120 SUCCESSFUL FARMING 



value can be determined only by examination by the chemist or by actual 

 field test. 



Gypsum or land plaster is frequently used on land, and while it will 

 supply calcium as a plant food, it has little or no effect in correcting soil 

 acidity. 



The rock phosphates and Thomas slag, used as sources of phosphorus, 

 contain considerable lime, and their liberal use may obviate the necessity 

 for applying lime to the soil. 



Forms of Lime.— Lime (CaO) does not occur in nature. It is pre- 

 pared by heating limestone (CaC0 3 ) in kilns; 100 pounds of pure lime- 

 stone thus heated loses 44 pounds of gas known as carbon dioxide (C0 2 ), 

 and results in 56 pounds of lime. This 56 pounds of lime may be slaked 

 with water and will combine with enough water to make 74 pounds of 

 hydrated lime. Therefore, 1120 pounds of pure lime equals 1480 pounds 

 of pure hydrated lime, which equals 2000 pounds carbonate of lime or 

 pure pulverized limestone. When lime and hydrated lime are exposed 

 to the air they slowly combine with the carbon dioxide of the air until 

 finally reverted to the original form of carbonate of lime. The only 

 difference between the original lime rock and completely air-slaked lime 

 is that of fineness of subdivision, the one being in the form of large rock 

 masses and the other a very fine powder. It is this fine state of sub- 

 division that makes air-slaked lime valuable to apply to the soil. If the raw 

 limestone could be made equally fine it would be just as good as air-slaked 

 lime for the same purpose. If used in generous amounts it need not be 

 so fine as air-slaked lime, but in order to be prompt and effective, pulver- 

 ized limestone should be so fine that 90 per cent will pass through a 100- 

 mesh screen. Where abundant and cheap, larger amounts of coarser 

 material may be used because of the considerable amounts of finely divided 

 active material it carries. The coarse portion may become available in 

 later years. Lime is generally sold in one of five forms: ground lime- 

 stone, freshly burnt or lump lime, ground burnt lime, hydrated lime and 

 air-slaked lime. Some deposits of lime are nearly pure carbonates of lime, 

 while others contain much magnesia and are known as dolomite. The 

 presence of magnesia slightly increases the neutralizing power of a given 

 weight of lime. 



FUNCTIONS OF LIME 



Lime as Plant Food.— The absence of lime prevents a normal develop- 

 ment of plants. Lime is, therefore, essential as a plant food. Most 

 soils contain sufficient lime to meet the food requirements of plants. 

 Some soils, however, may contain so little, or it may be so unavailable, 

 that plants that are hungry for lime may suffer from a lack of it. 



Chemical Action of Lime. — The chemical effect of lime on most 

 soils is of minor importance. It varies somewhat with the form in which 

 it is applied to the soil. Freshly burnt or caustic lime is the most active 



