Oct. 8, 1917 
Nitrate-Nitrogen Accumulation in Soil 
53 
of soil is, of course, the same as in the loose condition—that is, 41 c. c. 
This leaves 39 c. c. to be occupied by air and moisture. From Tables 
IV and V it appears that 25 c. c. of water gave approximately opti¬ 
mum moisture conditions for nitrification. This leaves 14 c. c., or 17.5 
per cent, of free pore space necessary for efficient aeration. Similarly 100 
gm. of compact soil occupied approximately 60 c. c. of space. Subtract¬ 
ing the 41 c. c. of soil volume leaves 19 c. c. for air and water. On refer¬ 
ring to Table V it is observed that 20 c. c. of water per 100 gm. of soil did 
not permit sufficient aeration in the lower strata, while 15 c. c. did. There¬ 
fore, with an air content of 4 c. c. per 100 gm. of soil, or 6.33 per cent of 
free pore space, oxygen was supplied in sufficient quantities for maximum 
nitrification at a depth of 12 inches. Stoklasa (13) says that if the free 
pore space falls below 2 per cent anaerobic processes set in. Even though 
these figures are meager and only approximations, it is evident that 
increasing the moisture content of a soil not only decreases aeration by 
actually decreasing pore space, but also increases the demands for 
oxygen, and, hence, the free pore space necessary to supply sufficient 
oxygen for maximum aerobic processes. This is probably due to a 
number of factors. According to Rahn (10), the soil-moisture film should 
be 10 to 20 fjL thick for optimum aerobic conditions. Therefore, increasing 
the moisture content until this thickness is reached should increase 
aerobic activity, provided saturation does not exclude aeration. Rahn’s 
views are in accord with daily observations that increasing moisture 
up to a certain point increases bacterial activity. In the soil under 
study saturation will result before this optimum film thickness is 
reached. One would expect, therefore, that the more moisture added 
until saturation inhibits aeration, the greater the aerobic activity will 
be, and consequently the greater the demands for oxygen. The writers 
observed that increasing the moisture content of a similar soil in loose 
condition from 12 to 20 c. c. per 100 gm. of soil, or from 20 to 33 per cent 
of saturation, increased nitrate accumulation 4.8 times. Increasing the 
same from 20 to 30 c. c., or from 33 to 50 percent of saturation, increased 
N 0 3 accumulation 2.6 times that which occurred at 20 c. c. A further 
increase of from 30 to 42 c. c., or from 50 to 70 per cent, increased the 
nitrate accumulation 1.4 times. There is no reason for not believing 
that other aerobic processes are increased in similar proportions. 
Then, too, increasing moisture content decreases aeration at a greater 
rate than it does porosity; especially is this true with relative high mois¬ 
ture contents. This is due to the fact that, as the water content increases 
the thickness of the films, the continuous air tubes upon which rapid 
exchange of gases is dependent are closed at the narrow points much 
faster than they are actually filled with water. When all such passages 
are closed and all available oxygen consumed, aeration will be* inhibited 
regardless of the free pore space remaining. 
