LIMING THE SOIL 365 



mercial slaked lime ranges in composition from 60 to 75 per 

 cent, of lime expressed as calcium pins magnesium oxides. 

 Both the burned and slaked forms of lime tend to absorb car- 

 bon dioxide from the air, producing calcium and magnesium 

 carbonate. This is called air-slaking.^ 



A number of lime compounds are sold under the head of 

 carbonated lime. Of these pulverized or ground limestone is 

 the most common. There is also bog lime or marl, oyster 

 shells and artificial carbonates. The latter are by-products 

 from certain industries. All of these are quite variable in 

 their content of calcium and magnesium carbonates. Pul- 

 verized limestone may vary in purity from 75 to 98 per cent., 

 90 per cent, being a fair average. Highly magnesian stone is 

 generally avoided, although stone carrying from 15 to 20 per 

 cent, of magnesium carbonate is often used. The magnesium 

 carbonate, however, usually makes up less than 5 per cent, of 

 the lime present. 



The figures ^ quoted in table LXXI (see page 366) show the 

 average composition of liming materials offered for sale in 

 Pennsylvania from 1916 to 1920 inclusive. 



198. Determining the need for lime. — The lack of lime 

 in the soils of humid regions is so universal that liming will 

 generally increase crop growth. For example, 72 per cent, of 

 the soils of Pennsylvania ^ are sour, while 75 per cent, of the 

 cultivated lands of Indiana* show acidity by the ordinary 

 tests. While it is safe to assume that the productivity of 

 three-fourths of the soils in the eastern part of the United 

 States would be raised by liming, it is a question in many cases 

 whether such treatment would pay. 



^ Ca(0H)2 + 002 = CaCOs + HA 

 Mg(0H)5 + GO, == MgCa + HA 

 =» Kellogg, J. W., Ume Report; Penn. Dept. Agr., Vol. 4, l<lo. 2, Feb. 

 1921. 



^WTiite, J. W., Lime Bequi/remeTits of Pennsylvania Soils; Penn. Agr. 

 Exp. Sta., Bui. 164, 1920. 



* Wianeko, A. T., Conner, S. D., and Jones, S. C, The VaUe of Lime on 

 Indiana Soils; Ind. Agr. Exp, Sta., Bui. 213, 1918. 



