surface such as plains, hills, and mountains; degree of Relief, 

 steepness of slopes; and other physiographic features. 



Topsoil 



The uppermost portion of the soil, often considered the 

 layer six or seven inches in thickness, which is richer in 

 organic material and lighter in texture than the material 

 below. In uniform material the topsoil includes the layer 

 that is usually plowed up. cf. Subsoil. 



Tornado 



(1) A violent vortex, with a diameter usually of about 

 0.25 mile, in the atmosphere, accompanied by a pendulous, 

 more or less funnel-shaped cloud. (2) In West Africa a 

 violent thundersquall. 



Torrential 



Refers to the rapid and violent movement of materials 

 such as water in a stream, heavy rainfall, sliding of gravel, 

 etc.; or to organisms living in swift streams. 



Torrid Zone 



See Tropics. 



Total Digestible Nutrients (T. D. N.) 



The standard evaluation of the digestibility of materials 

 in the feed of livestock, including proteins, fats, nitrogen- 

 free extract, and fiber content. 



Total Estimate 



The combined estimates of abundance and cover char- 

 acteristics of vegetation; used commonly in England. 



Toxin 



A poison produced by an organism. 



Trace Element 



See Essential element. 



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