54 GARDENING FOR THE SOUTH. 



in the soil, they furnish the roots with just the salts re- 

 quired to nourish the growing plant. 



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

 disappeared under the action of fire. The combustible 

 portions 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 up by the roots. 



After the gaseous constituents of plants are driven off 

 by combustion, the small percentage of ashes remaining, 

 as 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 sub- 

 stances formed by plants. Thus sulphur and the phos- 

 phates are, with ammonia, necessary for the formation of 

 albumen, fibrin, and caseine, which are essential con- 

 stituents of our blood. 



Limb generally occurs as a carbonate and sulphate, or 

 gypsum. Partially soluble in water, it is an important 

 ingredient in the soil to most of our cultivated plants. It 

 is indispensable to such plants as beets, potatoes, peas, 

 beans, fruit trees, grasses, and vines, but to Kalmias and 

 coniferous trees it is injurious. Lime in the soil enables 

 it better to absorb and retain heat, and also corrects 

 acidity. It is of great value as an application to cold. 



