Comparing Properties of Worm Casting Materials
Dr. J. Benton Jones, Jr. found considerable variance among the five worm casting (vermicast) materials. For this article, he explores three physical properties.
When I once examined the elemental content of five worm casting (vermicast) materials, I found that there was considerable variance among the five materials as to their water pH and total and extractable elemental contents. For this article, three physical properties: organic matter, moisture contents, and volume-weight for these same vermicast materials were determined and reported.
Organic Matter Content of Vermicast
The same procedure used for estimating the organic matter content of an inorganic soil was used: loss on ignition. An aliquot of vermicast was oven dried at 175°F for 12 hours, and then placed into a muffle furnace set at 750°F for 12 hours. The weight loss percent is considered due to the combustion of organic substances in the vermicast material. The percentages of organic matter content (due to loss on ignition) of the five vermicast materials were found to be:
Most vermicast materials are usually designated as being organic, which would suggest that the majority of its constituent contents would be primarily organic.
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Only one of the vermicast materials, sample D, had an organic content greater than 50%. Therefore, one might wonder, what is the criteria for designating a vermicast material as being organic? Does such a designation mean that at least 50% of its content must be organic constituents? If so, then only sample D fits that requirement. Therefore, we are then left wondering what the designations for samples A, B, C and E would be.
The source ingredients are only known for sample D-cow manure and silage-and yet its inorganic content seems high at 32%, higher than one might expect. So, where did all this inorganic material come from? It could be that this approximate 30% may be the threshold level, even though the source materials are considered to be, essentially, totally organic. It is the other materials that are not organic that need to be further identified as to their form and elemental composition.
Moisture Content of Vermicast
A weighed aliquot of each of the vermicast materials was oven dried at 175°F for 24 hours with the loss in weight being identified as its moisture content. The results are as follows:
Others have reported similar moisture percentages for vermicast materials to be around 30%, which is in agreement with four of the materials having that near percentage, the one exception being for sample D.
The moisture in vermicast can be of two possible forms, that which exists as free water in equilibrium with the surrounding atmosphere, and that which is bound water held within the material structure of the compounds constituting the vermicast-a form of water that is not in equilibrium with the surrounding atmosphere.
The level of free water will depend to some degree on the air temperature and relative humidity of the exposed atmosphere, and therefore, will vary with changing atmospheric conditions, while bound water will be a fixed property of the vermicast itself.
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It is uncertain whether vermicast is hydroscopic; that is, having an ability to absorb water from the surrounding atmosphere greater than that associated with equilibrium balance water.
Some believe that the moisture content is an essential quality that will determine the level of possible microbial activity within the vermicast as well as being able to easily blend into a rooting medium, i.e., easily able to wet.
The moisture content in a vermicast material may be a factor determined by the processing method used to generate the vermicast, and whether a moisture reduction step is included in the processing.
More details from vermicast producers are needed to determine the preparation procedures and better understand on what basis the moisture level is established for the finished vermicast product.
Volume-Weight of Vermicast
An aliquot of vermicast was placed into a 0.77-oz. volume scoop that was lightly tapped so that the vermicast particles settled into the scoop. The vermicast that filled the scoop was weighed and the volume-weight calculated. The volume-weight was also calculated by correcting for moisture content. The volume-weight values are as follows:
The volume-weight is an important value since it determines what volume of material is supplied by weight as well as vice versa. For the seller, the quantity of material supplied will depend on the unit of measure, volume or weight.
For the buyer, which unit of measure used for applying the vermicast to a rooting medium will determine the quantity of vermicast needed. Adjusting the volume-weight determination due to moisture content tended to group these five materials around the 0.50 value.
The variances in the physical properties among these five vermicast materials are not as great as that found for their pH and elemental concentrations reported in the earlier article, except for their organic matter content. Therefore, what defines a substance as being a vermicast cannot be based solely on its organic matter content, and moisture content and volume-weight seem to be less variable among these five vermicast materials.