134 
AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
particularly unfavorable. He further notes that by promoting a diseased 
and sickly condition in the roots, poor soil aeration may be a factor of great 
importance in facilitating the attacks of injurious root fungi of this class. 
On the other hand, in a few instances diseases have been reported as 
being less severe with the soil saturated than otherwise. Appel (i) reports 
that alder trees in Germany suffer more severely from the parasite Valsa 
oxystoma when they are growing in meadow-land than when they are in 
their natural swampy habitat. Peltier (13) states that when carnation 
plants growing in soil inoculated with Rhizoctonia were given a heavy 
watering and the soil was then allowed to dry out, they were killed more 
rapidly than were plants growing under the same conditions, except that 
the soils were continually over-watered. 
Concerning the effects of medium and low moistures there is very little 
which can be said definitely. Johnson found that whether the soil was 
one fourth, one half, or three fourths saturated made very little difference 
in the amount of Thielavia root rot which appeared. 
The relation of soil moisture to the growth of autotrophic plants has been 
the object of considerable investigation, and in general the results indicate 
that there is a very wide range of moistures at which plants will grow well. 
Fowler and Lipman (5), reporting on work with lemon trees, state that the 
range of optimum or nearly optimum conditions of moisture is relatively 
wide. They also found that, as the moisture content was raised above the 
optimum, the growth curve broke sharply. As moisture decreased from 
the optimum, however, the rate of growth fell off very gradually. Thus the 
^moisture curve was characterized by a gradual rise to the optimum and a 
sharp drop from the optimum to saturation. Kiesselbach (10) secured 
best growth of corn at 60-percent saturation. He noted also that the plants 
grew well from 20-percent to 98-percent saturation. Harris and Maughn 
(6), working with wheat, secured the highest yield of grain with a soil having 
a moisture content of 20 percent throughout the growing season, this being 
equal to approximately two-thirds saturation. The problem of soil moisture 
in relation to the growth and activity of soil fungi has received little atten¬ 
tion. Waksman and Cook (15), and more especially Coleman (4), have 
published results of experiments in which they grew a variety of soil-inhab¬ 
iting fungi in soil cultures of different moisture contents. The results 
showed that the moisture requirements of fungi differ considerably. Thus 
a dry medium favored maximum growth of Aspergillus niger , while Tricho- 
derma koningi grew best on a moist medium. 
Methods of Controlling Soil Moisture 
Prior to beginning these experiments, a survey was made of the literature, 
with the object of securing information on the technique of controlling soil 
moisture. In what might be called the “practical” experiments, involving 
economic crops, the custom has been to weigh the plants, container, and 
