494 SOILS: PROPERTIES AND MANAGEMENT 



In swamp and inundated soils the nitrogen of ammonium 

 salts and nitrites forms a larger proportion of the soil 

 nitrogen than does the nitrate nitrogen, but in well 

 aerated soils these compounds exist in very small quan- 

 tities. 



416. Forms in which nitrogen is absorbed by plants. — 

 The utilization of atmospheric nitrogen by leguminous 

 plants and by a few others that have nodule-bearing roots 

 . has been established beyond question ; but the extent 

 to which this form of nitrogen may be utilized by other 

 plants, or the identity of the plants that participate in 

 its use, are subjects on which opinions differ, and which 

 are still being investigated. 



417. Use of nitrates by plants. — Boussingault first 

 demonstrated the importance of nitrates for higher 

 plants. Previous to that time ammonia had been con- 

 sidered the chief source of nitrogen, and at a still earlier 

 time humus had been considered the source. Liebig 

 gave the weight of his influence in favor of ammonia 

 as the supply. He was unaware, of course, of the trans- 

 formation of ammonia nitrogen into nitrates in the soil. 

 Since the publication of the experiments by Boussin- 

 gault and the later work on nitrification, there has 

 been a tendency to consider nitrate nitrogen as the 

 only available supply of nitrogen for agricultural plants. 

 While this is an extreme view of the matter, the 

 fact remains that all. the higher plants, including the 

 legumes, appear to be able to absorb nitrates, and this 

 form of nitrogen has frequently proved of greater benefit 

 to plants than other forms of nitrogen tested at the 

 same time. 



418. Ammonia as a plant-food. — That rice plants on 

 swamps use ammonia nitrogen rather than other forms 



