EFFECTS OF ORGANIC MATTER 129 



of the original volume, and that of humus to be 20 per 

 cent; while others report the shrinkage of muck samples 

 to be more than twice this amount. 



60. Plasticity. The crude organic matter exhibits no 

 striking peculiarities, but the humus substance has 

 many. One of these is its plasticity. Although very 

 fine, its plasticity is not great as compared with clay. 

 But it is sufficient to act as a weak cementing material 

 in soil, which is very important in binding together 

 light sandy soils, and in lightening up and holding apart 

 the aggregates or crumbs in clay soil. Thereby it greatly 

 promotes the granulation of clay soils which are properly 

 drained. 



61. Effects of organic matter. The effects of organic 

 matter on the soil, and thereby upon plant growth, 

 are so numerous and so far-reaching and generally so 

 beneficial, and further, its maintenance is so important 

 a part of good soil management that, at the risk of 

 anticipating some of the subsequent discussions, its 

 effects are here briefly summarized. They are of two 

 sorts, (1) Physical, and (2) Chemical. 



62. Physical effects. (a) Physically, it affects both 

 tilth and granulation. Owing to its weak plasticity 

 and its great contraction when dried, it is a very potent 

 factor in hastening the granulation process of clay 

 soils in the way that has been explained above. And 

 on light sandy soils which are loose and inclined to be 

 drifted by the wind or eroded by rains, it has the effect 

 of binding them together and imparts a much more 

 loamy character. 



(b) By its beneficial effect on the structure of the 



