GENERAL SUMMARY 185 



have been appointed to inquire into the causes of root 

 failure under such circumstances. The lupin has been 

 found to have a beneficial effect on soil about to be afforested, 

 and still better results have been obtained with the false 

 acacia planted in alternate lines with conifers. This tree is 

 unsuitable for mixing with larch, as it does little to improve 

 the soil surface, but it mixes well with any of the shade- 

 giving pines, such as Scots, Corsican, or cluster (maritime) 

 pine. The false acacia is not a popular tree in British 

 forestry, as it does not mature well here, but it apparently 

 does its work on the subsoil quickly, and it may be thinned 

 out as soon as it fails to maintain its vigour. Besides 

 improving the soil physically, the false acacia adds to the 

 nitrogen content of the soil through the agency of the 

 bacteria, which cause nodules to develop in its roots. 



These comments all tend to show that larch is not suitable 

 for a first rotation on agricultural land. Where, however, 

 it is grown it should be si)aringly mixed with other trees 

 which improve the subsoil, and if the intermixture of three 

 different species is not objected to, it would be as well to 

 mix the tree with Scots pine and false acacia, by which 

 means it is possible that an ultimate crop of healthy larch 

 might be obtained. 



(ii) Trees which are enumerated in the text-books under 

 the heading of soil improvers are mostly those which keep 

 the soil surface in good condition by producing a mass of 

 leaves which rot slowly on the soil and maintain a layer of 

 porous humus. The chemical composition of the humus is 

 not particularly favourable to trees, as is shown by the poor 

 natural regeneration which occurs where deep humus is 

 present ; but it remains open and porous in the sunniest 

 weather, and besides allowing free access of oxygen to the 

 soil below, in which the roots grow, prevents excessive loss 

 of moisture through evaporation. These soil-improving 

 trees also keep out the sunhght which stimulates the quick 

 decomposition of the humus, and prevent the growth of 

 weeds which mat the upper soil layers, and may have, as 

 well, a toxic effect on tree roots. 



