Lab 6 – Soil Ecology

 

 

 

 

 

Objectives: 

To learn about soil horizons and factors that affect soil fertility

To learn how to measure bulk density of soils

To learn how land uses affect soil and stream health

 

Background:

Soil is an important, living component of terrestrial ecosystems.  Clearing of land for agriculture and urban development can negatively affect soil health.   Natural topsoil depends on decaying vegetation for fertility, therefore the removal of vegetation can reduce the amount of fertility in soil.  In agricultural areas, rainstorms can cause high amounts of erosion.  Urban development can also radically affect a watershed’s ability to maintain soil, resulting in high amounts of soil and subsoil entering streams during rainstorms, disrupting aquatic ecosystems.  Steeply sloped and heavily used soils are likely to be highly susceptible to erosion.  Compaction also reduces overall fertility of soil.

 

Fertility

The fertility of soil is essential for healthy growth of forests or agricultural crops.  Fertility is shown by the color of the soil – in general, the darker the soil, the more fertile it is.  Soil fertility naturally decreases at increasing depth, nutrients are either taken up by plants and microorganisms or leached from the system, leaving paler horizons dominated by the mineral composition of the parent bedrock material.

 

Horizons 

Soil horizons are distinct layers are formed over long periods of time, hundreds or thousands of years.  When land is cleared and farmed, as much of the land in the US has been, the soil horizons are mixed and much topsoil can be lost to erosion.

 

Each horizon possesses unique physical and chemical properties.  Many of the properties of the soil can be deduced by measuring the soil’s texture and color, which can give us an overall measure of soil fertility.  The bulk density of soils is a way scientists describe soil compaction.  Soil can become compacted through walking, use of heavy machinery (for example, in farming) or construction of impervious surfaces such as parking lots.

 

Leaf litter is a natural component of forest soils, and processes of decomposition function to return fertility to the soil as leaves and other plant matter erodes.  Soil in areas from which trees and leaves have been removed, such as lawns, may suffer reduced fertility.

 

Texture of soil is an important physical property of soils that affects other soil properties such as nutrient and water retention.  Larger sand and silt particles have reduced ability to hold water and nutrients.  Smaller, clay particles hold many soil nutrients and can form soil microaggregates.

 

Materials:

Ziploc bags

Rulers

Soil Corers

Coffee Cans

Scale

Squeeze bottles and water

 

Method:

 

Collect soil samples from a campus lawn, a forested area, and a walking trail (other sites acceptable also, just be sure to note what the site is used for).  From each site, take three cores of the soil using your soil corer. This can also be carefully done with a simple spade. Simply did down about 20 inches keeping in mind that you need to be able to keep the soil in the correct layering as it was in situ.  Before you core the soil, measure the leaf litter layer of the area you are in with your ruler and record it.  After you get your core, measure each layer of soil and record the color of each.   The amount of leaf litter, if any, should be measured and recorded before taking the soil core.

 

To use a soil corer, push the corer into the soil and twist as you push down.  If the soil is too compacted at your collection site, you can also use a shovel, look at the side of the hole that the shovel has created to gather data about depth and color of horizon.  Soil this compacted is likely to have little to no O and A horizons.

If you encounter a rock or stone you may wish to choose another area to core.  Once you get your core, measure the depth of each distinct layer (in centimeters) and record the soil layers and depths on the chart below.  Try practicing once or twice before you record.  When you are done recording, tap the soil onto the ground and sweep it into the hole in the soil.  Note – You may not be able to find all of the soil layers despite your best efforts. Please be sure to fix any holes you have made during this lab.

 

Step 1.  DetermineSoil Profile.  Draw the soil profile on the attached sheet.  Use Figure 1 for guidance.  Color or shade the appropriate square in the diagram to indicate the look of the soil, and note the depth next to the layer on the diagram.

 

Figure 1:   This figure represents a generic soil profile.

 

 

 

Organic Layer: Organic material

 

 

Horizon A: Dark soil, “humus”

 

Intermediate zone

Horizon B:

 

Horizon B “subsoil”: Clay, metal oxides

 

 

 

Intermediate Zone

 

 

Horizon C: Parent material, bedrock

 

 

 

 

 

 

 

Step 2. Determine the texture of your soil for each layer.  Moisten a small ball of the soil and squeeze it between your thumb and forefinger.

 

If it feels:                                                                               It is comprised mostly of:

Gritty                                                                                       Sand

Smooth, slick, not sticky                                                     Silt

Smooth, plastic, sticky                                                        Clay

 

 

 

Step 3. Record the color of your soil for each layer, and estimate the amount of organic matter in your soil and the erosion factor using the chart below.

 

 

	Dark (dark gray, brown, black)	Moderately dark (dark brown to yellow-brown)	Light (Pale brown to yellow)
Amount of organic matter	Excellent	Good	Low
Erosion factor	Low	Medium	High
Available nitrogen	Excellent	Good	Low
Fertility	Excellent	Good	Low

 

Step 4. Measure Soil Compaction/Bulk Density:

 

Bulk density is easily measured by taking samples of the same volume and weighing them.

 

Density = Mass ÷ Volume

 

If the volume of collections is kept constant, the sample with greater mass has higher density.  Greater density is a reflection of compaction and is not desirable in soils.

 

 

Results. Using the chart below, record your findings about each soil sample. Soils on slopes are more likely to erode than soils on flat land.

 

 

Table 1.

	Dominant Texture	Estimated amt of organic matter	Bulk Density	Slope (none, gentle, moderate)
Sample A Top Layer
Sample B Top Layer
Sample C Top Layer

 

 

 

Record depths and color of your 3 samples below:

 

 

 

 

Sample 1                                                 Sample 2                                                 Sample 3

 

 

 

Questions:

 

1. What were the main differences in soil among the different areas sampled? How do you explain those differences?

 

 

 

 

 

 

 

2. What are the historic and current uses of soil in the area where you are studying? Have those uses had positive or negative affects?

 

 

 

 

 

 

 

 

3. What methods might you recommend to farmers or urban developers to improve soil retention?

 

 

 

 

 

 

 

 

4. Why is soil fertility important? Where does soil derive fertility?