Soil Studies Lab

Soil is a renewable resource composed of unconsolidated mineral material (clay, silt, sand, and pebbles), decomposing organic matter, water, air, microbes and detritvores. The mineral material comes from the weathering of rock and sediments deposited by erosion via wind, water, ice and gravity, and is influenced by climate and topography. The physical properties of soil is dependent on the mixture and size distribution of mineral particles comprising the soil.

Soil is also a vital component of the hydrologic cycle. It acts as a natural filter by absorbing some chemicals that may be applied, such as fertilizers, herbicides, pesticides, and industrial waste chemicals. By filtering these products, the soil helps protect against groundwater contamination. The ability of soil to act as a natural filter is dependent on the mixture of particles in the soil, its pH, amount of organic matter, and the presence of microbes.

In this lab you will analyze some of the chemical and physical properties of soil.

Materials: soil auger, soil sample, hand lens, 2 100ml graduated cylinders, 2 250ml beakers, ruler, soil test kits (ph, nitrogen, phosphorous, potassium), Ziploc bag, sieve, sterile water,  25ml and 50ml conical tubes, nutrient broth, nutrient agar, vortex, pipettes, sterile swab, incubator, 10% bleach, newspaper, soil invertebrate ID key, Berlese Funnel, cotton, 16oz plastic water bottle, methylene blue dye solution, eosin y dye solution, aluminum foil.

Collection of Soil and Observation of Soil Profile

1. Collect a soil sample using the soil auger as demonstrated by instructor. Place the sample in a sealable plastic bag.
2. Observe the cross-section of the soil diagram that shows the soil profile. The profile is composed of layers or horizons. The top layer or O Horizon is the surface litter layer. It is comprised of freshly fallen and partially decomposed leaves, twigs, animal waste, fungi and other microbes. Active decomposition occurs here and this material is a primary energy source for the soil ecosystem. Below the surface litter is the topsoil layer or A Horizon. This layer is dark and rich in humus (decomposed organic matter). Humus is loose and spongy and binds the sand, silt and clay particles. It helps hold water and nutrients in the soil. This layer, along with the O layer, is abundant with detritivores and decomposers, and is where the roots of most plants are found. The next layer in most soils is the subsoil or B Horizon. This layer is typically lighter in color than the topsoil layer, and is composed of broken down rock, and a mixture of gravel, sand, silt and clay. It is often called the zone of illuviation because it collects leached minerals. The color is sometimes reddish yellow because of an accumulation of iron, aluminum and clay. The bottom layer of the soil profile is the parent material or the C Horizon. This layer is composed of weathered parent rock. It is sometimes saturated with groundwater and is below root level. Some soils contain an extra layer between the A and B horizons, the E Horizon or eluviation layer. This is a zone of leaching where the downward movement of water pulls soluble minerals into the B layer. It is often light in color.
3. Measure each layer of your soil profile. Draw a diagram and label each layer. Indicate the measurement of each layer and identify the color of each. List the components of each layer.

Identification of Biotic Components

The soil ecosystem is rich in detritivores and decomposers. Their primary role is to decompose the detritus, release nutrients into the soil, and to mix and aerate the soil. The CO₂ released during their respiration enhances the weathering process. They include a wide variety of bacteria, fungi, protozoans, and invertebrates such as sowbugs, millipedes, mites, beetles, ants, spiders, earthworms, roundworms, insect larvae, snails and slugs. A single gram of soil contains hundreds of millions of microbes!

Soil Invertebrates

1. Take the sample back to lab and place it on a piece of newspaper. Look closely at the sample and remove any visible living things such as worms and insects. Use the soil invertebrate key to identify the critters.
2. Place a sample of the soil in the Berlese funnel apparatus under a heat lamp. Observe after 24 hours.
3. Record the invertebrates found and identify the roles that they play in the soil.
4. How do the surface detritus and soil organisms contribute to the formation and characteristics of the topsoil?

Bacterial & Fungal Culture of Soil Sample

1. Obtain a Ziploc bag and add a few spoons full of soil.
2. Mix sample massaging the bag several times.
3. Sift sample through a small sieve to remove large particles.
4. Weigh out 1g of sample and place it in a 50 ml conical tube.
5. Add sterile water to the 25ml mark on the tube.
6. Shake vigorously for 1 minute to disperse the microbes.
7. Vortex the tube for 1 minute.
8. Add 4 ml of nutrient broth to a 25 ml conical tube.  Add 1ml of your soil prep from the 50 ml tube.
9. Mix by inverting the tube several times.
10. Add 0.5 ml of your dilution mix to each of 2 agar plates.  Use a sterile Q-tip to spread across the plate.
11. Label the plates around the edge of the bottom of the plate with your initials.  Cover with aluminum foil and place in a 25C incubator upside down. Observe after 24-48 hours.
12. Clean lab bench with 10% bleach and wash your hands!
13. What did you find growing on the plates?

Soil Texture

Soil texture is the way a soil feels and measures the proportions of each mineral portion of the soil. The texture is dependent on the amount of each size particle in the soil. Soil is made of a mixture of 3 different size particles: clay, silt and sand. Clay is the smallest particle size (<0.002mm) and feels sticky. Silt is medium sized (0.2 - 0.002mm) and feels soft and silky. Sand is the largest particle size (2 - 0.2mm) and feels gritty. Large particles allow empty space for air and water to enter the soil, and smaller particles help to hold water and nutrients in the soil. Sandy soils feel gritty and are characterized by good drainage and aeration, but do not bind nutrients or support root growth. They tend to leach nutrients out quickly. Silty soils are less permeable to air and water, but have a good capacity to hold mineral nutrients. Clayey soils are tightly packed soils with good water and nutrient holding capacity because of the small particle size and greater surface area. The large surface area makes clayey soils chemically active because it allows them to bind and store both mineral and organic nutrients. High clay content soils however are easily waterlogged and have a tendency to exclude air and become anaerobic. Loam is the most desirable agricultural soil and is composed of 20% clay, 40% sand, and 40% silt. (See the Texture Table below.) The soil texture determines the porosity and permeability of the soil, the nutrient and water holding capacity, the aeration, workability, and infiltration.

1.   Feel a sample of your soil by squeezing it through your fingers. If you can ribbon the soil, you have a clayey soil. What does your soil feel like? Does it ribbon?

2.  Fill a 100 ml graduated cylinder with 25 ml of soil.

3.  Add water until it reaches the 75 ml line.  Cover with parafilm.

4.  Agitate the cylinder vigorously for at least 1 minute or until the soil is thoroughly suspended in the water.

5.  Let the sample stand over night.

6.  When sample has settled out, measure the volume of each layer and the total volume of the sample. Record these values.

7.  Calculate the percent of each of the components (clay, silt and sand) and record results.

8.  Identify the type of soil in the sample from the soil texture triangle below. Each side of the triangle represents one of the three components, silt, clay or sand, on a scale from 0% to 100%. The graph is read by following the clay % 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.

Porosity measures the volume of pore space in the soil. Pore space between soil particles can occupy 35-60% of the soils volume. It fills with water and air. The water dissolves soluble minerals and is absorbed by plant roots. The air and oxygen is required for cellular respiration by soil organisms. Porosity and texture together determine soil permeability or the rate at which water and air moves through the soil from the upper layers to the lower layers. Typically, the finer the texture and the lower the porosity; the slower the permeability will be. Together, texture, porosity and permeability determine the water holding capacity, the nutrient holding capacity, the workability, the ability of the soil to hold air (aeration), the ability of water to penetrate the surface of the soil (infiltration), and the ability of water to move through the soil by gravity (percolation).

Determine Porosity

1. Fill 2 - 250 ml beaker to the 200 ml line with dried soil. Tamp the soil down gently, but do not compress it.
2. Fill a 100 ml graduated cylinder to the 100 ml mark with water. Slowly pour the water onto the surface of the soil until the soil is completely saturated and water just starts to pool up on the surface. Add the water slowly enough to give the water a chance to percolate down into the pores.
3. Measure the amount of water left in the graduated cylinder. The amount used is the amount of pore space in your sample. Record volume of soil and volume of water used.
4. Calculate porosity as a percent: % = (volume water added/200ml of soil) x 100.
5. Based on the texture and porosity data and the following tables, determine the relative permeability, water infiltration capacity, water holding capacity, nutrient holding capacity, percolation, aeration, and workability of the soil sample.


Average permeability for different soil textures in cm/hour

Soil Permeability


Organic Matter

Soil color is an indication of the amount of organic material present. Dark brown and black soils contain a high amount of organic matter. Brown to yellow-brown denotes a moderate amount of organic material, and pale brown to yellow denotes a low organic content. White colors indicate the presence of salts or carbonates, mottled colors indicate poor aeration, and blue, gray or green tingled soils indicate that the soil is water logged.

1. Identify the color and relative organic content of your soil sample. What does the soil smell like?
2. Why is it important to have organic material in the soil?


Soil Fertility

Soil fertility characterizes the ability of the soil to support plant growth. In order to grow, plants require sunlight, water and essential minerals obtained from the soil. These minerals include N, P, K, Ca, Mg, S, Cl, Fe, Mn, Cu, Zn, Mb and B. Of these minerals, the 3 primary nutrients required by plants are N, P and K.

There are 4 main factors that determine soil fertility: pH, the amount of nitrogen, the amount of phosphorous, and the amount of potassium. Acidic soils typically have lower fertility than basic soils because H+ ions in the acid displace the positively charged nutrient ions. These nutrients can then be leached from the soil into the groundwater. Normal pH for soil is between pH 4 - 8, but the uptake of N, P and K occurs more readily if the pH is between pH 5.5 - 7.5. Optimum soil pH varies for different types of plants. In acidic soils (less than pH 5), plants are more likely to uptake toxic metals, such as aluminum, iron and manganese that can kill the plants. In acidic soils, applied pesticides, herbicides and fungicides will not be absorbed or held in the soil, but will have a tendency to either runoff the land with rain water or percolate into the groundwater.


Nitrates are usually stored in the soil in the organic matter. Normal levels are between 60 -175ppm. Nitrates do not bind to soil particles and thus can easily be leached from the soil into the groundwater, especially during heavy rain. Phosphates in soils tend to cling to the surface of clay particles and organic matter, and are quickly absorbed by plants. High levels of phosphates can accumulate in the top layers of soil in the form of insoluble calcium phosphate, and subsequently can runoff into surface water producing phosphate rich sediments. Normal levels are between 5 - 15ppm. Normal potassium levels are between 75 - 200ppm.


1. Use the soil kits to test the pH, nitrogen, phosphorous and the potassium content of your soil sample. Record results.
2. Is the pH within the normal range?
3. What could you add to a soil that is too acidic? To a soil that is too basic? (Be specific)
4. Are any of the nutrients deficient in your sample? How could you increase the fertility of the soil and at the same time build the topsoil layer and quantity of humus?


Pollutants in the Soil

The motion of water and pollutants through soil is heavily influenced by the properties of the soil itself. Certain soils are very likely to trap and retain pollutants over long periods of time, while others provide for a great deal of vertical motion for both water and pollutants.


1. Cut the neck off four 16 oz water bottles. Invert the neck in the bottles to act as funnels. Plug the necks with a piece of cotton.
2. Fill the necks of two bottles with the soil sample to 1 cm from the top. Likewise fill the other two bottles with sand. Set the neck or funnel of each bottle into the bottom part of the bottle so that it can collect the pollutant as it passes through the soil sample.
3. Add 20 ml methylene blue solution to one of the soil bottles and one of the sand bottles until it just begins to pool at the surface.
4. Let the soil sit until the dye drains through to the bottom of the bottle.
5. Repeat steps 3 and 4 with the remaining two bottles (soil sample and sand) and eosin y solution.
6. Record the volume and color of each filtrate.
7. Is the filtrate color lighter than the beginning color? Does it appear that the dye was filtered out by the soil?
8. Which dye was retained by the soil? Explain why one dye was retained and why one dye moved through the soil. (Hint: one dye is cationic or positively charged and the other dye is anionic or negatively charged.)
9. How did the soil sample behave differently from the sand sample with each dye? How would a clayey sample behave?
10. What would happen to nitrates (negatively charged anions) in these soils? Would they be absorbed to the soil particles or have the tendency to leach into the groundwater?
11. How would this demonstration relate to potential pollution of groundwater if excess nitrate fertilizers were applied to the land?
12. Considering what you have learned in this activity, what potential remediation qualities do soils have to buffer against chemical pollution and to act as filters for water percolating through the soil?
 

Earth System Science Center at Penn State

Natural Resources Conservation Service

This lab can be used as a part of an AP Environmental Science Class.

Contributed by: Paula Wang, Sidwell Friends School