NITROGEN ASSIMILATION; THE NITROGEN CYCLE 89 



vided the necessary carbon dioxide and water (Fig. 5). There was 

 no source of nitrogen other than that in the air surrounding the 

 plant, and the amount of initial nitrogen in the seed was deter- 

 mined by the analysis of similar seeds. The apparatus was then 

 exposed to the light and the plant allowed to grow for two or three 

 months, when the nitrogen content was again determined; but 

 under these conditions no increase in the amount of nitrogen was 

 ever detected. 



Soil Nitrogen. — The nitrogen in the soil occurs in three dif- 

 ferent forms. It is found in organic compounds as the result of 

 the large masses of organic matter which are constantly being re- 

 turned to the soil after the death of the organisms of which it was 

 a part. These humus compounds are of various degrees of com- 

 plexity. The second source of nitrogen in the soil is formed by 

 the ammonium salts, which result either from the decomposition 

 of organic matter in the soil, from animal excreta, or to a small 

 degree from the natural processes which operate in the air, such 

 as the evaporation of water, during which a small amount of 

 nitrogen combines with hydrogen forming ammonium compounds. 

 Thirdly, there is the nitric nitrogen or that which is in the form 

 of nitrates and nitrites. In these inorganic compounds of nitric 

 or nitrous acid, the nitrogen occurs in the acid part of the salt, 

 where it is united to some base such as sodium or potassium form- 

 ing sodium and potassium nitrate. This nitric nitrogen is con- 

 stantly increasing as the other forms decrease. 



The organic compounds are not available for the higher plants, 

 which are quite unable to use complex nitrogen compounds such 

 as proteins until they have been broken down into amino acids 

 and nitrates. Whether this is due to the inability of the root to 

 absorb large molecules or to some other cause is not known. 



The ammonium salts are of doubtful value in the direct nutri- 

 tion of plants. In some cases they can be used but not so easily 

 as nitrates. In other cases they seem to be even more available. 

 Up to a certain concentration ammonium sulphate is said to be 

 better than potassium nitrate for peas and other legumes. Corn 

 is said to do best when young on ammonium compounds, and Nag- 

 aoka (1904) reported that rice did as well on ammonium salts as 

 on nitrates. The availability of ammonium is probably dependent 

 upon (1) the nature of the soil, (2) the concentration of the am- 

 monium salt, and (3) the nature of the salt. If the soil contains 



