124 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



Table 7. 



Class. 



Metric (Diameters of sieve perforations.) 



English. (Number of openings per inch.) 





Millimeters. 







Over 2 



4 to 10. 





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 



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). 



Clay 



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 down, 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- 



