42 TRAWSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



or washed out in a form unavailable to the roots of the trees, 

 and get into the subsoil to help in the formation of pan. If soil 

 conditions are unstable, drought alternating with rain (e.g. on 

 sands), the results are a similar leaching of salts and formation 

 of sour humus. But where decomposition is under typical 

 mesophytic conditions, fairly moist, well aerated and without 

 violent fluctuations of the conditions of aerobic bacterial 

 growth, then the salts are gradually made available to the 

 roots of trees, and the humus is mixed with the mineral soil by 

 the action of earthworms, and the soil tends to become healthier 

 as time goes on. 



The influence of the species of crop on soil may be summar- 

 ised in a few words ; since a tree reacts on the soil through the 

 shade it casts upon it, i.e. the amount of leafage it bears, pre- 

 venting insolation, drought, etc., and through the amount of 

 organic matter it deposits on the soil (roughly again its leafage), 

 the influence of any species in improving or mamtaining the 

 quality of the soil is proportionate to its shade-bearing capacity. 

 In the second place, the influence is proportionate to the amount 

 of salts it returns in leaf-litter, that is to say, in a general way, 

 that broad-leaved species are more effective than conifers. But 

 besides these general rules, certain peculiarities of the plant 

 itself may have an important bearing on the question. The soft 

 rich leaves of the ash rot too rapidly as compared with those 

 of the oak, the dry siliceous needles of the Scots pine form a 

 felted impermeable mat which is very apt to produce unsatis- 

 factory humus, and other examples might be quoted. 



These are a few of the fundamental edaphic facts upon which 

 silvicultural practice ought to be based. Facts of economic or 

 other nature may have equal or greater importance, but if these 

 natural phenomena are neglected, even although the results are 

 not felt immediately or during the present rotation, the practice 

 which does not give them due consideration is not economic 

 forestry, and may result in such soil conditions as render 

 forestry of any kind difficult if not impossible. 



One of the most important aspects of these effects of tree- 

 growth on the soil is their bearing on the question of the growth 

 of pure crops. In the earlier part of their life most species cast 

 sufficient shade to keep down weeds so long as canopy is kept 

 fairly close. But after a time light-demanding species, such as 

 larch and Scots pine, open out naturally and expose the soil to 



