260 TRANSACTIOXS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



Similarly, the greater the permeability of the ground, the 

 less can humus accumulate upon it. In fine-grained soils there 

 is not enough air, under normal conditions, for an active 

 eremacausis ; consequently the organic matter is but slowly 

 decomposed, leaving an abundant residue. 



In some cases the air-supply may be so meagre that the 

 disintegration is a process of putrefaction, and humus gathers 

 forming thick layers. According to the permeability of the 

 soil, there is every link between these extremes, and the 

 transitions are represented by very various types, on account 

 of the number of the agencies which are brought to bear upon 

 the eremacausis. 



Humus varies in colour from a dark brown to reddish yellow, 

 according to its mode of formation. It presents widely different 

 textures, porous in mould, spongy or friable in peat, earthy 

 or fibrous elsewhere. Old peat has a greasy appearance, and 

 forms an amorphous plastic moss. Humus consists of organic 

 substances with a greater or less amount of mineral matter. 

 Among the former are ulmin and ulmic acid, humin and humic 

 acid, crenic and apocrenic acids, which arise from subsequent 

 oxidation of humin and its acid. A comparative analysis of 

 living and humic matter shows in the latter an increase of 

 carbon and a corresponding decrease of hydrogen and oxygen. 

 The nitrogen varies from 3 to 20 per cent, of the total humus 

 weight, but is usually combined as insoluble compounds. 



Humus forms an almost waterproof layer ; but it absorbs the 

 water easily, and retains it even better than clay. On the other 

 hand, it is suVyect to a veiy great evaporation. On the whole, 

 humus contains more water than any other kind of soil, but 

 gives it off to the plants with greater difficulty. It must there- 

 fore contain a greater proportion of water than sand or clay 

 does, in order to meet the requirements of its vegetation. 



Humus dried up in air becomes more permeable to gases. 

 When water increases and the volume of the pores decreases, 

 the quantity of carbonic, ammoniacal, and other gases, which 

 is here always greater than in the open air, diminishes corre- 

 spondingly. The humic substratums are very hygroscopic, and 

 remarkaVjle l)y their power of condensing ammonia, marsh-gas, 

 sulphuretted hydrogen, etc. 



On account of its dark colour, humus has a strong absorbing 

 power for the sun's rays. On the other hand, it has but weak 



