Two efficient, semi-automatic elutriators for assaying soil samples for nematodes are described. The 1st apparatus is a 4-unit elutriator which combines conventional extraction methods with the major features: automatic mixing of 500-1500 cm3 soil samples with water (.+-. air); turbinate sample splitters from which fractions of 1/15 or greater are passed onto 26 or 38 .mu.m sieves for collection of larvae and adult nematodes; the capacity for collecting roots, intact egg masses and cysts on 250-425 .mu.m sieves; and a variable speed motorized sieve-shaker. Nematodes, after being collected on 38 .mu.m sieves, are separated from debris by centrifugation or by Baermann trays. Secondary features include air cylinders, solenoid valves and time clock for automatic dumping residual soil and water; relay-controlled coarse spray nozzles activated for 5 s every 30 s for washing nematodes through 250-425 .mu.m sieves; adjustable rates of water and air flow and timing. The 2nd type of elutriator operates on similar principles but costs less to construct. It requires somewhat more operator participation; sieve spraying is carried out by the operator and elutriators are dumped manually. Both elutriators also show promise for monitoring populations of certain other soil microorganisms.