32 GARDENING FOR THE SOUTH. 



and slightly in the same species when grown on different 

 soils; but they are always a valuable manure for the 

 species from which obtained, and, slowly dissolving in the 

 soil, they furnish the roots with just the salts required to 

 nourish the growing plant. 



But, in general, over nine pounds in every ten have dis- 

 appeared under the action of fire. The combustible por- 

 tions which have been expelled are carbon, hydrogen, 

 oxygen, and a little nitrogen, which have been derived 

 from carbonic acid, water, and ammonia, which are, as 

 elements of food, equally indispensable as the substances 

 of which the ashes of plants are composed. 



The incombustible constituents of the plant come from 

 the soil alone, and are taken tip by the roots. 



After the gaseous constituents of plants are driven off 

 by combustion, the small percentage of ashes remaining, 

 we have stated, consists of silicic and phosphoric acids, 

 potash, sulphur, lime, magnesia, iron, chlorine and soda, 

 (the two latter generally unite as chloride of sodium), all 

 of which, in greater or less proportions, enter into the 

 composition of our field and garden crops. These earthy 

 or saline constituents are found within the cells of plants, 

 or deposited as a lining to the cell-walls, or entering into 

 their substance. They are useful to the plant itself, and 

 useful in the plant's products as affording food to man. 

 Some of them are always present in the azotized substances 

 formed by plants. Thus sulphur and the phosphates are, 

 with ammonia, necessary for the formation of albumen, 

 fibrin, and caseine, which are essential constituents of our 

 blood. 



Of these substances Lime, Potash, Soda, Phosphoric 

 Acid, Sulphur, and Chlorine, are all the gardener will 

 have occasion to supply, the others being always present 

 in sufficient quantity in all cultivated soils. 



Lime generally occurs as a carbonate. Partially solu- 

 ble in water, it is an important portion of food to most of 



