124 BULLETIX 1059, U. S. DEPARTMENT OF AGRICULTURE. 
Table 7. 
Class. 
Metric. (Diameters of sieve perforations.) 
English. (Number of openings per inch.) 
Rocks 
Millimeters. 
Over 2 
4 to 10. 
Fine gravel 
2.00 to 1.00 
10 to 20. 
1.00 to 0.50 
20 to 40 
0.50 to 0.25 
40 to 60. 
0.25 to 0.10 
60 to 100. 
Very fine sand 
Silt 
Clav 
0.10 to 0.05 (settles in test tube in 30 seconds) 
0.05 to .005 (settles in centrifuge in 5 minutes 
at 800 revolutions per minute.) 
.005 to 0000 (does not settle in centrifuge: 
measured by deduction). 
Over 100 (settles in bottle in 30 seconds) . 
Same as metric. 
Turbid water evaporated and weighed. 
The following procedure is suggested as the result of a good deal 
of experience in treating forest soils : 
1. If the soil to be sampled contains a great deal of rock, say over 
25 per cent by volume, and of large size, it is desirable to determine 
the rock percentage by sifting a considerable quantity of the mate- 
rial through a screen having four meshes to the inch. This should 
be done only when the material is air-dry, and should be accom- 
panied by much beating and brushing to remove the fine material 
from the rock surfaces. After the process, a sample of about 100 
grams of the finer material may be taken. If rocks are few and 
small, it is better to sample and wash them with the other material, 
separating them when dry from the coarse gravel on the 2-milli- 
meter or 10-mesh sieve. 
2. The sample is placed in a wide-mouth 8-ounce bottle, which is 
then nearly filled with clean tap water, stoppered, and placed on the 
shaking machine, or attached to a pulley which is turning at the 
rate of about 100 revolutions per minute. The amount of shaking 
necessary will vary from two to eight hours with different soils, but 
should always be sufficient to break down every lump of whatever 
size. If the soil lacks gravel for its own pulverizing, place two or 
three round pebbles in the bottle. 
3. The lumps thoroughly broken clown, the contents of the bottle 
are placed on the coarsest screen, with the finer sieves in succession 
below it, and the whole nest standing over a can of 1 or 2 gallons 
capacity. The material is washed down through each screen by a 
tiny stream of water, until all silt and clay have been removed ; that 
is, until the water comes through perfectly clear. The nest of sieves 
may then be placed in the oven to dry, after which the separation of 
the sands is readily accomplished by a little jarring of each sieve; 
the material held on each is weighed promptly, before it can take up 
moisture from the air. 
4. The very fine sand which passes the sieves after drying is 
placed in the washing bottle. The water from the washing of the 
material several hours earlier may now be decanted off into a meas- 
