II Jan., 1909.] Nitrogen and Nitragin. 37 



When a soil dries up in which nitrification has been acti\e, the nitrates 

 may sometimes be so abundant as to form an efflorescence on the surface 

 of the soil. Since all these nitrates are, however, soluble in water, any 

 heavy shower of rain will tend to wash out the excess of nitrates from the 

 soil before the roots have time to absorb them. Hence there is far less 

 danger of loss, and a more permanent effect is produced when the nitrogen 

 needs of the plant are supplied by the bacterial oxidation of humus manures 

 applied to the soil, than when the same amount of nitrogen is directly 

 applied in the form of a dressing of Chilian saltpetre ^ sodium nitrate). 



Although the air, which contains four-fifths by volume of nitrogen, 

 represents an enormous total amount of this element, it is not directly of 

 use except in a few special cases. Ordinary green plants can make no 

 use whatever of the free nitrogen of the air. It enters the plant, and is 

 found dissolved in the sap in everv part, but cannot be assimilated or used 

 as food any more than horses can digest the sand which they often swallow 

 when grazing. Every thunderstorm, however, and also certain slow chemical 

 processes of oxidation like that of phosphorus in moist air cause the 

 oxygen and nitrogen of the air to combine, ultimately forming nitric acid, 

 which is washed down bv the first shower of rain. In the same way, when- 

 ever the air contains ammonia derived from manure heaps, or from de- 

 caying animal or vegetable matter, this is washed down by the rain, and 

 oxidized to nitric acid and nitrates in the soil. 



The amount of combined nitrogen available for the plant's use which 

 reaches the soil in this manner, although quite appreciable, is never more 

 than a small fraction of that removed from the soil by drainage and by 

 the crops. Under the most favourable circumstances it does not represent 

 more than a tenth or twelfth of the annual loss of nitrogen from a well- 

 drained, cultivated soil with an average rainfall, and is usually consider- 

 ably less. 



Certain organisms exist, however, in most soils which have the power of 

 assimilating the free nitrogen of the air and ultimately enriching the soil 

 with combined nitrogen available for the plant's use. The most important 

 of these organisms is Clostridium Pasteurianum, an anaerobic bacterium, 

 which is unable to exist in ordinary soils unless supplied with free nitrogen 

 and unless associated with certain other micro-organisms. The latter shield 

 it from the oxygen of the air, which is poisonous to this organism. If all 

 the required conditions were fulfilled, soils containing this bacterium might 

 gain from 20 to 30 lbs. of nitrogen (= 120 to 180 lbs. of sodium nitrate) 

 per acre per annum. As a matter of fact, the actual gain due to the 

 presence of this and similar micro-organisms appears usually to be com- 

 paratively small, partly because the conditions are rarely the best possible 

 and partly because denitrifying bacteria are usually also present which 

 set free the combined nitrogen of the soil and may in some cases cause 

 a loss instead of a gain to occur. In addition, the conditions which favour 

 the fixation of nitrogen by soil bacteria not dependent upon leguminous 

 plants are, in general, not the most suitable ones for the development of 

 ordinary crops. 



It is well known that leguminous plants differ from ordinary ones in 

 having the power of assimilating the free nitrogen of the air, and hence 

 can exist on the poorest and sandiest soils where nitrates are practically 

 absent or very deficient in amount. They have, however, this power only 

 when peculiar tubercles are developed on their roots which contain nitrogen 

 fixing bacteria derived from the soil or originally present on the coats of 



