THE ELEMENTS OF PLANT FOOD 53 



most soils, but some soils do not have enough to supply 

 plants with the lime that they need. In such cases 

 lime may be supplied in the form of ground limestone. 

 Carbonate of lime will correct the acidity of sour soils 

 and will unite with nitric add to form calcium nitrate, 

 an available form of plant food. 



Lime has also a good physical effect on soils. When 

 applied to light, sandy soils, it has a tendency to bind 

 the particles of sand together, while on heavy clay 

 soils it renders them more open and porous and helps 

 to admit air. Although carbonate of lime furnishes 

 an essential plant food, namely calcium, the soil may 

 have an ample supply of this element in other com- 

 pounds and still need carbonate of lime to improve 

 the physical condition of the soil, to correct its acidity 

 or to aid in rendering available other forms of plant 

 food. To obtain the best results, lime should be 

 applied with manure or other forms of plant food. 

 The old couplet, 



" Lime without manure 



Makes the father rich and the children poor," 



expresses a truth that should be heeded. 



Calcium oxide (CaO), quicklime, is made by heat- 

 ing limestone (CaCO 3 ) in a kiln. A kiln is often built 

 from rough stones in the side of a hill. The lime- 

 stone is then piled in such a way in the kiln as to make 

 a place for the fire, but piled so loosely that the heat 

 may pass up among the pieces of limestone. The 

 process of burning the lime requires several days. 

 CO 2 is driven off by the heat, leaving CaO, or quick- 

 lime. 



Quicklime may serve as a fertilizer the same as CaCO 3 , 



