Get Down and Dirty: How Does Soil Change with Depth?

1. Select your sites.
2. Describe each site in your lab notebook. Include in your description:
   1. slope,
   2. landscape position (e.g., hillside, summit, large flat area),
   3. cover type (e.g., trees, grass),
   4. land use (e.g., forest, lawn, beach),
   5. parent material (underlying bedrock, if you know it—a county soil survey may be available and would be helpful here).
3. Take soil core samples at each site.
   1. Have your helper hold the PVC pipe vertically, with one end firmly on the ground where you want to sample the soil.
   2. Hold the wood block in place on top of the PVC pipe, to distribute the force of the blows from the hammer or mallet. (Keep your fingers out of the way!)
   3. Pound the PVC pipe into the soil, until there is about 6 in remaining above the surface. (Alternatively, you can take the soil cores out in smaller chunks, carefully assembling them in order.)
   4. Holding the pipe straight up and down, rotate it back and forth to loosen the pipe in the hole.
   5. Pull up the pipe with your soil core sample inside.
   6. Use the 1 in dowel to carefully push the soil core sample out of the pipe. Use paper plates or sheets of plain paper as a background for examining the soil core samples.
4. Identify soil horizons. Use all of the information at your disposal:
   1. soil color and texture,
   2. root depth,
   3. evidence of worms and other soil organisms, etc.
   4. This is a good time to use your magnifying glass!
5. Using golf tees or nails (or markers on the background paper), mark the soil horizon depths.
6. Measure the horizon depths (top and bottom of each horizon) and record them in your data table (see example).
7. For each soil horizon, use the table to identify the soil structure and record it in your data table (GLOBE, 2005a).
   1. Use a trowel or other digging device to remove a sample of soil from the horizon being studied.
   2. Hold the sample gently in your hand and look closely at the soil to examine its structure. This is a good time to use your magnifying glass!
   
   

   Granular: Resembles cookie crumbs and is usually less than 0.5 cm in diameter. Commonly found in surface horizons where roots have been growing.
   Blocky: Irregular blocks that are usually 1.5–5.0 cm in diameter.
   Prismatic: Vertical columns of soil that might be a number of cm long. Usually found in lower horizons.
   Columnar: Vertical columns of soil that have a white, rounded salt "cap" at the top. Found in soils of arid climates.
   Platy: Thin, flat plates of soil that lie horizontally. Usually found in compacted soil.
   In certain cases, soil samples may have no structure. These would be classified as either "Single-Grained" or "Massive."
   Single-Grained: Soil is broken into individual particles that do not stick together. Always accompanies a loose consistence. Commonly found in sandy soils.
   Massive: Soil has no visible structure, is hard to break apart and appears in very large clods.

   Step 7. Identifying Soil Structure
   
   
8. For each soil horizon, identify the soil color by matching to the swatches in a soil color book (optional, but preferable), or by using descriptive color terms. Photographs would also be useful here for your display board.
   1. Take a ped (a ped is a small, naturally-occuring aggregate of soil—in other words, a small "lump" of soil) from the horizon being studied and note whether it is moist, dry or wet. If it is dry, moisten it slightly with water from your spray bottle.
   2. Break the ped and hold it next to the color chart.
   3. Stand with the sun over your shoulder so that sunlight shines on the color chart and the soil sample you are examining.
   4. Find the color on the color chart that most closely matches the color of the inside surface of the ped.
   5. Record the color of the ped in your data table, in the row for the appropriate soil horizon. Sometimes, a soil sample may have more than one color. Record a maximum of two colors if necessary, and indicate (1) the dominant (main) color, and (2) the sub-dominant (secondary) color.
9. For each soil horizon, use the table, Step 9, to identify the soil consistence (GLOBE, 2005a).
   1. Take a ped from the soil horizon being studied. If the soil is very dry, moisten the face of the core sample by squirting water on it, and then remove a ped for determining consistence.
   2. Holding the ped between your thumb and forefinger, gently squeeze it until it pops or falls apart.
   3. Record your findings for each soil horizon in your data table.
   
   

   Loose: You have trouble picking out a single ped and the structure falls apart before you handle it. Note: Soils with Single-Grained structure always have Loose consistence.
   Friable: The ped breaks with a small amount of pressure.
   Firm: The ped breaks when you apply a larger amount of pressure and the ped dents your fingers before it breaks.
   Extremely Firm: The ped can't be crushed with your fingers (you need a hammer!)

   Step 9. Identifying Soil Consistence
   
   
10. For each soil horizon, identify the soil texture using the instructions and the soil textural triangle, (GLOBE, 2005a).
    Soil Texture, Step 1
    1. Place some soil from a horizon (about the size of a small egg) in your hand and use the spray bottle to moisten the soil. Let the water soak into the soil and then work it between your fingers until it is thoroughly moist. Once the soil is moist, try to form a ball.
    2. If the soil forms a ball, go on to Soil Texture, Step 2.
    3. If the soil does not form a ball, call it a sand. Soil texture is complete. Record the texture for this horizon in your data table.
    Soil Texture, Step 2
    1. Place the ball of soil between your thumb and forefinger and gently push and squeeze it into a ribbon.
    2. If you can make a ribbon that is longer than 2.5 cm, go to Soil Texture, Step 3.
    3. If the ribbon breaks apart before it reaches 2.5 cm, call it a loamy sand. Soil texture is complete. Record the texture for this horizon in your data table.
    Soil Texture, Step 3
    1. If the soil:
       • is very sticky,
       • is hard to squeeze,
       • stains your hands,
       • has a shine when rubbed,
       • forms a long ribbon (> 5 cm) without breaking,
       then call it a clay and go to Soil Texture, Step 4.
       
       
    2. Otherwise, if the soil:
       • is somewhat sticky,
       • is somewhat hard to squeeze,
       • is at most slightly sticky,
       • forms a medium ribbon (between 2 and 5 cm) before breaking,
       then call it a clay loam and go to Soil Texture, Step 4.
       
       
    3. Otherwise, if the soil:
       • is smooth,
       • is easy to squeeze,
       • is at most slightly sticky,
       • forms a short ribbon (< 2 cm) before breaking,
       then call it a loam and go to Soil Texture, Step 4.
       
       
    Soil Texture, Step 4
    1. Wet a small pinch of the soil in your palm and rub it with a forefinger. If the soil:
       • Feels very gritty every time you squeeze the soil, go to b.
       • Feels very smooth, with no gritty feeling, go to c.
       • Feels only a little gritty, go to d.
         
    2. Add the word sandy to the initial classification.
       • If you've reached this point, the soil texture should be one of the following:
         • sandy clay,
         • sandy clay loam, or
         • sandy loam.
       • Soil texture is complete. Record the texture for this horizon in your data table.
         
         
    3. Add the word silt or silty to the initial classification.
       • If you've reached this point, the soil texture should be one of the following:
         • silty clay,
         • silty clay loam, or
         • silt loam.
       • Soil texture is complete. Record the texture for this horizon in your data table.
         
         
    4. Leave the original classification.
       • If you've reached this point, the soil texture should be one of the following:
         • clay,
         • clay loam, or
         • loam.
       • Soil texture is complete. Record the texture for this horizon in your data table.
         
         
    
    Figure 1. Soil Textural Triangle. This diagram shows the approximate percentages of sand, silt, and clay in the soil, according to how the soil feels in your hands. See instructions for step 10, above.
    
    
11. For each horizon, observe and record if there are none, few or many rocks. Use the #10 sieve to sift a soil sample from the horizon. Rocks will remain behind in the sieve.
12. For each horizon, observe and record if there are none, few or many roots. Use the #10 sieve to sift a soil sample from the horizon. Roots will remain behind in the sieve.
13. For each soil horizon, measure the free carbonates.
    1. Set aside a portion of the soil to use for the free carbonates test. Make sure not to touch it with your bare hands.
    2. Use the eyedropper (or disposable squeeze pipette) to add several drops of vinegar to the soil sample.
    3. Look carefully for the presence of effervescence (bubbles). The more free carbonates that are present, the more effervescence you will observe.
    4. From your observations, record in your data table one of the following notations:
       • None: if you observe no reaction, the soil has no free carbonates present.
       • Slight: if you observe a very slight bubbling action; this indicates the presence of some carbonates.
       • Strong: if there is a strong reaction (many, and/or large bubbles) this indicates that many carbonates are present.
14. Take a photograph or make a drawing of your soil profile. Here are some tips for getting a good photograph.
    1. Use a marker and a blank sheet of paper to make a sign that identifies the site. Take a photograph of the sign (get close enough so you can read it) so that you'll know where the following pictures came from.
    2. Place a tape measure or meter stick starting from the top of the soil profile next to where the horizons have been marked.
    3. With the sun at your back, photograph the soil profile so that the horizon markers and their depths (on the tape measure or meter stick) can be seen clearly.
    4. Take another photograph of the landscape around your sampling site.
15. Here is a sample data table.

Interpreting Your Data

How do the soil profiles compare between the different sites? What do they tell you about the differences in environmental history between sites?

Note: the following suggestions are taken from the Soil Characterization Protocol (GLOBE, 2005a).

• Soil Horizons. It is unlikely that large numbers of distinct horizons will be found in very young soils (recently deposited, or close to bedrock), or very highly developed soils (such as are found in tropical regions). More horizons are found in temperate climates under forest vegetation.
• Color. Dark colored soil is usually found at the surface, unless there has been intense leaching of organic material, such as in a coniferous forest, or deposition has occurred where new parent material has been deposited on top of a soil profile that was already developed.
• Texture. In general, soil texture is similar as you go deeper into the soil, with a gradual increase in clay. If there is a very sharp difference in texture (such as a clayey soil over a very sandy soil) this may also be an indication of a different parent material due to deposition. This may occur if you are in an area near a stream where flooding is common, or where human activity has disturbed the soil and fill has been added.
• Structure. Granular structure is generally found where there are many roots. Soils with high amounts of clay typically have blocky or massive structure.
• Consistence. When soil has single-grained structure, the consistence is always loose and the texture is usually sand or other very sandy texture such as loamy sand. Testing for the bulk density of the soil can act as a check for the consistence since the denser the soil, the more firm the consistence will be.
• Roots. Bulk density should be lower when there are many roots in the soil that add pore space to the horizon.
• Carbonates. If free carbonates are present, the pH should be 7 or above since high amounts of calcium carbonate decrease the soil acidity and increase the pH.