38 LIVESTOCK ON THE FARM 



should have corn fodder and several kinds of hay, as well as 

 a variety of grains. This calls for the sowing or planting of 

 different crops on a given piece of land every year for a 

 period of years. 



Different plants are made up of different combinations of 

 chemical elements. A crop will take out of the soil so much 

 of a certain substance in a season that the same crop ordinarily 

 will not do so well the following year. The partially exhausted 

 substance thus becomes the limiting element, and if the same 

 crop is continued the yield grows smaller from year to year. 

 Under a good system of rotation, however, the crops of one 

 season draw more heavily upon substances that the previous 

 crop has used the least of. One crop, also, may leave a waste 

 product in the soil which acts as a poison to a like crop but 

 not to a different kind of crop. Moreover, by a wise c oice 

 of crops for a rotation, some of the elements taken out of the 

 soil by one crop may be in part restored by another. This 

 supplemented by manure keeps the soil fertile. 



Cultivation. — One of the principal operations in agriculture 

 is the cultivation of the soil. As mentioned in Chapter I, 

 soil is improved by the movement of soil particles. This 

 refines the soil and liberates more of the material out of which 

 the particles are made for plant food. 



By keeping the surface loose, cultivation also checks the 

 evaporation of water. Of course, the surface soil becomes 

 very dry, but the loose portion which extends below the 

 extremely dry part does not allow the water to rise from be- 

 neath and escape into the air by evaporation. The capillary 

 action is broken. This simply means that the soil particles 

 are so far apart that the water cannot rise between them as 

 it rises in the fibers of a lamp wick. 



In order that plants may grow upon any soil there must be* 

 present a considerable amount of water. When a soil is loose 

 it will hold more water than when it is hard. A loose soil also 

 allows water which falls on the surface to soak in. This pre- 

 vents the washing away of the best particles of the soil. 

 Though a loose soil breaks capillary action, rain on a loose soil 

 packs it enough to restore capillarity. Of course, a very 

 coarse, sandy soil does not possess much capillarity, neither 



