ON THE IMPROVEMENT OF SOIL. 161 



Caroline. But how can it be rendered tepid without 

 exposing it to the sun ? And in that case it will be evap- 

 orated. 



Mrs. B. Exposure is not necessary : the heat of the 

 atmosphere gradually penetrates the soil, and the water 

 diffused in it acquires the same elevation of temperature. 

 Farmers conceive that the soil is injured by the action of 

 ploughing in summer ; but the injury proceeds from im- 

 peding the solutions requisite for the following crop. 

 In northern climates, where evaporation is less active, 

 ploughing is not so objectionable in summer. 



CONVERSATION XIII. 



THE ACTION OF SOIL ON PLANTS CONTINUED. ON THE 



IMPROVEMENT OF SOIL. 



Mrs. B. We may now proceed to examine the vari- 

 ous modes of improving the soil by mineralogical pro- 

 cesses. The first and most simple of these is to clear it 

 of stones ; when stones are injurious to cultivation. 



Caroline. But are not stones always injurious ? For 

 of whatever materials they may be composed, they are 

 such hard insoluble bodies that vegetables can acquire no 

 nourishment from them. 



Mrs. B. True ; but stones often perform a very useful 

 mechanical part in agriculture. They render a clay soil 

 less tenacious by separating its parts, and thus leave room 

 for water to drain off: they form, as it were, so many nat- 

 ural irregular conduits ; and, if you take them away, you 

 must employ them for the construction of artificial con- 

 duits to effect the same purpose. 



Caroline. This may be the case with stones buried in 

 the earth, but those lying on its surface must surely be 

 prejudicial to vegetation ? 



Mrs. B. Generally they are so, but not universally : 

 in some hot countries grass cannot grow, excepting un- 

 der shelter of loose fiat stones. I have seen pastures of 



884. In hot countries why should not ploughing be in summer 1 ? 

 885. How may the soil be rendered tepid 1 886. By mineralogical 

 process what is the first method of improving the soil"? 887. Of what 

 use are stones to a clay soil 1 ? 888. Of what use may stones be when 

 lying on the surface, and what is an example 1 



14 * 



