ORGANIC MATTER AND STRUCTURE 115 



would otherwise occur, and the large spaces thus pro- 

 duced constitute lines of weakness. (2) It is a property 

 of humus to undergo great change in volume when 

 dried out. This is another factor akin to the fineness 

 of the soil, and produces larger shrinkage crack. This 

 is noticeable in many black clay soils, which check 

 excessively. (3) The great capacity of humus for 

 moisture permits a wide range in moisture content, 

 which produces corresponding physical alteration. (4) 

 The color of the humus affects the color of the soil, 

 and thereby increases the rate of change from wet to 

 the dry state by increased evaporation of moisture. 

 The relative effects of crude muck, and the ammonia 

 extract from the same muck, upon the cohesion of the 

 soil, as indicated by the force required for a uniform 

 penetration of a knife-edge reduced to a basis of 100, 

 is shown in the following table. Samples dried and 

 rewetted twenty times. 



Crude Muck 



1 . Check 100.00 



2. Muck, 5 per cent 82.00 



3. Muck. 15 per cent 73.50 



4. Muck, 25 per cent 58.48 



5. Muck, 50 per cent 50.25 



Ammonia Extract of Crude Muck 



1. Check 100.00 



2. Muck extract, 1 per cent . 85.30 



3. Muck extract, 2 per cent 76.40 



4. Muck extract, 4 per cent 69.00 



This table indicates that the material represented 

 by the muck extract is the constituent of the muck 



