A METHOD FOR EXTRACTING NEMATODES FROM A TROPICAL FOREST SOIL

The centrifugal flotation method is a common method of extracting nema todes but it has been used infrequently for extracting free-living nem atodes from tropical soils. No estimate of its efficiency is available , so an experiment was conducted to optimize the extraction efficiency of nematodes of all sizes and shapes from soil samples previously fix ed by addition of formalin. Factorial combinations of the speed and du ration of centrifugation, the agent used in the extraction and the den sity of the agent were tested. Samples were drawn from a homogenized m ixture of soil from 24 sites in an experimental forest in Cameroon whi ch covered a range of forest types and ages. More nematodes of all siz es and shapes were extracted by Ludox-TM than MgSO4. A solution of den sity 1.15, speed of centrifugation of 1500 g and duration of 4 minutes extracted the most nematodes. Furthermore, the proportions of nematod es of certain sizes and shapes extracted were altered by using differe nt parameters. Using the optimum parameters, the efficiency of a singl e extraction in Ludox was about 70% of the total number of nematodes r ecoverable. Efficiency was not significantly different for sites under different management regimes, but declined for subsequent extractions after the first. The practicability of a method of extraction depends on the amount of extraneous matter extracted with the nematodes. This was also assessed for all the combinations of parameters. The method most efficient for nematodes retained more extraneous matter than some other combinations of parameters, but was not the worst. To obtain cl eaner samples would entail considerably reduced extraction efficiency and a sample of nematodes biased by size and shape.