PLANT NUTRITION 249 



ROLE OF THE SOIL 



Soil Formation 



Soil is the loose, easily crumbled part of the earth's crust in which 

 plants find support and nourishment. This loose, friable material is 

 a mixture of four major components, inorganic material, organic mat- 

 ter, water, and air. The process by which this mixture is formed and 

 becomes a suitable medium for plant nutrition is complex. A detailed 

 consideration of soil formation is beyond the scope of this tliscussion. 

 However, the principal forces involved in the formation of soil include 

 physical forces causing disintegration and mechanical subdivision of 

 rocks and rock particles, chemical forces causing decomposition, and 

 biological forces residting in both disintegration and decomposition. 



The physical forces acting on parent rock and smaller particles in- 

 clude the following: changes of temperature, causing expansion and 

 contraction; freezing of water in cracks and crevices; abrasion when 

 boulders, rocks, gravel, and sand are carried by streams of water; 

 wind-borne sand, which grinds away native rock and moves particles to 

 new locations; and glaciers, which pick up rock and rock particles and 

 cut, grind, and crush native rock over which they move. 



Simple solution is probably the first reaction which takes place be- 

 tween water and any kind of rock mineral. Among the materials most 

 readily dissolved are compounds of sodium, potassium, calcium, and 

 magnesium. When water dissolves a mineral such as calcium silicate, 

 hydrolysis takes place with the formation of calcium hydroxide and 

 silicic acid. 



CaSiOg -}- 2H,0 ^ Ca(OH).2 + HS'iOs 



The silicic acid may lose water, thereby forming SiOo, a very un- 

 reactive compound. The calcium hydroxide may be washed away as 

 such, but it is more likely to be changed by carbonation to calcium 

 carbonate. Carbonic acid, the carbonating agent, may arise from the 

 carbon dioxide produced by plant roots, by decomposing organic 

 matter or by microorganisms, from carbon dioxide in the air, or from 

 carbonates of rocks and minerals. Carbonation affects compounds con- 

 taining sodium, calcium, potassium, and ferrous iron. 



Hydration plays an important part in chemical decomposition by 

 forming hydrated products of minerals. A large number of silicates, 

 oxides, and carbonates will form hydrated compounds. These com- 

 pounds are more reactive than the original complexes. 



