264 IS SILICA A CONSTITUENT OF PLANT FOOD? 



which are found in the ash, it will then draw upon the atmosphere 

 for all the materials the crop ultimately contains. These views 

 of Liebig, coming at a time when great interest was directed 

 towards agriculture, and backed by his great scientific reputation, 

 aroused instant and widespread attention, being the foundation 

 both of practical experiment and scientific research. 



The ashes of a large number of plants were analysed, and 

 the natural conclusion was drawn that all these ash constituents 

 found were essential to plant growth, and were of equal import- 

 ance. In the majority of cases the percentage of silica in the 

 ashes was found to be very high, even amounting to as much as 

 50 and 60 per cent. 



From this it was generally thought that silica was one of 

 the chief constituents of plant food, and experiments were 

 made using sodium silicate as a fertiliser, and the following from 

 A. D. Hall's " Fertilisers and Manures," page 271-272, will show 

 how far people went in drawing conclusions. 



" Silica is so large a constituent of the ash of many plants, particularly of the 

 straw of cereals, that it was inevitably regaided as a necessary constituent of the 

 food of such plants, and was naturally enough supposed to contribute to the stiff- 

 ness of the straw. In his manure Liebig supplied the alkalies combined with silica, 

 and when Way discovered that certain strata of the Upper Greensand, near Farn- 

 ham, contained considerable quantities of silicates readily dissolved by acids, the 

 rock was for a time extracted and ground up as manure for cereals." 



Sir Humphrey Davy's theory regarding the part that silica 

 played in the plant was that it gave the plant a certain firmness, 

 but he overlooked the fact that the leaves of the grasses which* 

 do not help to support the plant contain much more silica than 

 the stems, and hence his theory cannot be correct. 



This fact is also clearly brought out on glancing at the 

 results of the analysis of the ashes of wheat given below. 



Leaves 



Stems 



At this time many exaggerated ideas were held as to the 

 the ammonia present in the atmosphere ; the rain was 

 believed to bring down as much as 30-40 lbs. per acre per annum 

 of combined nitrogen instead of the 3-4 lbs. which we now know 

 to represent the true quantity. This was due to imperfect 

 methods of analysis. A great deal of controversy took place, 

 some holding that it was unnecessary to use nitrogenous manures, 

 as sufficient nitrogen as ammonia in the rain was supplied for the 

 requirements of the plant. 



