ORGANIC MATTER OF SOIL 11 



tides. At times, the water may entirely fill the pore spaces, 

 pressing out all of the air. This film of water, or the free water, 

 carries in solution the minerals which are dissolved from the 

 inorganic soil constituents, the carbon dioxide and other sub- 

 stances produced from the decomposition of the organic matter. 

 This film of water forms the soil solution. The growing plants 

 obtain the nutrients necessary for their growth by absorbing 

 them largely from the solution by means of their roots and root 

 hairs, which penetrate between the inorganic soil particles. 



Some of the plant nutrients are only slightly soluble in water 

 or in the weakly acidified aqueous solution, and may be present, 

 therefore, in the soil solution only in small amounts at any one 

 time. However, as these minute quantities are removed by the 

 growing plant or by drainage waters they are replaced by further 

 solution from the crystalline or colloidal soil materials. 



Water moves in soils in many ways, depending upon its 

 abundance, the physical condition of the soil and the vegetation. 

 Where large amounts reach the soil in a short period of time, 

 much of it may disappear by running off from the surface into 

 drainage channels. Some of the water penetrates the soil and, 

 where there is more than the soil particles can hold about them- 

 selves, it sinks to the level of the water table by precolation. 

 When moisture is not descending upon the soil there is loss by 

 evaporation from the surface and transpiration from the leaves of 

 the vegetative cover. This water may be drawn up from the 

 lower levels of the soil by capillary forces as more and more water 

 is lost from the surface. Hea\y rainfall alone may not solve 

 the problem of supplying water to plants. The solution fies 

 rather in the proper distribution of the water in the soil over the 

 period of time in which the plants develop. In humid regions 

 the problem may become one of drainage; in regions of very low 

 rainfall, irrigation may be resorted to. A combination of ade- 

 quate drainage and irrigation systems makes possible almost 

 complete control of the soil water, but unfortunately such equip- 

 ment is too expensive for general practical application. 



Organic Matter of Soil. — In addition to the soHd inorganic 

 particles or the mineral framework, the soil also contains solid 

 organic particles, namely, roots and sloughed-off portions of 

 roots, residues of stems, leaves and branches, as well as numerous 

 organic complexes which have originated by the partial disin- 



