28 Nature- Study Agriculture 



called " humus." Most soils contain a mixture of the 

 five different kinds of particles. (Exp. 6.) 



An ' The following illustration may help us to understand 



riwoftiie the na ^ ure f the soil. " Let us in imagination suppose 



s il a cubic inch of ordinary garden soil to be enlarged to 



a cubic mile a magnification which no microscope 



would be able to give. Then we should see it composed 



mainly of a mass of rocks varying from several feet in 



diameter to the size of a pea and smaller. 



" Each stone would be seen to be covered with a film 

 of water, and some of the smaller spaces would be full 

 of water. Larger spaces would contain air. 



" Scattered throughout the rock mass we should see 

 quantities of water-soaked decaying vegetation, like 

 rotting logs in a mass of rock and gravel. Winding in 

 and out among the stones and penetrating the spongy 

 water-soaked humus is a network of plant roots, push- 

 ing aside the stones as they grow longer, and absorbing 

 the water which fills the small spaces and with which 

 the decaying matter, the humus, is saturated." 1 

 Clay, sand, Soil textures. Like cloth, a particular soil may be 

 Dam said to be either of coarse or fine texture. Pure clay 

 has a texture too fine to allow the easy circulation of 

 air and moisture, and it is too sticky to work. Pure 

 sand is of so coarse a texture that water drains out of 

 it too easily, leaving it dry, and plants find little nutri- 

 ment in it. A soil that has sufficient clay to bind the 

 particles together somewhat and sufficient sand and 

 humus to keep it from being too dense and sticky is 

 called " loam." Loams are said to be sandy, or clayey, 



1 W. J. Spillman. 



