THE SOIL 23 



Living forms in the soil. - - The living forms in the soil 

 are mainly microscopic plants called bacteria. These 

 soil bacteria, as you will learn in the next chapter, 

 add much to the fertility of the soil. 



Humus. - - The dead organic matter, the remains of 

 plants and animals, is called humus. 



The roots, leaves, and stems of plants furnish much 

 of the humus. The bodies of dead earthworms and 

 insects in the soil also add to the total amount of 

 humus. 



Soil particles. According to their size, we divide 

 the soil particles into three classes; clay, silt, and sand. 

 The very finest particles form clay. Larger particles, 

 but still quite small, form silt. Coarse soil grains, 

 usually particles of quartz rock, are sand. These 

 grains of sand are large enough to be seen with the 

 naked eye. 



Kinds of soil. Soils made up mostly of clay or silt, 

 or clay and silt together, are called clay soils. Soils 

 made up mostly of sand are called sandy soils. Soils 

 which contain large proportions of clay and silt and 

 sand are called loams. If the amount of sand is greater 

 than the amount of silt and clay, we have a sandy 

 loam. If the amount of silt and clay is greater than 

 the amount of sand, we have a clay loam. 



If a soil contains a large amount of humus, it is called 

 a humus soil. The soil of a swamp is of this kind. 



Topsoil and subsoil. - - The layer of soil which is 

 plowed and worked is called the topsoil. The part 



