IS SILICA AN INDISPENSABLE CONSTITUENT OF 

 PLANT FOOD? 



By Marshall Lundie. 



The use of farmyard manure as a fertiliser was known to 

 man long before letters existed by which it could be recorded. 

 We begin to get records of some agricultural practices in Roman 

 times. Even then not only the value of dung was known, but 

 also the virtues of other manures such as marl were established. 

 The great poet Virgil in his works makes reference to the fertilis- 

 ing effect of a crop of vetches or lupins upon the succeeding 

 wheat crop. To whatever point the knowledge of manuring had 

 reached in the time of the Romans, for a long time it made no 

 progress. In the mediaeval times agriculture took a step forward ; 

 the value of chalk, woollen rags, ashes was generally known. 

 The experience of men of an enquiring turn of mind had built up 

 a certain knowledge of manures and manuring, and had even 

 begun to reason a little on the mode of action of these manures. 

 In spite of the experience that was accumulating respecting the 

 fertilising value of this or that substance, no real progress was 

 made towards a theory of manuring until the beginning of the 

 nineteenth century. 



Before the development of the science of chemistry it was 



^naturally impossible to form any idea as to how a plant came to 



grow ; while the nature of the plant itself, of the air, water and 



earth were equally unknown, no correct opinion could be reached 



as to how the latter gave rise to the former. 



The true theory of the nutrition of plants begins soon after 

 the discovery of the composition of the air. Priestly observed 

 that plants possessed the power of purifying air rendered defec- 

 tive by combustion or by the respiration of animals, and he 

 having been the discoverer of oxygen, it was found that the gas 

 which leaves gave off was oxygen. 



It was further demonstrated that light was essential for the 

 development of these phenomena, while the oxygen was proved 

 to be derived from the carbonic acid taken up, and that the gain 

 in weight of the plant was practically represented by the carbon 

 combined with the elements of water forming chiefly starch and 

 sugar. 



Such great scientists as Sir H. Davy, de Saussure, and Liebig 

 took this important subject in hand, but it is to the latter that we 

 must attribute the chief impulse which agricultural chemistry has 

 received, for he drove home to the minds, both of scientific men 

 and of farmers, the true theory of plant nutrition. He laid down 

 the general principle that the carbon compounds which constitute 

 more than 95 per cent, of the dry substance of most plants are 

 derived by the plants from the atmosphere, and that if the plant 

 be supplied with the 2 per cent, or so of mineral constituents 



