188 SOTLS: PROPERTIES AND MANAGEMENT 



a relatively few centers. This process produces clods, 

 or " overgrown " granules. If there are numerous lines 

 of weakness, however, there will be many centers of con- 

 traction, and consequently a larger number of small 

 clods, or granules, will be formed. This is the desirable 

 condition and constitutes good tilth that is, the most 

 favorable physical condition for plant growth. 



Just what may be the effects of wetting and drying on 

 the colloidal matter of soil is a question. In general, 

 desiccation tends to flocculate colloids and in many cases 

 their binding power becomes highly developed thereby. 

 If such colloids are irreversible, as many in the soil un- 

 doubtedly are, this binding becomes more or less per- 

 manent, which explains the tendency for a crumb struc- 

 ture to persist. Wetting, on the other hand, tends to 

 develop colloidal matter which will become binding mate- 

 rial on the next drying. The desiccation and throwing 

 down of colloids, as well as their generation, thus be- 

 comes a very important factor in the wetting and drying 

 as related to granulation. 



The following figures 1 represent the relative force 

 necessary to penetrate puddled clay dried once, as com- 

 pared with the same puddled soil wet and dried twenty 

 times. The relative hardness may be taken as a rough 



measure of granulation : 



Percentage of pene- 

 tration 



1. Puddled clay dried once .... 100.0 



2. Puddled clay dried twenty times . 31.4 



3. Puddled clay dried twenty times . 30.6 



4. Puddled clay dried twenty times . 32.0 



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

 Amer. Soc. Agron., Vol. 2, pp. 106-121. 1910. 



