344 



PRINCIPLES OF SOIL MICROBIOLOGY 



with the 20-mesh, the residual material should be examined carefully. To 

 make sure that no animals remain in the liquid, the latter is passed several times 

 (5 to 10) through the finest sieve. When a portion of the final liquid is examined 

 and no nematodes are found, the liquid is finally discarded. The larvae of 

 some animals, like those of Heterodera, are caught on 

 the finest sieves. The animals are then washed away from 

 the sieves (kept at a slightly inclined position) by a small 

 amount of water. The washings with the animals are 

 either mixed or kept in separate vessels. 3 



The separation of the animals from the clay portion 

 of the soil which is kept in suspension, is based upon 

 the fact that they will settle quicker than the clay. Care 

 should be taken that no animals are floating on the sur- 

 face of the liquid. After the latter is allowed to stand 

 for 30 minutes, it is poured off and replaced by clean 

 water. The floating animals can be made to sink by 

 adding some alcohol to the run-off material (so as to 

 make 20 to 30 per cent alcohol), shaking well and adding 

 water immediately (fig. 10). This process can be modi- 

 fied greatly depending on nature of soil. 



The mixture of inorganic soil particles and or- 

 ganisms, lying in clear water, are then examined 

 by using an ordinary dissecting microscope. Nem- 

 atodes are fished out by slender, tapering, sharp 

 needles free from grease. A small portion of the 

 debris is placed in a watch glass, in clear water, 

 about | inch in depth. When a nematode is lo- 

 cated, the point of the needle is brought under it and 

 it is floated to the surface, lifted on the point of 

 the needle and transferred to a watch glass con- 

 taining a few drops of clean water. The final sus- 

 pension may be well mixed and only an aliquot 

 portion examined in a graduated watch glass. 

 Examination should be made soon after washing is 

 completed. 



ig. .j pp Certain members of the animal, non-protozoan 



for the separation ol . . ' c 



nematodes from the population can also be isolated by the use of agar 

 soil (after Cobb). media. Small worms can be isolated 4 by the fol- 



lowing method: 



3 See also Baunacke, W. Untersuchungen 7Air Biologie und Bekiimpfung des 

 Riibennematoden Heterodera schachtii Schmidt. Arb. Biol. Reichsanst. Land. u. 

 Forst, 11: 185-288. 1922. 



4 Shaw, C. Zuchtungsversuche zur Gewinnung von Reinkulturen kleiner 

 Wurmarten der Garten- und Ackererde. Centrbl. Bakt. II, 64: 41-45. 1925. 



