CHAPTER VI. 



PHYSICAL PROPERTIES OF SOILS. 



The soil affects the growth of the plant through both physical 

 and chemical properties, which react upon, and modify one 

 another. The chief physical conditions which affect the plant are 

 the depth of the soil, its temperature, the amount of water it 

 supplies, and the composition of the soil atmosphere. These are 

 modified by a variety of factors. The chief chemical condition 

 which affects plants is the supply of plant food, but this supply 

 is to some extent dependent upon physical factors. The chemical 

 composition of the soil has other effects upon plants. It affects 

 the physical character of the soil. It is related to the condition 

 of the soil as regards neutrality, and the absence or presence 

 of injurious substances, which also modify the relation of soil to 

 plants. 



The physical and chemical properties of the soil are closely 

 related, and are more or less dependent upon one another. They 

 cannot be entirely separated without presenting a very one-sided 

 view of the functions of the soil. 



Soil and Subsoil. Going down into the soil from the surface, 

 we generally find the following layers : First, the top, or surface 

 soil, varying from 3 inches to a foot or more in depth, and usually 

 darker in color than the layers below. Next is the subsoil, from 

 a few inches to several feet in depth. If the soil is sedentary, 

 below this is a mixture of rock fragments and soil, and then comes 

 the rock of the locality, the upper layers of which are decom- 

 posed and rotten. Sometimes the layer of soil rests directly upon 

 the solid rock. Sometimes the surface soil has been washed 

 away, leaving the unproductive subsoil. 



The surface soil is distinguished from the subsoil by its darker 

 color, due to the decaying vegetable matter contained in it, 

 derived from roots and plant residues. The depth of the surface 

 soil is, in humid climates, often determined by the depth of 

 plowing, and is generally from 4 to 12 inches. 



A different definition of soil and subsoil is used by the Bureau 



