?

Thank you!

Amy, if you look at lecture 30-31, the 1st slide after "Soil Classification and Geneis" shows the genetic evolution of soils under relatively good plant-growing conditions. You are correct in that soluble salts are often translocated early on in soil formation. Be clear however, that the loss of soiluble satls is NOT diagnostic for Regosolic soils.

Hello!

I am a little confused about the appearance of the LFH horizon in soils. In some of the 'identify the soil order' questions, there is no LFH horizon present, which confuses me. In my understanding, every well-drained soil with any biological activity on it should have some sort of an LFH horizon where organic litter decomposes into humic substances. Can someone explain to me what is wrong with my understanding? Is it just that the LFH layers in these soils are negligibly thin and are therefore ignored?

LFH horizons are ONLY present in forest ecosystems. Hence, soils that form in other type of ecosystems (eg grasslands, wetlands, tundra) will not have LFH horizons.
In real-life, on substantially disturbed forest sites, LFH horizons might not be present due to that disturbance. But we do not include those examples in the questions that are on our assignments or exams.

Why are LFH horizons only present in forest ecosystems? Is there insufficient litter for LFH horizons to form in non-forest ecosystems?

In forest ecosystems litter input is high and decomposition < input, thus litter accumulates forming LFH horizons

That makes sense, thank you!

For the phosphorous cycle true and false questions I got a few wrong but I'm not sure which ones are wrong... here are my answers: a)F b)T c) T d)T e)F Thanks!

For details on the P cycle see lecture #25
If you are uncertain on a specific process, please provide your thinking and we are happy to respond.
e.g. Q2a) Phosphorus can be lost from the soil by volatilization. False because.... (but also indicate why you are unsure)

specifically I was wondering about the following: Does chemical weathering of primary minerals release available P such as orthophosphate ions (H2PO4- and HPO42-)? Is is correct that the principle way of P uptake is in the form of Phosphate? Is organic residues plat available P? Because in the diagram it has a arrow going to soil organisms which makes me think its not plat available.

Q. Does chemical weathering of primary minerals release available P such as orthophosphate ions (H2PO4- and HPO42-)?
A. Yes, the weathering of primary minerals does release plant-available forms of P (ie H2PO4- and HPO42-), but this is a very slow process.

Q. Is it correct that the principle way of P uptake is in the form of Phosphate ions?
A. Yes, plants can ONLY take P (and any other element) in ionic form.

Q. Are organic residues plant available P?
A. No, organic compounds containing P, are not directly available to plants. ALL organic compounds need to be decomposed (ie mineralized) by soil organisms and through that decomposition organic forms of elements (eg P) need to be transformed into ionic (or mineral) form that is available to plants.

Hi Maja,

On Problem Set 3, I indicated that only inorganic P would be plant available (i.e., #2e - False) but this response was marked wrong. I am confused.

Thanks!

Prob Set #3 Q2e) Organic residues are a major source of plant available P in soil. The key word is source; i.e. the question does not say that organic P is plant available but that plant available P is derived from organic residues.