186 GENERAL SUMMARY 
The soil improvement considered under (i) may be com- 
pared with deep and thorough digging in the garden, whereas 
(ii) has its horticultural parallel in surface hoeing, which 
removes weeds and prevents the caking of the top layer. 
Formerly the continental literature which dealt with the 
sylvicultural aspect of soils has given great prominence to 
the maintenance of a good surface, and it is only recently 
that the subsoil has received the attention which it deserves. 
Beech, which has been planted so enthusiastically in Germany 
as a soil improver, is chiefly good for the surface, but it is 
a moderately deep rooter, and no doubt assists in opening 
the subsoil to some extent. 
If the chief trouble in planting agricultural land is the 
preparation of the subsoil, in the afforestation of heaths 
the first difficulty is the soil surface. The layer of peat 
which commonly collects on the surface of heather or heath 
moor is all but impenetrable to gases in any form, and the 
death of roots which is frequently observed in the first 
rotation on moors is probably chiefly due to this cause. 
The special treatment required in planting heaths is a subject 
outside the scope of the present book, but the susceptibility 
of larch to poor soil aeration makes the tree an unsuitable 
crop for planting where the layer of peat is sufficient to 
prevent a free circulation of gases. For such country Scots 
pine is probably the best crop, and the good results reported 
from Norway by Schotte (1917), which have been obtained 
with a mixture of larch and Scots pine, may be largely due 
to the improving influence exerted by the latter on heather 
peat, 
In the north of England larch is often mixed with spruce. 
This mixture is in keeping with the principle of growing 
a light demander with a shade bearer, but otherwise has 
little to recommend it. Spruce does little to improve the 
soil surface, and being a surface rooter does not work the 
subsoil. Consequently, the larch gains next to nothing from 
the mixture. The encouragement which spruce gives to 
the larch aphis and the needle-cast fungus (see p. 170) 
renders the mixture undesirable. 
