CHEMICAL CLASSIFICATION OF SOILS. 245 



spect, they are the opposite of sandy and gravelly soils. Clay 

 soils are of every quality, from a dead, barren mass, to the 

 rich clay loams, which are some of the most fertile and pro- 

 fitable soils which are cultivated. Hence their fertility will 

 depend upon the proportion of other earths, the quantity of 

 animal and vegetable matter they contain, and the character 

 of the sub-soils. Common clay is wholly barren. Mixed with 

 calcareous or siliceous earth, it is nearly so; but when, in 

 addition, it contains large quantities of manure, it becomes 

 comparatively fertile, if the sub-soil is sand, or such as to 

 permit the water to drain oft'; but if the sub-soil is impervi- 

 ous to water, they are always cold, wet, and unfriendly to 

 those crops which require the heat of summer to bring them 

 to maturity. 



The most fertile of these soils are the alluvial clay soils. 

 These are formed at the mouths of rivers, where the sea exerts 

 its influence upon the fine materials brought down by their 

 waters, as they flow over argillaceous rocks. They often be- 

 come mixed with animal and vegetable substances, and ap- 

 proach rich clay loams, of the most fertile and valuable qual- 

 ity. The common clay bottoms may be converted into fertile 

 clay loams, by cultivation. 



Mode of improvement. Aluminous soils are improved by 

 admixture of siliceous and calcareous sand and peat muck. 

 This renders them more friable and more easily tilled. 



Sand often forms the sub-soil, in which case sub-soil 

 ploughing may be resorted to, by which the sand and clay 

 will become incorporated. This is diflferent from trench 

 ploughing, in which two ploughs are used, the one to turn the 

 upper soil, and the other to bring up the sub-soil to the sur- 

 face. But in sub-soil ploughing no portion of the sub-soil is 

 brought to the surface, but merely loosened and pulverized. 

 By this process, the air and water exert a fertilizing influence 

 upon it, and then it is incorporated with the clay by trench 

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