148 
If we leave out of the calculation this last cause of loss, which it is 
impossible to estimate and which is doubtless of little importance 
under ordinary circumstances, we shall find that a piece of arable 
land of average quality loses, by exhaustion from the crops, the infil- 
tration of rain water and the ammonia which it disengages, an 
amount of nitrogen equal to a minimum of 120 kilograms per hectare 
annually. Therefore, as its soil contains scarcely ‘10. 000 kilograms, 
its exhaustion would be complete in less than a century if these losses 
were not compensated by gains of about the same extent. Let us 
now examine into these causes of gain. 
The soil receives nitrogen principally by the fertilizers given to it. 
Their proportion and richness are very variable, but experience 
shows that in general they do not suffice to supply the loss occasioned 
by cultiv ation alone. The difference which is found between the 
quantity of the nitrogen contained in the crop and that contained in 
the fertilizers is sometimes very great. Boussingault, to whom we 
are indebted for very precise researches on this subject, mentions a 
field where lucerne grass and wheat were cultivated, which having 
originally received 225 kilograms of nitrogen in the form of manure, 
furnished in a space of six years 44,000 kilograms of dry hay and 
5,550 kilograms of wheat, straw, and grain, containing altogether 
1, ‘078 kilograms of nitrogen. The total excess, 854 kilograms, amounts 
in this case to a little more than 140 kilograms per hectare annually. 
In general, this difference is less, but, i repeat, it is always in the 
came direction and may be estimated on an average at 30 or 40 kilo- 
grams annually; it remains then for us to provide for this excess, 
increased as it_is by the losses caused by drainage of nearly 100 kilo. 
grams per hectare annually. 
The most diverse and sometimes the most improbable reasons have 
been brought forward to account for this fact. It has even been sug- 
gested that the atmospheric dust acted as a natural fertilizing agent; 
but let us go on to more serious hypotheses. It has been thought that 
the rain water in taking from the air its soluble compounds might fur- 
nish a certain proportion of ammonia or nitric acid to the soil. 
Analysis has shown that this proportion is extremely small; water 
caught in a rain gauge contains, indeed, only a mere trace of nitric 
substances, scarcely 2 grams of ammonia and less than 1 gram of 
nitric acid per cubic meter, which corresponds to a maximum of 5 to 
§ kilograms of nitrogen a year per hectare. This quantity would, 
then, be barely sufficient to compensate for the losses due to the gase- 
ous ammoniacal emanations from the earth. 
On the other hand, Schloesing admits that the earth, and the plants 
by means of their foliage, directly attract the ammonia existing in 
the air. This ammonia, according to the learned agronomist, is con- 
stantly emitted by the sea water, which thus restores to us under 
another form the nitrogen which is constantly being brought to it 
by the drainage water. 
It is certain that humid soil can attract the ammoniacal vapors, 
but it is also certain, as proved by the experiments of Boussingault, 
that such soil can also emit them; there is, therefore, a tendency to 
establish, in this respect, an equilibrium between the soil and the 
atmosphere, the result of which is probably not far from a perfect 
compensation. 
If, then, it is true that the leaves of plants can assimilate gaseous 
