o Labs submitted without advised instructions will result in a 4 point deduction: Proper document
name (LastName_SoilsLab6), name included in document, legible numbering and spacing
including questions with answers.
o Early lab submissions will receive feedback with option to resubmit. Do not miss out on a great
opportunity to be sure you understand the materials and increase your lab grade.
Lab 6 – Plant Available Nitrogen and Colorimetric Assays
Nitrogen is the most complex and studied of the nutrient cycles in soil due to its importance as a plant
macronutrient and its complex cycle mediated by microbial transformations. Nitrogen is often the most
limiting of the macronutrients and is needed in the largest supply to maximize yield. The cycling of
nitrogen is largely mediated by the microbial population which facilitates the fixation of atmospheric
nitrogen into the soil system, mineralizes nitrogen from the organic to inorganic forms, then transforms
the inorganic ammonium to nitrate in nitrification, and finally returns the nitrogen back to gaseous
forms in denitrification.
Nitrogen mineralization is the conversion of organic nitrogen sources into inorganic sources in a process
also called ammonification. Ammonium (NH4+) is liberated from organic nitrogenous compounds such
as proteins, nucleic acids, and the breakdown of soil organic matter. A wide variety of heterotrophic
organisms including bacteria, actinomycetes, and fungi can carry out this process under a wide variety of
environmental conditions. The microbial community gains energy from the reduction of nitrogen
compounds. The ratio of carbon to nitrogen in the soil system dictates whether soil nitrogen will be
mineralized or immobilized.
Immobilization is the opposite of mineralization and is the conversion of inorganic nitrogen back to
organic nitrogen. Soil conditions especially the amount of carbon dictate whether nitrogen is
mineralized or immobilized. If C:N ratios are less than 20 and all other things are in good order like
aeration and water status, net mineralization will occur as there is ample carbon and nitrogen in the
system to mineralize organic nitrogen into microbial biomass and plant available ammonium. If the
ratio is above 30, net immobilization can occur where the microbial community actually scavenges
nitrogen from the system at the expense of plant available nitrogen.
The next step in the transformation of nitrogen is the microbial conversion of the inorganic nitrogen
between ionic forms in a process called nitrification. Nitrification is the conversion of ammonium (NH4+)
first to nitrite (NO2-) and then to nitrate (NO3-). This process is also microbially mediated in a two-step
process. First, chemoautotrophs, Nitrosomonas spp. oxidize ammonium to nitrite and then Nitrobacter
spp. oxidize nitrite into nitrate. If conditions are favorable, including reactants and bacterial species are
present, this reaction occurs quickly in soils. Thus, ammonium is a relatively transient nitrogen ion in
soils. Plants can utilize both ammonium and nitrate but generally prefer nitrate. Unfortunately, nitrate
is readily leached out of the soil profile or under certain conditions can also be lost to denitrification.
For this reason, products called nitrification inhibitors have been developed to slow this process.
Denitrification is the anaerobic transformation of nitrate into gaseous forms of nitrogen gas. The
nitrogen gas is lost to the atmosphere and no longer plant available. In this process, nitrate replaces
oxygen as a terminal electron acceptor and can be carried out by a number of facultative anaerobic soil
bacteria including Pseudomonas, Bacillus, and Micrococcus. When wet waterlogged conditions occur
even over short periods of time and at microsites in soil, nitrogen can be lost from the soil system.
Agronomically, this is an important loss of nitrogen, but environmentally it can be a positive. If nitrogen
lost in overland flow from runoff or erosion events can make its way through a wetland where anaerobic
conditions persist prior to reaching surface water, it can be significantly decreased in wetland
environments. Denitrification in this case is an absolute positive and is very helpful in decreasing the
load of nitrogen in the downstream surface water body. The denitrification process though is what is
called a ‘leaky pipe’ in that in some situations the conversion of nitrate all the way to dinitrogen gas can
be shunted and the intermediary nitrogen gases including nitric and nitrous oxide gas can be produced
which are considered greenhouse gases and contribute to the warming of the earth’s atmosphere.
Soil sampling is the gold standard for testing levels of macro and micronutrients in soil as well as pH,
CEC, and even SOM. But even though nitrogen is typically the nutrient recommended at the highest
rates of application, nitrogen is actually not included in the routine analysis of extracted nutrients in a
soil test panel. Nitrogen recommendations are typically made based on previous crop and
management, i.e. whether legumes were utilized and manure was applied, as well as crop and target
crop yield to be planted in the coming season. Ammonium is generally transient in soils due to the rapid
microbial conversion to nitrate. So nitrate is most relevant for soil analysis. Nitrate levels in soils
fluctuate tremendously based on time of year, temperature, water conditions, and other soil variables
due to the nature of its cycle. Thus, nitrate testing in humid regions is especially problematic. Nitrate
also is very easily leached out of the soil system with a rain event. Due to this fluctuation, nitrogen
testing is not very informative especially when tested long before planting season.
Corn is the row crop with highest demand for nitrogen and is the center of a large portion of the
nitrogen utilization research including nitrate testing. A nitrate test has been developed called the
presidedress soil nitrate test (PSNT) to help in determining potential nitrogen available for crop uptake
during the growing season. The PSNT is sampled at twelve inches depth when the corn is approximately
12 inches tall, tested just prior to adding a second application of nitrogen prior to when nitrogen needs
are highest. Each state has developed their own standards for testing and generally, if there is greater
than 25 ppm of nitrate available additional nitrogen may not be needed to maximize yields. If the corn
is not following a legume (soybeans), cover crop (also with legumes), soil is high in organic matter, or
had a recent manure addition, soil nitrogen tend to still be recommended, i.e. needed, to maximize
yield. But in cases where soils do have ample nitrogen to meet plant and yield goals, this test can be
utilized to reduce the amount of additional nitrogen added which reduces both cost and environmental
burden. UT soil testing labs and others in the region offer this service, but utilization of this tool has
been somewhat slow by producers.
Research efforts to characterize the nitrogen cycle in the lab, in research trials on small and very large
scales, and even by producers in fact test all of the various forms of nitrogen routinely. Total nitrogen,
nitrate, and ammonium can be readily tested using analytical methods while ammonium and nitrate can
also be quantified using colorimetric assays as well as test kit methods in the field. Analytical methods
are much more accurate but the test kits are easy to utilize, can be done quickly in the field, and offer a
general range of the amount of nitrate in a soil or water sample. The field kit strips measure both
nitrate and nitrite. The strips contain indicators which in the presence of nitrate cause a color change.
The darker the color, the more nitrate in the sample. Figure 1 below is a picture of the color change and
their corresponding amounts of nitrate from a Hatch Water Quality test strip canister. Again, the larger
the color change, i.e. the darker the color, the more product is in the sample. A video demonstration of
this procedure is included in the link for testing for nitrate using this strip test
(https://www.youtube.com/watch?v=4gMVoLTRMvY).
Figure 1: Hatch, Water Quality Test Strips for Nitrate/Nitrate-N. Color block on the side of the bottle.
(Note, this is one of many of these type kits on the market and is not a product endorsement.)
The use of color change to quantify a product is can be quantified in what is called a colorimetric assay.
Many, many variables have indicator tests designed for color change analysis including pH, phosphorus,
and many enzymes in soils just to name a few. These results can be quantified utilizing a spectrometer
and the basic principles of the Beer-Lambert Law which states that the absorbance of light is directly
proportional to the concentration of absorbing species when the path length is fixed. A colorimeter or
spectrometer is used to quantify the color change. These instruments are relatively simple in that a
specific wavelength of light is directed through a sample. If the sample contains a species that absorbs
that particular wavelength of light, the intensity of the light that can pass through the sample will be
diminished. The absorbance of the light can then be related to the concentration of the chemical
species in the sample. The darker the color, the more absorbance (or less transmittance) which equates
to a higher concentration of the species in the sample. A dilution series is created with a standard,
known quantity of the species to be tested which used to make what is called a calibration curve. A
range of concentrations is created to fit the range of potential concentrations in the samples to be
tested including the maximum concentration and minimum (zero) made of the diluent. The known
concentrations and the absorbance values for each can then be used as x, y pairs to create a regression
line. The absorbance is plotted on the y axis and concentration on the x axis. The equation of the line
can readily be calculated by hand or using a computing program like Excel. Concentrations of unknown
samples can then be quantified using the absorbance values (y) and solving for concentration (x) based
on the equation of the line from the calibration standards.
There are several reagents and extractants commonly utilized in colorimetric assays to test for
ammonium and nitrate. Nessler’s reagent is commonly used in soil microbiology labs to detect
ammonium which yields a yellow to orange-brown color while Griess’ reagent can be used to detect
nitrite and nitrate and yields a pink to red-violet color.
See below an image of a dilution set used to develop a standard curve, an example of standard
concentrations and absorbance values in table form, and finally the creation of a standard curve plot
line with the equation included (y = mx + b where y = absorbance, x = nitrate concentration, and b =
slope of the line.)
Low Nitrate Concentration High Nitrate Concentration
No color change Dark Purple
Figure 2. Dilution series in cuvettes (standard tubes utilized for spectrometer) for the creation of a
standard curve for nitrate-nitrogen. Nitrate concentration increases from left to right with no color
change on the left to a dark purple on the right.
(Photo credit: http://public.iorodeo.com/docs/colorimeter/lab_3.html)
To calculate the concentration of unknown samples, utilize the equation of the line with the absorbance
(y) values determined via spectrometer and solve for nitrate concentration (x). For instance, if the
unknown absorbance value was 0.7 (y), plug that into the equation of the line and solve for x. (The R2
value is how well the data ‘fit a line’ so a perfect fit would be 1.0)
Equation of the line: y = 0.02x – 0.02
Insert your absorbance value for y: 0.7 = 0.02x – 0.02
Solve for x: (0.7 + 0.2) = 0.02x
0.72 = 0.02x
x = 0.72/0.02
x = 36
You can also get an estimated value by locating the absorbance value on the y-axis, moving over to the
line, and then moving straight down to the x axis to see the corresponding nitrate concentration. For
this example, an absorbance value of 0.7 would yield an approximate concentration of 40 ppm of
nitrate. The known absorbance and concentration values for the standard curve are simply x,y pairs
plotted on a graph and then utilizing the equation of that line as the more exact relationship between
the two variables. To put this into perspective, the 50 ppm standard sample had just over double the
absorbance value than the 25 ppm sample (50 ppm, 0.9 absorbance vs 25 ppm, 0.4 absorbance). The
standard sample with no nitrate had an absorbance value of zero; the sample had no nitrate, so no color
change and thus no absorbance. The 50 ppm sample was just over double the intensity of the dark pink
color than the 25 ppm sample; the more color, the more absorbance, the more concentrated the nitrate
is in the sample. This is the exact same concept as the color wheel utilized in the test kit strips just using
actual data to calculate the concentration rather than a visual determination.
Colorimetric assays are relatively easy to conduct in the laboratory with standardized extraction
procedures and indicators for many, many chemical ions seem in soil and water. Producing a
standardized curve with known quantities of the ion in question allows an easy determination of the
y = 0.02x – 0.02
R² = 0.99
0
0.
1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50 60
A
b
so
rb
a
n
ce
(
5
4
0
n
m
)
Nitrate (ppm)
Nitrate Standard Curve
concentration of unknown samples by utilizing the calculated relationship of absorbance and
concentration described by the equation of the standard regression line.
1.) Utilize the example dataset in the lab to calculate nitrate concentration (ppm) in the
following samples:
a.) Sample A Absorbance: 0.09
b.) Sample B Absorbance: 0.25
c.) Sample C Absorbance: 0.54
2.) Why is nitrogen not included in routine soil test analysis? How are nitrogen recommendations
determined?
3.) Nitrogen is generally most important macronutrient needed in high quantities to achieve even
modest yield goals. Describe at least two ways nitrogen can be lost from the soil system and
some management practices producers can utilize to decrease this loss.
4.) How can denitrification be both a positive and a negative?
Matching Review Section with Answer Bank Below (2 points each, 28 points total)
1. Bacteria spp. which converts NH4+ to NO2-
2. Bacteria spp. which converts NO2- to NO3-
3. Enzymatic catalyst for the biological fixation of N2 to NH3
4. Bacteria spp. involved in biological N fixation
5. Conversion of organic to inorganic forms
6. Conversion of inorganic to organic forms
7. Name of the human induced process by which nitrogen gas is fixed to ammonia
8. Type of plants best known for their ability in concert with bacteria to fix their own nitrogen
9. Macronutrient where fixation is a major limitation to plant availability
10. Macronutrient needed to combat environmental stress
11. Secondary macronutrient that can be sorbed through the plant leaves
12. Bacteria spp. which facilitates acid mine drainage problems
13. Only micronutrient that becomes more available with increased pH
14. Complexation with organic compounds, common with cationic micronutrients
Answer Bank for Matching
a. Molybdenum
b. Sulfur
c. Nitrobacter
d. Phosphorus
e. Bradyrhizobium
f. Haber-Bosh
g. Chelation
h. Nitrosomonas
i. Potassium
j. Mineralization
k. Thiobacillus
l. Nitrogenase
m. Legumes
n. Immobilization
Intro Soils – Lab 7
Nutrient Management – Soil Test Recommendations – Commercial
Fertilizers
o Lecture Materials: Nutrient Management (Chapter 16)
o Labs submitted without advised instructions will result in a 4 point deduction: Proper document
name (LastName_SoilsLab4), name included in document, legible numbering and spacing
including questions with answers.
o Early lab submissions will receive feedback with option to resubmit. Do not miss out on a great
opportunity to be sure you understand the materials and increase your lab grade.
Equation 2: Potassium
K x 1.2 = K2O
K2O x 0.83 = K
The fertilizer grade includes values for all three of the macronutrients in a three digit code which is the
percentage by weight for N – P2O5 – K2O, respectively. For example, a fertilizer with a grade of 13-13-13,
is 13% nitrogen, 13% phosphate (P2O5) and 13% potash (K2O). Another way to describe the fertilizer
grade, is that 100 lbs. of 13-13-13- would contain 13 lbs. each of N, P2O5, and K2O. Some fertilizers come
as a complete fertilizer and contain all three nutrients like the one just described (e.g. 13-13-13), but
others are mixed and contain just two of the three (e.g. 18-46-0), or just one of the three, called a
straight fertilizer (e.g. 46-0-0).
The text (Table 16.13) lists several of the more common inorganic fertilizer materials, their percentages
of N,P,K, acid formation values, and some other comments. For nitrogen, common amendments include
urea (45% N) and UAN (32% N) which is a liquid formulation of urea and ammonium nitrate, as well as
anhydrous ammonia which is a gas. Many producers are utilizing liquid formulations of nitrogen due to
its relative ease and uniformity of application and more rapid potential for plant uptake. Further, safety
and liability concerns for storing and handling ammonium nitrate have curtailed its use recently as it is
hazardous material routinely used to make explosives. For phosphate, common amendments are
diammonium phosphate (DAP) or triple super phosphate (TSP). And finally for potassium or potash,
muriate of potash (MOP) is the most commonly used amendment. Sulfur is another routine
recommendation regionally and is often applied as ammonium sulfate. Many of the micronutrients like
zinc, boron, and others are typically needed in very small amounts and are often included as a foliar feed
alongside herbicide application.
Soil test recommendations for row crops are typically reported in lbs./acre and for turf and garden
varieties often in lbs./square feet. Depending on the grade of the fertilizer, one must calculate how
many pounds of product are needed to deliver the recommended rate. Large agronomic providers
utilize computer programs to readily generate blends of fertilizers to meet the recommendations often
times with mixing several different fertilizers. For instance, the recommendation calls for phosphorus
and nitrogen, so DAP (18-46-0) might be used to supply the P and at the same time some, but not all of
the nitrogen. Urea (45% N) or ammonium nitrate (33% N) might be blended in with the mixture to fulfill
the N recommendation; another route might also be to apply liquid UAN (32% N) in a split application
for wheat or corn.
The calculations for supplying recommended rates of nutrients are the same whether calculating for
your garden or large row crop operation. For instance, if the recommendation calls for 150 lbs. per acre
of nitrogen and you have ammonium nitrate readily available (32-0-0). Rate to add is 150 lbs./acre,
there are 32 lbs. of N in every 100 lbs. of ammonium nitrate, so you will need to add 441 lbs./acre
(Equation 3).
Equation 3: lbs. product to apply = recommendation lbs. x 100 lbs. product
acre acre amount nutrient per 100 lbs. product
lbs. NH4NO3 to apply = 150 lbs. N x 100 lbs. NH4NO3 = {(150 x 100)/32} = 441 lbs./acre
acre acre 32 lbs. N
If utilizing another fertilizer like DAP (18-46-0), both N and P2O5 will be applied, but at different rates, so
it is important to include all of the nutrients when calculating fertilizer application prescriptions. When
making blends, it is extremely important to note all of the nutrient sources and not over apply nutrients
needed in lesser quantities (typically P and K) to meet the larger needs (typically N).
Lawn and garden recommendations are generally much, much lower overall additions and are typically
recommended at rates at equal than to less than a pound per 1000 square feet. These
recommendations tend to be met with complete fertilizers readily available at lawn and garden centers
rather than potentially complex mixtures utilized in large ag production operations. In these cases, it
may be more difficult to meet the exact recommendations for all three nutrients with a complete
fertilizer (one with N,P, and K). It is important to get it as close as possible to the recommended rate,
and if you over/under apply P and K just adjust during your next application. Another approach, also
utilized on large scales, is to utilize the complete fertilizers (fertilizers with more than one nutrient) for
your lowest needed nutrients (typically P and K) and straight fertilizer to fill the remaining requirement
for your larger needs (typically N).
Variable rate technologies utilize GPS and grid sampling to variably apply nutrients across a field rather
than just applying one rate to the entire field. Soil test recommendations from the individual grid or
zone samples are integrated with GPS on the fertilizer application unit and prescription rates are applied
to each of those grids or zones individually. It is not a perfect science by any means, but it can greatly
reduce the overall amount of fertilizer added to a particular field and also identify grids or zones that
may need additional nutrients to be successful. Variable rate and precision ag technologies have
dramatically changed how fertilizers are applied and made a huge contribution to nutrient management
goals overall.
Careful consideration of commercial fertilizer needs is an important part of nutrient management. Soil
testing and utilizing recommendations based on crop response rather than set amounts based on crop
needs and uptake have significantly changed how nutrients are managed for the positive.
Comprehensive nutrient management can and should be a win-win situation were yields are maximized,
input costs are minimized, and natural resources are maintained or even enhanced.
Utilize Lab, Lecture and Text Materials: Nutrient Management (Ch. 16)
1.) Farmer Johnson’s soil test recommendation calls for 80 lbs./acre of P and 25 lbs./acre of K. His
consultant though has provided him with calculations that include P2O5 and K2O, the oxides,
instead of the actual elemental units. For ease of comparison how many lbs. of P2O5 and K2O
were recommended? (4 pts)
2.) Farmer Smith soil test recommendation calls for 150 lbs./acre of N, 90 lbs./acre of P2O5 and 80
lbs./acre of K2O. DAP (diammonium phosphate) and potash (KCL) are both readily available at
her local supplier. Farmer Smith plans on applying DAP (18-46-0) for N and P2O5 needs and
potash for K2O needs (0-0-60). (8 pts)
a.) Scenario A: Farmer Smith uses the recommended N rate to calculate how much DAP (18-46-
0) to apply. Calculate how many lbs./acre of DAP to apply to meet the recommended N rate
of 150 lbs./acre?
b.) Scenario B: Farmer Smith used the recommended P rate to calculate how much DAP (18-46-
O) to apply. Calculate how many lbs./acre of DAP to apply to meet the recommended P2O5
rate of 90 lbs./acre.
c.) Why do these values differ? What nutrient management issues might arise if Farmer Smith
utilizes scenario A? What crop management issue might arise if Farmer Smith utilizes only
Scenario B?
d.) Famer Smith also needs to add some potash (0-0-60). Calculate how much potash to apply
to meet the K2O recommended rate of 80 lbs./acre.
3.) Explain the general idea behind a soil test. Why should we soil test, what type of results might I
expect to receive, what do the ppm or lbs/acre values of the various nutrients on the data sheet
actually mean, and then how were these recommendations for nutrient additions made? (8 pts)
4.) What are the four main goals of nutrient management? (4 pts)
5.) What is IPM? (4 pts)
6.) What are some pros and cons of utilizing animal manures in a farming operation? What are
some measures to take to ensure you are meeting both nutrient and environmental goals? (4
pts)
7.) Why is Liebig’s Law of the Minimum an important concept to keep in mind when thinking about
comprehensive nutrient management? How does the philosophy for soil testing also ring true
for this law? (4 pts)
8.) What is the P Index? Why was it necessary and how is it utilized by producers? (4 pts)
Did you know ….
Did you know that proper management of nutrient applications, whether occurring in your corn field, golf course,
or lawn, can help prptect our drinking water? Chapter 15 is a very important chapter to round out our nutrient
discussion and detail how to manage nutrients to maximize yield, but protect our natural resources.
Lecture content notes are accompanied by videos listed below the notes in each submodule (e.g. Nutrient
Management (Chapter 16) Videos A thru 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.
suppression strategies to maintain pest populations below economically damaging levels and to minimize
harmful effects of pest control on human health and environmental resources.
Pest
Any species that directly or indirectly causes damage or annoyance by destroying food or fiber products,
causes structural damage, or creates a poor environment for other organisms; undesirable
Source: NRCS: Nutrient and Pest Management, Conservation Planning (http://www.nrcs.usda.gov/)
Integrated Pest Management
PAMS
Prevention
Avoidance
Monitoring
Suppression
Economic Threshold
Moving Target
Crop prices
Input prices
On-the-ground conditions
Overall goal – NOT to stop pesticide use, but to manage use to meet the economic, environmental, and
production goals!
Source: NRCS: Nutrient and Pest Management, Conservation Planning (http://www.nrcs.usda.gov/)
Content Video B
Best Management Practices
Pesticide Application
Follow label directions
Follow application directions
Be a good neighbor!
BMP Mainstays
Buffer strips – dense vegetation along water body edge
Cover Crops – vegetative cover
Conservation Tillage – keep at least 30% of soil surface covered
Soil Calcium, Magnesium, and Micronutrients (Chapter 15) Notes
Did you know ….
Did you know that some nutrients for plant growth are only needed in atom level quantities, but without them
yields can and are limited? Chapter 14 finalizes our discussion on the macro and micronutirents in soil, and
finishes with an important discussion on nutrient management.
Lecture content notes are accompanied by videos listed below the notes in each submodule (e.g. Soil Calcium,
Magnesium, and Micronutrients (Chapter 15) Videos A thru C). 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 Calcium, Magnesium, and Micronutrients
AGRI1050R50: Introduction to Soil Science (2020S) LH
Plants – Major component cell walls, growth, enzymes
Animals – Bones and teeth
Not generally a problem unless VERY acidic
Magnesium
Plants – Photosynthesis, enzymes, bridge for ATP
Animals – Grass Tetany
Not overly problematic – weathering/dissolution soil minerals
Availability linked to soil pH and amount of weathering
Lime – adds back from plant uptake/leaching
Calcium and Magnesium Cycles
Content Video B
Trace Element
Trace Element – Based on abundance in soils
< 100 mg/kg in soil solids or < 100 mol/L in soil solution
Micronutrient Nutrients required for plant growth but in small quantities
Micronutrient – Nutrients required for plant growth but in small quantities
‘C-HopKinsCafe’ micronutrients (17 total)
Non-Mineral (3): C, H, O
Macros (6) : N, P, K, S,C a, Mg
Micros (8): Fe, Mn, Cl, Zn, Ni, Cu, B, Mo
Why the big fuss?
Intensive ag – depleted micronutrients
Less ‘contamination’ of commercial fertilizers, less organic/manure application
Better technology – Assess deficiency/toxicity
Functions Micronutrients
Micronutrient Symptoms
3/29/2020 Soil Calcium, Magnesium, and Micronutrients (Chapter 15) Notes – AGRI1050R50: Introduction to Soil Science (2020S)
pH – more available with lover pH
Oxidation – more available in reduced conditions
CEC – involved in CEC reactions
Chelation – complexation with organic compounds
More or less plant available
Microbial or Synthetic (EDTA)
Anions – Cl, B, Mn
Cl – not huge issue except in alkaline soils, easily leached
B – Boric acid, readily leached
Mo – pH dependent: odd ball, more available at high pH, insoluble at acidic pH
Review Ca/Mg/Micronutrients
What is the major source of Ca/Mg in soils?
What is the difference between a trace element and a micronutrient?
What are the main micronutrients?
What are the ionic forms of the micronutrients found in the soils solution?
Did you know ….
Did you know there is an area in the Gulf of Mexico where animals and plants cannot live due to high levels of
nutrients? Chapter 14 moves us down the nutrient line to discuss phosphorus and potassium in soils including
eutrophication which contributes to the Dead Zone in the Gulf.
Lecture content notes are accompanied by videos listed below the notes in each submodule (e.g. Soil
Phosphorus and Potassium (Chapter 14) 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.
Plant
Photosynthesis, Flowering/Fruiting, Root Growth, Meristem
Deficiency: Stunted growth, weak stem, dark blue/green
Mobile in plant – moves old to young tissues
Plant Uptake – Low, Low P in soil solution
HP042- or H2PO4-
Phosphorus and Soil Fertility
Conundrum:
Total P low (200 to 2000 kg/ha in upper 15 cm)
P Compounds in soil not plant available and insoluble
Added P (fertilize, manures) become unavailable too
Environmental Quality
Underdeveloped Countries – Lack of P fertilizer, limits crop production
P Enrichment and Loss – Eutrophication
Soil and Environmental Quality, 2nd Edition, Pierzynski, et al.
Phosphorus Cycle
Weathering P minerals primary source – Inorganic
Parent materials and weathering degree dictate sources
Apatites (Ca-Alkaline) vs Fe/Al Oxides (Hi Weathered –Acidic)
Soluble P in solution – Low – 0.001 to 1 ml/L
Ion Dictated by soil pH – HP042- or H2PO4-
Fixation P – Strong – Not exchangeable
Movement to roots is SLOW
Mass Flow
Water
Mycorrhizae
Losses P
No real gaseous loss
Plant Uptake
Leaching not huge issue – strong affinity colloids
Erosion/Runoff – major environmental loss mechanism under right conditions
Phosphorus and pH
3/29/2020 Soil Phosphorus and Potassium (Chapter 14) Notes – AGRI1050R50: Introduction to Soil Science (2020S)
Forms of P in Runoff
Form of P Name Description
TP Total Phosphorus Total P in Runoff Volume
TSP Total Soluble P Orthophosphates and Organic P
SP Soluble P Soluble forms of Inorganic P
SOP Soluble Organic P Soluble forms of Organic P
PP Particulate P Total P in Sediment
Managing P
Apply P to meet plant needs – Do not over apply P
Apply in combination with N fertilizers
N fertilizers create acidity locally, keeps soluble, plant available
Apply in band as Starter Fertilizer
Minimizes surface area for fixation
Make plant available early, minimize loss
Enhance cycling of organic P
Cover crops – Build SOM, etc.
Mycorrhizal Symbiosis
Control soil pH – Near-neutral for max solubility
Minimize tillage – Decrease loss runoff/erosion
Buffer/Filter Strips – Catch P/N in runoff water/sediments
Plant
Osmotic water potential – aids with water loss
Photosynthesis, Protein Synthesis, N Fixation
*Environmental Stress – Drought tolerance, winter hardiness, better resistance to fungal disease and
insect pest.
1 to 4% Healthy Plant Tissue (Similar to N)
Deficiency: Yellow spots on leaf edge, look burned/ragged
Plant Uptake – Generally large total quantities in soil, low availability
K+ ion in soil solution
Potassium Cycle
Liming and Soil K
Maintaining Neutral pH with Lime
Maintaining Neutral pH with Lime
K+ more readily replace Ca2+ and Mg2+ (neutral) than Al3+ (acidic)
Keeps K+ in soil profile
Potassium (K) Cycle
Additions
Weathering parent materials and exchange on soil colloids
Not associated with SOM
Much total K not available: low solubility and fixation
Fertilizer needed in cropping systems
Losses
Leaching – Greater in acidic conditions
Crop Uptake – Big loss
Luxury Consumption
No gaseous loss
Forms of K in Soils
Pools dependent on parent material, clay content, CEC
Primary Mineral Structure – Micas/Feldspars
90-98% total soil K
SLOWLY available – relatively unavailable pool
Non-Exchangeable K – Fixed in Clays
3/29/2020 Soil Phosphorus and Potassium (Chapter 14) Notes – AGRI1050R50: Introduction to Soil Science (2020S)
Non-Exchangeable K – Fixed in Clays
1-10% total K
Slowly available
Readily Available
0.1 to 0.2% Soil Solution K
1 to 2% Readily Exchangeable K – Soil Colloids
K Fixation
Amount determined clay and pH
1:1 (Kaolinite) – Very little fixation
2:1 Clays (Vermiculite especially) – High levels of fixation
pH
Liming increases fixation
Closer to the soil colloids – 2:1 clays fixation
Managing Soil K
Crop uptake – major removal
SOIL TEST – Know what need!
Commercial Fertilizer Addition – Potash (KCl)
Maintain adequate soil solution K for particular plant needs
Many soils just need ‘maintenance K’
Highly weathered, acidic soils, or sandy soils – generally need more commercial K
Maintain soil pH
Review P and K
What is P utilized for in the plant?
What are the plant available forms of P?
What are the challenges for P and soil fertility – both in developed and developing countries?
What is the P Index?
Can you describe Figure 14.9 – P and pH?
Can you describe the mechanisms of P fixation – soil pH, orders, soil colloids, etc.?
Why are organic forms of P important? Why are they more likely to be lost to leaching than inorganic P?
Point Source vs Non-Point Source pollution
Why is erosion and runoff loss a greater issue than leaching of P unlike N?
Define Eutrophication – why is it environmentally important?
What are some management strategies to manage soil P?
What is a buffer strip?
What role does K play in plant production?
What form of K is available for plant uptake?
What are the major additions and losses of K from the soil system?
Did you know ….
Did you know that nitrogen is commonly the most limiting nutrient in agronomic settings? Chapter 11 begins
our journey through the major plant limiting nutrients and their cycles in soil discussing why nitrogen is most
limiting, how nitrogen cycles in soil, and then reviewing similar information for sulfur.
Lecture content notes are accompanied by videos listed below the notes in each submodule (e.g.
Soil Nitrogen
and Sulfur (Chapter 14) 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 Nitrogen
Nitrogen
Generally most limiting macronutrients
Yellow, Chlorotic Tissue
AGRI1050R50: Introduction to Soil Science (2020S) LH
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