Apr., 1922] HOPKINS — HYDROGEN-ION CONCENTRATION I67 
agar. The deepest minimum appears after 19 hours. After 43 hours there 
is but a shght depression. The minima appear at a pn of 5.0-5.5. 
3 H S <> 1 t 
Pj, Potato Agar 
Fig. 6. Hydrogen-ion concentration and growth of G. Sauhinetii on potato-dextrose 
agar after nineteen, twenty-four, and forty-three hours — lactic-acid series. 
The Relation between the Hydrogen-ion Concentration of the 
Soil and Seedling Infection of Wheat 
BY Gibberella Saubinetii 
Gillespie and Hurst (3, 4) have investigated the acidity of soils in which 
potato scab was prevalent and of those relatively free from scab. They 
found the mean value of the exponent for the Washburn type of soil, on 
which scab is prevalent, to be pn 5-93, while that of the Caribou type, which 
is generally free from scab, was pn 5-2. This agrees with the results of 
Gillespie (lo), previously discussed, that the growth of the potato-scab 
organism is inhibited at hydrogen-ion concentrations between pH 5.2 and 
Ph 4-8. 
More recently Martin (15, j6), in working on the effect of inoculated 
and uninoculated sulphur on the development of potato scab, determined 
as well the effect of the sulphur on the hydrogen-ion concentration of the 
soil. He found that increasing the amounts of sulphur applied to the 
soil increased the hydrogen-ion concentration of the soil and correlated 
this increase with the decreasing percentage of scabby tubers. The in- 
