270 MATERIALS OF INDIRECT VALUE [chap. 



as a manure, but there is no evidence to show that its 

 action is in any way different from that of calcium 

 carbonate, i.e.^ it behaves as a base but is not of any 

 further value as supplying magnesia to the plant. 



Sulphate of Iron. — It is well known that iron is one 

 of the essential constituents of all green plants ; for 

 example, in water cultures it is easy to show that 

 seedling plants become blanched, chlorophyll does not 

 form in the leaf, and the plants soon die, unless a 

 small quantity of some soluble iron compound is 

 added to the culture liquid. Such an addition is 

 followed by a rapid return of the green colour to the 

 leaf and by the renewed growth of the plant. A 

 very widespread opinion has been based upon such 

 experiments and is specially current in horticultural 

 literature, that high colour in fruit and flowers 

 is to be associated with an abundance of iron com- 

 pounds in the soil, and that in consequence sulphate of 

 iron is valuable as an adjunct to manures. One argu- 

 ment advanced in favour of this opinion is the bright 

 colouring of apples, roses, etc., grown on the red sand- 

 stones and loams of Herefordshire and Worcestershire, 

 the red hue of which is admittedly due to oxides of iron. 

 When the facts are more closely examined, they afford, 

 however, little support to such a theory. In the first 

 place, the plant requires very little iron indeed : as a 

 rule, not more than i per cent, of the ash of a plant 

 consists of oxide of iron, 2 per cent, might be taken as 

 an outside limit, so that the amount of oxide of iron 

 taken from the soil by a heavy crop of mangolds (the 

 leaf of which is specially rich in iron) only amounts to 

 about 10 lbs. per acre. Now it is very rare to meet 

 with a soil that does not contain 2 per cent, (or 20 tons 

 per acre in the top 9 inches) of oxide of iron soluble in 

 hydrochloric acid, and of this a considerable proportion 



