98 CHEMISTRY OF FARM PRACTICE 



matter, corrects acidity, furnishes the element calcium, 

 and by means of its chemical activity reacts with other 

 bases in the soil. An example of this last effect is found 

 in the reaction between lime and the zeolites of the soil, 

 which are the double silicates of aluminium and some other 

 base, the base being changed by substitution of calcium 

 due to the action of lime. In this way, lime may serve to 

 liberate potassium, which must be present in available form 

 in a fertile soil. Lime may 'also bring about reactions with 

 the phosphates of iron or aluminium, the product being 

 a phosphate of lime, which is more soluble and therefore 

 more available for plant food than the phosphates of iron 

 or aluminium. 



Calcium hydroxide is produced by the action of water 

 upon quicklime and is similar in its action to calcium 

 oxide. When applied to the soil, calcium oxide is quickly 

 converted into calcium hydroxide by moisture 



CaO+H 2 = Ca(OH) 2 , 



which, in turn, is rapidly converted by the carbon dioxide 

 of the air into calcium carbonate 



Ca(OH) 2 +CO 2 = CaC03+H 2 O. 



Calcium carbonate is not caustic, and consequently it is 

 much less drastic in its effects than is the oxide or hydroxide. 

 Carbonates are easily decomposed by acids, therefore the 

 application of ground limestone serves to correct acidity 

 in soils. In humid regions probably it is rapidly converted 

 into calcium silicate. Calcium carbonate fulfills most of 

 the functions performed by calcium oxide and calcium 

 hydroxide, but it acts in a much milder manner. Calcium 

 silicate (CaSiOs) and calcium sulphate (CaoSC^) or " land 

 plaster," serve most of the functions of the other forms 

 of lime except the correction of acidity, the effectiveness 

 of the salts varying with their solubilities. 



Lime modifies the physical structure of soils. It tends 



