Apr., 1922] HOPKINS — HYDROGEN-ION CONCENTRATION I7I 
Fig. 10. Soil acidity and seedling infection — pn 4.65, pn 4.8, pn 5.5, check. 
Infection in the seedlings was first observed ten days after planting, 
and frequent counts were made of the total number of seedlings as well as 
of those known to be infected. Figures 9 to 13 show the appearance of the 
seedlings as contrasted with one of the uninoculated, untreated control 
flats. The thinness of stand and the wilting of the plants in the first three 
flats (the most acid) show a striking contrast to the condition of the control. 
At a Ph of 5.5 the stand is more nearly perfect, but as we approach the more 
alkaline soils the stand again becomes poor until at a Ph = 9.0 there are no 
seedlings up. 
Fig. II. Soil acidity and infection — pn 5.7, pn 5.8, pn 5.9, check. 
At the end of three weeks all seedlings were removed and the number of 
infections was recorded. Many seedlings which appeared healthy were 
found to be infected below the surface of the soil. In the most acid and 
most alkaline soils many seedlings which had germinated were rotted before 
reaching the surface of the soil. In most cases the greater amount of injury 
was due to stem rot, although in the very alkaline soils more root rot was 
observed. 
