56 AGRICULTURE 



Soil and subsoil. We see that the soil is the finely 

 divided surface layer of the earth in which higher plants 

 can grow. It consists of two portions, which are not 

 always alike, (i) the looser, upper layer, or soil proper, and 

 (2) the more compact layer under this, called the subsoil. 



The soil consists chiefly of sand and clay, but neither 

 one of these is food for plants. Sand is useful in keeping 

 soil from packing too closely, and from being too wet and 

 sticky. Clay is useful in holding moisture and cementing 

 the sand grains together. A small part of the clay, under 

 proper conditions, can finally be changed into plant-food. 



Much more useful to plants as food are the decaying re- 

 mains of earlier generations of plants. These remains of 

 plants are usually spoken of as vegetable, or organic 

 matter, or humus. There is much more vegetable matter 

 in the soil than in the subsoil. Hence the soil produces 

 much larger crops than the subsoil can when it is first 

 brought to the surface. This is because the vegetable 

 matter in the soil supplies plant-food, holds moisture, and 

 makes the soil loose and mellow, permitting the roots 

 and air to penetrate it. The subsoil, when first thrown 

 up from the bottom of a ditch, is unsuitable for plant 

 growth, but after it has been exposed to the air for several 

 years, plants grow on this changed subsoil as well as 

 anywhere else. 



Available and unavailable plant-food in the soil. The 

 soil contains all of the chemical elements found in plants, 

 and many more besides. All cultivated soils are rich 

 enough in most of these elements, so only those elements or 

 compounds which are sometimes scarce need be considered. 



