1 6 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



needed in such small quantity that it need hardly be considered as 

 a plant food by the agricultural chemist. 



Silicia not Essential but Useful. Silica is present in all plants 

 grown under normal conditions, and makes up- a considerable 

 proportion of the ash of some plants. The ash of cereal straws 

 contain 20 to 40 per cent. It was formerly thought that silica 

 was essential to the strength of cereal straws. Plants have been 

 grown to maturity without silica, though they secured traces from 

 the glass vessels in which the solutions were contained. 1 The 

 plants attained a normal development, and produced seed well. 

 Silica is, therefore, not essential to plant life. Though not 

 essential, silica is useful. In certain water culture experiments 

 by Kreuzhaga and Wolff 2 with oats, the presence of the silica 

 increased both the number and the weight of the seed. The 

 silica appeared to aid the plant to mature and form seed. The 

 following table shows the results of an experiment in which all 

 conditions were constant except the silica: 



The silica is supposed to cause the leaves to die off during the 

 ripening of the fruit, allowing essential elements to be withdrawn 

 and utilized in formation of seed. According to Wolff, if silica 

 is absent from the solution, oats will produce empty seed heads 

 unless an excess of phosphoric acid is present. The silica thus 

 economizes phosphoric acid, and this is a highly useful function. 

 Hall and Morison, 3 at the Rothamsted Station, show that silica 

 used as a fertilizer causes an increased yield and earlier forma- 

 tion of the grain of barley, but causes the plant to take up more 

 phosphoric acid from the soil. 



Soda Not Essential. Soda is never absent entirely from any 



1 Sachs, Jahresber. Agr. Chem., 1862, p. 97. 



2 Landw. Versuchs-stat., 1884, P- T 6i. 



3 Proc. Roy. Soc., 1906, p. 445. 



