240 HUMUS — HUMOGEN. 



actions on plant residues." He states that " numerous studies 

 have proved that this group jwssesses at least six properties not 

 shown by Group (a) — 



( 1 ) It gives a dark brown or black colour to the soil. 



(2) It can withdraw various ions from their solutions. 



(3) It causes the soil to puff up and so leads to an in- 



crease of pore space. 

 - (4) It increases the water-holding capacity of the soil. 



(5) It swells when wetted. 



(6) Although essentially transitional, it has a certain 



degree of permanency, and only disappears slowly 

 from the soil." 



Russell then goes on to say that " the group of substances 

 possessing these properties is generally called ' humus.' From 

 these half-dozen properties we may infer that humus is a brown 

 slowly oxidizable colloid, but unfortunately we cannot get much 

 further." 



That several types of humus exist there is not the least 

 doubt, as witness the studies of P. E. Muller and of the r.ritish 

 Vegetation Committee. 



(8 ) P. E. Muller has examined the types of humus occurring 

 in Danish forests, and descril)es two principal t}pes, i.e., " mull "' 

 and " torf ." or mild humus and peat. The former is free from 

 all trace of acidity, and l)ears a characteristic and varied vegeta- 

 tion, while the latter is acid and not so favourable to plant 

 growth. In fact, " torf " so inhibits the growth of voung trees 

 that as the old ones die the forest tends to become open heath. 

 Muller seems not to attribute any great part in the formation of 

 type to soil or climatic conditions ; but to the nature of the living 

 organisms in the soil he assigns an important significance. He 

 would seem to consider earthworms the chief factor in the for- 

 mation of " mull." their absence and the presence of fungi the 

 chief fac^tors determining the production of " torf." The 

 British Vegetation Committee recognises at least live distinct 

 types of humus, i.e. — 



(i) Dry i)eat found on heaths on poor dry sandy soils. 

 (2) A peat formed under similar conditions, excepting 



that the rainfall is greater than in (_i). 

 (3} Wet peat formed in districts of high rainfall. The 



drainage water from them is acid and very deficient 



in mineral salts. 



(4) hVn, formed in the presence of calcium carbonate 

 and other mineral salts. Its drainage waters are 

 alkaline 



(5) Can-, closely related to (4). 



The work oi' the liritish Committee, while not discrediting 

 Mullcr's observations, seems to show that climatic and soil con- 

 ditions have much to do with the type of humus formed. In this 



