MINERAL CONSTITUENTS 



135 



game matter, and in general by anything, as lime, that will produce 

 granulation. 



(b) Shrinkage. Clay possesses the property of shrinkage to a 

 remarkable degree, due to the loss of moisture from the particles 

 in general but the colloidal constituent particularly. This shrinkage 

 is emphasized when a large amount of humus is present, because 

 the humus is partly colloidal. Clay has been found to shrink 31.9 

 per cent, and peat 32. (J per cent (see accompanying table). Hence 

 a soil composed of both of these will possess the property of shrink- 

 age to a great and sometimes injurious degree. 



Shrinkage of Soils of Varied Physical Composition, with the Moisture and 

 Organic-Mattel Content 8 



This property is frequently detrimental to crops, because of the 

 formation of large cracks that tear the roots of the plants as well as 

 cause excessive loss of moisture (Figs. 72 and 73). The property of 

 shrinkage is a primary cause of granulation, and this is only pos- 

 sessed by soils which contain colloids. It is also an aid to percola- 

 tion and drainage, because the cracks produced by shrinkage do not 

 close entirely, thus leaving passageways for water. During the 

 dry summer of 1914, a clay loam shrank to such an extent that an 

 inch auger could be pushed into cracks without any effort to a depth 

 of 24 to 28 inches. The cracks undoubtedly extended to a depth 

 of 30 inches. 



(c) Plasticity. A moist clayey soil may be molded into any 

 form or pressed into thin plates, retaining the shape indefinitely. 

 The property permitting this is called plasticity. The degree of 

 plasticity varies directly as the amount of colloids present. Tf is not 

 a desirable quality for soils to possess, ns such are liable to be more 

 readily puddled. The amounts of shrinkage, hygroscopic water and 

 adsorption are approximate indications of the plasticity of day soils. 



