# Determining Bulk Density of Different Soil Samples and Data Analysis

#### Summary

This is a field based lab experiment where students work in groups to come up with a collection strategy and lab analysis to test differences in soil bulk densities. Students may collect soil from agricultural fields, school forest settings, or right on the school grounds. Students bring the soil samples back to the classroom to conduct a test that measures soil density for their different soil types and create a density curve. Density curves are graphed by hand and also generated in Microsoft Excel. Data analysis includes discussion of slope, 1:1 data ratios, and R2 values.

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## Learning Goals

1. Practice designing data collection and lab experiment techniques to test a hypothesis.
2. Learn how to create a graph of soil densities by hand and in MS Excel.
3. Reinforce a line equation (y = mx + b) paying particular attention to slope.
4. Compare and contrast different data sets and be able to draw conclusions.

Higher order thinking skills include critical analysis of experimental design and data analysis. Students will become familiar with using electronic balances to measure mass.

Concepts to be discovered:
1. Density = mass / volume
2. Bulk density line should be linear because data has a 1:1 ratio, slope should be ~1.
3. If data has a 1:1 ratio you should expect your R2 value to be close to 1 as well, indicating a linear data set.

Vocabulary Words:
1. Bulk density
2. Slope
3. R2

## Context for Use

Grade Level: 8-12, depending on extent of data analysis and guided inquiry.
Field Exercise: Soil collection is done in the field. Provides opportunity to use school forest if one is available.
Lab Exercise: Students design data collection and lab analysis to determine soil density. They carry out their experiment, record and graph their results, and analyze data.
Time Needed: Two to four 50-minute class periods depending on extent of soil collection and data analysis, and level of inquiry instruction.

This exercise gives students an opportunity to think like a scientist to answer a question. Students gain experience in designing data collection and lab techniques, including use of an electronic balance to measure the mass of a sample. Some discussion of the concept of density as a property of matter is useful before starting this lab. This lab can be done when the initial concept of density is introduced and may be the student's first exposure to graphing using computer software and analyzing data.

Subject: Environmental Science:Soils and Agriculture, Geoscience:Soils
Resource Type: Activities:Lab Activity
Special Interest: Field-Based Teaching and Learning
Grade Level: High School (9-12), Middle (6-8)
Theme: Teach the Earth:Enhancing your Teaching:Teaching in the Field, Teach the Earth:Course Topics:Soil, Environmental Science

## Description and Teaching Materials

Materials (in no special order):
• Electronic balance
• Soil core or spade shovel
• Plastic baggies
• Sharpie or other permanent marker
• Chemical hood or drying oven (if available)
• Volumetric cylinder (at least 100 mL)
• Mortar and pestle
• Massing boats
• Spatula
• Graph paper
• Microsoft Excel (if available)

Soil Collection:
1. Remove Sod layer (if present), exposing soil
2. Insert soil corer, turn clockwise 180°, pull core up to surface. If using a shovel, dig a hole 6-12 inches deep and take a sample of soil.
3. Empty core into plastic zip-lock bag and label

Density Determination:
1. Crush 100 - 200g soil with mortar and pestle.
2. Measure out 10mL of soil using volumetric cylinder, record in table
3. Weigh the 10mL sample using an electronic balance, record in table
4. Repeat steps 2 and 3 increasing the volume of soil used by increments of 10 until a final volume of 60mL soil is reached.

Data Analysis:
1. Graph data
Independent variable: Volume (mL), x-axis.
Dependent variable: Mass (g), y-axis
**Should be a straight line**
2. Determine Slope (rise/run)
3. Graph same data using MS Excel
4. Have Excel generate the line equation and R2 value
5. Compare slopes and R2 values for different soil samples

Discussion:
1. Construction of a density curve
2. Meaning of slope
3. 1:1 ratio of data = straight line
4. What does R2 measure? R2 of a straight line should be what?
5. Similarities/differences of data.
• Did all the density curves end up linear?
• Are there differences in the densities of our different soil types? If so, what could account for these differences?
• What types of things affect density?

Closure:
1. Reinforce density equation: D=M/V
2. Reinforce the meaning of slope, including units!
3. State possible sources of differences in data:
• Bad data points due to human error
• Experimental error
• Pitfalls in experimental design
• Differences among soil types

## Teaching Notes and Tips

1. Idea and interpretation of slope needs reinforcing.
2. When collecting soil, try to collect all soil from a uniform depth as a way of standardizing collection methods across varying soil types.
3. Can make this more inquiry based by encouraging students to come up with their own data collection and laboratory procedures rather than giving them the above outline.
4. Familiarity with graphing in Excel is most helpful.

This activity is different from what I have done in the past because:
1. Uses soil to introduce the density concept.
2. Allows students time to work outside.
3. Integrates some environmental science with chemistry
4. More student driven as far as data collection and experiment design.

## Assessment

1. Before actual data collection and lab procedure, students should get their experimental design approved by the teacher. This ensures that students are on the right track and are going to carry out an experiment that is testing what they are interested in; in this case it is soil density.
2. Students record all work including their procedure and graphs in a lab notebook. The lab notebook is collected and reviewed by the teacher before the concept is tested.

## Standards

8.I.B.1 - Experiment design
8.I.B.3 - Excel graph and slope analysis
9-12.I.B.2 - Qualitative vs. quantitative data
9-12.I.B.3 - Excel graph and R2 data analysis