DETERMINING SOIL TEXTURE The Soil Density Method
INTRODUCTION: The amount of sand, silt, and clay ultimately makes up the class of the soil. To determine the class type of an unknown soil we will have to determine the ratio of sand, silt, and clay particles in a specific volume of soil. Soil particles are categorized into groups according to size. Clay = less than 0.002 mm, Silt = 0.002 to 0.06 mm, Sand = 0.06 to 2.0 mm, Gravel = greater than 2.0 mm. We will be using comparative volumes to determine the ratio of these particles, based upon the fact that different sizes of particles will fall out of solution at different rates.
PURPOSE:
The purpose of this lab activity is to determine the amount of clay, silt, and sand particles in a given soil. Soil class will be determined, based on the USDA soil survey manual.
MATERIALS:
3 – 15 ml graduated centrifuge tubes Tube rack Soil dispersing agent - Calgon, powder* Soil samples
*Calgon contains a dispersing agent called sodium hexametaphosphate. In this process, the Na+ ions replace the polyvalent cations (usually Ca++) that form interparticle linkages; once the polyvalent cations are free, they react with phosphorus and precipitate out.
PROCEDURE:
1. Label the centrifuge tubes A, B, and C.
2. Break up the soil into individual particles and add soil to the level of the line marked 5 ml in centrifuge tube A. Tap the bottom of the tube gently against the table to pack the soil and eliminate any large air pockets. Add more soil, if necessary, to the 5 ml mark.
3. Add a few drops of the soil dispersing agent (Calgon) to the soil, and add tap water to the level of 15 ml.
4. Place the cap firmly on the tube. Holding the cap, shake the tube for two to 5 minutes. Make sure all the soil is mixed with the water.
5. Remove the cap and place the centrifuge tube in the stand for 30 seconds. The time is critical. If you allow more than 30 seconds to pass, shake the tube again and allow the tube to stand for another 30 seconds.
6. Carefully pour all the solution from the centrifuge tube A into centrifuge tube B (leaving the soil particles that settled out). Gently tap tube A on the table to level the soil left in the tube and return to the stand.
7. Allow tube B to stand undisturbed for 30 minutes. At the end of the 30 minute standing time, carefully pour the solution from centrifuge tube B into centrifuge tube C (again leaving the particles that settled). Centrifuge tube C.
8. Read the volume of soil particles, as accurately as possible, for tubes A, B and C. Record in Table 1. Particles in Particles in Particles in Sample % Sand % Clay % Silt Soil Type tube A (ml) tube B (ml) tube C (ml)
Table 1: Soil Types
CALCULATIONS:
The mineral particles in separation tube A are sand. They are the largest and heaviest particles. Therefore, they settle out first. The particles in separation tube B are silt. Since they are lighter than sand, they take longer to settle out. The particles remaining in the final tube are clay. Clay particles swell when placed in water, and they tend to remain in water. This tube is not an accurate indication of the amount of clay in the sample. The amount of clay is more accurately determined mathematically.
9. Calculate the Percent of Sand in your soil sample: volume in tube A(sand) divided by 5 ml (total sample) times 100 = ______% Sand.
10. Calculate the Percent of Silt in your soil sample:
Volume in tube B(silt) divided by 5 ml (total sample) times 100 = ______% Silt.
11. Calculate the Percent of Clay in your soil sample:
Add the volumes of tubes A and B then subtract that answer from 5 ml (total sample). This is the volume of clay.
Volume of clay divided by 15 ml times 100 = ______% Clay.
12. Now determine the soil type for your sample by comparing your answers in steps 9, 10, and 11, to the following table.
SOIL TYPE:
Sands: Soil that contains 85% to 90% or higher sand, with no more than 10% clay and with the rest silt.
Loamy Sands: Soil that contains between 70% to 85% sand and with clay being 14% or below.
Sandy Loams: Soil with 52% or more sand and 20% or less clay.
Loam: Soil with 7% to 27% clay, 28% to 50% silt, and less than 52% sand.
Silt Loam: Soil that contains 50% or more silt and 12% to 27% clay, or 50% to 80% silt and less than 12% clay.
Clay Loam: Soil that contains 27% to 40% clay and 20% to 45% sand.
Clay: Soil that contains 27% to 40% or more clay, and less than 45% sand and less than 40% silt.
Soil type for sample = ______
SOIL LAB WATER-HOLDING CAPACITY INTRODUCTION: Water-holding capacity is dependent upon the organic matter in the soil and soil particle size. Normal field soil water-holding capacity is 60-80% of its total capacity; that is, 60-80% of the water filled pore spaces are filled. This corresponds to the optimal biological activity for water-holding capacity. When the water-holding capacity falls below 55-60%, organisms could suffer from dryness; and when the capacity is over 80%, they begin to suffer from a depletion of soil oxygen.
PURPOSE: The purpose of this experiment is to determine the water-holding capacity of your soil sample.
MATERIALS:
Soil sample (oven dried) Humid chamber (a bell jar or similar will work) Shallow pan Hilgard soil cup (make from vegetable can and screen wire) Spatula Balance Filter paper
PROCEDURE:
1. Record on the data table the soil sample number.
2. Place a Whatman #2 filter paper on the screen inside the Hilgard soil cup (as illustrated) and find the mass to the nearest 0.01g. Record on Table 2 the mass of the cup and filter paper.
3. Fill the cup gently with oven dried soil and find the mass to the nearest 0.01g. Record on Table 2 the mass of the cup, filter paper, and dry soil sample.
4. Place the cup into a shallow pan of water allowing only the bottom few centimeters of the cup to become wet.
5. Allow the soil to become saturated from the bottom of the cup to the surface.
6. Remove the cup from the pan of water and place it in a humid enclosure until drainage is complete. Find the mass and record on Table 2 the mass of the cup, filter paper, and saturated soil sample.
Mass of cup Mass of cup, Mass of cup filter paper, filter paper, Mass of Mass of water Percent water Sample and filter Mass of dry and dry soil and saturated saturated soil in saturated holding paper (g) soil (g) sample (g) soil sample (g) (g) soil (g) capacity Table 2: Water Holding Capacity
CALCULATIONS:
7. Calculate the mass of the dry soil by the following:
mass of cup, filter paper, and dry soil - mass of cup and filter paper
Mass of dry soil = ______g
8. Calculate the mass of the saturated soil by the following:
mass of cup, filter paper, and saturated soil - mass of cup and filter paper
Mass of saturated soil = ______g
9. Calculate the mass of the water contained in the saturated soil by the following:
mass of the saturated soil - mass of the dry soil
Mass of water contained in saturated soil = ______g
10. Calculate the percent water holding capacity by the following: mass of the water contained in the saturated soil X 100 mass of the saturated soil
Percent water holding capacity = ______%
DETERMINING SOIL ORGANIC MATTER CONTENT BY IGNITION INTRODUCTION: Soil organic matter (SOM) is an extremely important part of any soil because it influences the plant, animal, and microbial life in the soil. Soil OM influences the water holding capacity of the soil as well as the rate of water loss through percolation. In addition, OM is an important reserve for nutrients, especially, N, P, and S. In most mollisol soils, the organic matter content is between 1-6%, and its level of presence in soil is an indicator of the amount of plant (and animal) residue that is returned to the soil. The black color in surface soils is mostly from humus. If you burn the humus off of several soils by heating them red hot over a bunsen burner or on a stove, you will see the iron color (tan, yellow, or red) that was hidden by the humus. Humus is a very stable form of organic matter that persists in the soil for long periods of time; its turnover rate can be quite long, often up to 100 years.
PURPOSE: The purpose of this activity will be to determine the SOM in the soil.
MATERIALS:
Crucible and lid Bunsen burner Clay triangle Tongs Ring stand Desiccator Oven-dried soil sample
PROCEDURE:
1. Pre-heat a crucible, allow it to cool in a desiccator to room temperature and find its mass to the nearest 0.01g. Record on Table 3 the mass of the crucible and lid.
2. Fill the crucible 1/2 full of oven dried soil, and find the mass to the nearest 0.01g. Record on Table 3 the mass of the crucible, lid, and soil sample before heating.
3. Place the crucible and contents on the triangle and heat with a moderate flame. Increase the temperature until a red glow is evident within the crucible. Rotate the crucible periodically. Maintain the red glow for 30 minutes. After the 30 minutes, remove the crucible from the flame and allow to cool in a desiccator to room temperature.
4. When cool, find the mass to the nearest 0.01g and record on Table 3 the mass of the crucible, lid, and soil sample after heating.
Mass of Mass of Mass of crucible, lid Mass of soil Mass of soil Mass of Percent crucible, lid Sample crucible and and soil before after heating organic organic and soil after lid (g) before heating heating(g) (g) matter g) matter (g) heating (g)
Table 3: Soil Organic Matter
CALCULATIONS:
6. Calculate the mass of the soil sample before heating. mass of crucible, lid, and soil sample before heating - mass of crucible and lid
Mass of soil sample before heating = ______g
7. Calculate the mass of the soil sample after heating. mass of crucible, lid, and soil sample after heating - mass of crucible and lid
Mass of soil sample after heating = ______g
8. Calculate the mass of the organic matter. mass of the soil sample before heating - mass of the soil sample after heating
Mass of organic matter = ______g
9. Determine the percent organic matter by the following equation.
mass of organic matter % organic matter = / mass of soil sample before heating x 100.
Percent organic matter = ______%
SOIL LAB SOIL pH
INTRODUCTION: As plants, animals, and microbes inhabit the soil, their actions on the soil, along with the effect of physical factors, will change the soil solution and therefore the soil pH. It is important that we have a basic understanding of the rain that comes down and waters the soil. Rain water is not necessarily at pH 7.0. Because it picks up CO2 as it passes through the atmosphere, it becomes acidic as the CO2 reacts with water to form carbonic acid (CO2 + H2O = H2CO3). Since air has always contained CO2, rain has always been acid. Modern day rain can be pH 4.6 or lower because we contaminate it with sulfur dioxides and nitrogen dioxides which for sulfuric and nitric acids and give it additional acidity. PURPOSE: The purpose of this lab activity is to determine the pH of a given soil using a pH meter.
MATERIALS:
Soil samples Balance 100-ml beaker pH meter with probe
PROCEDURE:
1. Weigh out 20g of dry soil in a 100-ml beaker.
2. Add 20 ml of distilled water and stir intermittently for 20 minutes allowing for the paste to reach equilibrium.
3. Measure the liquid phase of the soil-water mixture using the pH meter. Record your results in Table 4.
Sample pH
Table 4: Soil pH