KINDS OF SOIL 95 



Thus we can see why it is said that to the farmer each 

 field has a problem of its own. The fertility of a field 

 depends largely upon the physical and chemical nature 

 of its soil; it may be good for some crops, but poor for 

 others; it may be good for a certain crop one season, 

 but poor for that crop the next season, because of the 

 changes in the soil which that crop produces. Many 

 crops gradually "poison," or reduce the fertility of soil 

 for crops of the same kind. So we hear of what is called 

 rotation of crops; by changing his crops from season to 

 season the farmer really changes his soil and so helps to 

 keep up its fertility. The changes which one kind of 

 crop makes are counterbalanced by the changes which 

 another kind makes; one kind will succeed another kind 

 better than it will succeed itself. This is because the 

 materials which one kind of plant takes from the soil 

 may be different in amount from those taken by another 

 kind, and the changes which one kind produces in the 

 soil may also be different from those produced by another 

 kind. 



Soil as to Origin. As to origin, there are evidently two 

 great kinds of soil. There is the kind which has been 

 formed just where you find it, and there is the kind which 

 was formed somewhere else, and then moved, usually by 

 water, to the place where you find it. The former is called 

 residual soil; the latter, transported soil. 



The fertility of residual soils varies more than the fer- 

 tility of transported soils; some residual soils are fertile, 

 others are sterile; this depends chiefly upon the nature 

 of the parent rocks from which they were formed. Trans- 

 ported soils, on the other hand, are much more uniformly 



