After drying the soil, we will want to determine how much of it is due to organic matter. We can find this out by combustion analysis:
1. Purpose: to determine the amount of organic matter in the soil.
2. Materials: well mixed soil sample (from sampling ring) -- about 50 grams, hot plate, balance, evaporating dish (watch glass)
3. Procedure: Scoop out or scrape out about 200 grams of soil. Mix it well. Then take about 50 grams of this portion and weigh it accurately. Put it on the hot plate set at mid heat -- not HOT -- and note its location on the hot plate. There is room for about 9 dishes or watch glasses on the hot plate. That is why I said to note its location. Weigh it periodically. When the sample is at constant weight, it is dry. Weigh it again and record this weight. Then transfer all or a weighed portion to a crucible. It is important to know exactly how much you have placed in the crucible! Then, heat it with a bunsen burner until all organic materials have burned off. You can tell when combustion is complete because soot which deposits on the crucible in the early stage of burning will be oxidized and gone. If possible, do the combustion in the fume hood. Let it cool and reweigh. If we have a desiccator in which to cool the sample, use it.
4. Data: record the weights at each step.
The difference between the initial dry weight and the final weight is the mass of organic material which was present.
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