1921.] The Need for Lime and How to Meet it. '201 



The Stone. — The fust considiM-atioii is obviously the discovery 

 of a bed or outcrop of liriiestone of suitable composition upon 

 the estate or near enough to it, to make hauhige not too serious 

 an item. All the Upper Chalk is entirely suitable for hurniii.^ and 

 for grinding; the Lower, or Gi-ey Chalk is by no means so good, 

 and often contains so much combined silica that when burnt 

 it forms a strongly hydraulic lime of doubtful use to the agricul- 

 turist. Limestones offer a much wider range of texture and 

 chemical composition than chalk, and before any decision is 

 arrived at with regard to grinding or burning a limestone, 

 samples should be analysed. It may be decided at once that 

 any stone which contains over 92 per cent, of calcium carbonate 

 (CaCOg) is suitable either burnt or ground. Many rocks which 

 fall far short of this percentage of calcium carbonate are also 

 good, but a little consideration of their other constituents is 

 necessary before deciding upon their use. 



Magnesian Limestone » — IMany deposits of limestone, some 

 of them of considerable extent, and consequently of importance, 

 contain the element Magnesium in varying proportions. 

 Magnesium has a close chemical relationship to calcium, and 

 also occurs in limestone in the form of a carbonate. Magnesium 

 carbonate (MgCOg) is met with in all proportions from mere 

 traces up to 45 per cent., at which point it is in chemically, 

 equivalent proportion to the calcium carbonate; such limestones 

 containing a high proportion of magnesium carbonate are 

 termed Dolomites. 



There has been much controversy as to whether, to what 

 extent, and in w^hat circumstances dolomitic limestones are 

 injurious to plant growth when applied to soils either before or 

 after burning. Trials have shown that an excess of magnesia 

 in a soil has a toxic action on crops, but the extent of such 

 action depends on the type of soil, the condition of the soil, 

 and particularly on the amount of lime present in the soil with 

 the magnesia. It may, how^ever, be safely assumed that a stone 

 containing up to 10 per cent, of magnesium carbonate can 

 always be used on any soil; higher proportions of magnesium 

 carbonate should be regarded with some suspicion where it is 

 intended to apply the ground stone to land already heavily 

 charged with magnesia.* Ground limestone has been particu- 

 larly mentioned, for w^here we are dealing with burnt lime 

 made from dolomitic limestone, we are faced with a new source 

 of danger not connected with any toxic action of the magnesia, 



* Furtlier information on the use of Lime or Limestone rich in Magnesia 

 will be found in Leaflet No. 170, p. 13. 



