IX.] FUNCTION OF IRON SALTS 271 



is soluble in the weakest acids and must be regarded as 

 available for the plant Moreover, the red sands and 

 loams mentioned above show rather less than the normal 

 amount of iron on analysis ; the bright red colour is 

 due to some variation in the mode of deposition of the 

 oxides of iron and not to any excess in their amount. 

 These facts alone render the theory improbable, but the 

 chief point is that no direct evidence has been adduced 

 for the beneficial effect of an application of iron salts, 

 either on colour or yield. From time to time experi- 

 ments with iron sulphate have been quoted, but they have 

 never been conducted in a manner to raise the supposed 

 increase due to the iron beyond the range of experi- 

 mental error. Even had the results been positive they 

 would have required futher examination, because the 

 application of sulphate of iron to the soil would result 

 in a variety of secondary effects, due to the precipitation 

 of the iron and the solution of a corresponding amount 

 of other bases present. As far as colour goes, no 

 evidence has ever been adduced to show that iron plays 

 a part ; experiments made by the author upon apples 

 gave purely negative results ; and though some effects 

 upon the colour of carnations were seen, no positive 

 conclusions could be drawn. In practice the employ- 

 ment of sulphate of iron for either farm or garden 

 crops may be dismissed. 



Manganese appears also to be a constituent of all 

 plants, and recently experiments have been put forward 

 to show that small quantities of manganese salts have 

 a stimulating effect upon the growth of crops. The 

 experiments are, however, by no means conclusive, and 

 pending further investigations, the use of manganese 

 salts cannot be recommended in practice. 



Silicates. — Silica is so large a constituent of the ash 

 of many plants, particularly of the straw of cereals, that 



