22 VEGETABLE GARDENING. 



Phosphates, potash, lime, magnesium, sulphur, 

 and iron, obtained from the soil ; carbon, 

 hydrogen, and oxygen, obtained from air 

 and water ; and nitrogen from both air 

 and soil. 

 2. Less essential, which include 



Salts of sodium, silicon, chlorine, and man- 

 ganese. 



The functions performed by the substances in- 

 cluded in the first group are well known, and al- 

 though these materials are interdependent, they each 

 operate in the building up of plant growth along 

 fairly distinct and well-defined lines. The sub- 

 stances named in the second group are present in a 

 smaller proportion in the ash of all crops, and, though 

 they doubtless perform useful functions, these are 

 less evident and are not so well understood. 



Although all these materials enter into the economy 

 of plant nutrition in a greater or less degree, it has 

 been abundantly proved from practical experience, 

 and by careful and extended experiments, that there 

 are only four substances which require to be con- 

 stantly added to the soil in manuring namely, 

 nitrogen, phosphates, potash, and lime, and these 

 must be maintained in the soil in suitable proportions 

 according to the special needs of the crops. The 

 absence or insufficiency of any one of these com- 

 ponents of plant food at once reduces the fertility 

 of the soil, and is soon revealed in the reduced 

 quantity and quality of the crops grown. 



