98 THE PRINCIPLES OF SOIL MANAGEMENT 



tion. The real significance of colloidal material is some- 

 what doubtful, and, further, the amount present in 

 even the most plastic clays is so small as hardly to be 

 given credit for the effects noted. It seems probable 

 that plasticity and cohesiveness of the material is due 

 to several uniting causes, but for all practical purposes 

 of the farmer it may be identified with fineness of tex- 

 ture. Associated with plasticity is a certain amount of 

 shrinkage upon drying, and expansion upon wetting. 

 The checking of the clay soil is an example of this. 

 As the water dries out of the soil, the surface film draws 

 continually closer about the particles, and, if these 

 are small enough, may move them closer together. 

 Then, if the whole mass is not drawn together as one 

 unit, there will be cracks developed as a result of the 

 shrinkage. The cracks occur where there is a weakness, 

 from whatever cause, in the structure of the soil. War- 

 ington reports the results of Schiibler, which show that 

 a very pure clay, when dried from a thoroughly puddled 

 condition, contracted 18.3 per cent of its original vol- 

 ume; a sandy clay contracted 6 per cent, and a sample 

 of humus, 20 per cent of its volume. Gallagher found 

 a shrinkage of over 30 per cent in drying out a sample 

 of muck. These figures illustrate the general fact that 

 the finer the texture the greater the shrinkage. Con- 

 versely, on wetting, there is a similar though smaller 

 degree of expansion. 



The checking of soil resulting from this shrinkage 

 may be very injurious to crops. Where large checks 

 or cracks are formed, the roots of plants may be injured 

 or broken. And, further, these cracks greatly hasten 



