FOODOF PLANTS. 21 



found that some kinds of plants in a soil of pure silex, moistened 

 with distilled water, were capable of growing and forming nitro- 

 geni^ed compounds. In this case the nitrogen must have been de- 

 rived either from this gas as it exists in the atmosphere, or from 

 the ammonia which is present in very minute proportion. The 

 latter explanation is on many accounts the most probable. 



According to Dumas, nitrate of ammonia is also generated by a 

 combination of its elements, by the action of electric sparks in 

 thunder storms, which is carried to the earth by the rain. 



From these sources ammonia is constantly jiassing into the at- 

 mosphere for the support of the vegetation in the earth. But this 

 general supply does not furnish a sufficient proportion for cultivat- 

 ed plants, and hence the necessity of manures, which are capable 

 of producino: it in greater abundance. 



The manures which furnish the most abundant supply of ammo- 

 nia, are urine and the excrements of animals, but particularly the 

 former. But owing to the volatility of the carbonate of ammonia 

 which is generated by the manures, it is liable to pass at once into 

 the atmosphere, instead of contributing to the nutrition of the 

 plants around whose roots it is deposited. Different modes have 

 been proposed for fixing this ammonia, by converting the carbo- 

 nate into a salt which is not volatile. This is effected by adding 

 some mineral acid, such as sulphuric or muriatic, which gives rise 

 to a sulphate or muriate of ammonia, which is not volatile, and 

 remains permanent, so that the whole of it may be absorbed into 

 the plant. The addition of these acids has the further advantage 

 of destroying the ammoniacal smell of putrid urine, and for these 

 two reasons it is resorted to in the man\ifacture of poudrette. 



According to Liebig* " the evident influence of gypsum (sulphate 

 of lime) upon the growth of grasses — the striking fertility and 

 luxuriance of a meadow upon which it is strewed, depends only 

 upon its fixing in the soil the ammonia of the atmosphere which 

 would otherwise be volatilized with the water which evaporates. 

 The carbonate of ammonia contained in rain water is decomposed 

 by gypsum in precisely the same way as in the manufacture of sal 

 ammoniac. Whether sulphate of ammonia and carbonate of lime 

 are formed, and this salt of ammonia possessing no volatility, is 

 consequently retained in the soil." 



• Agricultural Chemistry. 



