RESEARCH METHODS IX STUDY OF FOREST ENVIRONMENT. 123 



combinations, such that variations between a specific acidity of 3,000 

 and a corresponding u superalkalinity " may be detected with not too 

 great refinement, yet probably with all the precision necessary in 

 studying the distribution of plants. These extremes correspond, re- 

 spectively, to hydrogen-ion concentrations of P H = 3.5 and P H = 10.5. 

 For the most precise determinations of the degree of alkalinity or 

 acidity the potentiometer is undoubtedly the last word. A number 

 of such instruments are on the market and should require practically 

 no adaptation for the treatment of soil extracts. Xo reason appears 

 why they might not be readily used in the field. Apparently the 

 apparatus devised by Briggs (1 1 1 "» for determining the ''soluble 

 salt content of soils" w^as of very similar nature, though its relation 

 to hydrogen-ions was probably little understood at the time. 



THE MECHANICAL ANALYSIS OF SOILS. 



A mechanical analysis of any soil which is being studied exten- 

 sively is probably worth while if only to give a convenient and 

 approximately correct name for the soil. Thus may be avoided the 

 <>rror of speaking of a soil as a ''clay' when, in fact, it contains 80 

 per cent silt and only a very little clay, or perhaps even a large 

 component of very fine sand and small amounts of the finer mate- 

 rials which make it as stiff as clay. With accumulated analyses of 

 -oils, too, comparison will show whether the mechanical analysis of 

 two are very similar, approximately what water-holding capacity a 

 new soil may have, what wilting coefficient, etc. However, in this 

 calculation the humus plays a very important part and its effect is 

 difficult to estimate. 



The method of mechanical analysis which may be considered 

 standard has been recently described by Fletcher and Bryan (120). 

 It employs a number of sieves, with perforations of successively 

 smaller size, which separate the particles of various sizes but allow 

 the very fine sand, silt, and clay to pass through. These three grades 

 are then separated in water under the action of gravity. 



The standard soil grades recognized by the Bureau of Soils, 

 United States Department of Agriculture, are indicated by the fol- 

 lowing table of diameters (Table 7) , which also indicates the diam- 

 eters of the circular perforations in the standard sieves. Opposite 

 these values have been set the approximately corresponding sizes 

 of screens which are adapted for handling larger samples in the 

 study of forest soils, under what may be called the "English' 7 rather 

 than the metric system of classification. 



