Today is 27 September 2007 - Analyzing soil for total organic content

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.

Today is 28 October 2007 -- Here is how we start analyzing soil

The weight of an empty container is the TARE WEIGHT. The weight of the container WITH a sample is the GROSS WEIGHT. By subtracting the tare weight from the gross weight, you have the weight of the sample itself. This is the NET WEIGHT. When you begin an experiment, you have a STARTING or INITIAL WEIGHT. At the end of the experiment, you have a FINAL WEIGHT. When you express a fraction gained or lost in terms of the whole sample, you calculate a PERCENTAGE. Please answer the following questions:

"The tare weight of a watch glass is 3.025 grams. The gross weight (wet) of a soil sample is 9.034 grams. After drying the gross weight (dry) is 7.084 grams."

A. What is the weight of the water in the sample?

(All you have to do is subtract the dry gross weight from the wet gross weight. You don't have to account for the weight of the watch glass since it has not changed in weight.)

B. What is the dry weight of the soil in the sample?

(Subtract the tare weight from the dry gross weight.)

C. What is the percentage water in the sample?

(You want to know how much water was in the dry soil, as a percentage. Divide the amount of water by the amount of soil and multiply by 100.)