Biological Sciences, Environmental Sciences, Agricultural Sciences
Soil Organic Matter provides numerous and essential ecosystem services, including supplying soil organisms and plants with carbon, nitrogen, and energy; stabilizing soil particles; serving as a reservoir for plant nutrients; serving as a sink for CO
2
, thus reducing greenhouse gases; providing the basis for high soil biodiversity; binding pesticides and heavy metals, thus reducing water pollution; and enhancing the water and nutrient holding capacity of soils, thus improving plant productivity. In this lesson, students will discuss the importance of soil organic matter in ecosystem function, and will measure the organic matter content of different soils. It’s an excellent lesson in experiment design, and can be combined with the following lessons:


Biodiversity of Soil Invertebrates
Global Warming and Carbon Sequestration .
 Soil Chemistry and Physics
Several STEEP research projects focus on soil organic matter and soil biodiversity, because both are essential for healthy soils and productive ecosystems. Scientists at Washington State
University, University of Idaho, Oregon State University, and with the U.S. Department of
Agriculture have conducted studies to define the role of soil organic matter and soil biodiversity in agroecosystems, to figure out ways to preserve and enhance soil organic matter so it can be used to improve plant growth, and to see how organic matter can be used as a carbon sink to minimize the effects of global warming. Here are some highlights of their research:
 Tilling soil can greatly decrease soil organic matter, soil carbon, soil nitrogen, the activity of microorganisms, earthworms, and populations of beneficial insects.
 One way for farmers to improve soil quality (e.g., increase soil organic matter) is to use
 no-till methods where seeds are planted directly into the ground without tilling the soil.
This also reduces soil erosion from wind and water, reduces air pollution, improves water quality, can reduce global warming by keeping carbon in the ground, and enhances biodiversity.
For additional highlights, see: STEEP Research Summaries

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
Students will define soil organic matter and its function in terrestrial ecosystems
Students will measure the soil organic matter content of different soils
Nutrient Cycling, Scientific Method, Soil Ecosystems, Terrestrial Ecosystems
AAAS Benchmarks
Idaho State Science Standards
Oregon State Science Standards
Washington State Science Standards
Print Resources
 Choosing a Sample Site and Preparing Soil Samples
 Soil Organic Matter Fact Sheet
 Simple Soil Analyses (optional)


Scientific Experiments and Lab Report Format
 Constructing Bar and Line Graphs
Student Handout
Collecting Soil Samples
 soil and litter samples from 2 or more habitats (1 sample per student group)
 shovels, trowels, and/or soil corers

 ruler
 2 plastic bags per sample (1 for litter and 1 for soil) markers
Analysis of Organic Matter
 small containers for air drying ca. 20 g of soil (1 per sample)
 mortar and pestle (or any other means of grinding soil samples)
 1-mm mesh sieve or screen for sifting soil
 balance with 0.01-g accuracy
 crucible or ceramic boat to hold ca. 5 g of soil
 kiln or combustion oven
 USDA National Resource Conservation Service - Soil Biology Primer . An introduction to the living component of soil, intended as a resource for farmers and ranchers,
 agricultural professionals, scientists, students, and educators.
USDA National Resource Conservation Service – Soil Organic Matter

Preliminary
1.
Present students with information on soil organic matter or allow them to explore the topic on their own (see Web Resources, above).
2.
Present the class with one or more testable hypotheses. Alternately, have students work in small groups to develop their own testable hypotheses
 example – there is a positive relationship between soil organic matter and the amount of litter collected on the soil surface because litter is the main source of organic matter.
 example – the organic matter content of soil collected from a tilled field is less than the organic matter content of a no-till field because tillage increases decomposition.
3.
Hand out the Student Handout . Alternately, with guidance, have students develop the experiment methods to test the hypotheses.
Collect Soil Samples
4.
Choose 2 or more sites to collect soil samples (see: Choosing a Sample Site and Preparing
Soil Samples ). Either collect the soil samples (1 per group) a day or two ahead of time, or allow students to collect samples, if nearby sites are available.
5.
Remove the litter from an area of ca. 1 ft
2
and place in a plastic bag; label.
6.
Remove a cupful of soil from a depth of 1-5 cm from each sample site. This will be used for
Organic Matter analysis.
Organic Matter
7.
Air dry the soil samples collected for the Organic Matter analysis. This will take about 2 days.
8.
Using a mortar and pestle, break up the soil and pass it through a 1-mm mesh sieve.
9.
Place about 5 grams of air-dried soil in a crucible (or ceramic boat) and weigh; this is the preweight. Weigh to the nearest 0.01 g and record the weight in the data table.
10.
Place the crucible with soil in a furnace or kiln and heat to about 360 o
C (680 o
F) for 3 hours to burn off all the organic matter, leaving just the mineral soil.
11.
After cooling, reweigh the sample again; this is the post-weight. Record the value in the data table.
12.
Calculate the % organic matter:
% Organic Matter = (pre-weight – post-weight)/pre-weight x 100.
Data Analysis
13.
Record and analyze data. Have students discuss results and conclusions.
14.
Have students prepare a formal lab report (see: Scientific Experiments and Lab Report
Format )

Standard assessment for Lab Reports