THE CHEMICAL COMPOSITION OF SOIL.S III 



about by the loss of the plant food combined with the 

 humus. 



119. Loss of Humus. Loss of humus from soils is 

 caused by oxidation and 'by fires. Any method of 

 cultivation which accelerates oxidation reduces the 

 humus content. In many of the western prairie soils 

 which have been under continuous grain cultivation 

 for thirty years or more, the amount of humus has been 

 reduced one half. Summer fallowing also causes a loss 

 of humus. When land is continually under the plow, 

 and no manures are used, the humus is rapidly oxidized, 

 and there is left in the soil only the organic matter that 

 is slow to decay. 



Forest and prairie fires have been very destructive 

 to the organic compounds of the soil. A soil from 

 Hinckley, Minn., before the great forest fire of 1893, 

 showed 1.69 per cent humus and 0.12 per cent nitro- 

 gen. 17 After the fire there were present 0.41 per cent 

 humus and 0.03 per cent nitrogen. The forest fire 

 caused a loss of 2500 pounds of nitrogen per acre. In 

 clearing new land, particularly forest land, there is 

 frequently an unnecessary destruction of humus mate- 

 rials. Instead of burning all of the vegetable matter, it 

 would be better economy to leave some in piles for 

 future use as manure. When all of the vegetable mat- 

 ter has been burned, two or three good crops are 

 obtained, but the permanent crop-producing power of 

 the land is reduced because of the loss of nitrogen and 



