DAMPIXG-OFF IN FOREST NURSERIES. 77 
plats on which the seedling counts were conducted, each determina- 
tion representing two, and in some cases four, points. The deter- 
minations for the upper one- fourth inch of soil, made more frequently 
than for lower levels, are connected in figure 20 by a dotted line. 
which gives some idea of the amount of moisture in. the surface soil 
during the periods between determinations. The determinations were 
too infrequent to permit anything more than an estimate of the 
moisture conditions between determinations, but the writer, having 
before him the records of the times and amounts of rainfall and 
artificial watering as well as the evaporation and soil-moisture de- 
terminations, is in a better position to make such an estimate than 
the reader. The dotted line which gives this estimate should not be 
depended on to show what the percentage of moisture was at any one 
time, but is believed reasonably reliable as showing whether in gen- 
eral the soil was wet or dry. In interpreting the soil-moisture rec- 
ords, it should be kept in mind that the soil was very sandy, the 
wilting coefficient of composite samples from various parts of the 
nurser} T , as determined by the indirect method of Briggs and Shantz 
in the Laboratory of Biophysical Investigations of the Bureau of 
Plant Industry, being only 3.4 per cent. The hygroscopic moisture 
in dry air for the soil of the plats actually under consideration was 
indicated by repeated determinations for the surface soil on dry days 
to be in the neighborhood of or slightly below 2 per cent. The 
nursery is located in a region of large temperature fluctuations, where 
the air during the day is generally dry, and consequently the dew 
is heavy at night. 
The first result of interest is the difference between the damping- 
off for the day and the night periods. In the records of every 
day but two, more seedlings went down during the day period than 
during the night, the differences in most cases being large. As the 
evaporation and temperature showed similar day and night fluctua- 
tions, it is difficult to say whether temperature or moisture condi- 
tions were responsible. The other interesting result brought out by 
the graphs is the sudden drop in the general level of the damping- 
off graph following the rains of June 15, June 19-20, and July 3. 
In each of these three cases the damping-off came up again only 
after the soil moisture came down. 
The fact that in the daily fluctuations the damping-off varied 
directly with the evaporation rather than inversely is an apparent 
contradiction of the generally accepted doctrine that moisture favors 
the disease. This contradiction is, however, only apparent. Dur- 
ing the first part of the damping-off period, when the seedlings are 
still soft, the recognition of damping-off depends on the decay of 
that part of the stem just above the soil surface which allows the 
seedling to fall over. This usually takes place at this nursery as 
