RESEARCH METHODS IX STUDY OF FOREST ENVIRONMENT. 123 
combinations, such that variations between a specific acidity of 3,000 
and a corresponding " 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. No reason appears 
why they might not be readily used in the field. Apparently the 
apparatus devised by Briggs (111) for determining the "soluble 
salt content of soils " was 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 ap- 
proximately correct name for the soil. Thus may be avoided the 
error 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 
soils, 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 " rather 
than the metric svstem of classification. 
