THE SOIL PARTICLE 95 



weighed. The sands, which are obtained in bulk, are 

 » further separated by sieves into the grades desired. 

 Where a large quantity of organic matter is present, it 

 must be determined and included in the final report on 

 the sample. 



This method of mechanical analysis as perfected by 

 the Bureau of Soils has been very generally adopted by 

 soil workers. It has many advantages over other methods. 

 In the first place, it is rapid, often requiring only hours 

 where other methods take days for completion ; secondly, 

 it is simple, and the technique of the separation is easily 

 acquired ; thirdly, in the decantations no very large 

 amount of water is accumulated with the separates, 

 except for the clay, and thus the time and cost of evapora- 

 tion is eliminated. The clay, moreover, may be as ac- 

 curately determined by difference as by direct methods, 

 thus allowing a further saving of time. The cost of the 

 equipment for this method is low. The apparatus itself 

 is simple, and is carried by all standard chemical com- 

 panies. The same cannot be said of the various elutriator 

 mechanisms. While the method is accurate only within 

 one per cent, it is sufficiently precise for practical pur- 

 poses, especially in class determination, for which me- 

 chanical analysis is generally utilized. 



71. Classification of soil particles. — With the large 

 number of different methods of mechanical soil analyses 

 there has arisen a large variation in textural groupings 

 expressed in diameter of particles. This would naturally 

 occur because of the differences in degree of refinement 

 which the various methods of separation allow, and also 

 because of the uses which the investigators wished to 

 make of such analyses. Some of the best-known group- 

 ings are given below : — 



