

HISTORY OF MANURING 5 



ance in sunlight ; while Jean Senebier discovered that 

 instead of these leaves taking in oxygen and breathing 

 out carbon dioxide gas, as animals always do 

 (respiration), they actually carried on during sunlight 

 the reverse process, viz. : that of absorbing carbon 

 dioxide gas and breathing out oxygen gas. 



Minerals. 1 Digby found that saltpetre gave wonder- 

 ful results when applied to hemp plants, due, he thought, 

 to this substance fertilising the air. Later on, Liebig 

 propounded his great mineral theory, that the minerals 

 in the soil were essential in the growing of farm 

 crops. 



In recent years this mineral theory has been 

 followed up. Plants were grown in water to which 

 was added most mineral substances found in the ash 

 of plants, and it was found that the plants grew and 

 matured. One by one these minerals were eliminated, 

 until it was ultimately discovered that for the successful 

 growth of plants it was necessary to supply in suitable 

 form nitrates, sulphates, and phosphates of potash, 

 lime, magnesia and iron, so far as the minerals are 

 concerned. 



Further trials with farm crops showed that, in 

 ordinary farm practice, it was only necessary to supply 

 in suitable form, nitrogen, phosphates, potash, and 

 occasionally lime. Seeing that lime only needs to be 

 applied every four to eight years, manures supplying 

 nitrogen, phosphates, and potash are called "complete 

 manures," so far as the mineral requirements of the 

 plant go. 



The above discoveries are all very important. Even 

 to-day it is still necessary to supply farm crops with water 



1 The term " minerals " is taken to include the nitrogen which 

 the plant obtains from the soil in the'_form of salts (nitrates chiefly). 



