74 SUBSOIL AND TRENCH PLOUGHING. 



soil is thus broken and mellowed, air finds entrance, 

 injurious substances are washed down lower, or, if 

 there are drains, carried away, and the whole soil to 

 a greatly increased depth is fitted for the sustenance of 

 plants. It should be repeated once in five or six years. 

 It is difficult to go down more than a few inches below 

 the old furrow at the first subsoiling; at the second, 

 one or two more can be gained, and so on till the 

 greatest possible depth is attained. In some parts of 

 England they dig over the whole soil as deep as two 

 feet, but that is too expensive an operation for most 

 parts of this country. 



Trench ploughing is also practised in certain situa- 

 tions. A very heavy plough is used, of the same 

 shape as the ordinary plough, but much heavier; this 

 brings the lower soil to the surface. Such an opera- 

 tion is only to be advised when the subsoil is of good 

 quality, as otherwise the poor earth would be left on 

 the top, and the richer surface soil buried deep beneath 

 it 



SECTION VI. ON THE RELATIONS BETWEEN THE SOIL AND 

 THE PLANT. 



We now come to a new department of our subject, 

 in considering the connection which exists between the 

 soil and the plant. The attentive reader will already 

 have perceived that the inorganic substances in both, 

 show a certain marked coincidence. The source of 

 the organic part in plants has been shown in a pre- 

 ceding chapter to be partly the soil and partly the air. 

 The inorganic substances can of course only come from 

 the soil, and thus it is at once easy to perceive why 

 the differences indicated by Table I. constitute fertility 

 or barrenness. It is because the jUant needs these 

 substances, that their absence is so destructive to the 

 value of a soil. 



