24 TROPICAL AGRICULTURE CHAP. 



animals, but its principal sources are rivers, lakes and seas, 

 from whence it is obtained by evaporation during the heat 

 of the day. 



The ammonia, nitric acid, chlorine and sulphuric acid of 

 the atmosphere are carried to the earth by the rains, and so 

 made available for the use of plants. 

 How plants Nitrogen is a very important constituent of plants. It is 



obtain . . 



nitrogen. contained, as we have seen, in the atmosphere in a free con- 

 dition, as well as in combination with other elements in the 

 form of ammonia and nitric acid which are carried down to 

 the earth by rains. It also exists in the soil in chemical 

 union with other substances, as nitrates. Ammonia, too, is 

 formed in the soil by the decomposition of dead plants and 

 animals ; it does not remain free, however, for when it comes 

 in contact with the potassium salts, it acts on them and forms 

 compounds of nitric acid which are readily taken up by the 

 delicate hairs on the roots of plants, and it is in this way 

 that the nitrogen used by plants is obtained. 



Food taken ACTIVE AND DORMANT CONSTITUENTS OF SOILS. All the 



rooSof * substances taken up by the plant from the soil, must be 



plants must capable of being dissolved in water aided by carbonic acid 



and some organic acid. If a portion of dry soil be soaked 



in pure water for a time, the water that is poured off will be 



found to be altered in character and taste, for the reason 



that it has taken up from the soil some of the active or 



soluble substances used as plant food. Perhaps, in a fertile 



soil, this will not be more than two parts in every thousand, 



so that nine hundred and ninety-eight parts are insoluble in 



water. But, if acids be added to the water with which the 



soil is soaked, a very much larger quantity of plant food will 



be dissolved out, and this is what occurs in the case of 



Acid sap of plants, for the delicate root-hairs dissolve out from the soil 



nalrs. 001 " the active constituents by means of an acid fluid which 



f always permeates the walls of these finest divisions of the 



x roots. If the'soil, however, from which all the plant food 



