THE SOIL AND SOIL FORMATION 6l 



fire, so that great holes or pits are often left, making the 

 surface very uneven. This is of especial importance in 

 regions where drouths occur of sufficient duration to 

 allow the peat to dry out to considerable depth. 



Soil body and soil fertility. The solid body, or frame- 

 work, of the soil usually makes up 90 per cent, or more, 

 of the weight, and is composed of the solid particles of 

 clay, sand and stone from which the soil was made. 

 Spillman says : 



" In order to understand the methods necessary for 

 restoring worn-out soils, let us consider what occurs in 

 a fertile soil that is growing a large crop. Imagine a 

 cubic inch of ordinary field soil magnified into a cubic 

 mile. It would then present very much the appear- 

 ance of a mass of rocks varying from the size of a pea 

 to masses several feet in diameter. Scattered among 

 these rock masses would be many pieces of decaying 

 plant roots and other organic matter, resembling rotting 

 logs in a mass of stones and gravel. The masses of 

 organic matter would be found to contain large quan- 

 tities of water, and to somewhat resemble wet sponges, 

 while every mass of rock would have a layer of water 

 covering its surface. The open spaces between the solid 

 masses would be filled with air. 



" If a crop were growing on this soil, its roots would 

 be found threading their way among the masses of rock 

 and decaying roots, and pushing these aside by the pres- 

 sure exerted by the growing root. From the surface of 

 the growing root, near its tip, root hairs extend into the 

 open spaces and suck up the water covering the rock par- 

 ticles. The plant food is dissolved in this water, but is 

 usually present in exceedingly small quantities. While 

 the plant is growing, a constant stream of water flows up 

 through it and evaporates at its leaves. For every pound 

 of growth in dry matter made by the plant, from 300 to 

 800 pounds of water flow up through it." 



