SOIL STRUCTURE 191 



ture is even more marked. While the actions of humus 

 are mam;, as has already been shown, its relationship to 

 physical condition is always particularly emphasized. 



Humus contains much colloidal material and in this 

 way possesses a certain degree of plasticity. It is, how- 

 ever, of a very loose structure and the large spaces con- 

 stitute lines of weakness. Another property of humus is 

 that it undergoes great change in volume when dried 

 out a property akin to the fineness of the soil, producing 

 larger shrinkage cracks. This is noticeable in many 

 .black clay soils, which check excessively. The great 

 capacity of humus for moisture permits a wide range in 

 moisture content, which produces corresponding physical 

 alteration. This wide swing from one extreme to another 

 is a potent factor in granulative influences. The color 

 of the humus affects the color of the soil, and thereby 

 increases the rate of change from the wet to the dry state 

 by increased evaporation of moisture. The relative 

 effects of crude muck, and the ammonia extract from the 

 same muck, on the cohesion of a puddled clay, as indi- 

 cated by the force required for a uniform penetration of 

 a knife-edge, is shown in the following table ; the samples 

 were dried and rewetted twenty times : 



PUDDLED CLAY PLUS MUCK 1 



Percentage 

 of penetration 



1. Clay 100 



2. Clay plus 5 per cent of muck .... 82 



3. Clay plus 15 per cent of muck .... 73 



4. Clay plus 25 per cent of muck .... 58 



5. Clay plus 50 per cent of muck .... 50 



1 Fippin, E. O. Some Causes of Soil Granulation. Trans, 

 Amer. Soc. A-jron., Vol. 2, pp. 10G-121. 1910. 



