LIME 



247 



But whatever the cause may be, permanent soil acid, as 

 a rule, should be neutralized by certain calcium compounds, 

 loosely called "lime." 



187. Lime. — The chemical term lime is used for calcium 

 oxide, CaO. Agriculturally, however, lime is any compound 

 of calcium which will neutralize acids. The compounds 

 having this power are the carbonate, hydroxide, and oxide. 



; ^- * ■ ,3 





-"kAf *- 





a& 



Fig. 60. — Limestone quarry. Department of Experimental Agricultural 



Chemistry, Pennsylvania Station. 



(a) Calcium Carbonate, CaC0 3 . — This form of calcium 

 occurs in limestone, o} r ster shells, shell marl, and chalk, 

 all of which when pure may contain as much as 95 to 98 

 per cent, calcium carbonate, but frequently contain less 

 (Fig. 60 shows a limestone quarry). Limestone very often 

 is composed of magnesium carbonate as well as calcium 

 carbonate, in amounts varying from a few per cent, up to 

 45 per cent. When the latter content of magnesium car- 

 bonate is present the mineral is called dolomite. To be used 

 on the soil all forms of calcium carbonate should be ground 

 very fine, preferably so that 75 per cent, at least passes a 

 hundred mesh sieve. It must be remembered that it is the 



