CHAPTER XI. 



LIME, GYPSUM AND GREEN MANURES. 



Lime has been used for agricultural purposes for many cen- 

 turies, but for how long we do not know. Records show that it 

 was used on land before the Christian Era. During the sixteenth 

 and seventeenth centuries the practice of liming the land was 

 common in Great Britain and at that time lime was one of the 

 principal fertilizers and large applications were often supplied. 



Forms of Lime. — Lime is obtained by burning limestone, chalk, 

 or shells. These are all substances rich in carbonate of lime 

 (CaCOs). When they are burned the carbonic acid (CO2) 

 passes off, leaving the oxide of lime (CaO) , which is called quick- 

 lime, caustic lime, store lime and burned lime. The oxide of 

 lime is usually known as lime. When water is added to this pro- 

 duct it is readily absorbed and high heat develops forming hy- 

 drate of lime (Ca[OH]2) which crumbles to a powder. This 

 is known as slaked lime. Quicklime readily absorbs water and 

 therefore slakes when exposed to the air. This is known as air 

 slaked lime and is not as completely slaked as when treated with 

 water. Quicklime is apt to change to limestone on standing as 

 it absorbs carbonic acid from the atmosphere. When quicklime 

 is applied to the soil it changes to carbonate of lime. 



One hundred pounds of limestone make about 50 to 56 pounds 

 of quicklime which produce about 75 to 85 pounds of water 

 slaked lime. The purer the limestone, the more quicklime and 

 water slaked lime are obtained.* 



When Soils Need Lime. — A certain amount of calcium carbonate 

 should be present in soils as this compound helps to make plant 

 food available and keeps the soil in a condition favorable for pro- 

 ducing crops. When there is a deficiency of calcium carbc-nate, 

 the soil will most likely be acid or sour. ■Most farm crops do not 

 grow well on sour soils, but certain weeds seem to thrive on 

 them, and so it is important to keep soils sweet or stocked with a 

 sufificiency of carbonate of lime. The addition of ordinary fer- 

 tilizers will not benefit crops on sour soils because the nitrifying 



