SOIL 191 



what loosely held together in a mass, and when dry 

 it does not bake nor crack, but returns to its loose 

 granular form. 



The larger particles of sand afford larger spaces 

 between their particles, thereby permitting air to cir- 

 culate among them quite freely. 



The roots of plants have no difficulty in penetrating 

 sand, but because of the looseness of its particles and 

 the ease with which they shift about, most plants are 

 unable to get started in it. Thus sand alone is an 

 unsuitable soil for plants. To verify this you need 

 but recall the almost lack of vegetation on sand dunes. 



The material in the soil which comes from the decay 

 of organic matter is called humus, or leaf-mold. It is 

 dark in color and makes up a large part of the soil of 

 heavily shaded forests. The humus part of soil is very 

 important. It teems with microscopic plant and animal 

 forms so necessary in the soil to the life of green 

 plants. 



From the fact that pure clay is difficult for the roots 

 of plants to penetrate and sand alone may shift too 

 frequently for plants to become established, it is appar- 

 ent that neither alone is an ideal soil for plants, though 

 either may be abundantly supplied with the minerals 

 and water necessary for plant life. 



For most plants, the best soil is a combination of 

 clay, sand, and humus. 



A soil of equal parts of clay and sand with some 

 humus is called loam. 



