30 TRANSACTIONS OF ROVAL SCOTTISH ARBORICULTURAL SOCIETY. 



of physical conditions assume a much greater importance 

 relative to that of the mere chemical content, and since physical 

 soil conditions are somewhat correlated to climatic conditions 

 there is a tendency to ascribe to climate many effects which are 

 essentially produced by soil conditions. 



Arising from these widely divergent grounds — the chemical 

 nature of soil science as applied to agriculture, and the ultimate 

 importance of weather in the growth of trees — the subject of the 

 soil in relation to tree-growth has received less attention from 

 the scientific forester on the one hand and the plant physiologist 

 on the other than its importance warrants, with the result that 

 so far as scientific investigation of forest soils is concerned, only 

 a meagre beginning has been made. In fact, could the obser- 

 vations of practical foresters throughout the country be tabulated 

 and brought to their scientific foundations, more would be 

 accomplished simply by that means than has already been 

 deduced from the fundamentals of plant physiology. 



Before considering the forest soil in particular, it may be 

 useful to sketch out a few of the principal facts underlying plant 

 nutrition as a soil phenomenon. 



In the growth of plants, so far as their roots are involved, 

 three material essentials are concerned — water, air (oxygen), and 

 nutrient salts. If these are present in sufficient and proportionate 

 amounts, there are few plants which would not show approxi- 

 mately normal growth in water itself as a medium for the roots. 

 Besides acting as an anchorage for the support of the plant, soil 

 is simply a medium by and from which these essentials are 

 supplied to the roots. Its value for the growth of any species 

 depends in the main upon the exactness with which its structure 

 and salt-content enable it to satisfy the requirements of the 

 species for these essentials of growth, and as excess of any one 

 of these essentials may, and usually does mean not only relative 

 but absolute diminution of one or more of the others, plant 

 vitality may be rapidly affected by comparatively small changes 

 in soil conditions. 



The soil is composed of various sized particles upon the 

 surface of which lies the water containing salts, and between 

 which circulates the necessary air supply. If the water-supply 

 is too great, air circulation is reduced, the roots become 

 suffocated, the plant unhealthy. (It may be noted, however, that 

 free running water, whether percolating through the soil or in 



