52 
Journal of Agricultural Research 
Vol. XI, No. 2 
When the moisture content was increased to 25 c. c. per 100 gm. of 
soil, the total nitrate accumulation in the loose and medium samples 
was greatly increased, indicating the possibility of less loss of ammonia 
by volatilization with the higher moisture content. Here again the 
nitrification was incomplete in the surface soil of both the loose and 
the medium samples. The nitrate content also increased as the depth 
increased, except in the lower strata of the medium packed, where the 
nitrification apparently was not so active. In the compact samples the 
nitrate accumulation was inhibited except in surface soil. This soil, 
however, was in a saturated condition. 
The evidence thus far submitted seems sufficient to show that increasing 
the depth of a column of soil, up to at least 2 feet, increases the for¬ 
mation or rather the accumulation of nitrate nitrogen from ammonium 
sulphate, provided the moisture content is not so high that aeration is 
prevented. In the type of soil under study aeration will be sufficient 
for optimum nitrification, at least up to two-thirds the total amount of 
water the soil will retain with any degree of compactness tried. Pack¬ 
ing, of course, decreases the water-holding power almost in the same 
proportion as the volume of soil is decreased; hence, the quantity of 
water necessary to limit aeration is correspondingly less. In as com¬ 
pact condition as it was possible to obtain by the methods used and 
with a moisture content at least up to 15 c. c. per 100 gm. of soil, nitri¬ 
fication was more active a foot below than nearer the surface. - With 
20 c. c. of moisture per 100 gm. of soil the nitrate accumulation a foot 
below the surface on very compact soil was far in excess of any nitrifi¬ 
cation ever observed under field conditions. Compacting this soil type 
to two-thirds its loose volume increased the nitrate accumulation with 
any moisture content up to 25 c. c. per 100 gm. of soil. 
From the data available one can estimate roughly the free pore space 
necessary to supply oxygen in sufficient quantities for maximum nitrate 
accumulation at the depths to which these experiments were carried out. 
They have not been conducted to greater depths because under our 
field conditions practically all the nitrate formation takes place in the 
first foot of soil. One hundred gm. of this soil in loose condition occupies 
approximately 116 c. c. of space. The specific gravity of this soil is 
2.4. Therefore 100 gm. of soil actually occupy approximately 41 c. c., 
thus leaving 75 c. c. of space to be occupied by water and air. From 
Tables III and IV it appears that 36 c. c. of water (which is two-thirds 
that the soil will retain) is approximately the highest moisture content 
permitting sufficient aeration for optimum nitrification in loose condi¬ 
tion. Thirty-eight c. c. did not permit sufficient aeration, while 36 c. c. 
permitted complete nitrification of added nitrogen. Accordingly 39 c. c., 
or 33-6 per cent, of pore space were required for optimum aeration. 
On applying similar calculations to the medium-packed samples, 100 
gm. of soil occupied approximately 80 c. c. of space. The actual volume 
