ACTION OF THE SOIL ON PLANTS. 153 



rock, that seeds bury themselves, and vegetation com- 

 mences. 



Emily. In attempting, then, to cultivate a barren soil, 

 we should follow those lessons which Nature points out, 

 and scatter seed in such crevices, which, if they did not 

 arrive at complete maturity, would, by their remains at 

 least, help to prepare a soil. 



Mrs. B. This is often done ; for instance, in the fis- 

 sures of the lava of Mount Etna. Indian fig or prickly 

 pear has been sown ; the roots of which insinuate them- 

 selves into every little cavity, and help to split the block. 

 This plant produces a great quantity of fruit, but its most 

 important recommendation is, that of forming a soil for 

 future vegetation ; but to proceed to soils of a less obdu- 

 rate nature. 



A stiff argillaceous soil is difficult to cultivate, on ac- 

 count of the resistance it opposes to the penetration of 

 roots. This description of soil attracts moisture, and is 

 so retentive of water as to be seldom dry, unless during 

 the heat of summer, when it splits ; and it is in the cre- 

 vices thus formed that vegetation commences. Such a 

 soil requires frequent ploughing, in order to break down 

 and pulverise the clods, when practicable, earth of a light- 

 er nature should be mixed with it. Plants having large 

 roots will not, in general, succeed in a soil of* this de- 

 scription, as they will not be able to penetrate it, 



Emily. Yet plants with small delicate roots will have 

 still less strength to do so. 



Mrs. B. True ; choice should therefore be made of 

 roots which have slender, but firm and strong roots : those 

 whose roots are of a dry nature are best adapted to hard, 

 impenetrable soils. , 



A sandy soil offers difficulties of an opposite descrip- 

 tion. If the sand be mixed with calcareous matter, these 

 are more easily overcome ; for a portion of the lime is 

 dissolved by rain, and its solution gives some degree of 

 stability to the soil : but if the sand be almost entirely 

 silicious, like that of the sea, the evil is well nigh insuper- 

 able ; for this species of sand is insoluble, and nothing 



843. How is soil formed in the fissures of lava on Mount Etna? 844. 

 Why ia it difficult to cultivate a stiff argillaceous soil? $45. What 

 is said of large roots in this soil 1 ? 846. What roots are suited to 

 it! 847. Why is it impracticable to cultivate the deserts of Arabia 

 and Africa! 



