AP Environmental Science 2012-2013 1
SOIL ANALYSIS LAB
INTRODUCTION
Composition of soils (texture): Soils are composed of three major constituents: sand, silt, and clay. A fourth component,
organic matter, although extremely important in the biological, chemical, and physical aspects of the soil, is not general
considered in the textural makeup of soils. The different components of a soil are referred to as fractions, namely, the
sand, silt, clay, and organic fractions. Soil types have been characterized by field and laboratory tests, which are based
on certain common chemical and physical properties. The texture of a soil is based on the relative proportions of these
constituents in a given soil. The different class names are shown in the soil texture triangle below.
The colloidal portion (sub-microscopic particle size, large surface area) of soils consists of highly decomposed particles of
clay and organic matter, and account for a soil’s capacity to hold nutrient elements. These minute clay and organic
colloids have a net negative charge, and therefore attract and hold iron, manganese, zinc, and copper. The positively
charged metals are called cations, and the capacity of a particular soil to hold such cations is called the Cation Exchange
Capacity (CEC). Hence, the capacity of a soil to hold metal cations varies directly with the CEC of individual soils.
The CEC varies considerably from soil to soil depending on the type and percent of colloidal clay and organic fractions
present in a given soil. For example, a soil in which sand is the predominant mineral component, would have a low CEC
as opposed to a clay soil with a relatively high CEC. Soils high in colloidal organic matter have an even higher CEC than
clay soils.
Since these metal cations of a soil can be displaced through chemical extraction, the CEC of a particular soil can be
determined with the proper soil test. As a result, the native fertility level of the soil can be assayed and evaluated in light
of the extractable nutrient content as it related to the CEC of that soil. For example, soils classified as loams, sandy clays,
Adapted from Acton-Boxborough Regional High School Soil Analysis lab
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silt loams, etc. have a much higher CEC than sands, sandy loams, or loamy sands, and are less subject to nutrient
leaching. Consequently, the extractable nutrient fraction from these soil classes are generally much higher, which results
in a higher fertility status and residual benefit. In cases where both fertility status and residual nutrient supplies are high,
the need for supplemental fertilizer treatment could be reduced or in unique cases eliminated entirely.
Keep in mind, however, that soils have limited nutrient reservoirs and must be replenished periodically in order to
maintain high levels with a range required for optimum yields. Remember, also, that soil fertility, as such, is only one
aspect of the overall soil management program. Crop rotation, tillage practices, insect and weed control, and water
management are other key factors involved in maintaining the productivity of a soil.
The clay content or colloidal fraction of soils has a pronounced effect on the nutrient holding capacity, water retention,
and ease of tillage. Soils high in clay have a high water retention which can cause tillage delays during wet periods. Clay
soils are not very easily crumbled as compared with soils which have a low clay content, namely the silt loams, loamy
sands, etc. The latter soil types are therefore much easier to till.
The clay fraction performs a very useful function in soils, and should be considered a complimentary component of the
soil. In addition to enhancing the nutrient and water holding capacity of soils, clay acts as a binding agent in the soil,
thereby, bring about a sort of stability in the soil. With this binding agent, many sandy soils would have very limited
agricultural value. Since the clay fraction accounts for much of the chemical reactivity in soils, it is beneficial in its effects
on texture, structure, and consequently, fertility status of a soil. An ideal soil is generally defined as a soil composed of a
mixture of sand, silt, and clay – all of which have their unique effect on the chemical or physical aspects of the soil.
Porosity: Porosity is the percentage of open spaces or pores in a given volume of rock or sediment. Porosity determines
the total amount of water a rock will hold, and varies from one material to another. The greater the volume of pore
spaces a rock or sediment contains, the higher its porosity, and the more water it can hold.
Porosity is largely influenced by factors of particle size, shape, and assortment. Sorting refers to the amount of
uniformity in size and shape of the particles, or grains, composing rocks or sediment. A well-sorted sediment possess
particles with are all about the same size and shape. The overall size of the grains is unimportant. In fact, sediment
characterized by large grain size can have the same porosity as one composed of much smaller particles.
A sand deposit composed of well rounded quartz grains of fairly uniform size would possess a high porosity. In contrast,
poorly sorted sediments contain particles of many sizes, and are often of low porosity. This is because the smaller
particles fill up the pore spaces between the larger grains, making the sediment or rock less porous. For example, if we
mixed a well-sorted sand deposit with finer, poorly sorted particles of silt and clay, the porosity of the sand deposit will
be lowered.
Porosity is also influenced by the way materials are packed together. Loosely packed unconsolidated sediments of sand,
gravel, silt, and clay may have relatively high porosities of 20% to 50%. However, when these materials are compacted,
cemented, and consolidated to form sedimentary rocks, their porosity is greatly reduced. Overall porosities in various
materials can vary greatly with the level of compaction, fracturing, and cementation. In general, though, a porosity of
less than 5% is considered low, 5% to 15% reflects a medium porosity, and over 15% is thought to be high.
Adapted from Acton-Boxborough Regional High School Soil Analysis lab
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Permeability: The permeability of a rock or sediment refers to its ability to transmit groundwater freely. The rate at
which a material transmits water depends not only on its total porosity, but also on he size of the passageways between
its openings. To be considered permeable, the open spaces in a rock must be connected. The size and sorting of the
particles compositing the rock or sediment will affect its permeability.
Generally, materials of larger particle size that are consistently sorted will be more permeable. A rock can have high
porosity, but low permeability if the open paces are not well connected. For example, while clay may display a higher
porosity than sand, the clay particles are much finer and the spacing between them is very small. As a result, while the
cay may hold more water, the water is transmitted more readily through the sand than through the more porous clay.
This is principally due to the fact that the molecular attraction on the water is much stronger in the tiny openings
between clay particles. The passageways between particles in the sand are relatively large, and the molecular attraction
on the water is relatively low, allowing the water to move more freely in this material.
Friction slows down the flow of water through rock. Well-sorted, large grained rocks and sediments have less surface
area to cause friction during the water’s passage than do finer grained materials. Rocks composed of coarse-grained
particles, like sandstone, are often the most permeable. However, even smaller grained rocks, like limestone, can be
permeable due to the presence of interconnected cracks and enlarged fractures due to solution.
Regardless of how large the spacing is between particles in a rock or sediment, they must be interconnected for water to
pass through. Materials through which water cannot flow are called impermeable. Clays, shale, and most metamorphic
and crystalline igneous rocks are often impermeable or are poor aquifers. The most effective aquifers are typically
unconsolidated sand and gravel, sandstone, and some limestone. Permeability, then, plays a critical role in determining
the rate at which groundwater flows through an aquifer. Where permeability is high, the groundwater passes quickly
though the ground; when permeability declines, the flow is reduced.
Nutrients: The major essential nutrient elements supplied through the soil are Nitrogen (N), phosphorous (P), and
potassium (K). Nutrients absorbed from the soil by plants are supplied by decomposition of rock, decomposition of
organic matter, deposition by floodwaters, application of commercial fertilizers, and the use of animal or plant manures.
Nitrogen is the most abundant element in the atmosphere (about 80%), but the gaseous form cannot be absorbed by
plants. However, a relatively large group of plants, the legumes, have root nodules that contain Rhizobium bacteria
which convert gaseous nitrogen into a form usable by plants (nitrogen fixation). Nitrogen promotes succulence in forage
crops and leafy vegetables. It stimulates aboveground growth, hastens crop maturity, and is very influential in fruit sizing.
Phosphorous is necessary for the hardy growth of the plant and activity of the cells. It encourages root development,
and by hastening the maturity of the plant, it increases the ratio of grain to straw, as well as the total yield. It plays an
important part in increasing the palatability of plants and stimulates the formation of fats, convertible starches and
healthy seed. By stimulating rapid cell development in the plant, phosphorous naturally increases the resistance to
disease.
Potassium is not a component of the structural makeup of plants but it plays a vital role in the physiological and
biochemical functions of plants. The exact function of potassium in plants is not clearly understood, but many beneficial
factors implicating the involvement and necessity of potassium in plant nutrition have been demonstrated. Some of
Adapted from Acton-Boxborough Regional High School Soil Analysis lab
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these factors are: it enhances disease resistance by strengthening stalks and stems, activates various enzyme systems
within the plants, contributes to a thicker cuticle which guards against disease and water loss, controls the turgor
pressure within plants to prevent wilting, enhances fruit size, flavor, texture, and development, and is involved in the
production of amino acids, chlorophyll formation, starch formation, and sugar transport from leaves to roots.
It is important to note that plants need more than these three nutrients to grow and reproduce successfully. Carbon,
hydrogen, and oxygen from water and atmospheric carbon dioxide are used in abundance and other macronutrients like
calcium, magnesium, and sulfur are crucial. Calcium is a component of cell walls and is known to stimulate root and leaf
development as well as activate several enzyme reactions. It also maintains optimum pH levels in the plant by
neutralizing many of the organic acids generated by plant respiration. Sulfur and magnesium are both involved in the
formation of chlorophyll. Photosynthesis cannot occur without these nutrients. Yellow spots on the leaves of
houseplants are commonly the result of a lack of magnesium. Furthermore, there are a number of nutrients that plants
require in much smaller quantities. These include manganese, iron, boron, copper, zinc, molybdenum, and chlorine. A
healthy, balanced soil should contain appropriate levels of all of these nutrients.
Soil pH: A pH measurement of soil is a measure of acidity that is present in the soil solution, and does not address the
total acidity present in the soil. Total acidity includes the acidity of the soil solution as well as the hydrogen which is held
on the soil colloid. These two forms of acidity have been broken down into two categories, active acidity (measured with
a pH meter) and reserve of exchangeable acidity (acidity held on the soil colloids). The latter form of acidity must be
displaced by chemical means (generally a neutral buffered salt) before it an be effectively measured. Since these two
forms of acidity are in equilibrium with each other in the soil, a pH depression will be observed as the reserve acidity in
the soil increases. The reserve acidity is used in determining the amount of limestone required for optimum plant
growth.
Since the makeup of soils is alumino-silicates, aluminum is a major contributor to soil acidity. Therefore, the rate of lime
required to achieve optimum plant growth can be determined by measuring exchangeable aluminum in the soil.
Exchangeable aluminum, like exchangeable acidity, decreases with the increases in the soil pH. The pH at which
aluminum is rendered ineffective in generating acidity is around pH 5.5.
The pH measurement is a simple means by which the production potential of a soil can be evaluated. For example, soils
in which the pH is extremely low, have correspondingly low calcium and magnesium levels with high levels of
exchangeable acidity. All of the above factors have adverse effects on plant growth.
In addition, at low pH levels, metal cations such as aluminum and manganese are much more soluble and can reach
levels which are toxic to plants. These toxic components can be eliminated entirely by application of limestone to raise
the pH. In addition to the reduction in the solubility of toxic metals, increased pH levels also enhance microbial activity
within the soil. Hence, the key to good crop production is to maintain the pH within the range where plants and
microbial activity within the soil can function at their optimum level. For most soils this requires a pH between pH 6.0
and 6.5. Soil pH levels higher than 6.5 can create problems with certain micronutrients (manganese in particular).
Adapted from Acton-Boxborough Regional High School Soil Analysis lab
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SOIL ANALYSIS LAB
MATERIALS
100 ml graduated cylinders
50 ml graduated cylinders
electronic balance
filter paper
funnels
test tubes
test tube racks
water bottles
parafilm
LaMotte soil macronutrients kit
Soil sample
PROCEDURE
Experiment 1: Soil texture by fractionation
Soil texture can be determined by fractionation. Sand is larger so it will settle out faster in a suspension, silt is the next in
size so it settle out next. Clay is made up of the smallest particles so they will settle on the top.
1. Fill a 100 ml graduated cylinder with 25 ml of your soil sample.
2. Add water until there is about 75 ml in the cylinder.
3. Cover the cylinder with film and invert the several times until the soil is thoroughly suspended in the water. Place the
cylinder on the lab station and leave it to settle overnight
4. When the soil has settled out, there should be 3 distinct layers. Measure the volume of each layer and the total
volume of the sample.
5. Calculate the percentage of each component.
Amount of each component x 100 = % component
Total volume of soil
6. Identify the type of soil in your sample by using the soil texture triangle.
7. Record your result in Table 1 in your lab notebook.
Experiment 2: Density and Porosity
Determining soil density is one way of quantifying porosity. Dense soils have low porosity. Soils can become dense due o
the compaction of heavy equipment or traffic.
1. Weigh out 15 grams of your sample.
2. Pour it into a dry 50 ml graduated cylinder. Tap gently on the table to settle the particle.
3. Determine the volume of soil in the cylinder.
4. Add water until the soil is completely saturated. Make sure to keep track of how much water is added.
5. Determine the final volume by subtracting the volume of water added from the volume of soil in the graduated
cylinder.
6. Use the table below to calculate your results and record the final density values in Table 1 in your lab notebook.
Adapted from Acton-Boxborough Regional High School Soil Analysis lab
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Experiment 3: Water Holding Capacity of Soil – Permeability
1. Fold a piece of filter paper and place it in the funnel.
2. Put 20 ml of your sample into the funnel.
3. Hold funnel of soil over a beaker and pour 50 ml of water into the funnel. Time how long it takes for the water to
begin coming out of the bottom, and the times it takes the water to stop coming out of the bottom. The time
interval indicates the permeability of the soil.
4. Record your results in Table 1.
Experiment 4: Nitrogen and Phosphorus
Test for Nitrogen:
1. Fill a test tube to line 7 with Universal Extraction Solution.
2. Use soil measure to add one level measure of soil. Cap and shake for one minute.
3. Fold a piece of filter paper in half. Fold in half again. Gently push the corners together to form a cone. Place cone in
funnel.
4. Place funnel in clean filtrate tube and filter suspension through the filter paper. The clean solution is the extract.
5. Use the 1 ml pipet to transfer 1 ml of soil extract to one of the depressions on the spot plate.
6. Use the 0.5g spoon to add one measure of Nitrate Reagent #2. Stir thoroughly with plastic rod. Wait 5 minutes.
7. Match sample color to a color standard on Nitrate-Nitrogen color chart. Record as lbs/acre nitrate-nitrogen.
Test for Phosphorous
1. Fill a test tube to line 7 with Universal Extraction Solution.
2. Use soil measure to add one level measure of soil. Cap and shake for one minute.
3. Fold a piece of filter paper in half. Fold in half again. Gently push the corners together to form a cone. Place cone in
funnel.
4. Place funnel in clean filtrate tube and filter suspension through the filter paper. The clear solution is the extract used
in the test.
5. Fill the Phosphorous “B” tube to the mark with the soil extract.
6. Add 6 drops of Phosphorous Reagent #2 and swirl to mix.
7. Add one Phosphorous Reagent #3 Tablet. Cap and swirl until the tablet disintegrates.
8 Remove cap. Hold tube one half inch above the white area of the Phosphorous Color Chart.
Looking down through the solution, match sample color to a color standard. Record as lbs/acre phosphorous. The color
comparison should be done in natural light.
Adapted from Acton-Boxborough Regional High School Soil Analysis lab
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SOIL ANALYSIS LAB
Experiment 5: Test for pH and Potassium
Test for pH
1. Fill test tube to line with Tricon Flocculating Solution
2. Use 0.5g spoon to add three level measures of soil. Cap and gently shake by inverting the tube back and forth slowly
for one minute. Allow soil particles to settle. In most instances, the solution will be clear in several minutes. However,
clay soils may require additional time.
3. Add 9 drops of Tricon Soil pH Indicator. Cap and invert the test tube gently one time to mix the contents. Allow soil to
settle.
4. Place test tube in the Tricon pH Comparer. Match sample color to a color standard. Record as soil pH.
Test for Potassium
1. Fill a test tube to line 7 with Universal Extraction solution
2. Use soil measure to add one level measure of soil. Cap and shake for one minute.
3. Fold a piece of filter paper in half. Fold in half again. Gently push the corners together to form a cone. Place cone in
funnel.
4. Place funnel in clean filtrate tube and filter suspension through the filter paper. The clear solution is the extract used
in the test.
5. Fill a clean test tube to line 3 with the soil extract.
6. Add one potassium Reagent B Tablet. Cp and shake until the tablet disintegrates.
7. Add Potassium Reagent C to line 7. Allow it to run slowly down the side of the tube. Cap and mix gently. A precipitate
will form if potassium is present.
8. Place a Potash Tube “B” on the plastic plate directly over the black line. NOTE: THE REMAINDER OF THE PROCEDURE
SHOULD BE PERFORMED FACING A WINDOW OR SOME OTHER SOURCE OF DAYLIGHT.
9. Use a pipet to add the treated test sample slowly to Tube “B” allowing it to run down the sides of the tube while
observing the black line through the solution. Continue until the line just disappears.
10 The height of the column of test sample is measured against the potassium scale on the tube. Record as lbs/acre
potassium.
Adapted from Acton-Boxborough Regional High School Soil Analysis lab
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SOIL ANALYSIS LAB
DATA (copy this table into your lab notebook)
Table 1: Soil Properties
Sample
Texture
Density
(g/ml)
Permeability Nitrogen
(sec)
(lbs/acre)
Phosphorus Potassium pH
(labs/acre)
ANALYSIS
1. Identify and explain the relationship between porosity and soil texture.
2. Corn grows best at a pH of 6.0-7.0 and is known to be a heavy nitrogen consumer with moderate potassium and
phosphorous needs. Would your soil be suitable for growing a field of corn? Explain your answer with reference to your
data.
3. Would your soil be suitable for lining a landfill? Explain your answer with reference to your data.
4. Would your soil be suitable for siting a septic tank and leaching field? Explain your answer with reference to your data.
Adapted from Acton-Boxborough Regional High School Soil Analysis lab
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SOIL ANALYSIS LAB
SOIL ANALYSIS LAB – Pre-lab questions (To be completed in your lab notebook)
1. Complete the table below:
Point A: Sandy Loam
______% Sand, ______% Silt, ______% Clay
Point B: ___________
______% Sand, ______% Silt, ______% Clay
Point C: ___________
______% Sand, ______% Silt, ______% Clay
Point D: ___________
______% Sand, ______% Silt, ______% Clay
2. How will we be determining soil texture in this lab?
3. What is Cation Exchange Capacity (CEC) and what types of soils have a high CEC?
4. What is the difference between porosity and permeability?
5. How do particle size and sorting affect porosity?
6. What is the relationship between porosity and permeability?
7. What are the three most important plant nutrients? Briefly describe the importance of each in plant growth.
8. What is the ideal pH for most soils? Why is this the ideal pH?
Adapted from Acton-Boxborough Regional High School Soil Analysis lab