Two Labs. Labs 4 and 5. Do end of labs file questions.

I attached the chapter notes too

Intro Soils – Lab 5
Soil Microorganisms – Enumerating Heterotrophic Soil Bacteria

o Lecture Materials: Soil Organisms (Chapter 11)

o Labs submitted without advised instructions will result in a 4 point deduction: Proper document
name (LastName_SoilsLab5), name included in document, legible numbering and spacing
including questions with answers.

o Labs submitted early will receive feedback to aid in exam preparation with the opportunity to
resubmit the lab. Do not miss out on a great opportunity to be ensure understanding of the
materials and increase your lab grade.

Lab 5 – Enumerating Heterotrophic Soil Bacteria

Soil microbiology laboratory exercises are designed to familiarize students with the basics of
microbiology in general including the use of a compound microscope, sterile technique, the preparation
of materials including growth media, and even molecular methods including DNA extraction, the
polymerase chain reaction, and molecular marker screening. These skills are then utilized to better
characterize and understand soil microbial populations including but not limited to bacteria, fungi, and
nematodes, protozoa, and cyanobacteria. Today, we will highlight a mainstay in soil microbiology: how
to enumerate cultivable bacteria from soil.

Bacteria are generally the most abundant and diverse organisms in soil on the range of 106 to 109
bacteria per gram of soil. The soil bacterial population is dominated by species of Pseudomonas,
Arthrobacter, Clostridium, Bacillus, Micrococcus, Flavobacetrium and others. These bacteria can be
difficult to classify as many appear the same as seen with a microscope or on culture plates. Means for
classifying bacteria are vast and include their physical characteristics like size, shape, and color of their
colonies, nutritional requirements, metabolic products (gas, enzymes, etc.), serology, and more modern
techniques which compare their genetic relatedness by characterizing their ribosomal RNA.

There are many methods for estimating numbers of bacteria in soils and include various staining
techniques to directly count bacteria using a microscope, plating techniques employing a multitude of
various culture media, a statistical technique called most probable number, and molecular approaches
characterizing the bulk DNA extracted from soils or monitoring active RNA genes in soil.

Many soil scientists when looking to enumerate the aerobic, heterotrophic bacterial population from
soil are content to use the dilution plating technique on a non-selective agar media. It is well known
that this technique only measures a small portion of the actual bacterial population due to the inability
to adequately replicate soil conditions where these bacteria reside and thrive. Even with this
knowledge, it is very useful to be able to characterize cultivable organisms and how they change over
time and on various growth media with any number of research and/or management objectives.

The goal for a growth medium is to provide the bacterial population with the carbon and energy sources
it needs to grow. Media can either be non-selective or selective. Non-selective media look to provide
wide ranging nutrients and cultivate any and all organisms capable of growing on a solid agar plate or
liquid medium. Selective media are used for the growth and cultivation of specific groups of organisms
and generally include or exclude nutrients, particular metabolites, or even antibiotics to support the

growth of a population of interest. For plating techniques, media is prepared with agarose, a natural
gelling agent, to provide a solid surface where the bacteria can grow and contained on a petri plate
routinely 90 mm wide.

As soil bacterial numbers are in the billions if not trillions, it is necessary to dilute these samples to
reduce the number of colonies on the growth medium down what is called a countable range. Preparing
and plating the dilution series is illustrated below. To prepare the dilution series, first place 10 grams of
soil into 95 ml of water; accounting for pore space this is 1:10 dilution. The sample is shaken to mix the
soil and water. Then 1 ml is added to a 9 ml dilution tube for another 1:10 dilution; over all in this tube
is a 1:100 from the original sample. This is the basis of a serial dilution, each step down the line is
another ten-told dilution. For instance, if you started out with 1000 organisms, the 10 fold dilution
would net 100 organisms, next dilution down would be 10 organisms, and then another would net 1
organism. Depending on the range of bacteria in a soil sample you might need more or less dilutions to
achieve colony counts on the plate that are in the countable range. If colonies are crowded on the plate
as to not be able to see them individually they are said to be ‘too numerous to count’. For this
procedure, between 30 and 300 individual colonies is the target on at least one dilution to calculate the
colony forming units per gram of soil.

To this end, serial dilutions are made of the soil sample in water and then plated or spread evenly onto
the agar media, placed in an incubator at normal growth conditions (approximately room temperature
or slightly higher), and then enumerated or counted approximately 24 hours later. The goal is to be able
to count individual colonies on the agar plate of at least one dilution range. Each plate is enumerated
and data recorded. The dilution series plate (routinely these series are done in triplicate to gain an
average for each dilution) which meets the ’30 to 300’ criteria is used to calculate the number of ‘colony
forming units per gram of soil’ (CFU/gram). It is difficult to know whether each of those individual
colonies counted on the plate are from one or more than one actual bacteria, so to account for this
ambiguity, the term colony forming units is used. Simply multiply the count on the plate by the
reciprocal of the dilution plated (swap the sign on the exponent). For instance, in the example below,
the 10-6 plate had 81 colonies counted, so this soil had 81 x 106 CFU/gram of soil. Adjusting for proper
scientific notation, you move the decimal one place over so your figure is less than ten, and add one to
the exponent: 8.1 x 107 CFU/gram of soil.

Lab Reference: Laboratory Exercises in Soil Microbiology, Texas A&M University, Agronomy 405 – Soil
Microbiology, Dr. David Zuberer.

Images from a Bacteriology course at the University of Wisconsin (Link provided below). Items of note:
(1) Diversity in color, shape, and size of bacterial colonies, (2) Reduction in number of colonies as go
from least dilute (top left) to most dilute (bottom right), (3) Individual colonies are too close together to
be able to count in top two plates (TNTC) and countable in lower dilutions.
(http://inst.bact.wisc.edu/inst/index.php?module=book&type=user&func=displayarticle&aid=273)

http://inst.bact.wisc.edu/inst/index.php?module=book&type=user&func=displayarticle&aid=273

Intro Soils – Lab 5 –Assignment Questions
Soil Microorganisms – Enumerating Heterotrophic Soil Bacteria

Utilize Lab, Lecture and Text Materials: Soil Organisms (Ch 11)

1.) Farmer Jim’s daughter was taking soil microbiology and decided to enumerate the heterotrophic
bacteria from the alfalfa field her family limed back in Lab 4. As she collected her soil sample,
she noticed a neighbor had recently also added lime to his field which was in corn last fall, but
this family routinely utilizes tillage in their operation and had incorporated the lime into the soil.
She decided it might be interesting to see if there was a difference in the two cultivable bacteria
counts. Below are the results, using the illustrations and information provided in the lab,
determine the CFU/gram of each soil. Discuss some reasons why the two soils might not have
similar bacterial counts.

Alfalfa Field:
Plate 10-3 – TNTC
Plate 10-4 – TNTC
Plate 10-5 – TNTC
Plate 10-6 – 98
Plate 10-7 – 9

Corn Field:
Plate 10-3 – TNTC
Plate 10-4 – 65
Plate 10-5 – 6
Plate 10-6 – Zero
Plate 10-7 – Zero

2.) Rank these soil organisms in order of overall abundance (number per gram) in soil: Earthworms,
Bacteria, Actinomycetes, Fungi, Nematodes

3.) Fill in the following table with the appropriate metabolic group (4 in grey):

Source of Energy

Source of Carbon Reduced Inorganics /
Biochemical Oxidation

Light / Solar Radiation

Organic Carbon /
Combined Organic Carbon

Carbon Dioxide

4.) Name at least one positive contribution to soil health for each of the following soil organisms:
a. Earthworms
b. Bacteria
c. Fungi
d. Protozoa

5.) Name at least one negative contribution to soil/plant health for each of the following organisms:
a. Ants/Termites
b. Fungi
c. Nematodes

6.) What is the rhizosphere and why is it such an important area of activity in soils?

7.) Discuss some defining characteristics of soil Actinomycetes. Why are they culturally and

economically important?

8.) Why is an active soil microbial community so important to soil health and productivity? What
are some managerial activities to help promote this community?

BONUS: In your own words, describe the Universal Phylogenetic Tree.

IntroSoils – Lab 4

Soil pH: Acidity and Liming

o Lecture and Text Materials: Soil Acidity (Chapter 9) with Review Questions also included from Soil
Alkalinity (Chapter 10) and Soil Organic Matter (Chapter 12)

o Labs submitted without advised instructions will result in a 4 point deduction: Proper document
name (LastName_SoilsLab4), name included in document, legible and professional numbering and
spacing including questions with answers.

o Labs submitted early will receive feedback to aid in exam preparation with the opportunity to
resubmit the lab. Do not miss out on a great opportunity to be ensure understanding of the
materials and increase your lab grade.

Lab 4 – Soil pH: Soil Acidity and Liming

pH
Soil pH is considered a master soil variable due to its wide ranging effects on other soil variables.
Whether a soil is neutral, acidic, or alkaline is a measure of the relative concentration of hydrogen (H+)
and hydroxide (OH-) ions. pH is technically the negative algorithm of the hydrogen ion concentration:
pH = – log10 [H+] (Equation 1). Thus, for example, when the concentration of H+ ions in a solution is
0.0004 M of 10-4 M the pH of the solution is 4. It is important to note these values are on a log scale,
meaning that every unit on the pH scale is a ten-fold change; soil with a pH of 5 is ten-times more acidic
than a soil with pH of 6. pH values below 7 are considered acidic, pH values above 7 are considered
alkaline, with pH 7 being neutral. Soils have pH generally range from 4 to 9 (Figure 1, Text Figure 9.2).

Figure 1 (Text Figure 9.2). pH scale including common items and ranges for various types of soil.

Many soil properties are tied to soil pH, but most importantly nutrients are generally either more or less
plant available with more acidic or alkaline pH values. Many nutrients, especially aluminum and iron,
are relatively unimportant to plant growth due to low nutrient requirements for productivity at neutral
pH ranges, but can become toxic at acidic pH. As a general rule, most plant nutrients are most available
at ranges of 6-7 where they remain soluble and in plant available form. Additionally, the soil microbial

community works most efficiently in the more neutral pH values (6-8) rather than the extremes; so for
productivity purposes for nutrient cycling, residue decomposition, root nodulations, herbicide
breakdown, and other microbial activities, it is important to keep soils in this neutral range.

Soil Acidity
Most soil activities either consume or produce H+ ions. Soil parent material as well as weathering
conditions especially climate are a major determinants of soils potential to become acidic in addition to
human influences like adding nitrogen based fertilizers. In highly weathered soils with lower CEC values,
activities that produce H+ tend to outpace their counterparts and create soil acidity. Weathering leaches
base cations, i.e. calcium, magnesium, potassium, and sodium, from the soil profile leaving behind
aluminum (iron to an extent) and hydrogen on the exchange complex which lowers the soil pH, can
create toxic plant levels of aluminum in soil solution, and makes other soil nutrients less available for
plant uptake. Soil acidification is somewhat of a natural process from many soil activities, (1) as soils
weather they lose base cations leaving aluminum an acid producer on the exchange, (2) carbonic acid is
created when soil respiration produces carbon dioxide, (3) as nutrients including nitrogen, sulfur and
iron, are reduced H+ ions are produced, (4) the deprotonation of pH dependent charges on soil organic
matter produces acidity, (5) when plants take up cations for growth and production they tend to exude
H+ ions to maintain ionic balance, as well as cause an overall loss of base cations from crop removal
during production, and finally (6) soils gain acidity through the deposition of acidic products via
precipitation. Alfisols and Ultisols generally are acidic in nature due to their highly weathered nature
and thus lack of base cations; forest soils generally tend to also be acidic due to the nature of the
organic matter from leaf and conifer deposition. In agronomic settings, soils utilized in production
agriculture also tend to be acidic. The addition of nitrogen based fertilizers create large amounts of soil
acidity through the nitrification process. Ammonium and ammonia based fertilizers added to soil are
microbially transformed into nitrate through nitrification for plant availability and in the process create
acidity (2 H+ ions are produced for every NH4+ ion added to the soil). The addition of nitrogen-based
fertilizers are a necessity for crop production and exceed the amounts generally seen in routine nitrogen
cycle and must be counteracted with soil amendments to maintain relatively neutral soil pH.

Soils have the capacity to buffer or resist large changes in pH. Many activities in soil can either be
consumers or producers of H+ or OH- ions depending on soil conditions. Most of these activities are
reversible and are weak acids, so depending on amount of product or reactant more or less acidity can
be created or consumed. These properties greatly enhance the soils ability to buffer itself against
change. Further adding to the buffering capacity of soils are the various pools of acidity. There are
three pools of acidity in soils, active, exchangeable, and residual. The active acidity is the smallest pool
of acidity and is the hydrogen ion concentration out in soil solution; this pool of acidity is also the easiest
to counteract with soil amendments. The exchangeable acidity readily exchangeable aluminum and
hydrogen on the soil exchange complex and the residual acidity is the acid producing cations tightly
bound to the soil colloids. As active acidity is counteracted, the exchangeable and residual pools release
additional ions to keep the soil solution at equilibrium; this activity contributes to the buffering capacity
of soils (Text Figure 9.9 and Lecture Material Slide 15).

Aluminum and hydrogen are the acid producing cations while the base cations, which do not promote
acidity, include calcium, magnesium, potassium, and sodium. All of these same cations contribute to
CEC, the more base cations there are in the soil to counteract the acidifying cations, the stronger the
buffering capacity the soils have. Hence why soil pH is also an indirect indicator of the amount of
weathering that has occurred in a soil and the amount of CEC available. The acid saturation percentage
(the percentage of the CEC held by acid producers, Al3+ and H+ ions) as well as the base saturation

(percentage of the CEC held by non-acid producing cations, Ca2+, Mg2+, K+, Na+) are also important values
to know and understand when evaluating CEC values. The higher the base saturation and the lower the
acid saturation the better for soil productivity. If acid saturation exceeds 15-20% of the total CEC,
aluminum toxicity can occur and soil amendments are generally recommended to counteract that
acidity. Soil pH will also have a great effect on pH dependent charges on soil colloids including clays and
soil organic matter. Even with the capacity to buffer the system, highly weathered soils with lower CEC
and agronomic soils over time tend to be acidic necessitating amelioration using soil amendments.

Counteracting Soil Acidity – Lime
Generally speaking, to improve soil acidity one needs to increase the pH of the soil from acid to more
neutral pH by altering the ratio of H+ and OH- ions in the soil profile. On agricultural soils, this
improvement tends to come in the form of soil amendment like limestone or lime for short. Liming as
whole is less of a precise science than fertilizer additions as this amendment is working to overcome the
soil buffering capacity and to change the chemical nature of the entire rooting zone for the plant. For
these reasons, it generally takes large quantities of these materials to force a change in soil conditions,
usually in the tons per acre quantities. Liming agents for these reasons need to be relatively
inexpensive, readily available, as well as be safe and easy to handle.

Several compounds fall under the generic term ‘ag lime’ and are listed in table 1. The main
characteristic of a liming product is that is provides large quantities of base cations to counteract the
acid producing cations on the exchange complex. Calcium carbonate (CaCO3) is the mainstay for ag lime
products. The neutralizing capacity of all other liming products is routinely compared to calcium
carbonate on a percentage basis which is the calcium carbonate equivalent (CCE). Dolomitic lime,
(CaMg(CO3)2), is often used in areas that are deficient in magnesium as a source of the cation for plant
nutrition. Wood ashes can also be used as a liming material and are often used in homeowner or small
garden settings. Table 1 (Text Table 9.4) includes the chemical formula, calcium carbonate equivalent,
as well as some comments on the product.

Table 1 (Text Table 9.4). Common liming materials and their compositions.

Again, these soil amendments are added to the soil to increase the pH by changing the rooting zone
environment to make nutrients more available and limit other elemental toxicities for maximum plant
and microbial production. First, lime readily counteracts the small pool of active acidity with the
increase in base cations to produce carbon dioxide and water. Next, in the largest, most important
change, base cations (Ca2+ and/or Mg2+) in mass flow action replace Al3+ and H+ on the exchange
complex and send them into the soil solution. With water, Al3+ will ultimately precipitate as the
insoluble gibbsite (Al(OH)3). Ultimately, the goal is to raise the pH of the soil system to the target pH
recommended for a particular crop which generally range between 6 and 7 where most plant nutrients
are most available. The calcium and/or magnesium from the liming materials added also serve as a base
cation for plant nutrition during the growing season.

Liming requirements and their calculations vary depending on soil test methods and state and testing
facility guidelines. Ideal pH and thus liming needs are also specific to plants with some requiring more
acidic or neutral pH to maximize yield. Testing facilities take two different measurements to gauge the
need to lime soils. A soil pH with water and a buffer soil pH. Briefly, pH is determined using a pH
electrode routinely called a pH meter. The meter is placed in a solution of soil and water (1:1 or 1:3
ratio) or soil and buffer. The meter has a standard reference electrode where the difference in activity
of the H+ in the soil and the reference create an electrometric potential which is converted into the pH
scale.

The soil water pH (pHwater) is a measure of the active acidity in the soil solution. This measurement can
act as a guide in determining whether lime is needed or not. The exchangeable and reserve acidity, the
most important pool, is determined using a buffer (pHbuffer). The buffer pH helps determine how much
capacity the soil has to resist change in pH, or buffer the soil system. The buffer is meant to resist
change, so if the soil has the capacity to change the pH of that buffer by considerable margins, the soil
will require more lime to produce a change in soil pH. The reasoning behind this is based on CEC and

ultimately soil texture. Generally, soils with greater amounts of clay have higher CEC and thus base
saturation, and contain more cations in the system to buffer change and will require larger amounts of
lime to change the soil pH. More coarse textured soils high in sand are just the opposite with lower
amounts of clay, lower CEC and thus less base cations in the system to buffer pH and require less lime to
produce a change in the soil pH. Depending on several factors including typical soil organic matter
levels, typical parent materials, and CEC, different buffers have been designed specifically for use in soil
testing facilities. Two common buffers used for liming estimated are SMP (Shoemaker, McLean, and
Pratt) and Adams-Evans which is used in most soil testing facilities in TN and is the basis of the
recommendations from the University of Tennessee soil testing facility.

As mentioned previously, each state has varying recommendations for lime applications based on
previous research as well as knowledge of the soil systems in that area. The University of Tennessee
Agricultural Extension Service utilizes regression equations combining the water and buffer pH as well as
target pH for the various crops in TN to create easy to use approximations in tabular tables to
recommend lime additions (Table 2). For instance, for corn production (middle, b section) with a target
pH of 6.5 (middle, b section), with a soil water pH of 6.0 (left side column) and buffer pH of 7.4 (top row)
a farmer would need to add ~ 2 tons of lime with greater than 75% CCE.

Table 2. UT Ag Experiment Station Lime Recommendations (Essington, ‘Soil and Water Chemistry: An
Integrated Approach’)

A popular private soil and tissue testing facility in our area, A&L Laboratories in Memphis, TN, utilizes
the following regression equation to calculate lime recommendations for soil test results (personal
communication, Ruiz, A&L, Memphis):

Lime = { 1250 + ((pH goal – 0.3) – pH) * 1820)) + ((6.95 – buffer pH) * 5260)

For example:
Soil pH= 5.0
Buffer pH= 6.7
pH goal= 5.3

Lime = { 1250 + ((5.3- 0.3) – 5.0) * 1820)) + ((6.95 – 6.7) * 5260)

Lime = {1250 + 0 + 1315} = 2565 lbs. lime recommended/acre or ~ 1.3 tons/acre

Other Quality Factors for Lime Application
Several other factors besides overall quantity of lime are included in the quantification of lime
requirements and include calcium carbonate equivalent, depth of incorporation, and size of the lime
product applied. These characteristics are ultimately utilized to calculate how much of a particular
liming product will be required. Calcium carbonate is the standard for ag lime and other products ability
to neutralize soil acidity are referenced to this standard using a percentage called calcium carbonate
equivalent (CCE). Pure calcium carbonate or limestone is the standard and has a CCE of 100% while
other products may have more or less neutralizing capabilities with CCE of above or below 100% (Table
1). It is important to check the CCE of all liming materials as they can have a range of values and thus
effectiveness. A CCE of less than 100 generally also indicates impurities in the product which increases
the total amount of amendment needed to meet recommendations.

The speed at which limestone reacts in a soil to neutralize acidity is largely determined by particle size.
Smaller particles have more surface area to contact soil acidity, thereby producing more rapid change in
pH. Crushed limestone is screened through a series of sieves to determine its particle size range. Sieve
size (mesh) indicates the number of wires per linear inch, thus a larger sieve number (more wires) yields
smaller particle size in the lime product. The percentage of product in a sample of the liming product
that fits mesh size is used to calculate efficiency ratings for the various liming products. The smaller the
particle size, the higher the efficiency.

Each state utilizes its own verbiage and classifications for liming materials, but in Tennessee particle size
efficiency and relative neutralizing values (RNV) are utilized. For instance: Table 2 lists the particle size
breakdown for a liming material; the table includes size range (various mesh ranges), the percentage of
that size range for each category, the efficiency factor for each size range and finally the particle
efficiency for each size rage (% x Efficiency Factor). The summation of those particle efficiencies is the
total particle size efficiency of your liming product. The relative neutralizing value (RNV) is simply the
particle size efficiency for the product multiplied by the CCE. So, for instance, if this liming product had
a CCE of 90%, the RNV would be 88.4 (particle size efficiency) x 0.90 (CCE) = 80.

Table 2. Example Particle Size Breakdown of potential Liming Material – Total Particle Size Efficiency
and Relative Neutralizing Value

Size Range Percentage Size Range Efficiency Factor Particle Efficiency

Coarser than 10 Mesh 5 0.33 1.6

10 – 40 Mesh 20 0.73 14.6

40 – 60 Mesh 40 0.93 37.2

Finer than 60 Mesh 35 1.0 35.0

Total Particle Size Efficiency 88.4%

Relative Neutralizing Value (PSE x CCE) 80

The Tennessee Liming Materials Act requires liming materials sold in the state meet several
requirement: (1) minimum calcium carbonate equivalent of 75, (2) ground so that at least 85 percent
passes through a 10-mesh sieve and at least 50 percent passes through a 40-mesh sieve, and (3) liming
materials sold must have a relative neutralizing value (RNV) of 65 or greater. All of these values can be

utilized to compare actual liming needs across difference liming materials based on their cost and cost
to spread.

Lime itself is relatively insoluble and thus requires water to move down into the soil profile to become
active. This process can be faster with the finer, large surface area lime particles and slower with the
larger particles. For this reason, some farmers utilize slow release products to lengthen the effective
time the lime stays in the soil profile. The fall and early spring are good times to apply lime to the soil as
the wetter winter months can help move that lime down into the soil profile where it can begin making
a change to that soil exchange prior to planting. Lime is generally spread across the fields utilizing
spreader trucks which in theory spread an even layer across the soil surface at the recommended
application rate per acre. Generally speaking, lime has traditionally been added in a more liberal fashion
than fertilizers due to its relative cost, ease of application and a more broad range and timeline for
results. The use of precision agriculture techniques to more closely assess soil needs on a smaller scale
has led to the utilization of variable rate lime.

Lime actually is most effective if it can be incorporated into the soil profile, but modern conservation
practices work to limit tillage and disturbance of the soil surface to build and maintain soil organic
matter and soil structure. These same no-till practices tend to build up materials right at the soil surface
actually intensifying soil acidity problems localized in the top few inches of no-till soils, but regular
addition of lime in favorable conditions keeps this problem in check. Soil pH generally is most acidic at
the soil surface and increases with soil depth as more base cations are still available deeper in the profile
and surface applications of nitrogen tend to cycle in the upper soil layers.

Most producers rely on ag professionals, ag retail dealers or certified crop advisors, to be very informed
and knowledgeable about the ins and outs of all of the products they sell and recommend, but having a
working knowledge of the recommendations and how they are produced is a valuable tool for producers
and students alike.

References abound for soil acidity, lime, liming recommendations, and general knowledge on the topic.
A few listed below were helpful in preparing this laboratory exercise and may be useful as a review of
the information:

https://ag.tennessee.edu/spp/Pages/default.aspx

http://utbfc.utk.edu/Content%20Folders/Forages/Fertilization/Publications/PB1096

http://www.utextension.utk.edu/mtnpi/handouts/Fertility/Soil_pH_Explained

https://extension.tennessee.edu/publications/Documents/PB1061

http://publications.tamu.edu/SOIL_CONSERVATION_NUTRIENTS/PUB_soil_Managing%20Soil%20Acidity

http://www.agry.purdue.edu/ext/forages/publications/ay267.htm

http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_051574

https://ag.tennessee.edu/spp/Pages/default.aspx

http://utbfc.utk.edu/Content%20Folders/Forages/Fertilization/Publications/PB1096

http://www.utextension.utk.edu/mtnpi/handouts/Fertility/Soil_pH_Explained

https://extension.tennessee.edu/publications/Documents/PB1061

http://publications.tamu.edu/SOIL_CONSERVATION_NUTRIENTS/PUB_soil_Managing%20Soil%20Acidity

http://publications.tamu.edu/SOIL_CONSERVATION_NUTRIENTS/PUB_soil_Managing%20Soil%20Acidity

http://www.agry.purdue.edu/ext/forages/publications/ay267.htm

http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_051574

Intro Soils – Lab 4 –Assignment Questions
Soil pH: Acidity and Liming
Utilize Lab, Lecture and Text Materials: Soil Acidity (Ch 9)
Review Questions also include: Soil Sodicity (Ch 10) and SOM (Ch 12)

1. Why is it important to maintain relatively neutral soil pH?

2. What are some of the natural sources of soil acidity?

3. How do nitrogen fertilizers produce soil acidity?

4. Farmer Brown’s CEC for his West Tennessee silty loam soil was 12 cmolc/kg soil. The
acid saturation percentage (aluminum and hydrogen) was 30% of the total CEC. As a soil
professional why might that value concern you? What issues might arise due to this
high acid saturation percentage?

5. Explain how CEC and soil texture in general effects the buffering capacity in soils. For instance,
Farmer John’s silty clay has a CEC of 25 cmolc/kg with soil water pH of 6.5 and Adams-Evans
Buffer value of 7.0 while his loamy sand has a CEC of 8 cmolc/kg with soil water pH of 5.5 and
Adams-Evans buffer value of 7.9. Explain how their difference in texture, clay percentage and
thus CEC help shape those values. What effects might this also have on the amount of lime that
will be required to alter the pH of each of those soils?

6. Why is it important to test both soil water pH as well as soil buffer pH? What pools of acidity do

each of those test, which one is most easily counteracted, and which one is the most important
long term in maintaining neutral soil pH?

7. Farmer Jim is liming his row crop acreage and ended up with some extra lime and would like to

potentially use it on his alfalfa field, but does not have time to send it off for official analysis;
Jim’s daughter is close and happens works in a soils lab on campus and reports back that his soil
water pH is 5.8 and his Adams-Evans buffer pH is 7.4. Based on UT recommendations,
approximately how much lime did his daughter recommend he add to his pasture?

8. Describe two additional lime quality metrics besides just the amount of product required

utilized to ultimately determine how much of a liming product will be needed to counteract soil
acidity.

9. What are some defining characteristics of saline soils? (Hint: moisture, pH, nutrient deficiencies,

CEC, clays, etc.)

10. Why does irrigation in arid regions contribute to salinity issues?

11. What is dispersion? What role does the ion on the exchange site (i.e. sodium vs

calcium/magnesium) play in the tendency to disperse?

12. Describe the three major components of soil organic matter.

13. The nutritional requirement for the microbial community is important in the degradation

process. Explain the concept of a carbon to nitrogen ratio (C:N). Why is it important? What C:N
ratios might enhance degradation and what rations might slow degradation?

14. Describe some agronomic management tools to help build soil organic matter.

3/19/2020 Soil Alkalinity, Salinity, Sodicity (Chapter 10) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

https://gotoclass.tnecampus.org/d2l/le/content/8094442/viewContent/60403404/View 1/6

Soil Alkalinity, Salinity, Sodicity (Chapter 10) Notes

Soil Alkalinity, Salinity, Sodicity (Chapter 10) Notes

Did you know ….
Did you know that even in the desert plants can grow and thrive? Chapter 10 highlights soil alkalinity, salinity,
and sodicity. Soil in these catergories tend to be very dry, but with property management can be productive and
even support the growth of plants and animals.

Lecture content notes are accompanied by videos listed below the notes in each submodule (e.g. Alkalinity,
Salinity, Sodicity (Chapter 10) Videos A though D). Print or download lecture notes then view videos in
succession alongside lecture content and add additional notes from each video. The start of each video is
noted in parenthesis (e.g. Content for Video A) within each lecture note set and contains lecture content
through the note for the next video (e.g. Content for Video B).

Figures and tables unless specifically referenced are from the course text, Nature and Property of Soils, 14th
Edition, Brady and Weil.

Content Video A

Soil Alkalinity, Sodicity, Sodicity

Characteristics DRY Soils
< 500 mm Rainfall Aridisols and Entisols

AGRI1050R50: Introduction to Soil Science (2020S) LH

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3/19/2020 Soil Alkalinity, Salinity, Sodicity (Chapter 10) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Aridisols and Entisols
Alkaline: pH >7
Salts
Islands of Fertility
Desert Pavement
Calcium Carbonate Accumulation – Calcic Horizon

Content Video B

Sources of Alkalinity
Alkaline vs Alkalinity

Alkaline: Soils with pH >7
Alkalinity: Concentration of OH-

OH- Producers – Carbonates
CO32- and HCO3-

Ion on Exchange:
Ca2+ on exchange:

Less water-soluble, ppt CO3
pH 7 to 8.4 – Tolerable

Na+ on exchange:
More water soluble, More OH-
pH > 8.4 – Toxic

! Salt Accumulation !

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Salt-affected Soils

Irrigation and Salinity Salt
Irrigation Water – Man Inducted Issues

Water – Naturally High in Salts
High Temps – High Evaporation Rates
Need even more water for crop production
Water Evaporates – Salts Left Behind
Perpetuates natural salt concentrations

Sodic Soils – Na+ on exchange Sites
Salinity – Salt affects crop production
Worldwide Issue – Food Production

Content Video C

Measuring Salinity/Sodicity
TDS: Total Dissolved Solids

Basic Test
All salt solids in solution: mg/L

EC: Electric Conductivity
More salts, more conductivity
Lab or Field

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3/19/2020 Soil Alkalinity, Salinity, Sodicity (Chapter 10) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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deciSiemens per meter (DS/m)
Sodium
ESP: Exchangeable Sodium Percentage

Percent Na+ of CEC
SAR: Sodium Adsorption Ratio

Considers Ca2+ and Mg2+
SAR lower than ESP

Estimating Soil Salinity

Classification
Salinization: Process of accumulating salts

Saline Soils: Starting to accumulate salts
ECe > 4 dS/m
SAR < 13 pH < 8.5

Saline-Sodic Soils: Intermediate accumulation
ECe > 4 dS/m
SAR > 13
Plant issues begin

Sodic Soils: Na+ on Exchange
ECe < 4 dS/m SAR > 13
pH > 8.5
Soils disperse – Hard for plants to survive

3/19/2020 Soil Alkalinity, Salinity, Sodicity (Chapter 10) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Degradation – Soil Dispersion
Na+ is the issue

Hydrated Na+ ions large, takes double to satisfy charge
Too large and too many between colloids for flocculation
Disperse – Break Apart
Infiltration low – puddles – no soil structure

Ca2+ or Mg2+ on exchange sites:
Smaller radii, cohesive forces, flocculation
Improved infiltration rates and gas exchange

Content Video D

Salt Tolerance of Plants

Reclaiming Salty Soils
Leach salts out of the profile!
Sodic Soils:

Add Gypsum – CaSO4
Move Ca2+ onto colloids
Na2SO4 ppt/leached
Increases soil structure Better infiltration

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3/19/2020 Soil Alkalinity, Salinity, Sodicity (Chapter 10) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Reflect in ePortfolio Download Print

Increases soil structure – Better infiltration
Plants – Promote soil structure

Continued management:
Irrigation Water
SAR, EC, etc.
Soil Amendments

Review
Name some characteristics of dry soils?
Pros/Cons of islands of fertility and desert pavement
Why is CEC generally greater on soils in arid regions?
Micronutrients generally more or less available in alkaline soils?
Main micronutrient with issues on alkaline soils?
Sources of alkalinity
Why is it important to know which ion is most prominent on the exchange sites??
Why does irrigation in arid regions cause soil salinity?
Do you know the common methods measuring soil salinity and their units of measure?
Difference between saline and sodic?
Dispersion – why does it happen, ion is issue?
What are some means of reclaiming saline and sodic soils?

Task: View this topic

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3/19/2020 Soil Organic Matter (Chapter 12) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Soil Organic Matter (Chapter 12) Notes

Soil Organic Matter (Chapter 12) Notes

Did you know ….
Did you know that by composting you can create your own soil organic matter? Chapter 12 highlights soil
organic matter and role in the global carbon cycle, creation of soil organic matter via decomposition, factors
influencing soil organic matter production, and finally soil’s role in the greenhouse gas effect.

Lecture content notes are accompanied by videos listed below the notes in each submodule (e.g. Soil Organic
Matter (Chapter 12) Videos A though F). Print or download lecture notes then view videos in succession
alongside lecture content and add additional notes from each video. The start of each video is noted
in parenthesis (e.g. Content for Video A) within each lecture note set and contains lecture content through the
note for the next video (e.g. Content for Video B).

Figures and tables unless specifically referenced are from the course text, Nature and Property of Soils, 14th
Edition, Brady and Weil.

Content Video A

Soil Organic Matter (SOM)

AGRI1050R50: Introduction to Soil Science (2020S) LH

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3/19/2020 Soil Organic Matter (Chapter 12) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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http://photogallery.nrcs.usda.gov/res/sites/photogallery/

Global Carbon Cycle

World Soil Carbon

Composition of Plants

http://photogallery.nrcs.usda.gov/res/sites/photogallery/

3/19/2020 Soil Organic Matter (Chapter 12) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Rates of Decomposition
Sugars, Starches, Simple Proteins (Rapid Decomposition)

Crude Protein

Hemicellulose

Cellulose

Fats and Waxes

Lignins and Phenollics (Very Slow Decomposition)

Content Video B

Decomposition
Breakdown of larger particles into smaller ones

Microbial Community – Work Force

Oxidation via Enzymes: Energy, CO2 gas, and H20

Enzymes – Catalyst – Easier/Faster

TONS of microbial enzymes

Important too for soil structure

Disassembly Plant – Aerobic and Anaerobic

Aerobic: CO2, NH4+, NO3-, HPO4-, SO4-, H2O

Microbial Biomass

Recalcitrant materials –Humus –SOM

Content Video C

Rates of Decomposition
Physical

Smaller particles – Faster Degradation

Closer material is to microbes – Faster Degradation

Nutritional

Need balance

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3/19/2020 Soil Organic Matter (Chapter 12) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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C:N Ratio – Ratio of Carbon to Nitrogen

C:N Ratio < 20 – Rapid Degradation, Mineralization

C:N Ration > 30 – Slow Degradation, Scavenge Nitrogen

Make additional N: Plant Available

C:N Ratios and

Litter Quality

Typical C:N Ratios

Litter Quality

Lignin Content

Low C:N: Low Lignin – High Quality, Faster nutrient availability, Less accumulation of SOM

High C:N: High Lignin – Lower quality, Slower release of nutrients, Accumulation of SOM

Manage residues and anticipate nutrient availabilities

Cover Crop Residue Decomposition

3/19/2020 Soil Organic Matter (Chapter 12) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Content Video D

Generating SOM
SOM – Broad term

Living Biomass: Bugs and Animals

Plant Litter: Dead roots, identifiable residue

Humus: Soil colloid, High MW, unidentifiable reside

Soil Organic Carbon – % of Carbon in SOM

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3/19/2020 Soil Organic Matter (Chapter 12) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Classifying SOM

Humus
Humic Substances

Polymerization of lignin and degradation products

COMPLEX Ring Structures and Ill-defined

High MW

Recalcitrant – Resist microbial attack

Dark in Color

60-80% of SOM

Nonhumic Substances

20-30% of SOM

Less recalcitrant, Less Complex

Biomolecules – Microbial Byproducts

Polysaccharides

Impact soil structure – aggregate stability and nutrient availability

3/19/2020 Soil Organic Matter (Chapter 12) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Humus Influence
Colloid

High Surface Area

High Water Holding Capacity

HIGH CEC: 150 to 500 cmolc/kg

Bank for nutrients

Promotes soil structure: air/gas exchange, infiltration rates, ETC.

Stable

Portions of SOM are very stable – Centuries old

Clay-Humus combination – further protection and stabilization

Continual, Slow Degradation: Must protect have and continually add residue to build new to replace loss

SOM Pools – Monitor and Predict

3/19/2020 Soil Organic Matter (Chapter 12) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Content Video E

Factors Influencing SOM
Soil Orders

Alfisols – Lowest

Histosols – Highest

Texture

Clays > Sands

Complexation > Fine vs Coarse

Drainage Rates > Fine vs Coarse

Plant Cover

Plants > Bare

Grasslands > Forrest

Climate

Wet, Cold > Hot, Dry

Histosols: Waterlogged

Management – DO NOT DISTURB!

Distribution of SOM

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3/19/2020 Soil Organic Matter (Chapter 12) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Managing Agronomic Soils – Building SOM
Conservation and No-Till

Tillage: SOM broken down by microbial community

Crop Rotation

Crop Rotation > Monoculture

Maintain Soil pH and Nutrients

Nutrient availability for plants

Good for microbial population

Maximize CEC and Complexation for SOM

Plant Residues – Keep soil covered!

Crop Residues

Green Manures

Animal Manures

Morrow and Rothamstead Plots

Farmer’s Challenge
Aren’t bugs the bad guy here – Break down SOM?

Microbial community drive ALL nutrient cycling

SOM: CEC, WHC, Nutrient Availability, Structure

BALANCE

Content Video F

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3/19/2020 Soil Organic Matter (Chapter 12) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Content Video F

Composting
Create own humus!

Produce soil amendment from decomposition of organic materials

Management of compost pile – Box 12.4

Composting – animal manure – minimize pathogens

Soils and Greenhouse Effect
Soils natural source GHG: CO2,CH4, NOx
Loss of SOM – Net Loss CO2
Manage Soils SINK most gases:

Stop draining Histosols
Manage wet soils
Manage for increased SOM

Carbon Credit Industry

Review
What part does soil play in the global carbon cycle?
Where is most of the carbon in soils located?
What is the primary source for the production of SOM?
Are soil nutrients a large portion of the total plant material?
Do all plant materials decay at the same rate?
What is decomposition?
What group drives decomposition?
What is an enzyme?
Which is more efficient aerobic or anaerobic degradation?
Why is the nutrient content of plant material so important in is degradation status?
What is a C:N ratio – why is it important – what C:N ratio would cause mineralization and not
immobilization?
What is litter quality?
What qualities of litter promote SOM?
Can you describe Figure 12.6?
Name the major components of SOM?
Of 100 grams of carbon the majority is lost to what?
What are some characteristics of humus?
Why is SOM so closely tied to soil fertility?
Understand the pools of SOM – active vs passive
What soil orders might have the highest and lowest SOM?
Generally would clay soils or sandy soils tend to have greater amounts of OM and WHY?
What climactic conditions are most conductive to maximum SOM?
D d t d Fi 12 21?

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3/19/2020 Soil Organic Matter (Chapter 12) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Reflect in ePortfolio Download Print

Do you understand Figure 12 21?

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3/19/2020Soil Organisms and Ecology (Chapter 11) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Soil Organisms and Ecology (Chapter 11) Notes

Soil Organisms and Ecology (Chapter 11) Notes

Did you know ….
Did you know that there are more organisms in a gram of soil than there are people on this planet? Chapter 10
highlights the terrific abundance of life that happens right under our feet! The chapter will discuss life in soil
from large to small and their incredible importance in soil quality and health.

Lecture content notes are accompanied by videos listed below the notes in each submodule (e.g. Soil
Organisms and Ecology (Chapter 11) Videos A though E). Print or download lecture notes then view videos in
succession alongside lecture content and add additional notes from each video. The start of each video is
noted in parenthesis (e.g. Content for Video A) within each lecture note set and contains lecture content
through the note for the next video (e.g. Content for Video B).

Figures and tables unless specifically referenced are from the course text, Nature and Property of Soils, 14th
Edition, Brady and Weil.

Content Video A

Soil Organisms and Ecology

AGRI1050R50: Introduction to Soil Science (2020S) LH

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3/19/2020 Soil Organisms and Ecology (Chapter 11) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Universal Phylogenetic Tree

http://openi.nlm.nih.gov/detailedresult.php?img=2793248_1745-6150-4-43-27&req=4

General Size Classifications

http://openi.nlm.nih.gov/detailedresult.php?img=2793248_1745-6150-4-43-27&req=4

3/19/2020 Soil Organisms and Ecology (Chapter 11) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Life in Soil

Table 1-2. Principles and Applications of Soil Microbiology, Second Edition, Silva et al.

Content Video B

Plant Breakdown – Soil Food Web

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3/19/2020 Soil Organisms and Ecology (Chapter 11) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Cell Types

http://www.bio.miami.edu/dana/106/106F14_2.html

Prokaryotes vs Eukaryotes

http://www.bio.miami.edu/dana/106/106F14_2.html

3/19/2020 Soil Organisms and Ecology (Chapter 11) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Cell Metabolism
Carbon Source: Heterotroph vs Autotroph

Heterotroph: Organic Carbon

Autotroph: CO2

Energy Source: Chemotroph vs Phototroph

Chemotroph: Reduced Inorganic Compounds

Phototroph: Light (photosynthesis)

Join names together – Carbon and Energy Source

Oxygen Requirements
Energy Generation – Requires TEA
Obligate Aerobes – Must have Oxygen
Obligate Anaerobes – Function w/o Oxygen
Facultative Anaerobes – Prefer Oxygen, but can do without (TEA: nitrate, sulfate)

Start Video C

Soil Animals
Burrowers

Moles, Voles, Mice, Prairie Dogs, Earthworms
Earthworms – Natural Tillers

Physically and chemically process OM
Distribute OM in soil profile
Casts – Excrement – Build soil structure
Burrowing channels – Water and Aeration
Prefer soils: High OM, neutral pH, moist conditions, no-till
Indicator of Soil Health!

Review: OM – Organic Material (plant detritus, tissues, etc.)

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g (p )

Ants and Termites
Tropical and Arid Environments
Soil Mixers – Destroy Soil Structure
Termites

Powerful gut microbes
Methane Gas

Nematodes and Protozoa
Nematodes: Microscopic Soil Worms

Saprophytic: Eat plant detritus
Parasitic: Plants and animals
Significant Plant Pathogen

Soybean Cyst Nematodes

http://www.ipm.iastate.edu/ipm/icm/2006/9-18/scn.html\

Protozoa: Single Celled
Capture and Engulf Food
Help maintain active and diverse bacterial population

http://www.ipm.iastate.edu/ipm/icm/2006/9-18/scn.html/%22%20data-mce-href=

3/19/2020 Soil Organisms and Ecology (Chapter 11) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Soybean Cyst Nematodes

http://extension.entm.purdue.edu/nematology/soybeannems.html

http://extension.entm.purdue.edu/nematology/soybeannems.html

http://extension.entm.purdue.edu/nematology/soybeannems.html

http://extension.entm.purdue.edu/nematology/soybeannems.html

3/19/2020 Soil Organisms and Ecology (Chapter 11) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Rhizosphere

Start Video D

Soil Fungi
Multicellular, Heterotrophs
Molds, Mushrooms, Yeasts
Most are filamentous – Molds and Mushrooms

Hyphae – filaments
Mycelia – matt or group of filaments

SOM Formation
Primary degraders of lignin and cellulose
Dominant in Forest Soils – Acidic Conditions

Soil Structure – Aggregate Stabilization
Glomalin
Hyphae platform

Fungi: Good, Bad, Ugly
Mycorrhizae – Symbiosis Plant Root and Fungus

Most plants have these relationships
Increased H2O/P uptake for plant – Carbon/Energy for Fungi
Plant less susceptible to other pathogens

Chemicals – Antibiotics – Penicillin
Mycotoxins – Aflatoxin – Aspergillus

Warm-Humid regions
Acute and Chronic Issues – Liver
Dry Crops: Corn, Sorghum, Nuts, etc.
Issue in human AND animal feed

Plant Pathogens – Agronomy and Horticulture
Billions $ Loss – Most crops, especially cereal grains
Root Rots, Rusts, Wilts, etc.
Fusarrium (Root Rots, Wilts) and Phakospora (Soybean Rust)

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3/19/2020 Soil Organisms and Ecology (Chapter 11) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Nematode Trapping Fungi

Start Video E

Bacteria
Prokaryotes – Unicellular
Carbon, Energy, Oxygen

Chemoheterotrophs – Large portion
Chemoautotrophs – Nutrient Cycling
Oxygen: Aerobes vs Anaerobes – TEA

Reproduction – Binary Fission
Geometric Growth
Resistant spores survival

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3/19/2020 Soil Organisms and Ecology (Chapter 11) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Importance of Bacteria
Decomposition of organic substrates
Formation of SOM – Humus
Stabilize soil structure
Nutrient Cycling
Plant Growth Promoting Bacteria

Soil Bacteria

Environmental Microbiology, Third Edition, Pepper et al.

Environmental Microbiology, Third Edition, Pepper et al.

Archaea
Unicellular Prokaryotes
‘Ancient’
Bacteria-like but own domain
Live in extreme environments:

Hot springs
Ocean vents
Salt Flats
Methanotrophs

Big research topic – Range of abilities

3/19/2020 Soil Organisms and Ecology (Chapter 11) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

https://gotoclass.tnecampus.org/d2l/le/content/8094442/viewContent/60403409/View 11/12

Actinomycetes
Bacteria that look like fungi
Geosmyns – Characteristic ‘soil smell’
Prefer alkaline conditions

Antibiotics
Tetracycline
Amoxicillin
Ciprofloxacin

Promoting Healthy Soil Organisms

Review
What three domains are utilized to classify all living things in the Universal Phylogenetic tree?
What are the most abundant (#s) group of organisms in soil?
What are the most abundant by biomass in soils?
What is the MAIN difference between a eukaryote and a prokaryote?
What does the term chemoheterotroph mean?
What metabolic category do most bacteria fall?
If an organisms prefers oxygen but can function without it what type of organism is this?
Why are earthworms considered an indicator of soil health?
How do ants and termites destroy soil structure?
What are microscopic worms in soil called? What agronomic crop is most susceptible to these worms?

3/19/2020 Soil Organisms and Ecology (Chapter 11) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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What are microscopic worms in soil called? What agronomic crop is most susceptible to these worms?
How do protozoa contribute to an active and diverse bacterial population in soils?
Define rhizosphere – why is it such an important zone of activity in soils?
What KEY ROLE do fungi serve in soil?
What is afflation and why should I bee concerned about it in my animal feed?
What soil organism is generally responsible for plant pathogens like root rots, rusts, and wilts?
Why are bacteria so important in soils?
What is an Archaea?
What soil organism gives soil its distinctive soil smell?
Whi h f il i i ibl f tibi ti lik t t li d i illi ?

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3/19/2020 Soil Acidity (Chapter 9) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Soil Acidity (Chapter 9) Notes

Soil Acidity (Chapter 9) Notes

Did you know ….
Did you know that wood ashes can actually help change the pH of your soil? Chapter 9 highlights soil acidity,
its sources, how it occurs in soil naturally as well as man-induced, why pH is important for nutrient availabilty,
and finally how to manage soil acidity.

Lecture content notes are accompanied by videos listed below the notes in each submodule (e.g. Soil Acidity
(Chapter 9) Videos A though E). Print or download lecture notes then view videos in succession alongside
lecture content and add additional notes from each video. The start of each video is noted in parenthesis (e.g.
Content for Video A) within each lecture note set and contains lecture content through the note for the next
video (e.g. Content for Video B).

Figures and tables unless specifically referenced are from the course text, Nature and Property of Soils, 14th
Edition, Brady and Weil.

Content Video A

Soil Acidity

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Chemistry Review

Reversible Reaction: double arrows going left and right

LEFT of Arrow: Reactant RIGHT of Arrow: Product

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3/19/2020 Soil Acidity (Chapter 9) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Law Mass action:

Add Reactant (to the left side) push reaction to the RIGHT–Make more product (right side)

Add Product (to the right side) push reaction to the LEFT –Make more reactant (left side)

Dissociation – Break apart into constituents, generally adds to acidity (H+)

See carbonation above

pH, Acidity, and Alkalinity

Kw = [H+] + [OH-] = 10-14
pH = – log10 [H+]

Example:

[H+]= 0.0001 M (10-4)

10-4 M = pH = 4

Log Scale – Every step, pH change is 10X

Chemistry Review: Moles (M) = g/L

pH and pKa
Acids – donate protons – H+

Acid dissociative constant – pKa

Dissociate – Break apart – Produce more H+

½ the acid dissociate and ½ stays in solution

pH > pKa – More likely to dissociate

pH < pKa – More likely to stay in tact (undissociated)

pH = pKa – in equal concentrations

Weak Acids –Reaction pKa between 0 and 14

Lots of weak acids in soils – Offer BUFFERING CAPACITY

Carbonate, Nitrate, Phosphate, Sulfate

Strong Acids – Reaction pKa < 0

Content Video B

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3/19/2020 Soil Acidity (Chapter 9) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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pH Scale

Sources and Consumers of H+

Sources of Acidity
Microbial/Plant Activity

Produces CO2 gas – Carbonic Acid

Uptake nutrients – Release H+

Nutrient Cycling – Reduction of N, S, Fe

Humus/pH dependent charge

Atmospheric Deposition

Weathering

3/19/2020 Soil Acidity (Chapter 9) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Weathering

Rain Water – pH 5.6

Based on # of exchangeable cations (CEC)

Leach ‘base cations’: Ca, Mg, Na, K, Al

Leaves Al on the exchange site

Aluminum and Soil Acidity
Aluminum – Major Constituent Soil Acidity

Lots of Al3+ in soils – minerals, clays, oxides

Weathering – displaces Al3+ with excess H+ in solution

Now both exchangeable AND in soil solution

Hydrolyses Water – More H+

Breaks water into H+ and OH-

Produces Al(OH)x

pKa in typical soil ranges – perpetuates issue

Aluminum Hydrolysis
Al(OH)x – dependent on soil pH

Looking to ppt soluble Al3+ into Gibbsite

Problem: Aluminum is highly toxic to plants!

Weathering – Loss of Cations
Excess H+ ions in system

Move ‘base cations’ off the exchange and into solution

Base Cations lost to leaching

Leaves Al3+and its hydroxides on exchange

More Acidity

3/19/2020 Soil Acidity (Chapter 9) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Content Video C

Concept of Base Saturation
CEC – Saturation Percentage

Percentage of the CEC taken up by particular ions

Acid-Saturation – Al3+ and H+

Non-Acid Saturation (Base saturation): Ca2+, Mg2+, K+, Na+

Do not Hydrolyze like Al (and Fe) to produce more H+

Not really ‘bases’ just not acidifiers

Utilized in Soil Taxonomy – Ultisols vs Alfisols

Relative measure of level of weathering as well as soil fertility

Saturation and pH
Acid Saturation vs Base Saturation (CEC)

Wide pH ranges – ph Dependent charge – variable CEC

Aluminum toxicity >20% acid saturation

Pools of Acidity – Resist Change
Active – [H+]

Concentration of H+ ions in solution

Smallest portion

Exchangeable Al3+ and H+

Salt Exchangeable

Important in acidic soils

Residual

Non-exchangeable Al3+ and H+

Tightly bound

Largest Pool

Soil

Buffering Capacity

Active and Exchangeable – Easily neutralized

Buffered by addition of Al and H from the residual

Resist major changes in soil pH

Generally: The higher CEC higher buffering capacity

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3/19/2020 Soil Acidity (Chapter 9) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Generally: The higher CEC, higher buffering capacity

Weak Acids – Buffering Capacity
Reversibility – Mass Flow = Buffering Capacity

Move either direction to keep things the same – Buffer the system

Aluminum Hydroxide Formation

Humus

Weathering – CEC

Buffering Capacity

Content Video D

Man-Induced Soil Acidification
Commercial N Fertilizers:

Acid Deposition

Industrial Activity

Fossil Fuel Burning

SOX –NOX Gases – Come back as Sulfuric and Nitric Acid

Mining activities – Acid Mine Drainage

Significant environmental issue

Metal Oxide Oxidation – LOTS of Acidity produced

Runoff/Rainwater destroy ecosystems

Importance of pH
Nutrient availability Figure 9 22

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3/19/2020 Soil Acidity (Chapter 9) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Nutrient availability – Figure 9.22

Maronutrients: Ca2+, Mg2+, K+ – LESS available at extreme pH

Micronutrients: Mn, Zn, Cu, Co, Fe – MORE available at extreme pH – even toxic

Nutrient Interactions

Plants and Microbes: Al (and Mn) toxicity

Generally: pH <5.2 major issues

Accumulates in plant roots – Test roots

Wide range of tolerance

Fungi: Acid Lovers

Bacteria: Neutral pH (adapted wide range)

Content Video E

Reducing Soil Acidity
Increase the pH

Add soil amendments – LIME

Change chemical nature of the root zone

Overcome soil buffering capacity

Tons per acre

Liming Materials

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3/19/2020 Soil Acidity (Chapter 9) Notes – AGRI1050R50: Introduction to Soil Science (2020S)

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Reducing Soil Acidity
Add Lime – CaCO3

Active Acidity: Increase

Exchangeable Acidity: {Equations 9.25, 9.26, and 9.27}

Increase pH – Promote movement Ca2+ onto exchange – precipitate Al(OH)3

How much lime do I need?
Soil pH in water – active acidity

Soil ph in buffer – exchangeable acidity

Adams-Evans for TN – buffered at pH 8

Change from pH 8 – amount of exchangeable acidity

Big drop – more lime

Small drop – less lime

Lime requirements – crop specific and target pH specific

Regression combination of soil water pH and buffer pH

Vary by state – parent material (aka clays)

Review
What is pH?

Define soil acidity? 2 main acidifying ions

Sources of soil acidity– natural and human induced

Acid and base saturation – importance?

How do soils buffer pH?

Problems that arise from soil acidity – at what pH should I be concerned – how might I diagnose the
problem?

How do we reduce soil acidity?

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