i68 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9, 
crease in acidity is due to the oxidation of the sulphur to sulphuric acid. 
He ascribes the advantage of the inoculated sulphur to its more rapid con- 
version to the acid. 
In the experiments reported in this paper, the effect of hydrogen-ion 
concentration upon seedling infection of wheat by Gihherella Sauhinetii is 
reported. The experiments were carried out under greenhouse conditions 
in soil in flats. The reaction of the soil was adjusted by means of sulphuric 
or hydrochloric acid and sodium hydroxide. 
Experimental 
Soil-Acidity Determinations. These were made according to the method 
of Gillespie (2). The soils were air-dried and passed through a millimeter 
sieve. Thirty grams of this soil were placed in a loo-cubic-centimeter 
centrifuge tube with 30 cubic centimeters of water, the top was closed with 
the palm of the hand, and the mixture was shaken violently about fifty 
times. The tube was then centrifuged for about fifteen minutes, and the 
acidity was determined colorimetrically in the supernatant liquid by means 
of Gillespie's drop-ratio method. Determinations were checked when 
possible by using more than one indicator. Gillespie's standards were 
checked against standard phosphate, phthalate, and acetate buffer mix- 
tures, which were in turn checked by electrometric measurements. The 
agreement was to o.i of a pH in all cases. 
Experiment 4. To a rich loamy soil about one fourth sand was added 
and the whole was well mixed. Eighteen kilograms were then weighed 
into each of nineteen flats. The dimensions of the flats were 12 x 18x6 
inches. A sample of the soil was divided into 250-gram portions, and to 
each was added a given amount of Nji H2SO4 or Nji NaOH made up to a 
volume of 40 cubic centimeters with water. After mixing the soil and the 
acid or alkali well and preparing a sample as described above, the hydrogen- 
ion concentration was determined as described above. From these data 
a rough preliminary titration curve was constructed. From this curve 
desired values could be obtained to use in adjusting the reaction of the 
larger portions of soil. The original soil had a pn of 5.9. The effects of the 
acid and alkali on the soil in the flats are shown in table 5. 
• Table 5. The Effect of Sulphuric Acid and Sodium Hydroxide on the Reaction of the Soil 
Used in Experiment 4 
Treat- 
ment 
Cc. Nil H2SO] per 250 G. Soil 
Cc. Nil NaOH per 250 G. Soil 
25 
30 
15 
10 
7 
3 
2 
I 
0 
2 
1 
5 ! 7 
10 
IS 
20 
25 
pH 
34 
3.6 
3.8 
44 
4.6 

5-3 
5-5 
5-6 
5-9 
6.4 
6.6 
6.9 7.6 
7.9 
8-3 
8.6 
9.0 
