Lab 9: Soil Organic Matter
Objectives
1. To learn basic concepts of soil organic matter
2. To conduct a procedure that estimates organic matter content of
mineral soils
Introduction
Organic matter represents one of the most important components of soil
fertility. It is that portion of a soil that includes animal and plant remains
at various stages of decay, e.g. fallen leaves, dead tree trunks, dead
animals, or decaying roots. Chemically, it is a complex material
containing a variety of carbon containing compounds including
carbohydrates, starches, lignins, and proteins. As organic matter
decays, it releases nutrients that may be used by plants, especially
nitrogen, phosphorus, and sulfur. Generally speaking soils that contain
larger amounts of organic matter are considered more fertile soils. the
organic matter content of different soils varies widely. In some cases
there may be a high organic content in the top few inches of soil, but a
very low content in deeper layers. In addition, organic matter content of
soils may be altered significantly by management. Organic matter
content may be as low as 1% in a sandy soil or as high as 50% in the
upper layer of an undisturbed forest soil.
Society benefits from organic matter in soils in at least 3 ways:
1. Nutrient storage (essential elements)
2. Water storage
3. Increase in nutrient availability
Organic Matter Determination
The determination of organic matter may be accomplished by chemical
means or estimated by combustion. Some procedures call for a
combination of both chemical treatment and combustion. For example
one procedure involves pre-treating soil with strong acids (e.g. HF and
HCl), followed by combustion at high temperature. For our purposes we
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will estimate organic matter content by simply combusting the
samples.
The rationale behind the combustion procedure is relatively
straightforward. When we heat a soil sample to a high enough
temperature the organic matter literally begins to burn off. The mineral
fraction of the soil is more resistant to combustion and remains behind.
Thus, the weight loss during combustion reflects the weight of organic
matter in the original sample. We can simply divide the weight change
by the original sample weight to determine the % organic matter on a
mass basis.
Note: This procedure is intended primarily for mineral soils, as
opposed to potting soils or other very high OM soils.
Materials
1. Muffle Furnace
2. 2 Porcelain Crucibles
3. 2 soil samples (small - about 0.5 g)
4. Tongs
5. Analytical Balance (preferably capable of weighing to 0.001 g)
Procedure
1. Please keep in mind that the key to obtaining accurate results in this
procedure is to determine small changes in weight as accurately as
possible
2. In many labs it is routine to process duplicate samples to maximize
the odds for good results. Obtain 2 crucibles and record the weight of
each in the table provided.
3. Weigh approximately 0.500 gram portions of each soil. Remember
you are going to measure weight differences and so the starting weight
only needs to be in the range of 0.5-1.0 gram. Label the crucibles with
a china marking pen.
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3. Using tongs, carefully place the crucibles into a muffle furnace. Draw
a diagram on a piece of paper to indicate the location of the crucibles in
the oven so you will be able to identify your samples later. [Labels may
burn off in the oven, so knowing the position of the crucible in the oven
is crucial.]
4. With the help of your instructor, set the oven to obtain a temperature
of about 400-500°C.
5. Turn on the oven and leave on for approximately 2 hours.
6. After turning off the oven, allow the oven to cool down at least one
hour before opening the oven and carefully removing the sample.
Caution is needed since the crucibles may still be quite hot and because
a sudden gust of air may blow ashes out of the crucible and invalidate
your results.
7. Weigh the cooled samples and enter the data in the table provided.
8. Calculate % organic matter based on weight difference.
Data - Organic Matter
Record Data in grams
Item
Sample 1
Sample 2
Before Combustion
Empty Crucible
Crucible + Soil
After Combustion
Crucible + Soil
Soil
Calculations
Once you have collected and recorded the data in the data table above,
compute % organic matter for each sample by using the following
formula:
Soil Wt Before Combustion - Soil Wt After Combustion X 100
Soil Wt Before Combustion
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Pre-Test
Soil Organic Matter
1. Briefly define organic matter
2. What is the rationale behind this procedure?
3. What important roles does organic matter play in soil fertility?
By my signature I affirm that I have read the laboratory, completed
the above pre-test, and have a reasonable understanding of the
procedures involved in this laboratory.
Students Signature & Date
Instructors Signature & Date
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Soil Organic Matter
Final Results & Interpretation
Results
Sample 1
Sample 2
% Organic Matter
Interpretation of Results
For the interpretation of these results, indicate which range of organic
matter content corresponds to your data, using the guidelines given
below.
Low <1.0%
Moderate 1-5%
High>5%
By my signature I affirm that I have completed this laboratory,
calculated and recorded the results above, and have checked the
results with the lab instructor. I have also consulted with the lab
instructor with regard to the above interpretation of the results.
Students Signature & Date
Instructors Signature & Date
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