CH. I 



SOIL 15 



' Other chemists consider that the soil is more complex, 

 containing colloidal decomposition products and a 

 solution which not only differs in composition in 

 different soils but also shows local variations in com- 

 position in different parts of the same soil." It is 

 therefore considered that while the infertility of a " sour " 

 soil may be due to the presence of toxic substances, 

 great importance should be attached " to the nutritive 

 functions of the soil constituents and of added 

 fertilisers." 



The same author considers that where the American 

 writers have gone wrong is in ignoring the biological 

 changes going on in the soil, and the functions of the 

 nutrient material which arises in the soil from the 

 decomposition of organic remains by the action of micro- 



organisms. 



The most important contribution to our knowledge 

 is the recognition of the fact that, more than chemical 

 composition, the physical properties of the soil, which 

 depend on the size of the soil-particles and the con- 

 sequent regulation of warmth, air, and water-supply to 

 the plants roots, are more important in determining the 

 value of a soil for plant life. 



In Forestry, artificial improvement of the soil by 

 means of fertilisers is hardly practicable, except over 

 limited areas, such as nurseries and plantations. The 

 crop has therefore to depend on the humus which is 

 formed mostly from fallen leaves, fruit, twigs, etc., and 

 from the decomposition of roots and root-fibres. The 

 timber in a tree happily contains only a small proportion 

 of the rarer nutrient constituents of the soil, and what 

 comes back to the latter in more concentrated form is 

 sufficient to keep up the nutrition of the crop, while at 

 the same time improving its physical properties. It is 

 for this reason that such customs as the removal of 

 leaves, dead or alive, for manure or litter is harmful to 

 the forest and should be discouraged. In the same 

 way, the too-sudden removal of the leaf-canopy by clear- 

 fellings not only stops the supply of vegetable debris, 



