ORGANIC COMPOUNDS OF SOILS 97 



103. Loss of Humus. — The loss of humus from the 

 soil is caused by oxidation and by fires. Any method 

 of cultivation which accelerates oxidation reduces the 

 humus content. In man}' of the western prairie soils 

 which have been under continuous grain cultivation 

 for thirtv vears and 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, organic 

 matter which 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. 

 nitrogen. 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 2,500 pounds of nitrogen 

 per acre. In clearing new land, particularly forest 

 land, there is frequently an unnecessary destruction of 

 humus materials. Instead of burning; all of the vege- 

 table matter it would be better economy to leave some 

 in piles for future use as manure. When all of the 

 vegetable matter 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. When the vegetable matter has been 

 only partially removed the crops at first may be 



