HORTICULTURAL SOCIETY OF LONDOM. 



3-29 



nitrogen — the atmosphere. Liebig who was the first to point out 

 the vast importance of this element in the j)rocesses of vegeta- 

 tion, has also investigated the source whence it is derived. He 

 has shewn that the nitrogen of the air cannot be assimilated, 

 but that the air always contains a minute trace of ammonia — a 

 compound of nitrogen, and therefore a substance in which it 

 exists in a form capable of being assimilated by plants ; by a 

 most ingenious series of experiments and deductions he has es- 

 tablished the theory that plants derive the nitrogen, which is 

 necessary to their growth, by decomposing ammonia and assi- 

 milating the nitrogen it contains, and that the ammonia which 

 thus supplies them with nitrogen is constantly being formed by 

 the decay of animal and similar organic matters. This theory 

 enables us readily to understand multitudes of practical facts and 

 observations, and gathers strength the more it is examined. It 

 bears directly on all the operations of the farmer, and is of 

 especial interest in connexion with the action of manure. 



The substances which according to the views of Liebig consti- 

 tute the FOOD of plants are water, carbonic acid, and ammonia, 

 these not only contain the four elements of organic matter, but 

 contain them in a condition capable of being assimilated by the 

 organs of plants. 



In addition to these three substances however it is well known 

 that plants absorb others, of an inorganic nature in far smaller 

 quantity ; the former are derived principally from the atmosphere, 

 whilst the latter are obtained wholly from the earth and therefore 

 of course consist of substances existing in the soil in contact 

 with their roots. As the composition of soils varies greatly in 

 different parts of the world, so the nature of the inorganic sub- 

 stances absorbed by plants growing in various localities is 

 different. Very little indeed is known of the office which these 

 substances perform in the economy of plants and hence the 

 investigation of this subject presents a wide field of inquiry. 

 Many analyses of plants have been made, but they are of compar- 

 atively little value because we have not at the same time careful 

 analyses of the soils on which they grew. It has long been known 

 that certain plants, contain small quantities of peculiar inorganic 

 substances, thus wheat contains phosphoric acid in combination 

 with an earthy base j and as this appears to be always the case, 

 it is inferred that wheat could not be made to grow well, in a soil 

 not containing any phosphates. Our knowledge of this part of 

 vegetable chemistry is very scanty, being entirely confined to 

 isolated facts, whilst the effect produced by the absorption of 

 these earthy substances, or the circumstances most favourable to 

 their absorption are unknown. Hence even when by analysis we 



