BY JAMES M. PETRIE. 803 



countries, is 1 pound per 26 million cub. yards. Rain water 



contains — 



Rain. Dew. 



Ammonia 1 to 9 1 to 3 ) parts per 



to 6 "i parts 

 to 16 i milli 



Nitrous and nitric acids 2*4 2 to 16 S million. 



The Rothamsted experiments have shown that there is carried 

 down by the rain every year and added to the soil 



2*4 lbs. N per acre, as ammonia. 



1 -0 ,, as nitrates and nitrites. 



10 ,, as organic nitrogen. 



4*4 ,, total combined N (average over 5 years). 



Now there is removed in a crop of wheat from unmanured soil 

 20 lbs. of nitrogen per acre yearly, and Gilbert has said that the 

 amount of fixed nitrogen received by the annual rainfall is 

 approximately balanced by the loss of nitrates through drainage, 

 it is evident that there must be other ways in which combined 

 nitrogen is added to the soil. 



In the first place (a) we know that an acre of clover will add 

 to the ground 200 lbs of N per annum, (b) Since Winogradsky 

 isolated the bacillus Clostridium pasteurianum from the soil, 

 many other organisms have been found which can utilise the 

 atmospheric nitrogen directly in forming their protein substances; 

 such are the group of Clostridia, Azotobacter, Radiobacter, B. 

 radicicola, B. prodigiosum, etc. It has been estimated that 

 70 lbs. of N per acre are yearly added in this way, and the 

 organic compounds formed made available to plants, (c) Every 

 drop of water that passes into vapour on the earth's surface is 

 accompanied by an energy-change, which combines the atmos- 

 pheric nitrogen to the water-molecules and produces ammonium 

 nitrite [N 2 + 2H 2 0= NH 4 N0 2 ]. This important fact, first 

 noticed by Schonbein, probably accounts for a large amount of 

 the nitrogen fixed and added to the soil, since there is constant 

 evaporation of water from the ground and even from the surface 

 of plants. In this way, says Thorpe, the plant may indeed 

 prepare for itself a portion of its nitrogenous food. 



