Electrostatic attraction is an efficient way to reduce particulate organic matter in a soil sample. The main advantage of this method is that requires minimal visual judgment and works on fine particles not visible to the eye. Removing undecomposed organic fragments improves accuracy and consistency in measuring long term soil carbon storage.
After collecting soil samples to the desired depth, thoroughly dry them at 40 degrees Celsius. Sift the soil through appropriate sides soil sieves to obtain approximately 10 to 25 grams of sieved soil. Place the soil in a clean dry metal or a glass flat bottomed pan that is large enough for the soil to be spread thin.
Gently shake the pain horizontally to distribute the soil in as thin a layer as possible. Hold a 100 mm diameter glass or a polystyrene Petri dish top or bottom in one hand and vigorously rub the outer surface with a clean piece of nylon cloth, cotton cloth or polystyrene foam several times. Perform the surface charging away from the sample.
When finished, inspect the surface of the Petri dish to make sure that it is clean. Lower the charged surface to within 0.5 to 2 centimeters above the soil and move it horizontally to pick up as much particulate material as possible. Attraction to the surface can be noted visually and audibly.
When the Petri dish no longer attracts additional particles, move the dish away from the sample. To clean the electrostatic surface, hold it over a collection dish and use a fine brush to transfer the electrostatically attracted material from the Petri dish surface into the collection dish. Repeat this process until the number of organic matter particles being picked up decreases.
Redistribute the soil sample by horizontal shaking of the soil pan to expose new material at the surface and continue electrostatic collection. Determine the end point by visual inspection of the charged surface to determine whether a significant amount of organic particulates has been removed from the soil. This protocol was used for the analysis of silt loam soils from agricultural sites in the Pacific Northwest.
About 1%to 6%of the total soil mass was removed. In all cases the proportion of total sample carbon removed was greater than the soil mass removed. The carbon concentration and carbon to nitrogen ratio of the electrostatically removed soil fraction was always greater than the remaining soil.
The ambient conditions and the combination of materials used to produce the charged surface affected the results. The affects of treatment endpoints were examined by collecting a series of three electrostatic samples one after the other from the same soil sample. The first treatment collected the greatest amount of carbon.
The following two treatments were still highly enriched in carbon compared to the remaining soil. Finally, the amount of particulate material that could be electrostatically removed from the fine silt-sized fraction that passed through a 53 micrometer screen was tested on five silt loan soils. The electrostatically removed fractions demonstrated very little enrichment of particulate organic matter.
Microscopic inspection revealed that particulate organic matter does exist in the fine soil fraction of the soils but in very small quantities. The C to N ratio tends to stabilize as in completely decomposed organic matter is removed. This might lead to better models of long term soil organic matter.
