Microorganisms are not distributed uniformly throughout compost; they commonly occur in clumps or colonies ranging from few to thousands of individual cells. The populations vary greatly depending upon the amount of undecomposed organic matter and the micro-environment in a specific location. How wet the sample is, and whether it contains anaerobic or aerobic regions, also will affect the types of microbial life that are found. Multiple samples therefore should be taken to determine microorganism numbers or activity in compost.
Calculating Dry Weight
Water content of different composts may vary greatly. When comparing how much microbial activity there is in a gram of compost, you must allow for the difference in water content so that you can accurately compare what is happening in equal amounts of two different composts while discounting the weight of the water.
To determine the ratio of wet to dry weight of a compost, a sample of wet compost is weighed and then dried for 24 hours in a 105-110C oven. It is then reweighed, and the ratio between wet and dry weight is calculated.
When using the actual (wet) compost in a study, the moisture ratio is used to calculate how much compost to use. For example:
Amount of vegetable waste compost needed = 5 g
Predetermined wet weight of a sample = 4.3 g
Measured dry weight of the same sample = 2.8 g
Ratio of wet/dry = 1.54
Actual amount of compost needed for the experiment would be:
5 g x 1.54 or 7.7 g (wet weight)
Preparing Slides of Microorganisms
These protocols were written by Elaina Olynciw, biology teacher at A. Philip Randolph High School, New York City, while working in the laboratory of Dr. Eric Nelson at Cornell University as part of the Teacher Institute of Environmental Sciences.
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