Lab: Soil Chemistry Lab Investigation (N-P-K)
We live in an era of increasing concern over the conservation and management of our renewable and nonrenewable resources. We seldom think of soil in these terms, and yet improper development or natural erosion can
devastate a landscape that nature took centuries to create. Similarly, the mineral elements in the soil that nourish
growing plants can be depleted through repeated cycles of cultivation and harvest, resulting in a exhausted soil
incapable of supporting healthy plant growth.
For this lab exercise you are taking on the role of a soil scientist. Soil scientists receive samples of soil from
individuals in the community and analyze the soil samples to make a recommendation to the individual about the
quality of their soil and what they can do to improve the fertility of the soil.
Background:
Soil texture describes the relative amounts of sand, silt,
and clay in a mass of soil – it is one of the most important
indicators of soil quality. The texture of soil determines how
coarse or fine the soil is, its porosity and permeability, and the capacity to store nutrients and bind waste products.
Soil is classified into three categories based on their grain size: sand, silt, and clay. (See Figure 1)
Sandy soils have excellent drainage and lots of air spaces, but they do not bind nutrients or support root
growth. Sandy soils feel dry and gritty, and nutrients leach out quickly.
Clay soils, on the other hand, consist of microscopic particles that clump together and retain water. Soils
with high clay content are easily waterlogged and have a tendency to exclude air and become anaerobic, killing off
the living organisms that are a necessary part of healthy soil. Clay has a large surface area, however, and is
chemically very active, binding and storing both mineral and
organic nutrients. The most productive soils have a balance of
sand, silt, and clay and are called loams or loamy soils. (“Rich”
soils also contain high concentrations of organic matter.)
The United States Department of Agriculture (USDA) has
identified 12 main textural classes of soil based on the percentages
of clay, sand, and silt. The textural class is determined using a
three-sided graph called the soil texture triangle (Figure 1). Each
side of the triangle represents one of the soil separates on a scale
from 0 to 100%. The graph is read by following the clay percent
line parallel to the triangle base, the sand line parallel to the right
side of the triangle, and the silt line parallel to the left side of the
triangle. For example, follow the arrows in Figure 2: The asterisk
marks soil containing 30% clay, 50% sand, and 20% silt, which is
classified as sandy clay loam.
The pH of soil indicates whether the soil is acidic or basic. The pH scale is defined from 0 (very acidic) to
14 (highly basic). Soil pH influences the solubility and availability of soil nutrients, the viability of essential
microorganisms, and the movement of toxic heavy metals into groundwater. A pH range between 6 and 7 is ideal
for most plants. When soil is too acidic (<5.6), the plants cannot utilize the nutrients they need, and excessive
amounts of aluminum and iron, which are harmful to plants, dissolve into the soil.
The elements carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, and sulfur are considered
macronutrients because plants need them in large amounts. Of these, C, H, and O come from the atmosphere and
Ca, Mg, and S come from the mineral content in the Earth. The nutrients that are most likely to be missing are N, P,
and K – these elements are commonly added to soils in the form of fertilizers. Nitrate ions are the most common
source of nitrogen for plants. Before the widespread use of nitrogen fertilizers, soil nitrogen was primarily provided
by legumes (soybeans, alfalfa, and clover). The root structures of legumes contain bacteria that are capable of
converting nitrogen from the air into ammonia and nitrate ions. Nitrogen is an essential component of proteins –
plants grown in nitrogen-rich soils provide higher yields and are richer in protein and therefore more nutritious.
Nitrogen is also needed to produce leaf growth and green leaves. Nitrate levels in soil of 10 – 25ppm are considered
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optimal for agriculture. Phosphorus, which occurs naturally in soil in the form of phosphate minerals, is important
for root growth and also aids in the production of flowers and fruit. Adequate levels of phosphorus (2 – 4 ppm) are
especially important for root crops (beets, potatoes, carrots, radishes, etc.). In addition to fertilizers, other sources of
nitrates and phosphates in soil include decaying vegetation, human and animal waste products, and industrial waste
discharge.
Nitrate and phosphate fertilizer runoff is a serious problem in some areas. Nitrates do not bind to the soil,
and therefore end up passing down through the soil or being washed away by rain, eventually ending up in ground
water or surrounding bodies of water, respectively. Excess phosphate ions added to the soil precipitate in the form
of insoluble calcium phosphate, which binds to soil particles and washes away due to erosion or irrigation run off.
High levels of nitrates and phosphates in groundwater may leave water unfit for drinking. In addition, excess
nitrates and phosphates in bodies of water may lead to algae blooms. Algae blooms may be detrimental to
ecosystems. Algae blooms can lead to a thick blanket of algae on the surface of bodies of water, blocking out
sunlight needed by other photosynthetic life inhabiting the water below the surface. As plants and microorganisms
die off from lack of sunlight, bacteria levels increase. As bacteria and algae consume dissolved oxygen, the oxygen
levels decrease. Dissolved oxygen is used by other life forms such as fish, turtles, amphibians, etc. and decreased
levels may cause populations to deplete.
Pre-Lab Questions:
1. Which soil particle seems to have the largest influence in determining the properties of a class of soil
based on the soil texture triangle? Explain.
2. Explain why pH, nitrates, and phosphates are important aspects of soil fertility.
Materials:
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Graduated Cylinders (10mL & 100mL)
Pipet, Beral-type, graduated
Ruler (metric)
Soil (3 samples)
Spoon/Spatula
Ammonium hydroxide (household ammonia)
Dropper
Distilled Water
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Vinegar
Color Charts (pH, Nitrate, Phosphate)
Permanent marker
Test Tubes & Test Tube Rack
Aluminum foil
Stopper (must fit test tubes)
Balance
TesTab tablets (pH, Nitrate, Phosphate)
----------------------------------------------------------------------------------------------------Soil Testing:
Part One: General Observations
Look closely at your soil sample. What do you see? Observe and comment on the various particle sizes. Do
any sizes dominate?
General Comments: _________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
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Part Two: Soil Texture
Soil is made of mineral particles belonging to three size categories: clay, silt, and sand. The size of soil
particles is important. Large particles of sand allow empty space for air and water to enter the soil. Smaller silt
and clay particles help hold the water in a soil so that it does not drain away too quickly to be of use to plants.
The ratios of these materials, or texture, can be determined qualitatively and quantitatively.
We will practice by using a soil texture triangle to identify types of soil. Practice using the triangle below and
identify the different types of soils
Point A: Sandy Loam
65 % Sand; 20 % Silt, and 15 % Clay
Point B: ____________
_____% Sand, _____% Silt, and _____% Clay
Point C: ____________
_____% Sand, _____% Silt, and _____% Clay
Point D: ____________
_____% Sand, _____% Silt, and _____% Clay
Point E: ____________
_____% Sand, _____% Silt,
and _____% Clay
Qualitative Test:
Soil Texture by feel:
Use 25 grams of your sample
to do the following experiment
using the instructions below.
1. What type of soil do
you think you have?
Why?
_________________________
_________________________
_________________________
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_________________________
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Quantitative Test: Soil texture by fractionalization: Sand has a larger particle size and so will settle out
faster in a suspension, silt is the next in size so it settles out next with clay the smallest size particles so they will
settle on top.
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8.
Fill a graduated cylinder with 25 mL of your soil sample.
Add water until there is about 75 mL in the cylinder.
Add 5-6 drops of ammonium hydroxide (household ammonia) to separate the sand, silt and clay.
Cover the cylinder with film and invert several times until the soil is thoroughly suspended in the water.
Place the cylinder on the lab station and LEAVE IT to settle for at least 30 minutes.
When the soil has settled out, there should be at least 3 distinct layers. Measure the volume of each
layer and the total volume of soil for the sample (should be 25 mL).
Calculate the percentage of each component (Sand, Silt, Clay)
RECORD your data in the table below next to YOUR sample.
RECORD the other group’s data for the other samples in your data table.
Sample
% Sand
% Silt
% Clay
Type of Soil
*use soil triangle
A
B
C
9. Using the soil triangle and the information on the previous page to determine what type of soil you have
for each sample. RECORD in the last column of your data table.
10. Looking at your data ONLY, how does your answer (type of soil) compare to the qualitative method?
____________________________________________________________________________________
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Part Three: Fertility Analysis
Four variables are important in determining the fertility of soils. They are pH and the amounts of nitrogen,
phosphorous, and potassium. The values of each of these components can serve as a limiting factor in the
growth of plants.
Follow the directions for the Flinn soil kit (#AP7184) to determine the values of each variable.
1. pH
a. Obtain a TesTab for pH
b. Measure out 1mL of distilled water in your graduated cylinder and pour into a test tube.
c. Draw a line on your test tube and label it 1mL
d. Measure out 9mL of distilled water in your graduated cylinder and pour into your test tube.
e. Draw a second line on your test tube and label it 10mL
f. Discard the water
g. Using a clean spatula/spoon, add soil to the 1mL level in your test tube
h. Label this test tube pH
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i.
j.
k.
l.
m.
n.
Add distilled water to the 10mL mark
Add a pH TesTablet to the pH test tube
Stopper the test tube and shake vigorously for 30seconds.
Place the pH test tube into your test tube rack and allow 2-3min for the soil to settle.
Compare the color of the liquid in the pH test tube to the colors on the pH comparison Chart
RECORD the approximate pH value in the data table
2. Nitrate
a. Obtain a TesTab for Nitrate
b. Measure out 1mL of distilled water in your graduated cylinder and pour into a test tube.
c. Draw a line on your test tube and label it 1mL
d. Measure out 9mL of distilled water in your graduated cylinder and pour into your test tube.
e. Draw a second line on your test tube and label it 10mL
f. Discard the water
g. Using a clean spatula/spoon, add soil to the 1mL level in your test tube
h. Label this test tube Nitrate
i. Using a graduated Beral pipet, add 1mL of vinegar to your test tube
j. Add distilled water to your Nitrate test tube until the water level reaches the 10mL line
k. Stopper the test tube and shake vigorously for 1 minute.
l. Place the test tube in your test tube rack and allow 3-5min for the soil to settle.
m. While waiting, pour 5mL of distilled water into a clean test tube
n. Draw and label the 5mL line and discard the water
o. Decant (gradually pour liquid from one container into another without disturbing the sediment) 5mL of liquid
from your Nitrate test tube into your newly marked 5mL clean test tube.
p. Add a Nirate TesTab tablet to the clear liquid.
q. Stopper the test tube and shake vigorously for at least 1 minute or until the tablet dissolves
completely.
r. Place the test tube in the test tube rack and let it sit undisturbed for 5minutes.
s. Compare the color of the liquid to the colors of the Nitrate Color Comparison Chart.
t. RECORD the Nitrate concentration value in the data table
3. Phosphate
a. Obtain a TesTab for Phosphate
b. Measure out 1mL of distilled water in your graduated cylinder and pour into a test tube.
c. Draw a line on your test tube and label it 1mL
d. Measure out 9mL of distilled water in your graduated cylinder and pour into your test tube.
e. Draw a second line on your test tube and label it 10mL
f. Discard the water
g. Using a clean spatula/spoon, add soil to the 1mL level in your test tube
h. Label this test tube Phosphate
i. Using a graduated Beral pipet, add 1mL of vinegar to your test tube
j. Add distilled water to your Phosphate test tube until the water level reaches the 10mL line
k. Stopper the test tube and shake vigorously for 1 minute.
l. Place the test tube in your test tube rack and allow 3-5min for the soil to settle.
m. While waiting, pour 5mL of distilled water into a clean test tube
n. Draw and label the 5mL line and discard the water
o. Decant (gradually pour liquid from one container into another without disturbing the sediment) 5mL of liquid
from your Phosphate test tube into your newly marked 5mL clean test tube.
p. Add a Phosphate TesTab tablet to the clear liquid.
q. Stopper the test tube and shake vigorously for at least 1 minute or until the tablet dissolves
completely.
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r. Place the test tube in the test tube rack and let it sit undisturbed for 5minutes.
s. Compare the color of the liquid to the colors of the Phosphate Color Comparison Chart.
t. RECORD the Phosphate concentration value in the data table
4. Obtain results for each test from each of the other sample groups and RECORD in your data table.
Sample
pH
Nitrate
Phosphate
A
B
C
Analysis Questions:
1. Based on the results of these tests, describe the quality of the soils.
(Ex. which nutrients are low? pH acidic or basic? are the Nitrate & Phosphate levels suitable for plant growth?)
a. Sample A = ___________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
b. Sample B = ___________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
c. Sample C = ___________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
2. If you were a farmer planning to plant crops in the soil samples from your data table, which samples
would require fertilizer? Why?
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3. What type of area do you think the soil samples came from?
After guessing, check with your teacher for the correct areas.
a. Sample A = __________________________________ Actual = _________________________
b. Sample B = __________________________________ Actual = _________________________
c. Sample C = __________________________________ Actual = _________________________
4. Were you correct in your guesses? If not, why do you think you guessed incorrectly?
5. If you were to collect soil samples from the same hill but two different locations, the top and the bottom,
which do you think would have a higher concentration of nutrients? Explain why.
6. Why do excess nitrates and phosphates from fertilizers often end up as runoff in natural bodies of water
and groundwater? Why is this problematic?
7. In general, what conclusion can you draw about how soil texture affects land use and about how land
use affects soil texture?
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IMPORTANCE OF SOIL CHARACTERISTICS
Additional Info…
Consistence
 Can affect growth of plants…soil may be too hard for some roots to grow in.
Texture
 Affects soil’s ability to hold water…ability to hold water affects plants
 Clay – holds water –water does not pass through
 Sand – does not hold water – water passes through easily
Particle Size – measured in mm
 Affects permeability
 Larger particle size (sand) = higher permeability
 Smaller particle size (clay) = lower permeability
Permeability / Porosity
 How quickly water passes through the soil – affects plants.
 High permeability (sand) = water passes quickly
 Low permeability (clay) = water passes slowly
Absorbency / Moisture Content
 How much water the soil HOLDS.
 Can hold water for plant’s roots.
Clarity
 Can show how well the soil performs as a filter.
 Cloudy color = water was dirtier after passing through soil = poor filter
 Clear color = water was cleaner after passing through soil = good filter
pH
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Most plants cannot grow in soils too acidic or basic (pH of 5.5-7.5 range is good)
pH too high or low prevents plants from using nutrients like N, P, or K.
Nitrogen (N)
 Main element that affects plant productivity.
 Directly responsible for vegetative growth and green color in plants.
Phosphorus (P)
 Essential for strong roots, flower and fruit development.
 Boosts plant’s resistance to disease.
Potassium (K)
 Essential to the plant’s ability to produce sugars.
 Increase plant’s ability to survive cold temperatures, and survive droughts.
Changing N–P–K
 Fertilizer = replaces lost nutrients
 Packages contain 3 numbers = % of N-P-K
o Ex. 5-10-10 = 5% N, 10% P, 10% K
 he key is to match the correct fertilizer to the specific deficiencies of the soil
o Ex. Soil with N-P-K results of L-L-H would require 15-15-5 fertilizer
o Ex. Soil with N-P-K results of H-M-L would require 5-10-15 fertilizer
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