10 THE PRINCIPLES OF SOIL MANAGEMENT 



the moisture film as it contracted. The small particles 

 are moved into the spaces between the large ones, 

 thereby reducing the volume, as is shown by the checks. 

 The checks which result from shrinkage are due to the 

 unequal contraction. There comes a time when the 

 general film around the whole mass must rupture. It 

 breaks, along the line of least resistance, through a large 

 pore space independently of how this space may have 

 been formed. If the soil mass is very uniform, there will 

 be few breaks, and the shrinkage will be, as a whole or 

 at most, around a relatively few centers. This process 

 produces clods or overgrown granules. But, if there are 

 numerous lines of weakness, there will be many centers 

 of contraction, and consequently a larger number of 

 small clods or granules will be formed. This is the desir- 

 able condition, and constitutes good tilth, that is, the 

 most favorable physical condition for plant growth. 



While once drying produces some checks, a few 

 large ones and many small ones, such a structure 

 does not constitute good tilth. The process must be 

 continued further. When the soil is remoistened, it 

 expands, but usually not to its original wet volume. 

 Therefore the checks remain as lines of weakness, and, 

 upon a redrying, are effective in further reducing the 

 size of the granules. When this process is repeated 

 a number of times, as occurs under field conditions, 

 it results in a small and very desirable size of soil granule. 

 Further, the drying out of the water in the granule 

 deposits the salts in solution, which binds the particles 

 together in a somewhat permanent and stable aggre- 

 gate. The following figures represent the relative force 



