938 
Journal of Agricultural Research 
Vol. V, No. 20 
that source. As seen in Table V, the loam soils and muck show a higher 
count six months from the time of inoculation than do the clay and 
sand. During the first six weeks all five soils contained an amount of 
moisture sufficient for bacterial growth, and during the last two months 
only were the soils in the air-dry state. The amount of activity in the 
period intermediate between the optimum and minimum supply of 
moisture shows a gradual decrease, the rate varying in the different 
soils. 
While there was not a great difference in the initial counts, the oppor¬ 
tunity for bacterial growth in the five types of soil was by no means 
the same. This is clearly evidenced by the contrast between their 
counts during the first period, when the moisture content was yet suffi¬ 
cient to permit multiplication. Since the sand was saturated with the 
amount of soil solution used as an inoculum, it at first presented condi¬ 
tions more favorable to anaerobic than to aerobic species. As this 
amount of moisture diminished and the oxygen supply increased, oppor¬ 
tunity for the growth of aerobic types was given, but the extent of this 
favorable period was limited not only by the small amount of organic 
food material but also by the extremely rapid evaporation of moisture. 
Conditions in the clay were at first comparable with those in the sand, 
it being practically waterlogged. With the gradual reduction in mois¬ 
ture and increase in aeration, the growth of aerobic and facultative 
bacteria proceeded. The smaller size of the grains produced two notice¬ 
able effects—viz, a limited oxygen supply, inhibitory to the extensive 
multiplication of aerobic species, and a prolonged retention of moisture, 
which favored the longevity, if not the activity, of non-spore-bearing 
bacteria. As in the sand, a low content of organic nutrients acted as a 
natural limit to the growth of saprophytic species. In the clay loam, 
sandy loam, and muck multiplication was possible from the start, for 
the amount of solution used for inoculation was just sufficient to mois¬ 
ten the soils without saturating them. Their higher content of organic 
substance also gave them an advantage in respect to nutrition. 
However, in these soils also differences in size of grain, thickness of 
moisture film, and oxygen supply proved to be factors of more influence 
than the mere abundance of organic food substance. The muck, for 
instance, although containing the highest percentage of such organic 
materials, proved to be of a less favorable medium for bacterial growth 
than did the clay loam. The grain size of the clay loam appeared to be 
that which was most advantageous with respect to aeration, thickness of 
moisture film, and retention of hygroscopic water. Its content of 
decomposable substances, while not so great as that of the muck, was 
more than sufficient for microbial development. The sandy loam, with 
a smaller amount of organic materials, somewhat larger grain size, and 
consequently less hygroscopicity, did not show as large numbers of living 
