188 SOILS: PBOPURTIBS 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 higlily 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^ 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 Pippin, B. 0. Some Causes of Soil Granulation. Trans. 

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



