Advance Placement

Soil Analysis:Physical Methods

Objectives After this lab you should be able to do the following:

Determine soil texture and judge cohesive and adhesive soil properties. Determine capillary action and water­holding capacity of soil. Determine dry and wet permeability of soil.

Materials Materials for this lab are:

Spray Bottles

3 Plastic Cups

Rulers

Wax Marker

Soil Analysis Card

Bag of Sand

Bag of Clay

Soil Sample

Tap Water

4 Plastic Cylinders

4 Plastic Vials

4 Plastic Cups

Cheesecloth

Rubber Bands

60cc plastic medicine cups

humus

scissors

balance

Clock with Second Hand

Twist Ties

Introduction Soil Texture, Structure and Consistence

Soil texture is determined by the ratio of sand, silt and clay in the sample. Sand, siltand clay are all mineral components of soil and are defined by their particle size.Particles with the diameter greater than 0.05mm are considered sand; between0.002mm and 0.05mm, silt; and less than 0.002 mm, clay. Soil scientists group soilinto three broad classes based on texture: the sands, the clays, and the loams (amixture of sand, silt and clay).

A common field test is the ribbon test. In this test, a small amount of soil ismoistened, formed into a ball, then squeezed and pinched to form a ribbon. Thebehavior of the sample during the test determine its classification. You will be trying

this method in this lab.

Soil Structure

Primary soil particles are arranged into secondary units called peds. The shape ofthe peds and the way in which they aggregate in a soil is referred to as soilstructure. Soil structure affects how easily air, water, and plant roots move throughsoil. Human activity such as repeated trampling or plowing when wet can alter thestructure.

Soils that separate easily into rounded peds are called granular. Granular soils havehigh permeability and therefore do not pack tightly. They are usually found near thesoil surface where organic matter is abundant. Granular soils are particularly suitablefor plant growth, because their structure permits air, water, and plant roots to easilypenetrate the soil. Clay and loamy soils often have blocky peds, which are angularand somewhat irregular in shape. Their irregularity ensures that soils composed ofblocky peds contain pores that permit passage of air and water. Soils withplate­shaped peds, which can resemble stacked sheets of ice, are tightly packedand difficult for air and water to penetrate. Platy soils usually have a high claycontent and tend to be found in frequently flooded areas. These soils are oftencalled “clay pan.” On the other hand, sand itself is a structureless soil; the primaryparticles do not aggregate but instead fall apart.

Soil Consistence

The degree to which soil resists pressure is referred to as it consistence. Form andconstruction machinery and even a herd of cattle can put a great deal of pressureon the soil, so consistence is important when considering how land should bemanaged. The terms sticky, loose, friable, soft, firm, very firm, and hard are usedto describe the consistence of soil and how well the soil resists effects of wind, waterand machinery.

Water­Holding Capacity and Capillary Action of Soil

The way in which water moves through and is held by soil is of critical importanceboth to plant life and human activities. Soil particles have electrically charged sites ontheir surfaces. Most of these sites are negatively charged, but some are positive.Water molecules, though not charged, are strongly polar and form hydrogen bondswith one another. When water molecules encounter perfectly dry soil particles, thestrong attraction of the electrically charged soil particles for the polar water moleculesresults in the spreading of a thin film of water molecules over the surface of the soilparticles.

This water is called adhesion water and is several molecular layers thick. Theadhesion water molecules rarely move and are thought to exist in an almostcrystalline array, similar to the structure of ice. Under normal conditions adhesionwater is always present in soil particles, even dust in the air. Adhesion water is notavailable to plants. To remove it requires oven heating.

Since water molecules are strongly attracted to one another by hydrogen bonding,the regular array of adhesion water molecules provides sites for additional watermolecules to associate. Further away from the charged soil particle, these moleculesare held only by hydrogen bonding. They have more energy than the adhesionwater molecule and move about more. This water is called cohesion water. Cohesionwater fills and coats coil particles about 10­15 molecules more thickly than adhesionwater and fills micropores because surface tension is sufficient to hold it there.Cohesions water constitutes the major source of water for plant growth.

Cohesion water and adhesion water are retained in the soil by forces that exceedgravity. Water in macropores, however is less strongly held and moves down andout of the soil unless an impermeable barrier stops it. Macropore water is calledgravitational water because gravity attracts it more strongly than does soil. Whensoil is saturated, as in the determination of total pore space, all its macropores arefull. If the soil is allowed to drain, the macropores will lose their gravitational water,but, the waters of cohesion and adhesion will be retained. In a field or pasture, oncethe gravitational water has been drained, the areas is said to be at field capacity;you will determine the water­holding capacity of samples of soils in small columns.

Water flow in soil

When soil is saturated, all pore space is filled with water. The water moves rapidlythrough larger pores and more slowly through smaller pores. The situation isanalogous to water flow from a tank on the roof of a house down though a pipe toa faucet. The flow is a function of the height of the roof and the size of the pipe.Water flow in pipes and pores is directly related to the fourth power of the radius.Thus water can flow 10,000 times faster through a pore with a radius 10 timesgreater than another one. If water draining down through a relatively permeable soillayer encounters a relatively impermeable layer, it will collect in the permeable layer.The rate of gravitational flow through soil is sometimes referred to as the infiltrationrate.

Soils with a high rate of infiltration are easily leached, losing nutrient ions from thelayer of soil where roots are the most abundant. Such soils can quickly becomeinfertile without the addition of chemical fertilizers. The leachates may containfertilizer salts and pesticides that pollute the water table and rivers.

In addition, water can move through soil independent of gravity. Since there is astrong attraction between soil particles and water molecules, water can be drawnupward from the water table by capillary action when the soil becomes dry. For thisto take place, soil particles must be packed closely enough to provide a continuousfilm of surface for water molecules to climb. On the other hand, as pore sizesbecome smaller, the rate of capillary movement will slow. Soil with a high rate ofcapillary action loses water more quickly through evaporation than does soil in whichcapillary action is slower.

Permeability and soil compaction

Soil compaction primarily affects macropores, and thus can have significant effectson permeability and drainage. One study showed the permeability of soil on a loggingroad in Washington was only 8% of the permeability of undisturbed soil. Byeliminating macropores, compaction reduces availability of both air and water toplants; by decreasing permeability, compaction also make soils more vulnerable towater runoff and erosion.

Permeability and septic tanks

Homes that are not connected to municipal sewage treatment systems rely onseptic tanks for purifying water from drains. A septic tank is a large, buried tank intowhich waste flow at one end. In its middle, a divided wall reaches nearly to the topof the tank. Since most solid material in the waste stream sinks, the dividing walltraps most of it in the first half of the tank. A rich community of bacteria breaks itdown. The mostly liquid waste that flows over the dividing wall settles again in thesecond chamber.

Near the top of the second septic chamber is a one­way outlet to a system of

underground, perforated pipes called field lines. The now mostly clear liquid from theseptic tank trickles out of the perforations into the soil. Soil microbes break downremaining organic material and ions are absorbed on the surface of soil particles. Asthe liquid drains through the soil, it is purified.

If soil into which a septic tank were placed were highly impermeable, the liquidtrickling from the field lines might not be absorbed into the soil and instead mightaccumulate and ooze out of the ground. Such a situation would obviously be highlyundesirable, if not potentially a health hazard. Consequently, health departmentsensure that any proposed septic tank site has adequate soil permeability.Sometimes this is done by considering the composition of the soil. Other times,health department workers go to the proposed site and conduct what is called aperc test.

Pre­lab Questions 1. Define the following:a. Porosity

b. Permeability

c. Water holding capacity

d. Solution

e. Suspension

2. What industries would find it important to know the structure of the soil?

3. Using what you know about North Carolina now, would large scale use of septic tanks work well?

4. Use the soil triangle to decide what type of soil the following are.a. 10% Clay, 60% Sand, and 30% Siltb. 60% Clay, 20% Sand, and 20% Siltc. 20% Clay, 20% Sand, and 60% Siltd. 20% Clay, 40% Sand, and 40% Silt

Procedure Determining Soil Texture

1. Using the soil assigned to your group, follow the directions to classify the type ofsoil you have. Then, record the results in the table below, including the percentagesof each type of soil.

Hydrologic Soil Analysis1. Using a mortar and pestle, crush enough soil to fill about 1/3 of the jar provided.2. Remove any pieces of rock larger than about 2mm (between 1/16” & 1/8”).3. Put the soil into the jar and fill it with water, leaving about an inch of space at the

top, and put the lid on the jar.4. Shake the jar for a couple minutes.5. Allow the contents of the jar to settle.6. After about a minute, put a line at the top of the sediment that settled first. This is

sand.7. After a couple hours put a mark at the top of the sediment that settled next

(someone from the group will have to come back later to do this). This is silt.8. After two days mark the top of the last later of sediment. This is clay (it will

probably not all settle, some of it can float in there for many days.)9. Measure the total height of the sediment in the jar in cm. Measure the thickness of

the sand, silt and clay layers and calculate the percentage of each. Record thesemeasurements in the table below.

10. Make sure to include a picture of your final results.11. Use the soil triangle to determine the type of soil you have. Compare this to what

you determined by feel.

Data Table 1: Soil CompositionTest Soil Type

Determined% Clay %Silt %Sand

Texture by feel

Hydrologic test

Determination of Permeability

1. Suspend the soil column about 1 inch above a beaker.

2. Put 100mL of water into a graduated. Designate one member as the timekeeperand one as the recorder. When everyone is ready, pour the water onto the wetsand. The timer should time how long it takes until the first drop of water comes outof the bottom of the column. Set up a data table to record this information. Watchthe column until there is not more water standing above the soil and no moredripping through the column. Record this time as well.

3. Repeat with the pebbles column.

Data Table 2 Problem: How does the porosity of a soil affect its permeability?Hypothesis:

*Calculate the permeability by dividing water recovered by flow time.

Material Flow time (s) Waterrecovered (mL)

Permeability(mL/s)

Sand

Pebbles

Questions 1. Considering all the samples analyzed by your class in the first threeparts, do you find any relationship between texture and composition?

2. Compare and contrast the texture analyzed by feel and hydrologically. How doyour results compare? Explain any discrepancies.

3. Do you believe one type of soil composition test (by feel or with water) is morereliable than the other? Explain your answer.

3. How might the composition of soil affect the growth of plants? Think about wetand dry conditions.

5. What characteristic of soil is most important in determining water holdingcapacity?

6. Imagine a sloping field of very sandy soil and a sloping field of soil with a very highclay content, each with an identical drainage ditch at the bottom. In a prolongedheavy downpour, do you think one ditch will be more likely to flood then the other?Why?

7. If you have two fields of crops, one in which the soil was mostly sand and theother mostly clay, which would you have to water most often and why?

8. Use the information you have collected about the local soil samples and suggesthow this would affect agriculture and building in the area.

Soil Analysis: Physical Methods