Causes of Fertility or Barrenness of Soils. 
195 
the very substances we are looking for may be introduced, and a 
trace indicated in cas(!S Avhere the soil was quite destitute of it. 
We are more likely to arrive at correct conclusions as to the de- 
ficiencies of soils and the requisite materials to add, by studying 
the composition of the crops which flourish or will not grow upon 
them. In the ashes of plants we have a much more concentrated 
mass to work upon ; the soluble portions are not diluted or dis- 
persed, as in the soil, amongst a large mass of insoluble matter, 
and consequently our analysis, as proved by the similarity of 
result in different specimens of the same kinds, will be much 
more worthy of credit. 
III. — Effect of Depth of Soil on Vegetation. 
The deeper a soil is, or can be made by good tillage, provided 
it contains the elements of fertility, the more productive it must 
become, not only by causing a large supply of actual food, but 
also by presenting an increased surface for the action of chemical 
forces to retain those valuable substances, which being in solu- 
tion would otherwise pass away in the subsoil, where, though 
they might be retained, the roots could not get at them. The 
atmosphere penetrates more freely, warming and exciting the 
whole mass ; the roots instead of merely throwing out laterally, 
and creeping along just beneath the surface, as in shallow soils, 
push boldly out in all directions in search of food, and thus 
strengthen the plant. It is our belief that the wheat crop re- 
quires a deeper soil than is generally thought necessary, and that 
root-fall is often due to the lateral direction into which the 
roots are forced by the impassable pan a few inches beneath the 
surface ; at the same time the surface should be made as firm as 
possible, for there is no doubt that the wheat crop likes a firm, 
though not a shallow bed. How often we see cases of surface- 
rooted trees, such as lime or beech, being blown up by the wind, 
roots and all, while the deep-rooted oak stands firm ! The great 
use of the modern cultivators, so generally employed in fallowing 
operations, is to stir and lighten up those portions which, too 
deep to be reached by the plough, are yet pressed down by its 
weight passing over them. The alluvial tracts so frequently 
found surrounding our principal rivers, are all deep and very 
fertile ; though generally dry, they never suffer from drought, 
owing to their powers of absorption, both from the air and subsoil. 
Depth being so important, we should do all in our power to 
increase it by artificial means when required ; as frequent 
ploughing at a uniform depth produces even in the deepest soils 
a hard bed, through which neither I'oots, air, nor moisture can 
readily penetrate, the use of the subsoil plough, once in each 
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