IS SILICA A CONSTITUENT OF PLANT FOOD? 265 



The necessity of a supply of combined nitrogen was 

 demonstrated experimentally, the yield being in fact roughly 

 proportioned to the amount of combined nitrogen added as 

 manure. If only the mineral constituents of the ash were 

 supplied, the crop fell away rapidly as soon as the reserves of 

 active nitrogen in the soil had become exhausted, and it was 

 further demonstrated that the ordinary plants were incapable of 

 utilising the full nitrogen of the air, but only took up nitrogen 

 in a combined form from the soil as manure. 



In addition to establishing the value of nitrogenous manures 

 experiments also settled in a practical fashion the question of 

 which of the ash constituents were indispensable to the plant and 

 were necessary constituents of a complete manure. 



The fundamental necessity of phosphoric acid and potash, 

 and the non-essential nature of soda, magnesia, and silica as 

 manure constituents were soon established. The synthetical 

 method of determining the character of the plant food was carried 

 out first by Liebig by means of water cultures, with the result 

 that the following nine substances were found to form the 

 indispensable constituents of plant food, vis. : — 



He further proved that each constituent of plant food performs 

 certain definite functions in the system of the plant, which 

 cannot be performed by the other constituents. 



It was at this time that Liebig enunciated the great Law of the 

 Minimum, which states that if one of the necessary ingredients 

 of plant food is provided in insufficient quantities the plant takes 

 up of the other necessary constituents only so much as is in a 

 definite proportion to the quantity which is supplied in the 

 minimum amount. 



From these results it was clearly shown that silica, sodium 

 and chlorine were not indispensable, although it is a well estab- 

 lished fact that barley, figs and buckwheat thrive on a brack soil. 



The author has carried out a series of water culture experi- 

 ments with wheat, in order to demonstrate the non-essential 

 nature of silica. 



The experiment was carried out in the following manner : — 

 A number of wheat grains were soaked overnight, then put into 

 a germinating dish — Professor Nobbe's apparatus — on May 31st, 

 191 1. It was observed on June 3rd that they had commenced to 

 germinate. Three of the most healthy were selected for the 

 experiment. Numbers 1 and 2 were put into the water culture 



