190 SOILS: PROPERTIES AND MANAGEMENT 



figures, 1 expressed as the relative force necessary to 

 penetrate a puddled clay treated in various ways : 



Percentage 

 penetration 



1. Puddled clay dried once 100.0 



2. Puddled clay frozen once and dried once . . 30.3 



3. Puddled clay frozen three times and dried once 27.3 



4. Puddled clay frozen five times and dried once . 21.8 



Freezing probably affects the colloidal material in 

 the same general way as does drying. This has been 

 indicated by the work of certain investigators, 2 in which 

 it was found that lowering the temperature of a soil below 

 freezing lowered the hygroscopic coefficient. 



125. Addition of organic matter. Soils rich in humus 

 or decomposed organic matter are generally in better 

 physical condition than soils low in organic content. 

 The marked effect of the absence of this material in many 

 long-cultivated soils is well known. For example, in 

 much of the southern New York hill regions, the soils 

 are now recognized to have a very different relation to 

 crop growth from what they had for a few years after 

 they were cleared. Their color has become lighter, and 

 with the decay of the humus a decided physical change 

 has taken place in the soil, which is to some extent cor- 

 rected by the restoration of the organic content. In 

 certain prairie soils the effect of humus depletion on struc- 



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

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



2 Czermak, W. Ein Beitrag zur Erkenntnis der Veran- 

 derungen der Sog. physikalischen Bodeneigenshaften durch 

 Frost, Hitze, und die Beigabe einiger Salze. Landw. Ver. 

 Stat., Band 76, Heft 1-2, Seite 73-116. 1912. Also, Ehrenberg, 

 P., und Romberg, G. F. von. Zur Frostwirkung auf den Erd- 

 boden. Jour. f. Landw., Band 61, Heft 1, Seite 73-86. 1913. 



