98 Elementary Principles of Agriculture 



cation of lime may be due to the establishment of bal- 

 ance between the calcium and magnesium as just men- 

 tioned; to the effect on insoluble potassium or phos- 

 phorus compounds (^ 90) ; to a mechanical effect on the 

 texture of the soil (f 73) ; to the effect of lime in taking 

 up an excess of acid in soils (^ 141) ; or in neutralizing 

 some forms of alkali. 



140. The Mechanical Condition of the soil may be the 

 cause of unsatisfactory crops. Some crops, like wheat, do 

 best with a settled sub-surface soil, while beets, potatoes 

 and many other crops do best with a very loose soil. To 

 have the proper mechanical condition of a soil for a 

 particular crop is of great importance. It is in this par- 

 ticular that the farmer makes the greatest effort to im- 

 prove the productiveness of his soils. Herein lie the most 

 important problems of preparing and cultivating the soil. 

 In improving the mechanical qualities, the important 

 effects to be considered are: 



(a) The absorption of the rainfall; 



(b) The retention and movement of the water in the 

 different layers of soil; 



(c) The circulation of air in the soil; and 



(d) The absorption and retention of the heat of the 

 sun, and its loss by radiation. 



While these properties are fixed, in a large degree, by 

 the nature of the substance composing the soil, they 

 may be greatly improved by the ordinary means of 

 tillage. To know when to plow is just as important as 

 to know how the soil should be plowed. Who can tell 

 when and why, and how and why for plowing a partic- 

 ular piece of soil to prepare for a particular crop ? 



140a. The following topics are suggested for discussion : How 

 many kinds of soils are in the school district? What crops are 



