22 LIVESTOCK ON THE FARM 



two or three years. Then the available plant food is reduced 

 to such a degree that not enough can be grown upon the soil 

 to pay for the work involved. Such soils must thereafter be 

 fertilized or fed. This simply means putting on something 

 that the plants need for food. Many good soils, on the other 

 hand, have been known to grow crops abundantly for from 

 twenty to forty years. But even the best of soils will in time 

 become so reduced in plant food that they must be fertilized. 

 Or, if farming is to be continued indefinitely on the same soil, 

 a system of farming must be adopted that will put back into 

 the soil as much as the plants take out. 



In a great number of cases people have taken plant food 

 out of the soil and sold it in the form of grain and hay until 

 their farms became unprofitable and then have' moved else- 

 where. But that can no longer be done because the farming 

 lands of the country are now practically all occupied. Grain 

 farming of the kind mentioned, which has been called soil 

 robbing, must cease. It is not real farming. 



Limiting Element. — Some soils have all that is needed for 

 profitable crop production with the exception of one ingredient. 

 This missing ingredient is called the limiting element. Plants 

 cannot grow because they do not have it. The plant is like 

 the animal in this respect. Both grow with a definite com- 

 position or will not grow at all. The way to make such soils 

 productive is to supply the lacking material. 



Soil Washing or Erosion. — Soils that are not level will 

 wash. Washing, or Erosion, takes away the finer and best 

 particles. The delta at the mouth of the Mississippi river is, 

 in fact, made up of some of the best soil of the whole Mississ- 

 ippi Valley. Grain farming destroys the sod and uses up the 

 humus which helps to hold the soil particles together. It, 

 therefore, puts the soil into condition to wash badly. 



When a worn-out soil is washed away leaving the good sub- 

 soil exposed for crop production work, erosion is a good thing. 

 But this holds true only where there is a good sub-soil. In 

 general, soil washing should be prevented if possible. 



Soil washing can sometimes be prevented by deep plowing. 

 This loosens the soil to such a depth that rain, unless it is 

 exceptionally heavy, is absorbed and not allowed to run off on 



