14 
The nature of the reaction accompanying the absorption of am- 
monium compounds is very similar to that of potash salts ; namely, 
the replacing of calcium in humus, double silicates, and in some cases 
calcium carbonate. Hence the application of ammonium salts as 
fertilizer tends to deplete the soil of its basic constituents. 
It may be seen from a comparison of the preceding tables that the 
fixation of nitrogen is far in excess of that of potash in every instance 
except soil No. 292, which is the highest in magnesia content. The 
fixing power of the four soils in the series agrees more closely than in 
the potash series, but in each instance the clay soil fixed the least. 
Attention is called to soils Nos. 428, 448, and 474, which absorb much 
more nitrogen than potash. In case of two of the soils (428 and 474) 
this may be accounted for by the high content of organic matter. 
In the last two, fractions of percolate nitrates and nitrites were de- 
termined and both were found to be present in one case to the extent 
of 14.4 parts per million N as N0 3 and 3.1 parts per million N as N0 2 . 
This indicates the rate at which nitrification was going on at the close 
of the experiments. 
As in the potash series, the highly basic soils fixed much more 
nitrogen at the beginning of the experiments and a much larger total 
amount than the less basic. On the other hand, the decrease in fixing 
power was much slower and more gradual in the other soils. 
SODIUM NITRATE. 
Of the salts commonly used as fertilizing materials all are strongly 
fixed by the soil except nitrates. However, nature has made a wise 
provision for retaining nitrogen in an insoluble form, which becomes 
slowly available for growing plants. Determinations of the amount 
of nitrate nitrogen removed from the original soils gave the following 
results : 
Nitrate nitrogen removed from the soils by distilled water. 
[Expressed in parts per million nitrogen in the percolate.] 
Percolates of 100 cc. each. 
Soil 
No. 292. 
Soil 
No. 448. 
Soil 
No. 428. 
Soil 
No. 474. 
100 
4.2 
2.4 
.0 
8.6 
.0 
.0 
5.9 
.0 
.0 
106 
200 
2 
300 
.4 
These data indicate a condition found to be true in all soils, namely, 
the readiness with which nitrates are leached from the soil by rains. 
Soil No. 474 is a very porous, floury soil, containing a high percentage 
of organic matter, and under the existing climatic conditions would 
be expected to have a high nitrate content. 
