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 sparingly 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 sunlight 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. 
