[Vor. 7 
40 ANNALS OF THE MISSOURI BOTANICAL GARDEN 
days in n/25,600 HCl the growth rate was but little more than 
one-half that of similar roots in distilled water. Thus we may 
assume that a hydrogen ion concentration approximately Py 4 
depressed the root growth of this plant. The same seedlings 
survived n/400 potassium hydroxide during 24 hours, but with 
this reagent no growth measurements after an interval of several 
days, comparable to those with HCl, were made. The results 
of Heald (96) also show that while roots of Pisum sativum 
survived n/6400 HCl, and those of Zea Mays n/1600, still in 
the former the growth rate was low in n/12,800 and for corn 
in n/3200, during an interval of 48 hours. No comparisons 
with distilled water were included. 
Loew (03), working with the seedlings of Zea Mays, and 
Miyake (14), using Oryza sativa, have both shown a relatively 
greater resistance of these plants to alkali than to acid, all of 
which emphasizes the importance of devoting special considera- 
tion to the initial acidity of the culture solution. 
Respecting the reaction of soils, there is considerable evidence 
indicating that acidity alone is not necessarily a limiting factor 
in the growth of many crops. With a method considered ade- 
quately aecurate as applied to field conditions, Gillespie (16) 
has examined air-dried samples of 22 crop soils and found the 
Py exponent to vary from 4.55 to 7.1 in the case of 18 soils from 
Maine, Maryland, and Virginia, and a variation of from 8.1 to 
8.7 with 4 soils from Utah and Montana. It is also reported 
by Gillespie and Hurst (18) that the highest acidity (Pu 4.5) 
was not in the least injurious to potato culture in Caribou and 
Washburn loams, the two main potato-soil types in the region 
in which they worked. 
Plummer ('18), employing the soil-suspension method, ex- 
amined 68 samples of a variety of soil types of humid regions, 
especially of the southern states. Untreated sandy loam or 
clay soils exhibited a range of acidity .1x 107? to 1x 1075, while 
peat gave in one instance a hydrogen ion concentration .2 x 107. 
Evidence was gained to the effect that the surface film emphasizes 
the direction of the reaction, that is, in acid soils the surface 
film is more acid and in alkaline soils it is more alkaline. This is 
scarcely in corroboration of certain work of Sharp and Hoagland 
(16). 
