Soil Analysis Lab
Name _____________________________________
Per. _________- Date ___________
As you have found from your readings on soils, there is much complexity in understanding the
nature of soils and their capacity to produce healthy plant growth, and therefore support
ecosystems. You will be examining some of the physical and chemical features of soils which
soil scientists routinely analyze in this laboratory.
I.
Soil Texture is determined by the ratio of sand, silt, and clay in the sample. Sand,
silt, and clay are all mineral components of soil, and are defined by their particle
size. Particles with a diameter greater than 0.05 mm are considered sand;
between 0.002mm and 0.05 mm, silt; and less than 0.002 mm, clay. Soil scientists
group soil into three broad classes based on texture: the sands, the clays, and the
loams (a mixture of sand, silt, and clay). You will use a Fractionation Test to
determine your soil’s texture.
Fractionation Test
Soils tend to separate on the basis of particle size. This principle may be used to
determine the percentage of sand, silt, and clay in a soil sample.
Materials:
100 ml graduated cylinder
Soil sample
Parafilm
Procedure:
1. Fill a 100 ml graduated cylinder with 25 ml of your soil sample.
2. Add water until there is about 75 ml total volume in the cylinder.
3. Cover the cylinder with film and invert several times until the soil is
thoroughly suspended in the water. Place the cylinder in a location where it
will remain undisturbed overnight.
4. The next day, without disturbing the cylinder, examine the soil that has
settled. There should be 3 distinct layers. Record the volume of each layer
and the total volume of the soil sample (not water) in the data table and
calculate the percentage of each component. Using your soil triangle,
determine the soil texture.
Component
Sand
Volume (ml)
% of total
Silt
Clay
Total soil volume (ml)
By fractionation, Soil Texture = ___________________________________
Record your data on the board for comparison with other students’ samples. Indicate where the sample
was collected.
Analysis Questions:
1. What is the importance of soil texture to a farmer or gardener?
2. What texture do you think is the best for supporting plant growth and why?
II. Moisture Content of Soil
The following method for measuring the moisture content of soil involves comparing the mass of a
soil sample before and after it has been dried in an oven. From this information, the percent of
moisture can be calculated.
Materials:
Beaker or evaporating dish
Filter paper
Soil (50 – 75 g)
Drying oven set to 100oC
Procedure:
1.Determine the mass of the beaker.
Beaker = __________g
2.Add the soil to the beaker and determine the new mass. Beaker + original soil = __________g
3.Heat the beaker of soil in the oven at 100oC for approximately 24 hours.
4.After 24 hours, measure the mass of the beaker with the dried soil.
Beaker + dried soil = ___________g
Calculations:
Using the following relationships, calculate the percent of moisture in the soil sample.
In a – c you are subtracting and in d you are dividing!
a.
Mass of beaker + original soil
--mass of beaker
_________
= Mass of soil sample before drying
b. Mass of beaker + dried soil
--mass of beaker__________
Mass of dried soil
Record your results on the board.
c. Mass of soil sample before drying
-- mass of dried soil_____________
=mass of water in soil sample
d. Mass of water in soil sample X 100 = % moisture
Mass of dried soil
Moisture content = _____________%
Analysis Questions
1.
What factors determine the moisture content in soil?
2. Why is soil moisture important?
III.
Selected Soil Chemistry
pH is a measure of the concentration of hydrogen ions in a solution. It is measured in a scale that
runs from 0 to 14, with a pH of 7 representing a neutral solution. A solution with a pH below 7 is
acidic. The more acidic the solution, the lower the pH. A solution with a pH above 7 is basic
(alkaline). The more basic the solution, the higher the pH.
Many elements are more soluble in an acidic medium than in an alkaline. As a result, the pH of a
soil determines which elements will remain in the upper soil layers and which will be leached
through to the lower layers.
The elements nitrogen, phosphorus, and potassium are all essential to plant growth. Phosphorus
and potassium are released from minerals by weathering. Nitrogen is fixed from the atmosphere by
bacteria and other processes.
To test for pH and the elements listed above, you will use a Rapitest colorimetric kit. Follow the
directions on the sheet provided. Record your results below and on the board.
pH
Nitrogen *
Phosphorus*
Potassium*
*
IV. Water Holding Capacity (using oven-dried soil from Part II)
The water-holding capacity of soil is mainly dependent upon the amount of humus in the soil and the
size of the soil particles. Although some soils can absorb their own weight or more in water, ideally the
water content should be only 60-80% of the soil’s capacity. If soils contain less than 60% of their
capacity for water, there is not enough water for the cellular needs of many organisms. If soil contains
more than 80% of their capacity for water, there is too little oxygen available for the growth and activity
of many microorganisms.
Materials:
Can with both ends removed
Coffee filter
Rubber band
Procedure:
1. Attach the filter to one end of the can with a rubber band as shown in the diagram.
2. Determine the mass of the can with the paper attached.
A.
3.
Can + paper = ___________g
Place the oven-dried soil in the can and determine the new mass.
B. Can + paper + oven-dried soil = _____________ g
4. Slightly moisten the filter paper on the end of the can and determine the new mass.
C. Can + paper + oven-dried soil with moistened filter = ________________g
5.
Set the can in water so that the lower half is immersed and leave it overnight.
6. The next day remove the can from the water and transfer it to a rack where it can drain for
approximately 30 minutes. Wipe the surface of the can dry, blot the filter end carefully on a
paper towel (5 seconds), and determine the new mass.
D. Can + moist filter paper + moist soil = __________________g
Calculations:
Mass gained by water absorption in soil =
Mass of oven-dried soil =
D–C
B–A
Percent water-holding capacity of soil = mass gained by water absorption in soil X 100
mass of oven-dried soil
% water-holding capacity = __________________
Analysis Question:
1.
What do your calculations tell you about this aspect of your soil? How can your relate your
results to the composition of your soil?