NORTH-CABOLHTA GEOLOGICAL SUKVEY. 115 



exist. But this is not all ; the water of the soil being robbed 

 of its oxygen, its hydrogen is set free ; and being in its nas- 

 cent state, it is ready itself to combine with that body, for 

 which it has the strongest affinity. That body is nitrogen 

 contained in the air diffused in the soil ; and the body formed 

 by this union is ammonia. Now, ammonia is one of the most 

 essential bodies in the list of nutrients. Guano, as is well 

 known, owes its fertilizing properties in part to ammonia. 

 But I need not dwell upon this fact. By the interchanges oi 

 oxygen which take place with the oxides of iron, we are fur- 

 nished with an explanation of the origin of ammonia in the 

 soil. But the production of ammonia is only one of the 

 chemical changes which take place in a soil in which organic 

 matter, iron, water and air exists. The vegetable matter, 

 also, undergoes a change, for the oxygen which it has taken 

 from the peroxide of iron converts it into organic acids, which 

 are known by the names of crenic and apocrenic acids. These 

 acids being one of the series of changes effected through the 

 influence of the oxides, they in their turn become active, and 

 unite with the ammonia and form crenates and apocrenates 

 of ammonia. In the condition of a salt, this compound of 

 ammonia and the vegetable acids are taken up by the root* 

 of plants, and become their food. 



87. I have made these remarks for the purpose of pre- 

 paring the way for farther observations upon the action of 

 marls upon vegetation. The condition of the iron in a large 

 proportion of the marls, is that of a protoxide. Thus the iron 

 in the greenish marl upon the Tar River, is a protoxide. In 

 this condition, when it is spread upon land and mixed with 

 the soil which contains vegetable or organic matter, change* 

 first into a peroxide, it is then in an active state, and sezing 

 upon one of the elements of water, decomposes it. The ox- 

 ides of iron in the marl undergo the same changes in the soil 

 to which they are applied, as those which have been describ- 

 ed as taking place in all soils which have not been exhausted 

 o/ these organic matters. It will therefore be expected that 

 marls which contain a large percentage of iron, are more val- 

 uable than those which are destitute of it, and to the actioi. 



