KINDS OF SOIL 53 



western prairies, and in fact a large part of the 

 grain soils of the United States, are composed 

 mainly of silt. A high proportion of silt in a soil 

 has about the same effect upon it as a large amount 

 of clay, making it tenacious of water and of plant 

 food. Many soils said to be clayey have more fine 

 silt in them than clay. 



Humus is mostly decayed vegetation. All the 

 vegetable matter in a soil, however, is not humus; 

 the carpet of rotting leaves beneath a forest tree is 

 not humus. Not until this is entirely decayed and 

 has become a loose, black mould, in which neither 

 leaf nor stem may be discerned, is it humus. There 

 are all stages between this and the vegetation that 

 is just beginning to decay, and all have value. The 

 value of humus in a soil for increasing its capacity 

 to hold water, for making it mellow, and for fur- 

 nishing plant food has been stated in preceding 

 Chapters, and is considered yet more fully in 

 Chapter XII. 



From these four materials sand, clay, silt and 

 humus many kinds of soil have been made, dif- 

 fering widely in the proportion of each ingredient, 

 and in agricultural value. The relative amounts 

 of each material in a soil influence its texture, the 

 way it responds to heat and moisture, and its value 

 for cropping fully as much as its richness in plant 

 food. While nearly every fertile soil contains all 

 four, most soils are pronounced one way or another. 

 Thus we have, as a broad classification of .agricul- 

 tural soils, sandy soils, clayey soils (which include 

 soils that are mainly silt), and humus soils, in which 

 each of the respective ingredients predominates to 



