THE PLAJJT. 51 



When plan ^5 are destroyed by decay, or burning, 

 tlieir organic constituents pass away as water, am- 

 monia, carbonic acid, etc., ready again to be taken 

 up by other plants. 



The earthy matters in the soil can enter the plant 

 only with the aid of water. Potash, soda, lime, 

 and magnesia, are soluble in their pure forms. 

 Magnesia is injurious wlien present in too large 

 quantities. 



Sul])liuric acid is often used as a manure, and is 

 usually most available in the form of sulphate 

 of lime or plaster. It is also valuable in its pure 

 form to prevent the escape of ammonia from com- 

 posts. 



PhosphoriG acid is highly important, from its fre- 

 quent deficiency in worn-out soils. It is most readily 

 taken up by plants under certain conditions which 

 will be described in the section on manures. 



Silicic acid is common sand, and must be united 

 to an alkali before it can be used by the plant, be- 

 cause it is insoluble except when so united. 



Chlorine is a constituent of connnon salt (chloride, 

 of sodium), and from this source may be obtained in 

 sufficient quantities for manurial purposes. 



Oxide of iron is iron rust. There are two oxides 

 of iron, the peroxide (red) and i\\Q protoxide (black). 

 The former is advantageous in the soil, and the latter 

 poisons plants. 



Oxide of manganese is often absent from the ashes 

 of our cultivated plants. 



The food of plants, both oi-ganic and earthy, must 



