1905.] Function of Silica in the Nutrition of Cereals. 473 



extracted (1) with strong hydrochloric acid and (2) with a 1 per cent, 

 solution of citric acid. While there is no method of determining the real 

 amount of plant food in the soil which is at the service of the crop, the latter 

 method* gives comparative estimates which are of value when dealing with 

 soils of the same type. 



Table VI shows a series of determinations of the total phosphoric acid and 

 of the phosphoric acid soluble in 1 per cent, citric acid solution. 



Table VI. 





Total phosphoric acid. 



Phosphoric acid soluble in 

 1 per cent, citric acid. 



No silica. 



With silica. 



No silica. 



With silica. 



1 



2 

 3 

 4 



097 

 194 

 0-092 

 0-179 



096 

 0-199 

 0-089 

 0-183 



0-0086 

 -0495 

 0-0075 

 -0674 



-0067 

 -0721 

 0-0094 

 -0743 



Comparing the soils with and without silica, the use of silica has not 

 affected the amount of total phosphoric acid ; the greater draft it occasions year 

 by year from the soil of Plots 1 and 3, which are not supplied with phosphoric 

 acid, is barely visible as yet in the analyses. 



The silica has also little or no effect on the phosphoric acid soluble in citric 

 acid on the four Plots 1, 1 S, 3, and 3 S ; but the amount going into solution 

 is distinctly higher on Plots 2 S and 4 S than on Plots 2 and 4, all plots 

 receiving phosphoric acid in the manure. It is not, however, on these plots, 

 but on Plots 1 and 3 that the silica shows any effect on the crop, hence these 

 determinations support the conclusion that the sodium silicate has no action 

 upon the soil phosphates. 



Though the seat of the action is thus transferred from the soil to the plant, 

 it is by no means settled whether the stimulus which the silica gives to the 

 plant to enable it to take up more phosphoric acid from the soil reserves 

 is a general stimulus or a specific one confined to the phosphoric acid. In 

 other words, does the presence of a free supply of soluble silica so invigorate 

 the plant that it is enabled to repair any weak link in the chain of nutrition 

 and get as need be more nitrogen, phosphoric acid, or potash from the soil, 

 or is the beneficial effect confined to the phosphoric acid alone ? It is chiefly 

 towards the settlement of this point that the further experiments both with 

 silica and non-silica plants are now being directed. 



* Dyer, 'Chem. Soc. Trans.,' vol. 65, 1894, p. 115. 



2 M 2 



