188 SOILS AND FERTILIZERS 



of lime. When the materials are fairly, pure, the number of 

 pounds of each required to give approximately equivalent 

 quantities of lime are as follows : 



Quick-lime 56 pounds 



Water-slaked lime . . . 74 pounds 



Air-slaked lime, marl, ground limestone . 100 pounds 



When applying lime to land, these relationships should be 

 kept in mind. If it is a question of using quick-lime or ground 

 limestone one must provide nearly twice as much limestone 

 as quick-lime in order to apply an equal quantity of lime. 



242. Absorption of lime by soils. — In the forms in which 

 it is applied to soils, lime is not so soluble as potash fertilizers. 

 When brought in contact with soil, the lime is absorbed and 

 rendered still less soluble. It is, however, somewhat more 

 soluble than soil potash, and drainage waters usually con- 

 tain several times as much lime as potash. It is the soluble 

 part of the lime that has the beneficial effect on crops and 

 soils. The ways in which the benefit accrues are numerous 

 and will be described in a number of the following para- 

 graphs. Lime is usually applied in much greater quantities 

 than are fertilizers, but the treatment is given only at inter- 

 vals of four or five years. 



243. Lime requirement of soils. — It is possible, by means 

 of chemical methods, to ascertain how much lime a soil will 

 absorb before it shows alkalinity due to the presence of an 

 excess. Such a test is useful to indicate the quantity of lime 

 that should be applied to a soil in order that it shall be at 

 least temporarily adapted to the production of lime-loving 

 plants. 



The results of such a test are usually expressed in pounds 

 of lime required to satisfy the absorptive properties of a 

 certain number of pounds of soil, as for instance, 2,000,000 

 pounds. This will vary in different soils from none to several 

 thousand pounds. 



