Igt5] LIPMAN & SHARP—NITROGEN FIXING 405 
8 per cent is present, and amounts of moisture equivalent to 12. 
per cent (air dry basis) give about the same nitrogen fixation as 
32 per cent moisture. 
To comment on and summarize the interesting results above 
given we may make the following statements. 
1. Nitrogen fixation by a soil’s natural flora being the alge- 
braic sum of the activities of several classes of bacteria both aerobic 
and anaerobic, it must follow that the greatest fixation of nitrogen 
will occur at a moisture content very favorable for the most active 
forms of nitrogen fixing bacteria, and yet not entirely unfavorable 
for the less potent forms, or vice versa. In the soil in question 
that point seems to lie between the limits of 20 per cent and 24 per 
cent of moisture (air dry basis). It would appear just to conclude 
from these data that the aerobic forms of nitrogen fixing bacteria 
do best with a 20 per cent moisture content (the optimum for that 
soil on a physical basis). At higher percentages of moisture up to 
24 per cent the anaerobic forms become much more active, while 
the aerobic forms are depressed in their nitrogen fixing powers. This 
gives us two maxima of nitrogen fixation in that soil based on the 
moisture content or what appears from the table to be a curve 
which runs along at the same plane between rather wide limits. 
2. The case of nitrogen fixation with respect to soil moisture 
content, therefore, would seem to be analogous to that of ammoni- 
fication as studied by LrpMAN and Brown as referred to above. 
In both cases the end products which are measured represent the 
algebraic sum of the activities of both aerobic and anaerobic or- 
ganisms. 
3. We find in confirmation of the findings of Krarnsky, above 
mentioned, that nitrogen fixation is very active even with low mois- 
ture content of the soil. Thus, with a moisture content of only 
8 per cent very considerable quantities of nitrogen are fixed, and, 
to judge from the excellent agreement between duplicate nitrogen 
determinations in our work, appreciable quantities of nitrogen 
are fixed with a moisture content of only 4 per cent. 
4. Taken as a whole, the nitrogen fixing flora of the soil with 
which we worked, and which may be taken as a criterion for a large 
variety of sands and sandy loams, behave much more like the 
