8 4 
Journal of Agricultural Research / 
Vol. 26, No. 3 
close agreement in the values obtained by the use of the electrometric 
and the colorimetric methods. The method of expressing this value 
by the symbol P H and some number is well known. Extensive bibli¬ 
ographies relative to hydrogen-ion concentration are given by Schmidt 
and Hoagland (40) and by Clark (7). 
ADSORPTION AND ABSORPTION 
By adsorption is meant a combination of chemical substances such 
that the resulting product is neither a definite chemical compound nor 
a homogeneous mixture. When C 0 2 combines with CaO the product 
at equilibrium is a definite chemical compound. When HC 1 is added 
to water the product is a homogeneous substance. When this acid 
mixes with water there are no such surface phenomena as are found 
when a solid phase is present and absorption is used as the descriptive 
term. With adsorption are associated the ideas of surface and heter¬ 
ogeneity. When varying quantities of Ca(OH) 2 are added to soil in 
the presence of water a series of definite chemical compounds may be 
formed even though their presence would be difficult to prove or disprove. 
That the amount of Ca(OH) 2 taken up by a given quantity of soil is 
governed by various factors, one of which is the amount of clay, indi¬ 
cating a large extent of surface, was shown in a former publication (44). 
That the combination of soil and Ca(OH) 2 when mixed in various 
amounts is neither a homogeneous mixture nor probably a definite 
chemical compound will be evident from the data to be presented. 
No attempt has been made to determine chemical equilibrium in this 
investigation; it is very probable, however, that definite chemical 
compounds form at equilibrium. Since there are surface phenomena 
and heterogeneity, the term adsorption rather than absorption is used, 
although some writers, notably Lyon and Buckman ( 31 , p. 263) prefer 
the latter term. * 
HYDROGEN ELECTRODE IN USE WITH SOIL 
A large amount of work has been done and many papers published 
on the subject of soil acidity. Mclntire {33), Fisher (13), Ames and 
Schollenberger ( 2 ), as well as others, have discussed this subject fully 
and given extensive citations to literature. A study of the data pre¬ 
sented by different investigators will show that the results obtained are 
dependent upon the methods used in making the determinations. The 
results from one method are therefore comparable with those of another 
only in a general way. The hydrogen electrode, as a means of measur¬ 
ing the hydrogen-ion concentration in soil, has not yet been used as 
extensively as some of the other devices. It has been employed to a 
limited extent in soil investigations by Gillespie (74), Plummer (57), 
Knight (26), Sharp and Hoagland (47, 23), Swanson et al. (44), and 
others. 
To indicate qualitatively the hydrogen-ion concentration in a soil 
suspension or a soil extract, the hydrogen electrode offers a comparatively 
simple and rapid procedure, and the results are probably more accurate 
than those obtained by any other method. Electrometric titration as a 
means of determining the amount of acid or alkali required to titrate 
to the neutral point, or to change the P H present to that of any other 
desired P H can be used rapidly and successfully with many substances % 
