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OBSERVATIONS ON THE NATURE OF SOILS. 



nearly pure silica, containing not more than one-fortieth part of other matters. 

 Soils of this description are deficient in that degree of cohesion which is neces- 

 sary to the support of healthy vegetation ; they are liable to be dispersed by the 

 wind, or washed away by heavy rain ; they are, moreover, incapable of retaining 

 (except for a short time) a proper degree of moisture ; the rain which falls upon 

 them being rapidly absorbed and permitted to sink to a depth far beyond the 

 roots of the crop which they are intended to bear, while the surface becomes 

 immediately dried by the sun. 



ALUMINOUS SOIL. 



The earth which, next to silica, is found most abundantly in soils, and which 

 chiefly contributes to their tenacity, is alumina. It has a great affinity for 

 water, of which it imbibes and retains a large quantity. Soils, therefore, which 

 contain too large a proportion of this earth soon become saturated in wet 

 weather. Such soils are, consequently, too cold and adhesive in winter, on which 

 account the roots of plants having their temperature diminished below the 

 natural standard, are apt to perish ; in dry summers, on the contrary, they 

 shrink in an equal degree, becoming stiff and unmanageable, and presenting on 

 their surface large clefts of considerable depth. Such soils, from their hardness 

 in dry weather, are with difficulty penetrated by the delicate fibrous roots of 

 many plants. An aluminous, or clayey soil, prevails in most of the coal districts ; 

 though sometimes moderately fertile, it is for the most part poor, and requires 

 a great deal of nice management by draining, liming, &c, to make it tolerably 

 productive. The Plastic, the Weald, the Kimmeridge, and the Oxford clays, are 

 well-known examples. The Kimmeridge clay is favourable to the growth of 

 the oak. 



CALCAREOUS SOIL. 



This soil is distinguished by the presence of lime, combined generally with 

 carbonic acid, and is found in the state of chalk, limestone, shells, or marl ; the 

 latter consisting of carbonate of lime mixed in various proportions with clayey 

 sand ; it occurs also occasionally in combination with sulphuric acid, in which state 

 it is known by the name of gypsum, a substance which, when deprived of its water 

 of crystallization by exposure to a strong heat, forms the well-known article 

 called plaster of Paris. It has been ascertained that the most fertile soils 

 usually contain a considerable proportion of lime ; while, on the other hand, a 

 soil which contains too much calcareous matter is always sterile. Lime has a 

 great affinity for water, of which it is also very tenacious ; its power of absorp- 

 tion, however, depends very materially upon the degree of fineness of the cal- 

 careous matter as found in soils. According to the experiments of Professor 

 Schiibler, 100 parts of calcareous sand retained 29 parts of water, while the 

 same quantity in a state of fine powder retained 85 per cent. 



Some of the chalk districts, particularly where they come in contact with 



