The Nutrition of Plants. 165 



spring or well ! In the meantime, however, we may best content ourselves 

 with the food supply gained from plants, and be thimkful accordingly to our 

 tireless benefactors of the vegetable world. 



The continued supply of carbonic acid in ihe air is secured through the 

 various forms of decomposition and combusticm— the decay of animals and 

 plants, the exhalation of the former in the breath, and the products from all 

 domestic and commercial fires. The atmosphere serves as a great reservoir 

 which is continuously filled and as continuously emptied in an even and 

 ceaseless balance of account. It is rare indeed that carbonaceous manures 

 need be added to soils for the food supply of crops growing thereon. These 

 fertilizers have other important uses in the various physical or mechanical 

 properties of soils; but plants ordinarily have abund: nt opportunity to help 

 themselves to the carbon required from the air, providing other things are 

 favorable. 



In the combustible substances of plants there is one more element which 

 demands our attention — nitrogen. 



This exists in a free state in immense quantity in the air, composing four- 

 fifths of its bulk. But plants are not capable of making use of this free 

 nitrogen. This question has been long in dispute, but the weight of evidence 

 is as stated. Bathed in an ocean of the substiuice, plants, like ourselves, must 

 perish for want of it, unless supplied in combination with other chemical 

 elements, forming the so-called nitrates. Now these nitrates are not spon- 

 taneously formed in nature, or rather are formed only under certain condi- 

 tions and through the mediation of peculiar agencies. No problem in plant 

 nutrition has been so difficult to solve, nor so faithfully wrought, as this of 

 the source of nitrogen as a food material for plants. Important knowledge 

 has only recently been gained upon the subject, and it hrs not been until 

 very recent years that anything like positive assertion could be made on 

 many debatable points. Our knowledge now may be summarized as follows : 



1. Plants can not in any way use free nitrogen as food ; neither can any 

 plants whatever cause a combination of free nitrogen with other elements so 

 as to produce assimilable nitrogenous matter. 



2. The soil does not in any way fix the free nitrogen of the atmosphere so 

 as to render it useful for plant food. 



3. The unique, original source of assimilable nitrogen, capable of serving 

 for the nutrition of plants, and in consequence the original source of the 

 nitrogen of all animal foods, is in the chemical combinations, induced by 

 electricity, of free nitrogen and of oxygen or hydrogen (of waterj'^ vapor) in 

 the atmosphere. 



4. The nitrates formed in the soil — the results of the so called nitrification 

 of soil - are due to a fermentation of nitrogenous organic substances by which 

 ammonia and nitric acid are set free and the latter fitted for uniting with 

 the alkalies or alkaline earths. This fermentation, like all other such pro- 

 cesses of decomposition of organic matter, is due to living organisms. The 

 particular species of microscopic plants (bacteria) which produces this par- 



