Midterm Exam Discussion

This post is intended to give everyone an overview of the midterm exam, to highlight the areas that were done well and to indicate areas that need work.  I will also highlight questions that were similar to clicker questions used in classroom quizzes.  The information is presented in the same order that the exam was set up.

Midterm mark distribution

First of all, the overall distribution of marks is presented in Figure 1.  Although, the majority of students passed the midterm, 23 students out of 60 had marks 50% and lower and only two students out of 60 had a mark above 81%.  The class average was 58%.

Figure 1. Distribution of marks for the SLSC 240 midterm.  Percentage of students is out of 60 students and the midterm exam percent ranges are out of a total of 55 marks.

Part I. Multiple choice questions

Overall, most students did relatively well on the multiple choice section.  The average mark was 7.7 correct answers out of 10 possible marks.  There were four questions that people struggled with though, these included questions b, e, g and h.  At least 30% of the class selected the incorrect answer for these four questions.

The correct answer for b is 3 “soil forming factors are not expressed equally in all parts of the profile”.

Question e was actually a question that was previously used in a clicker quiz.  The question appeared in exact same form, but on the midterm 37% of the class selected the incorrect answer.  The correct answer for e is 3 “air filled porosity”.

The correct answer for g is 4 “the weight of soil/unit volume of soil solids”.

The correct answer for h is 1 “Aluminum Al³⁺”.

On the positive side, question c was very similar to a clicker question presented on CEC and 97% of the class got it correct.  As well, question f (pH values) was very similar to a clicker question presented in class, but the pH values had been changed slightly and 75% of the class selected the correct answer, option 5.

Did you find the multiple choice questions difficult?  Do you think the clicker quizzes prepared you for the type of questions you saw on the midterm?

Part II. Matching

Again, for the most part, the matching part of the exam was done relatively well.  On average, students got 7.1 marks out of a possible 10.  There were three terms that stood out as being difficult concepts.  Soil dispersion, negative charge, and exchangeable H+ were terms that 50% or more the class matched incorrectly.  These are all concepts associated with cation exchange capacity.  In regards to negative charge, some of the problem may have been that half of the answer “isomorphous substitution” didn’t show up on the exam.  However, it appears as though, CEC is still an area that people are struggling with.

Do you find cation exchange capacity a topic that is difficult to understand?

Part III.  Short answer and calculations

Questions 3 and 5 were calculation questions.  These questions were typically done relatively well; however, there were some clear problems with both calculations.  Units, units, units.  Almost everyone who knew how to do the calculations in question 3 forgot to put the answer in kg/hectare furrow slice.  Almost 50% of the class got 2.5 out of 3 on this question because they forgot the correct units. Question 5 was similar too,  a lot of people forgot to express the answer in cm³.

Another issue encountered was converting units.  It’s always a good idea to double check the number of zeros you have when dealing with metric conversions.  As well, if you aren’t sure how to proceed on a calculation question, it’s a good idea to try to draw the dimensions out or at least try something.  Many students left question 5 completely empty and that meant losing 4 possible marks. 

Bulk density.  This is a concept that has been stressed in class a lot and several clicker questions have focused on bulk density.  The majority of students were on the right track when it came to question 4, but just needed to fill out their answer.  The average mark was 1.6 out of 3 possible marks for that question.

Part IV. Short answer (Question 6-8)

This is where the midterm started to go quite poorly for most people.  For questions 6 to 8, the average mark out of 5 was less than 2.5.  This is an area where people lost a lot of marks.  The majority of marks lost were due to lack of information provided.  In general, one sentence is not a sufficient answer for 5 marks.  Individuals who used point form typically did better because they usually had 4 or 5 points listed.  The basis for the answers for each of these three questions comes directly from the class notes.

Question 6.

Briefly explain how topography can influence the soil formation process.

These two slides are directly out of the topography lecture notes.

Question 7.

What was the last period of glaciation (Wisconsin period) such a fortunate event in Saskatchewan prehistory?

Slide is from the glaciation lecture.

Question 8.

What is soil structure? What are the benefits of good soil structure?

Last two slides from the soil structure lecture

On questions 6, 7, and 8, students averaged approximately 7 marks out of a possible 15.  This was a huge loss of marks for most people, especially when the exam was only out of 55 marks.

Part V. Long answer

The long answer question was quite interesting because 40 students out of 60 selected Option A, probably thinking it would be simpler.  However, in terms of clicker questions presented in class, 3 questions provided the direct basis for the answer of Option B and only 1 clicker question dealt with Option A.

Option A.

The clicker question presented on this topic was, which soil genesis process does water NOT participate in?

-physical weathering

-chemical weathering

-biological weathering

-leaching

-parent material deposition

There was no correct answer for this question because water participates in all the processes listed.  For full marks, you could have listed the 5 processes and briefly described each one.  The average mark was 5.1 out of 10 for Option A.

Option B.

For the 20 brave souls in the class who attempted this option, there were plenty of hints dropped in the clicker quizzes.

Clicker question 1 on CEC

Which soil characteristic does CEC not influence?

-soil fertility

-soil moisture

-soil texture (correct answer, CEC does not influence texture, but texture DOES influence CEC)

-soil structure

-soil pH

Clicker question 2 on CEC

What soil property is NOT influenced by the negative charge on clay and organic matter (CEC)?

-soil fertility

-soil water holding capacity

-soil pH

-soil structure

-soil temperature (correct answer)

Clicker question 3 on CEC

A soil with a high cation exchange capacity will have

-a high clay content

-ability to resist leaching loss of cations from soil

-ability to supply more nutrient elements to plants

-all of the above (correct answer)

The correct answer for Option B could have included listing soil fertility, soil moisture/water holding capacity, pH, soil structure, etc. and briefly describing how they are affected by CEC.  The average mark for Option B was 4.5 out of 10 but that is hard to compare to Option A because so few people even selected Option B.

Overall impressions and suggestions

I’ve included a few suggestions for writing exams just based on what I saw in the results from the midterm. 

1. Take the time to write down anything you can think of for a topic, you never know, you might get partial marks. 

2. If it takes you a long time to write, start with the long answer questions and leave the multiple choice and matching to the end because they go faster. 

3. If a question is worth 10 marks, chances are you need to list 10 things or list 5 things and describe each item in some detail. 

4. Make sure the length of your answer reflects the number of marks allocated to the question. 

5. When studying, try to focus on topics that are stressed repeatedly in class, guaranteed they will show up on an exam in some form (most likely a long answer).

Did you find the clicker questions or the blog helped provide information or hints as to what would be on the midterm?

Other soil parent materials found in Saskatchewan

In our previous blog we examined soils developed on glacial till and glacio-lacustrine parent materials. These materials are the foundation of the majority of agricultural soils in this province. Let's briefly look at two other parent materials that represent much smaller areas of the province and produce soils that have limited or no agricultural capability. These parent materials are called Fluvial and Aeolian materials. Fluvial refers to relatively coarse parent material deposited by fast moving water. This water was mostly produced during the melt phase of the last glacial period.  Aeolian materials, in contrast, are wind blown sediments mostly deposited by the prevailing winds at the end of the last glacial period. Aeolian materials, if coarse, are called Dune Sand, whereas, finer silty textured materials are called Loess. When the great ice sheets had melted and retreated there was a period where the exposed parent material was not vegetated and very susceptible to wind erosion.

Fluvial Parent Material

  

Soils developed on fluvial parent materials generally have a very limited agricultural potential.  The precise nature of these materials is determined by the speed of water flow during deposition. In general, the more swiftly that water flows, the coarser the resultant material. Figures 1 and 2 are examples of comparatively fine fluvial material which has some agricultural potential. Figures 3 and 4 provide examples of coarser materials with virtually no potential for crop production.

Figure 1. Field developed on comparatively fine fluvial parent material

Figure 2. Close up of Figure 1, courtesy T. Yates

Figure 3. Fluvial soil profile exposed, courtesy of K. Van Rees

Figure 4. Extreme example of fluvial parent material, courtesy of D. Anderson

                                            Aeolian parent material

Figure 5. Aeolian parent material, Great Sand Hills of SW Saskatchewan

In Saskatchewan, the only significant areas of Aeolian (wind blown) parent materials are found in the Great Sand Hills and the sand dunes of Lake Athabasca. This dune sand material has extremely limited agricultural potential even as pasture. Finer Aeolian material (silt sized soil material) can also be transported by wind. This material is call Loess.  There is very limited area of loess type soils in Saskatchewan however loess is an important parent material for agricultural soils of USA and China.  It is difficult to recognize soils developed on loess parent material because the deposit is generally thin and follows the undying topography.  A microscopic examination of a soil sample is really needed to confirm loess. In Saskatchewan, small areas of loess parent material are located adjacent to coarser sand dune deposits. Hence some loess type soils are found on the eastern side (downwind) of the Great Sand Hills. When these silty-textured soils experience beneficial climate they are capable of good crop production.

From glacial till and glacio-lacustrine parent material to soil

We have examined the general characteristics of parent materials common to Saskatchewan, let us now turn our attention to the agricultural characteristics of these soils.

a. Glacial till
Glacial till parent material produces the largest portion of agricultural soils of Saskatchewan; about 60% of total arable acres in the province. Glacial till parent material was carried and deposited by the southward advance of the great ice sheets which covered the province during the last ice age. Glacial till is very mixed or heterogeneous material because it includes all types and sizes of materials that were scooped up and carried in the advancing ice sheet. Glacial till parent material therefore contains all particle sizes from clay to boulders. Fortunately the glacier encountered a large quantity of relatively finer materials, hence the average texture of glacial till soil in Saskatchewan is loam to clay loam. The amount of stone included in the material is variable, and to some extent the luck of the draw. As I indicated to you in my first blog, my farm has some disagreeably stoney fields.  These fields still produce well when it rains, but to this day, need a lot of stone picking. The more or less uncontrolled (random) release of parent material from the melting ice sheets produced irregular topography ranging from gently (figure 1) to strongly (figure 2) rolling landscapes. The more rolling the landscape the more likely the fields are to be dotted with sloughs and wet spots. In addition, the steeper the slopes, the greater the likelihood of soil erosion. Notice the lighter coloured knolls in figure 2.

Figure 1, gently to moderately rolling glacial till landscape

Figure 2  Rolling to strongly rolling glacial till landscape

b. Glacio-lacustrine parent material

Lacustrine parent materials were deposited in glacial lakes created during the melt phase of the ice sheets. These glacial lakes were formed because the melt water could not move northward because it was dammed by the ice sheet nor could it flow southward because land to the south was of higher elevation. The water in these large bodies of water was calm, particularly in the winter when ice covered. Therefore the water only carried fine soil particles in suspension. These fine particles slowly settled to the lake bed. Soils developed on these "old lake beds" are fine textured, generally level and stone free (figures 3 and 4). They are particularly well suited for production of lentil, chickpea and other short stature crops

    
                     Figure 3  Lacustrine landscape located in the Black soil zone of Saskatchewan

Figure 4  Lacustrine landscape in Brown soil zone of southern Saskatchewan

In the end most producers would choose lacustrine parent material if they had a choice. Most of us however don't have that choice, we have to play the cards we are dealt. Don't give up on till soils, if they are fertilized appropriately and it rains accordingly, they are capable of excellent production. Don't forget to include the important health benefits associated with glacial till soil; high quality exercise while picking stones.

The nature of parent materials in Saskatchewan

  We have discussed the influence of parent material, climate, vegetation, topography, time and man on soil genesis or pedogenesis. This post is dedicated to exploring more fully the role of parent material in the pedogenesis of Saskatchewan soils. Parent material is unconsolidated rock and mineral material which subject to the forces of weathering produces a soil with its characteristic horizons. This slow paced weathering process (encompassing 100's to 1000's of years) commences at the surface, the interface between parent material and the environment. With time, the forces of weathering exert their influence to greater and greater depths, however the intensity of the process is reduced with depth. Soil layers or horizons emerge, first an A horizon and subsequently a B horizon. If the forces of weathering are more intense the depth of the A and B horizons will generally be greater.  At a certain depth, which is determined by weathering intensity, the pedogenic forces have little or no affect on the rock and mineral material. At this point we recognize the C horizon, the more or less unaltered parent material.

                               
       Figure 1   Typical horizon development grassland soil                

  The last ice age in Saskatchewan's prehistory is responsible for deposition of most parent materials. A consequence of the glacial activity is the incorporation of large amounts of limestone into this parent material. Saskatchewan parent materials are therefore generally referred to as calcareous, with neutral to basic soil pH values. In the 10,000 to 15,000 years since the last glaciation, the lime content of the A and B horizons has slowly weathered away, however the C horizon which is relatively unaffected by weathering  has retained its lime content. The presence of lime is easy to detect by applying dilute hydrochloric acid (HCL).  The HCL releases CO2 from the limestone and produces a characteristic fizzing or effervescence. The depth to the C horizon can be easily determined by exposing the soil profile to dilute HCL. Using a plastic squirt bottle, apply a small stream of acid to the soil profile starting at the surface and proceeding downward. When fizzing appears it is an good indicator that you have reached the lime and therefore the C horizon. Visit SLSC 240 online and view the discussion by Dr. Anderson about soil horizons, He makes use of HCL to locate the C horizon. Go to the online course here then to module 6, section 6.1, and view the quicktime video discussing soil profile identification.

                                                       

Writing a term paper, the good, the bad and the ugly

One of the more difficult parts of my job as a teaching faculty is to train students in the production of good quality term papers. In any agricultural science, the production of  a quality term paper requires it conform to certain standards with respect to format, writing style and scientific content.  Student must master all three components in order to score well with their papers.

I hope to provide insight into the process I employ to arrive at a term paper grade. Perhaps by understanding my thought process during grading, you will produce a paper that achieves a better score. Come March of this term I will be required to grade more than 60 SLSC 240 term papers and this is not my only course this term. In addition, no two topic are the same, so there is no repeating content as is typical of a midterm or final exam. Any exercise or exam where all student respond to the same question(s)  is definitely easier and faster to grade. It is important to me that I provide a fair assessment for each students term paper whether it be the 3rd or 55th paper I have read.  To achieve that, I must apply a rigorous marking scheme to each paper which segregates the marks into categories. I allocate marks in three general areas:
                                                   1. paper format - 30%
                                                   2. scientific content - 40%
                                                   3. writing style and accuracy - 30%

Based on this allocation of marks a student could theoretically make a mess of the format but still achieve a 70% grade. In practice that does not happen because almost invariably a student who does a poor job of formatting the paper will also have shortcomings in the science and writing components. The following is a list of questions and thoughts I go through as I grade your paper.

 A. Paper format
        1. Does the paper have all the necessary components; title page, abstract, table of contents, list of figures and tables and references?
        2. Are graphs and tables captioned properly, author and date cited?
        3. Is the reference section formatted correctly, alphabetical order etc.?
        4. Are all authors cited in the text listed in reference section and vice versa?

B. Scientific content 
        1. Is the stated topic, as articulated in the introduction, covered completely, accurately and logically?
        2. Is the topic very general or specific (narrower focus)?
        3. Are the scientific concepts presented of a complex or simple nature?
        4. Are the number of references cited large enough to indicate that the topic has been thoroughly researched?
        5. Is the data presented useful? Does it support and clarify the concepts presented?
        6. Is there evidence of interpretation or synthesis of data as opposed to simply reporting it?

C. Writing and grammar
        1. Is the paper grammatically sound?
        2. Is sentence construction such, that meaning is clear?
        3. Does the paper contain complex and / or cumbersome sentences which cloud the meaning?
        4. Is scientific terminology used correctly?
        5. Are paragraphs constructed to provide a logical flow of concepts and ideas?

It is difficult to be more specific about the actual grading process, table 1 is an attempt to shed some light on the process.

Table 1. A general description of the mark allocation for SLSC 240 term paper.

Writing a well crafted term paper requires a lot of practice. In my own case I was 20 years into my career before I felt that my writing skills had really matured. To those students in 1st or 2nd year, I am not expecting miracles; however, if you pay attention to the key components of paper writing and ask for help if necessary, you can certainly achieve an acceptable grade.

How parent material influences soil pH

Although the pH scale ranges from 0 to 14, most soils have pH values between 4 and 9.  Many soils of the world are acidic for a variety of reasons including parent material, weathering and pollution.  Overtime soils have a general tendency to become more acidic.  The pH of a soil is often a limiting factor in plant nutrient availability. Soil pH may affect the water solubility of a nutrient and therefore plant uptake of that nutrient.  A common agronomic practice associated with highly acidic soils is liming.  Lime is a common amendment added to raise soil pH.  However, as was discussed in class, a soil may have high reserves of H+ ions which means that large amounts of lime would have to be added to have an impact on the soil pH.  The reserve acidity is dependent on soil texture and cation exchange capacity.  An example of this would be a high clay soil with high organic matter content which results in a high buffering capacity.  In a soil like this, it would be very difficult and expensive to change the soil pH.

Parent material can exert a strong influence on soil pH.  Materials such as shale, sandstone and granite generally produce more acidic soils.  In Western Canada however, soils are generally neutral to basic and resist acidification.  This is in large part due to the limestone parent material.  The soils in Western Canada therefore tend to resist acidification hence are buffered in the basic pH direction.  Saskatchewan has about 45 million acres of cultivated soil, of that 17.5 million has a pH > 7.0 and 1.5 million has a  pH value of > 5.5.  This is a result of parent material and the relatively recent glaciation of the soils.  The general bedrock geology (parent material) can be observed in Figure 1.  The yellow and orange areas on the map indicate areas of limestone parent material.  This band of limestone strongly influenced the distribution of pH values seen in Figure 2.

Figure 1. Bedrock geology of Saskathcewan.  Yellow and orange areas indicate limestone parent material.
Image from Atlas of Saskatchewan, 1969

Figure 2. pH values of Saskatchewan soils.
Image prepared by H.P.W. Rostad, J.J. Kiss, and A.J. Anderson, 1983, University of Saskatchewan

The soils on the east half of the province are dominated by basic and neutral soils while the soils on the western half of the province are neutral to acidic (Figure 2).  When the last glaciation occurred the thickest part of the glacier was on the east side of the province and scraped over the widest band of limestone parent material (Figure 1 - orange area).  Figure 3 illustrates the retreat of the last glaciation in Saskatchewan.  It was the advance and retreat of the glacier that incorporated the limestone parent material into the soil.  The glacier was thinner on the western half of the province.  The limestone band is also thinner in the northwestern part of Saskatchewan (Figure 1 - yellow area), so less limestone was incorporated into those soils.  The glaciation and parent material are the reasons that our agricultural soils are predominantly calcareous and have neutral to basic pH.  Once the lime has been exhausted though, Saskatchewan soils will lose the capacity to buffer against acidification.

Figure 3. Glacial retreat in Saskatchewan.

What constitutes desirable soil structure?

My previous blog discussed changes in Western Canadian cropping systems over the last 40 year period and the benefits that have accrued to soil organic matter, soil structure and water infiltration because of those changes. The question not explored thus far is, why have those cropping system changes had a beneficial impact on soil structure? The answer lies largely in the impact of these system changes to the biological processes that influence soil structure development. The other key issue is that tillage, which is so deleterious to soil structure, has been greatly reduced with the advent of  min-till /z-till systems.

Before discussing soil structure development further it is appropriate to describe what constitutes good soil structure. Fundamentally it involves the development of a relatively stable soil aggregate system that produces many crack/pores/fissures etc which support/enhance the normal vertical movement associated with water infiltration, gas exchange and root growth. In contrast, soil structures with a horizontal orientation produce barriers to the normal vertical processes and hence are considered undesirable.

Figure 1. A soil profile showing very good soil structure.

Figure 2. A soil profile showing undesirable structure in the second horizon.

Plant root systems, particularly dense fibrous root systems, produce beneficial soil structure.  Roots have the ability to enmesh or knit together soil particles into larger desirable soil aggregates. In addition the normal metabolic processes of roots involve the release of various slimes, gums and mucilages. These organic compounds  act as nature's cement, to further stabilize aggregate formation. Development of stable aggregate structure is desirable because well-aggregated soil can more rigorously resist the destructive powers of wind and water erosion. The forces of tillage however are likely to destroy even relatively stable aggregates.

Crops and cropping systems also enhance water infiltration because of the influence of old root channels associated with previous crop growth. These channels provide a rapid pathways for water infiltration
(see figure 3); however, tillage will destroy these channels or at least destroy its continuity. Pore continuity refers to a pore system connectedness. An optimum pore system will consist of cracks, pores, or root channels that support a continuous (uninterrupted) flow of water from the soil surface to subsurface. Intensive tillage tends to disrupt the entire pore system to the depth of tillage. The subsurface pore system is therefore no longer connected with the surface, reducing water infiltration to greater depth.

Figure 3. Microscopic view of an old root channel.                                                                                   

Another important component of water infiltration is preferential flow. Preferential flow describes water movement in soil via preferred pathways. In essence, soil water moves via the path of least resistance rather than moving  through the entire soil matrix. In this context I recognize two types of preferential flow, movement though macro pores and root channels.  This concept is illustrated in an experiment by Dr. Bing Si, College of Agriculture and Bioresources, University of Saskatchewan. Dr Si applied water, amended with a greenish dye, to the surface of a heavy textured soil to assess water infiltration. The irrigated area of the soil profile was later exposed with the use of a small backhoe to examine water movement. The path of the dye clearly shows the importance of the macro pore systems, particularly at depth (figure 5), in enhancing the naturally slow infiltration associated with clay textured soil.

Figure 4. Moisture movement into soil at surface.

Figure 5. Moisture movement in the 9 to 14 inch segment.                                 

In Figure 4, the dye pattern reveals a more general infiltration process through the bulk surface soil with some indication of preferential flow. Deeper in the profile (figure 5) the influence of preferential flow is more clear, water infiltration is more rapid and deeper than expected. Water and associated dye has taken the path of least resistance and has not infiltrated the entire soil profile. This concept is important when considering possible contaminant movement into a heavy textured soil. Extensive crack development supports preferential flow and thus has the potential to move a contaminate more swiftly and to a greater depth than expected based on its natural slow infiltration properties.

In summary, a reduction in number and intensity of tillage operations combined with a continuous cropping system, supports desirable soil structure development and superior water infiltration. Increased organic matter and reduced mechanical disturbance allows the development of stable soil aggregates which produce a vertical pore system that makes possible more rapid entry of water, even in fine textured soils. This improved infiltration is the result of stable soil structure, production and maintenance of root channels, enhanced pore continuity and the potential for preferential flow.