AP Biology
Soil Properties
Names____________________________________
Per.______________
How do the physical and chemical properties of soil affect soil quality? What does soil quality actually
mean? In this lab you will explore the physical and chemical properties of soil samples obtained locally.
Soil texture describes the relative amounts of sand, silt and clay in a mass of soil—it is one of the most
important indicators of soil quality. The texture of soil determines how coarse or fine the soil is, its
porosity and permeability, and the capacity to store nutrients and bind waste products. Soil is classified
into three categories based on their grain size: sand, silt, and clay. Sandy soils have excellent drainage
and lots of air spaces, but they do not bind nutrients or support root growth. Sandy soils feel dry and
gritty, and nutrients leach out quickly. Clay soils on the other hand consist of microscopic particles that
clump together and retain water. Soils with high clay content are easily waterlogged and have a
tendency to exclude air and become anaerobic, killing off the living organisms that are a necessary part
of healthy soil. Clay has a large surface area, however, and is chemically very active, binding and storing
both mineral and organic nutrients. The most productive soils have a balance of sand, silt, and clay and
are called loams or loamy soils. (“Rich” soils also contain high concentrations of organic matter).
The USDA has identified 12 main textural classes of soil based on the percentages of clay, sand, and silt.
The textural class is determined using a three-sided graph called the soil texture triangle. Each side of
the triangle represents one of the soil separates on a scale from 0 to 100%. The graph is read by
following the clay percent line parallel to the triangle base, the sand line parallel to the right side of the
triangle, and the silt line parallel to the left side of the triangle.
Part A: Physical Properties of Soil
The physical properties of soil texture will be determined by measuring the heights of the sand, silt, and
clay layers at the appropriate time intervals after mixing soil with “softened” water. The rate at which
the soil particles settle when mixed depends on their size. Large sand particles settle out quickly, within
a minute or so. Silt particles generally settle within 30 minutes, while tiny clay particles may take 24
hours to settle. Dividing the height of the respective soil layer by the combined height of all three layers
gives the percentage of each component.
1. Using a plastic spoon, add about 10 cm3 of air-dried soil to a plastic snap-seal vial. Gently tap the
vial on the table or countertop to eliminate air space and pack the soil down in the tube.
2. Carefully add 40 mL of distilled water to the vial.
3. Using a graduated Beral-type pipet, add 1 ml of sodium hexametaphosphate solution to the vial.
This is the softening agent which causes the colloidal clay particles to clump and settle.
4. Cap the vial and snap securely to prevent leakage. Shake vigorously for two minutes to
thoroughly mix contents of the vial.
5. Place the vial on the lab bench and immediately start timing. Set the vial down in a convenient
place for making observations and to measure the soil layers but where the vial will not be
disturbed for 24 hours. Do not jostle the vial
6. After EXACTLY one minute, measure the height in mm of the sand layer that has settled to the
bottom of the vial. Record the measurement in the data table for part A.
7. After 30 minutes, measure and record the combined height in mm of the sand and silt layers
8. Next day: After 24 hours, measure and record the total height of the clay, sand and silt layers.
Record the color and appearance of the water solution on top of the soil. Note: the clay will
probably look “congealed” and is usually lighter in color than the other layers.
Post Lab Questions, Physical Properties
1. Calculate the percentages of sand, silt and clay in the soil sample. Divide the height of each
respective soil layer by the combine height of all three layers and multiply by 100.
2. Using the soil texture triangle identify the soil texture class.
Part B: Chemical Properties of Soil
The chemical properties of soil will be evaluated by measuring the pH and testing for the presence of
macronutrients. The levels of these soil quality indicators are determined by mixing the soil sample with
water and analyzing the resulting solutions.
Soil pH influences the solubility and availability of soil nutrients, the viability of essential
microorganisms, and the movement of toxic heavy metals into groundwater. A pH range between 6 and
7 is ideal for most plants. When soil is too acidic (<5.6), the plants cannot utilize the nutrients they need,
and excessive amounts of aluminum and iron, which are harmful to plants, dissolve into the soil
solution.
The elements carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, and sulfur are
considered macronutrients because plants need them in large amounts. Of these, C, H, and O come from
the atmosphere and Ca, Mg, and S come from the mineral content in the Earth. The nutrients that are
most likely to be missing are N, P and K. These elements are commonly added to soils in the form of
fertilizers. Nitrate ions are the most common source of nitrogen for plants. Nitrate levels in soil of 10-25
ppm are considered optimal for agriculture. Adequate levels of phosphorus (2-4 ppm) are especially
important for root crops. In addition to fertilizers, other sources of nitrates and phosphates in soil
include decaying vegetation, human and animal waste products, and industrial waste discharge.
Nitrate and phosphate fertilizer runoff is a serious problem in some areas. Nitrates do not bind to the
soil, and therefore end up passing down through the soil or being washed away by rain, eventually
ending up in ground water or surrounding bodies of water, respectively. Excess phosphate ions added to
the soil precipitate in the form of insoluble calcium phosphate, which binds to soil particles and washes
away due to erosion or irrigation run off.
1. Obtain TesTab tablets for PH, nitrate and phosphate testing
2. Mark the 1-mL level in each test tube: measure 1 mL of water in a graduated cylinder and add
the water to one of the test tubes. Using a permanent marker, draw a line on the test tube to
mark the 1-mL level. Measure and add 9 mL of water to the test tube and draw a second line to
mark the 10-mL level. Hold two test tubes side-by-side with the marked test tube and draw lines
for the 1-mL and 10-mL levels on each. Discard the water.
3. Using a clean spatula or plastic spoon, add soil to the 1-mL level in each test tube and label the
test tubes pH, N, and P.
4. Add distilled water to the 10-mL mark in the pH test tube.
5. Stopper the test tube and shake vigorously for 30 seconds. Place the test tube in a test tube rack
and allow 2-3 minutes for the soil to settle.
6. Compare the color of the liquid in the pH test tube to the colors on the pH Color Comparison
Chart. Record the approximate pH value in the data table for part B. Rinse the stopper with
water.
7. Using a graduated Beral pipet, add 1 mL of vinegar to both the N and P tubes.
8. Add distilled water to the N and P test tubes until the liquid level in each is at the 10-mL mark.
Stopper the test tubes and shake vigorously for one minute. Rinse the stopper with water and
repeat for the P test tube.
9. Place the test tubes in a test tube rack and allow 3-5 minutes for the soil to settle.
10. Decant 5 mL of liquid from the N test tube into a clean test tube and add a nitrate TesTab tablet
to the clear liquid. Stopper the test tube and shake vigorously for at least one minute or until
the tablet dissolves completely.
11. Place the test tube in a test tube rack and let it sit undisturbed for 5 minutes. Compare the color
of the liquid to the colors of the Nitrate Color Comparison Chart. Record the nitrate
concentration in the data table.
12. Repeat steps 11 and 12 with the P test tube, using a phosphate TesTab tablet and the Phosphate
Color Comparison Chart. Record the phosphate concentration in the data table.
Post Lab Questions, Chemical Properties
1. Describe the quality of soil using your results from Part B:
a. Is the pH of the soil acidic or basic?
b. Are the nitrate and phosphate levels suitable for plant growth?
c. If you were a farmer planning to plant crops in this soil, would fertilizer be necessary?
Why or why not?
2. Why do excess nitrates and phosphates from fertilizers often end up as runoff in natural bodies
of water and groundwater? Why is this problematic?
3. Using the information learned in this activity, explain why few plants grow on the beach or in a
sandbox