Extraction from soil
Many species of invertebrate inhabit the soil. Since the majority are small, under 1mm in length, it is necessary to remove them from the soil in order to observe them. Several methods exist. For the microarthropods (e.g. mites, springtails and spiders) the most common is the use of Tullgren funnels or the improved design of MacFadyen. Although these systems are almost always referred to as extractors it is important to appreciate that they actual expel rather than extract. The animals must be alive in order to leave the soil sample.
In the original funnel as described by Tullgren a mesh is fitted inside a funnel on to which the soil sample is placed The soil sample is always positioned up-side-down in the funnel. A collecting vial is positioned under the funnel and a light bulb over the funnel. The heat of the light bulb progressively dries out the soil. The soil animals start to move downwards away from the dry and warm soil (using the same tunnels that they used to get into the soil, hence the reason why the soil is placed up-side-down in the funnel). Eventually, when the soil is completely dried out the animals drop out of the soil and into the collecting vial. The whole process usually takes around 24 hours depending on the size of the soil sample, the power of the light bulb and how wet the soil was to begin with.
This technique is fast but also fails to remove all the animals since many cannot move fast enough and die within the soil. MacFadyen therefore suggested an improved design. Here the soil is heated gradually over several days with the temperature applied controlled by a thermostat. With this system the soil is dried over several days, the temperature being progressively increased as the soil dries. In addition a cooling system is employed to ensure that that the lower side of the soil core is maintained at a low temperature further encouraging the soil animals downwards. Often, the cooling system is switch off for the last period ensuring that the soil core is baked dry.
While these techniques are efficient for animals such as mites and Collembola they are less suitable for animals that require a moisture film in which to move, for example the nematodes. Here the O'Conner funnel, a modification of the Tullgren funnel, is often used. The apparatus is effectively a Tullgren funnel but with a tap on the bottom. The tap is closed, soil is positioned in the funnel and water added until the soil is almost, but not entirely, covered. The light is then switched on and the water warmed. The worms then "swim" out of the sample after a few hours and are collected in the water just above the tap. Judicious opening of the tap allows just water with the worms to be removed into a collecting vial.