170 MANURES. VII. 



consisting mainly of oxide, hydrate, or carbonate of lime, but 

 always containing small and varying quantities of magnesia, 

 phosphoric acid, and iron oxide are employed as manures. 

 The important function performed by calcium carbonate in the 

 process of nitrification has already been discussed. * 



The chief effect of the application of lime or its carbonate to 

 a soil is to accelerate nitrification and thus to enable the crop 

 to draw upon the nitrogenous stores already present in the 

 .soil. Another action of value is its neutralising effect upon 

 the organic acids in peaty soils, the presence of which is 

 unfavourable to any but coarse, undesirable plants. It also 

 acts by replacing the potash in the silicates. Although caustic 

 lime (i.e., CaO or CaH. 2 O 2 ) speedily becomes converted into the 

 carbonate when applied to the soil, it always has a more 

 energetic action than chalk or limestone. This is due to its 

 solubility producing a more uniform distribution throughout 

 the soil before precipitation as calcium carbonate occurs than 

 it is possible to obtain by the mere mechanical admixture of 

 the soil with even finely pow r dered chalk or limestone. 



Lime made from magnesian limestone and therefore contain- 

 ing magnesia is not so suitable for agricultural purposes as a 

 purer product. This is usually stated to be due to the fact 

 that so long as the bases are in the caustic state, i.e., as 

 hydroxide, they have an injurious effect upon vegetation, and 

 magnesia is said to combine with carbon dioxide much less 

 readily than lime does. Consequently a lime containing much 

 magnesia may retain its caustic condition (because of the 

 MgH 2 O. 2 ) in the soil for a much longer time than a pure lime 

 would under the same conditions. 



Too heavy or too frequent dressings with lime produce harm 

 by exhausting the stores of nitrogenous matters contained in 

 the humus of the soil. 



The very valuable flocculating effects of lime upon clay have 

 been already discussed. ! 



Ferrous Sulphate, FeS0 4 .7H./), known commercially as 

 "copperas" and "green vitriol," is not often used as a manure. 

 Iron is usually sufficiently abundant in a soil. Griffiths:!: 



* v. p. 67. t v. p. 51 J Jour. Chem. Soc. 1885, Trans. 54 ; 1886, Trans. 121. 



