143 



If wo leave out of the calculation this last cause of loss, which it is 

 impossible to estimate and which is doubtless of little imi)()i-tance 

 under ordinary circumstances, we shall find that a piece of arable, 

 land of avera<ie (piality loses, by exhaustion from the crops, the infil- 

 tration of rain water and the anunonia which it disen<»;a<jes, an 

 amount of nitrogen e(iual to a minimum of 120 kilograms j)ei- hectare 

 anmuilly. 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 experienc(^ 

 shows that in general they do not suffice to supply the loss occasioned 

 by cultivation 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 

 •VooO 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 kilogi'ams per hectare annually. 



In general, this diflerence is less, but, I repeat, it is always in the 

 same direction and may be estimated on an average at 80 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 hyijotheses. It has l)een 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 nitri(; 

 substances, scarcely 2 grams of ammonia and less than 1 gram of 

 nitric acid per cul)ic meter, which corresponds to a nuixinnnn of 5 to 

 8 kilograms of nitrogen a year per hectare. This quantity would, 

 then, be barely sufficient to compensate for the losses due to the gase- 

 ous annnoniacal 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 iM 

 the air. This annncmia, according to the learned agronomist, is con- 

 stantly emitted by the sea water, w4iicli 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 annnoniacal 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 resj)ect, an equilil^rium betAveen 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 



