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. 



