COMPOSITION OF SOIL. 27 



Sodium chloride (common salt). 



Carbonates, sulphates and phosphates of iron and 



manganese. 



The quantity of salts in the soil does not. as a rule, exceed 



one-half per cent., and rarely one per cent. Larger quantities 



appear only in certain localities, such as salt plains, in the 



vicinity of the sea coast or salt springs, and in some peaty and 



swampy soils ; in these cases they act injuriously on vegetation. 



Potassium carbonate is of importance, as forest trees 



require a fair amount of it. Sodium carbonate in moderate 



quantities does not act unfavourably. Sodium chloride acts 



favourably only if present in small quantities. Salts of iron 



often act injuriously. 



r. Aci(h. 



Carbon dioxide and humic acid are the two free acids which 



generally appear in soils. Other free acids, if they appear at 



all, do so only temporarily, until they enter into combination 



with a base. 



(L Metals. 



Of metals, only iron is of importance in sylviculture. It 

 appears as ferrous oxide and as ferric oxide. The former is 

 believed to be injurious to plant life. Ferric oxide may be 

 mixed with soils, and unless the quantity exceeds 10 per cent., 

 it does not act injuriously ; on the contrary, most fertile soils 

 contain a certain quantity of it. Oxides of iron frequently 

 assist in the formation of impermeable strata, or pans. 



B. Onjanir Matin-, or Hvmm. 



By humus is understood in sylviculture all organic matter 

 which, in contact with the soil, is gradually decomposed, and 

 forms in mixture with the upper layer of mineral substances 

 the mould or black earth of the forest. The sources of humus 

 are the annual fall of leaves and twigs (or even whole trees) 

 and dead plants. 



The continuous decomposition of humus furnishes several 

 important products. In the first place the soil receives all the 



