GENEEAL IDEAS. 13 



ably affects its physical properties. All soils are 

 composed of argillaceous, calcareous or silicious 

 elements, either pure or mixed in greatly varying 

 proportions, together with a certain quantity of 

 substances capable of assimilation by plants. If, 

 then, we study the physical properties of each of these 

 three elements separately, it will be easy to ascertain 

 those of the soil in which any one of them may 

 happen to preponderate ; similarly we can deduce 

 the properties of soils intermediate in character. 



Clay is exceedingly stiff ; it absorbs water slowly, 

 but in great quantity, and a very sticky paste is thus 

 formed ; when a clay soil is saturated, water is no 

 longer absorbed but has to remain on the surface. 

 It retains water a long time, and on drying up shrinks 

 considerably and becomes deeply cracked. A pure 

 clay soil is unsuited to vegetation, because roots do 

 not easily penetrate it ; nor, when it is saturated 

 with water, do they find enough air ; also because 

 they are torn asunder and wither up when the soil 

 cracks in dry weather. Moreover, clay contains no 

 nutritive elements. 



Limestone, reduced to an earthy state, yields a 

 very light soil which rapidly absorbs a large quantity 

 'of water forming with it a light-coloured mud. It 

 loses this water with equal facility, and becomes a 

 fine dust. Though yielding lime salts, it is unsuited 

 to vegetation, because it is either too wet or too dry, 

 and does not afford a sufficiently solid base for large 

 trees. Limestone presents yet another danger; if 

 caught by frost when in the state of mud, it swells 

 and lifts up with it the young plants ; when a thaw 



