Formation of Soil. 61 



from the field is always in proportion to the size of the 

 crop developed by the micro-organisms in the soil. 



Then again ; the soil is a medium in which plants may 

 place their roots in such a manner as to enable them to 

 stand erect in the open sunshine and moving air currents 

 above. 



Finally, the soil is a means whereby the sunshine is 

 changed into forms of energy available to the needs of soil 

 organisms and the roots of plants and without which this 

 life could not exist ; for all of its movements must originate 

 primarily from the sunshine altered in the soil or in the tis- 

 sues of the plant above the soil. 



67. Formation of Soil. There are many agencies at work 

 in the formation of soils and the processes of soil growth 

 are in continuous operation day and night, winter and sum- 

 mer. Since all soil material originates from the breaking 

 down of the various rock structures which make up the 

 earth's surface all of the agencies which are operative in 

 rock destruction may also contribute to soil formation. 



68. Influence of Hock Texture on Soil Formation. Nearly 



all kinds of rock are made up of fragments or crystals of 

 various sizes and shapes and these are held together by in- 

 terlocking, by some cementing material, or else by direct 

 cohesion when extreme pressure has brought the grains 

 close enough together to make this possible. It is seldom 

 true, however, that the structure is so close or the cement- 

 ing so complete as to make the rock impervious to water 

 and the closest granite or the finest marble may absorb 

 as much as .1 to .4 of a pound of water to 100 pounds of 

 rock. If this water is changing it will dissolve away the 

 cementing materials and the faces of the crystals them- 

 selves, making the rock still more open and the grains may 

 even fall apart as is frequently observed in those cases 

 known as "rotten stones." 



The water may freeze in the stone and by its expansion 

 cause it to crumble. Or again, when the sun shines 00 



