Phosphates and Nitrogen 



support of vital heat. But we could not eat 

 the carbon as the charcoal-burner produces 

 it in the forest, or the phosphorus as it exists 

 in the match. The former would be a 

 horrible mouthful, and the latter a fatal 

 poison. They must be prepared in a suitable 

 manner, as we find them in bread, in milk, 

 in meat, fruit and vegetables. In a like way 

 the plants require nitrogen, not as it exists 

 in the air but in combination with other 

 substances, of which the most important 

 are the compounds of ammonia. 



To sum up. In the nutrition of plants 

 there are four prominent substances. First 

 of all there is carbonic acid gas, which pro- 

 vides the carbon — the most abundant of all, 

 but about which we need not concern our- 

 selves, because the plants take it from the 

 atmosphere, to which it is constantly supplied. 

 There are also potash, phosphoric acid and 

 nitrogen, which the roo^s extract from the 

 ground in combination with other matters. 

 These are the substances which are removed 

 with the harvest, and which must be restored 

 to the earth if it is to remain fertile. This 

 is the office of manure, without which the 

 soil would be exhausted and would cease to 

 produce. 



113 h 



