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Yearbook^ of Agriculture 1949 



GOVERNING EFFECT OF RAINFALL-EVAPORATION RATIO ON VEGETATION 



Each vertical plane cutting the figure from front to back locates a geographic position 



between the prairie of western Iowa (left) and the Appalachian Mountains of West 



Virginia (right) with its rainfall-evaporation ratio by months. The undulations at the 



right represent contrasting evaporation rates on Appalachian ridges and coves. 



upper hardwood-forest soil mellow and 

 granular. The forest absorbs mineral 

 elements from the soil, and in turn 

 largely replaces them in the yearly leaf 

 fall. If the forest dropped more leaves 

 than decayed each year, it would even- 

 tually bury itself in its litter ; and if the 

 rate of organic-matter decay were 

 greater than the rate of organic ac- 

 cumulation, the soil would at inter- 

 vals be totally devoid of organic mat- 

 ter. Such conditions never occur; a 

 balance short of them is maintained. 



OF THE FACTORS that adversely af- 

 fect forest soils, burning alone usually 

 does not seriously influence a hard- 

 wood-covered soil. It destroys the 

 litter, which protects the mineral soil, 

 but subsequent erosion does the real 

 damage. Fire in a coniferous forest is 

 frequently more serious since shallow 

 soils over bedrock are more common. 



Overgrazing is injurious to any kind 

 of forest. In wet weather trampling 

 compacts the soil and makes it hard 

 and harsh when it dries. Trampling 

 breaks up the litter cover, thereby ex- 

 posing mineral soil to excessive drying 

 in summer. Heavy grazing destroys 

 forest soil structure, and eventually 

 lowers its site quality. 



Since soils are formed slowly, their 

 loss through accelerated erosion is 

 especially serious. Erosion is more 

 damaging to some soils than to others. 

 Many of the hardwood lands of south- 

 ern Illinois grew only a few good crops 

 after the trees were removed. The 

 porous, organic-rich loess mantle dis- 

 appeared quickly when the protective 

 litter was plowed under. 



Aspect and degree of slope greatly 

 affect the rate of site deterioration due 

 to erosion. Any disturbance of site by 

 erosion is much worse on dry south 



