THE NUTRITION OF THE ROOT 43 



2 I per cent, consisting mainly of oxygen. This uneombined 

 nitrogen is only available in any appreciable quantity for 

 leguminous plants, which possess, as we now know, the faculty 

 of growing in a soil entirely devoid of nitrogen ; for though 

 it may possibly take part in the nutrition of other cultivated 

 plants, at least to a very slight extent, it must be absorbed by 

 their roots in a combined form. On the other hand, the small 

 green Algffi, which are present in every soil, seem to have the 

 power of making use of and of elaborating the free nitrogen, 

 and may therefore be looked upon as nitrogen purveyors to 

 the soil. The nitrogen compounds which are most important 

 for the nutrition of plants are ammonia, nitrous oxide, and 

 nitric acid. This combined nitrogen, however, which alone 

 can be assimilated by the roots, is not derived from the atmos- 

 phere except to a very slight extent. The decomposition of 

 nitrogenous organic substances liberates ammonia into the air, 

 and the lightning Hashes of a thunderstorm cause some free 

 nitrogen to combine witli ozone to form nitrous acid (ozone 

 and hydrogen peroxide also being produced). Besides this, 

 ammonium nitrite is formed in every process of oxidation, and 

 even during the evaporation of water. 



When the water-vapour of the air condenses into drops, 

 ammonia is enclosed in them, and more is absorbed by the 

 drops when falling as rain through the air. Dew, hail, and 

 to a lesser extent snow, all contain ammonia. Eain, too, 

 especially during a thunderstorm, contains some nitric acid. 

 These available nitrogen compounds are thus washed into 

 the soil by natural agencies, or may be (but only to a very 

 slight extent) absorbed directly out of the air. But the 

 absorptive power of a soil for nitrogen depends upon its 

 composition. The several constituents of the soil do not 

 only vary in their capacity of absorbing nitrogenous com- 

 pounds, but they also vary according to the form in which 

 they occur. Humus compounds, gypsum, carbonates of lime, 

 and magnesia, for instance, take up very little nitrogen ; but 

 if humic acid combined with calcium occurs in the soil together 

 with decomposing silicates, a very considerable absorption of 

 nitrogenous compounds takes place. From experiments (of 

 Bretschneider) it has been calculated that if water absorbs 



