NON-PROTOZOAN FAUNA OF SOIL 343 



fastened to it at the top, and each of the three larger plates has two 

 projecting teeth at the bottom. The plates are driven into the ground 

 down to the required depth to form a box 9 inches square, the small- 

 est plate being on the side towards the outside of the plot. The plates 

 enclose a cube of soil, with a side dimension of 9 inches, giving a 

 total of 729 cubic inches. The soil is removed from the cube, in layers: 

 the first sample contains only the upper inch of soil, the second and 

 succeeding samples taken at a depth of 2 inches at a time, giving in 

 all five samples for each cube. 



For making a census of the soil population, Cobb 2 devised soil sam- 

 pling tubes, which are open cylinders of thin metal (tin or galvanized 

 iron) with an internal diameter of 72.1 mm. The rim of one end is 

 reinforced and the other sharpened. The area of the internal cross - 

 section of the tube is one-millionth of an acre. The tubes may be of 

 any length; for counting nematodes, 6 to 9 inch lengths are sufficient; 

 below that depth, only few nematodes occur in the soil. Since the ani- 

 mal population is unevenly distributed in the soil, a number of samples 

 are required, with a minimum of five. The various samples for one plot 

 can be mixed and the census made. After the sampling tube is forced 

 into the soil, enough earth is dug away to enable one to introduce a 

 knife or saw-blade beneath the lower end of tube. The tube is then 

 removed full of soil and capped at both ends. The samples of soil 

 from one field are sifted and thoroughly mixed; wire sieves of \ to \ 

 inch mesh may be used. Various methods of mixing and sampling of 

 the soil are described by Cobb. 



An aliquot portion of soil is placed in an abundance of animal-free water, 

 usually 10 to 20 times its volume. The soil is well suspended in the water by 

 proper stirring with compressed air, carried on fast enough not to allow the 

 particles to settle. The heavy particles are allowed to settle for about five 

 seconds and the supernatant liquid is rapidly poured into another vessel. The 

 residue is washed several times with clean water, so as to remove all adhering 

 animals, the washings being added to the original liquid. The sand and gravel 

 are discarded. The process may be repeated so as to remove another portion 

 of the heavy inorganic material, being sure that it is free from animals. The 

 liquid is then allowed to run through a series of superimposed sieves, ranging 

 from 16 to 200 meshes per inch; the sieves, especially the finer ones, are agitated 

 when the liquid is passed through them. The finer sieves may be made of mill- 

 ers bolting silk. The nematodes will all pass through the 16 mesh sieve; the 

 residual particles should be washed so as to remove the animals. Beginning 



2 Cobb, N. A. Estimating the nema population of soil. Bur. PI. Ind., U. S. 

 Dept. Agr., Agr. Tech. Circ. 1. 1918. 



