MANURES 115 



purposes as a purer lime, probably because magnesia doe^ not 

 absorb carbon dioxide from the soil water and gases so quickly 

 as lime docs ; consequently the magnesia retains its causticity 

 for a longer time, and this hinders the growth of plants. 



Gas Lime. — This is a waste-product from gasworks, being the 

 residue left after passing coal-gas over slaked lime in order to 

 remove carbon dioxide, COg, and sulphuretted hydrogen, HjS. 

 It consists of a complex mixture containing calcium carbonate 

 and hydrate, together with varying quantities of incompletely 

 oxidised sulphur compounds {e.g.^ calcium sulphite, CaSOg, 

 sulphide, CaS, thiosulphate, CaS^Og). These latter substances 

 are powerful plant poisons, and fresh gas lime is very injurious 

 to plants. It should be kept for some time freely exposed to 

 air, in order to convert the unoxidised sulphur compounds into 

 sulphate, CaSO^. 



Ferrous Sulphate, FeS04.7H20, Green Vitriol ; and Copper 

 Sulphate, Qw'^O ^.h'H.fi, Blue Vitriol. — These substances are 

 rarely used as manures, but are often employed in spraying for 

 the destruction of charlock or of fungoid diseases. The iron 

 compound has been claimed to act as a useful manure for 

 grass, beans, potatoes, mangolds and cereals. It is applied at 

 the rate of about J cwt. per acre, and is said to kill moss and 

 to act as a plant food, increasing the amount of chlorophyll 

 in the crop and to a certain extent acting as a substitute for 

 potash manures. 



Analysis and Valuation of Manures. 



The value of a manure is usually estimated from the 

 quantities of nitrogen, phosphorus pentoxide, and potassium 

 which it contains. 



In the trade the results of an analysis are often expressed 

 in percentages of "ammonia" (i.e., NII3), "phosphates" (by 

 which is meant tricalcium phosphate), and " potash." Now in 

 many manures — e.r/., nitrate of soda — the nitrogen is not present 

 as ammonia ; so, too, " phosphates," meaning tricalcium phos- 



