538 
AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
Hoagland and Sharp (13) explain that 
The criteria heretofore used for judging the reaction of soils do not always permit of an 
accurate distinction between soils of different reactions. Some soils may be judged as acid 
from the standpoint of certain lime requirement methods, when in reality their reaction 
may be alkaline. 
Hoagland (11) calls attention to the common assumption that most 
agricultural plants require a slightly alkaline reaction in the soil, and cites 
the fact that good crops of barley, oats, beans, potatoes, onions, corn, and 
asparagus were grown on a California peat soil having an acid reaction 
ranging from pH 4.5 to 5.4. In another paper Hoagland (12) reports experi¬ 
ments with the growing of barley seedlings in partial nutrient solutions of 
like osmotic pressure but having a considerable range of hydrogen- and 
hydroxyl-ion concentrations. For similar divergences from the neutral 
point, the hydroxyl ion was found to be more toxic than the hydrogen ion. 
A reaction of approximately pH 8.3 was found to be distinctly injurious, 
while a reaction of pH 5.0 produced no injury. 
Little information is available concerning the behavior of fungi in 
general toward varying degrees of hydrogen- and hydroxyl-ion concentra¬ 
tion. Webb (22) has recently reviewed the literature on the toxicity of 
acids and alkalies to the growth of fungi. Most of the information given 
in the literature is stated in such terms as “alkaline, ” “slightly acid,” 
“strongly acid,” or as some percentage of acid or base added. Webb (22) 
experimented with spore gemination in culture solutions adjusted to various 
hydrogen-ion concentrations. He found that increasing concentrations of 
hydrogen ions from approximately neutral to pH 3.1 influenced favorably 
the germination of spores of all the forms studied. Germination was not 
inhibited until a concentration of pH 2.8 was reached. Spores of a species 
of Fusarium were found capable of germinating over the entire range from 
pH 2.8 to 10.0. 
Meacham (16) reports that the growth of four wood-destroying fungi, 
cultured in synthetic and malt-extract media, was not inhibited until 
approximately pH 3.0, and that the limiting value was approximately pH 
1.7. Hawkins and Harvey (10) tested the growth of Pythium debaryanum 
potato-juice cultures adjusted to known hydrogen-ion concentrations with 
sodium phosphate buffer mixture. The fungus grew well in all cultures 
ranging from pH 3.4 to 6.8. Growth was poor in the cultures adjusted to 
pH 3.1 and in those below the neutral point testing pH 7.3 and 8.4. 
In his study of the growth of the common potato-scab organism at 
various hydrogen-ion concentrations, Gillespie (7) found that the growth at 
pH 5.2 was slower and generally less vigorous than at higher concentrations. 
While some of the strains tested grew at pH 5.2, the growth in these cases 
was accompanied with a marked decrease in acidity. Gillespie and Hurst 
(8) report the examination of a considerable number of soils from northern 
Maine and show that a very close correlation exists between the hydrogen- 
