PHYSICAL PROPERTIES OF SOILS, ETC. 93 



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. 18 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 1 , particularly forest land, there is 

 frequently an unnecessary destruction of humus mate- 

 rials. Instead of burning all the vegetable 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 and 

 humus. When the vegetable matter has been only 

 partially removed, the crops at first may be smaller, 

 but in a few years returns will be greater than if all 

 of the vegetable matter had been burned. 



109. Physical Properties of Soils Influenced by 

 Humus. The physical properties of a soil may be 

 entirely changed by the addition or the loss of humus. 

 The influence of humus upon the weight, color, heat, 

 and water-retaining power of soils is discussed in the 

 chapter on the physical properties of soils. Soils re- 

 duced in humus content have less power of storing up 

 water and resisting drought. This fact is illustrated 

 in the following table : 31 



PER CENT. WATER. 



After 10 hours 



exposure in 

 In soil. tray, to sun. 



Soil rich in humus (3.75 per cent.) 16.48 6.12 



Adjoining soil poorer in humus (2. 50 percent.) 12.14 3-94 



