[Chap. XXX ROOTS: PROCESSES AND SOIL RELATIONS 319 



After all these soil processes and activities have continued for hun- 

 dreds or thousands of years, a soil usually has attained maturity and cer- 

 tain characteristic chemical and physical properties and water-holding 

 capacities. Its constituents become arranged in layers or horizons of 

 varying thickness beginning at the soil surface and ending below in the 

 parent material. These layers mav differ greatly in composition, reac- 

 tion, and structure, but some parts of most root systems extend into each 

 of them (Figs. 131 and 132). 



Roots growing in a soil, then, are developing in a highly dvnamic 

 medium, in which chemical, phvsical, and biological processes are con- 

 tinuous, and in which the available soil water, soil solutes, and soil air 

 may be as variable as those of the atmospheric environment in which the 

 foliage and stems develop. 



Field capacity and wilting percentage of soils. With reference to the 

 actively growing plant there are two critical points in the water relations 

 of a soil. The first is the amount of water that is held in equilibrium 

 against further movement by gravity and capillaritv in a given soil under 

 field conditions. One may visualize this condition bv assuming that a 

 large mass of a uniform soil after a prolonged drought has become air 

 dry to a depth of 5 feet. This soil still contains some water — about 1 per 

 cent if composed of sand, and as much as 5 per cent if clav. Seeds do not 

 germinate and seedlings do not survive under these conditions. 



Suppose now that an inch of rain falls on this soil and it all penetrates 

 the interstices between the soil particles. If the soil were sandy it might 

 moisten the upper layer to a depth of a foot. If the soil were a fine clay 

 only a few inches would become moist. After a day or two the water in 

 the moist layer of soil has become uniformly dispersed. Neither capil- 

 larity nor gravity causes it to move farther downward. The soil water is 

 now at equilibrium, and films of water surround the soil particles but 

 do not fill the larger spaces among them. The water held at equilibrium 

 is commonly called the "field capacity" of a soil. Sandy soils have a field 

 capacity of about 5 per cent, and clay loams about 35 per cent ( Fig. 133 ) . 



If a second rain of one inch followed, capillary forces would cause 

 the water to move through the moist layer into the dry layer just be- 

 neath, and at equilibrium the fine soil would be moist to a depth of 

 several additional inches, and the sandv soil another foot. 



Under these conditions, seeds can germinate and seedlings grow rap- 

 idly. The roots of seedlings have a continuous water supply as their 



