1915] LIPMAN & SHARP—NITROGEN FIXING 403 
35 per cent of the weight of the soil. They found, however, that 
nitrification was most active in the same soil with a moisture con- 
tent of 15 per cent, was only slightly less active with 10 per cent 
of moisture, and even quite appreciable with 5 per cent of moisture. 
CoLEeMAN in working with a loam soil found nitrification in it most 
active with a moisture content of 16 per cent, thus agreeing with 
the results of Lipman and Brown. Contrary to the results of the 
latter, however, CoLEMAN obtained marked reduction in nitrifica- 
tion when the moisture content of the soil was reduced to 10 per 
cent, but again obtained similar results to those of the other investi- 
gators when the moisture content of the soil was increased to 26 
per cent. DEHERAIN’S findings, in work with the nitrifying flora, 
were in harmony with those of the foregoing investigations on 
nitrification. KRarinsky in working with the nitrogen fixing flora 
found nitrogen fixation considerable even in soil with less than 
one-fourth of the optimum moisture content. 
In our work on nitrogen fixation a light sandy soil from a wal- 
nut grove in Anaheim, California, was employed, and the natural 
nitrogen fixing flora thereof studied in its relations to moisture. The 
soil culture method was used in which 50 gram portions of soil were 
mixed in tumblers with 1 gram of mannite and water added in 
varying quantities as indicated in the table below. The mixture 
was stirred with a sterile spatula, the tumblers covered with Petri 
dish covers and incubated at 28°-30° C. for 21 days. Other ex- 
Planatory data along with the amount of nitrogen fixed in the cul- 
tures with varying quantities of moisture are given below in table I. 
It is evident from the figures in the foregoing table that nitrogen 
fixation in a sandy to sandy loam soil by means of its natural flora 
and under optimum temperature conditions takes place most 
actively with a water content varying from 20 to 24 per cent based 
on the air dry weight of soil, or 22.5 to 26.5 per cent based on the 
water free soil. Even with 28 per cent of moisture (air dry basis), 
nitrogen fixation manifests an activity but little less potent than 
that just mentioned. With a moisture content of 32 per cent a 
marked decrease in nitrogen fixing power of the soil is evident, and 
a still greater decrease is noted with the largest water content em- 
ployed, namely 36 per cent. None the less, it should be noted 
