136 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



forestry. Mild humus results from partial decay of the organic 

 matter, mainly by oxidation. It is rich in nitrogenous and other 

 compounds available for the growth of plants, it is not acid, it 

 generally contains earthworms and other organisms, and fungi 

 flourish on it. The usual form of mild humus in woods is forest 

 mould consisting of fine fibrous particles with a large proportion of 

 mineral matter or ash when burnt, hence it crumbles easily in 

 the hand, and does not swell up much or become slimy when 

 wet. Raw or acid humus contains little ash, and is made up 

 mainly of fragments of plant remains vvhich can be identified 

 under the microscope (bog moss, heather, and tree leaves or 

 needles) ; when dry it is closely matted and stringy so that it does 

 not crumble, when wet it swells considerably and frequently 

 becomes slimy. The commonest forms of raw humus are forest 

 raw humus in woods, and moor turf and peat on moorlands. 

 This humus rarely contains earthworms, fungi, or other living 

 organisms which would assist in its decomposition ; the absence 

 of these is probably due to lack of air and excess of acid in raw 

 humus, so that while it contains much nitrogen, that substance is' 

 not converted so as to become available as a plant food, "ihe 

 effect of a surface layer of raw humus is to cut off the underlying 

 soil from aeration and other atmospheric action, and as the felted 

 mass absorbs water like a sponge, the lighter rains fail to reach 

 the soil below. Raw humus itself soon loses its water by 

 evaporation, but it prevents natural evaporation from the soil 

 below which thus tends to become acid. So that the effect of 

 the raw humus is not limited to the surface, but gradually leads 

 to deterioration of the whole soil which thus becomes unfavour- 

 able to plants requiring a fresh and constantly moist soil.^ Raw- 

 humus being at first a surface layer does not affect the older 

 trees, but it is fatal to natural regeneration because the seedlings 

 arising in it sooner or later succumb to drought before the)- 

 become rooted in the deeper soil. Much of the labour involved 

 in preparing heathy ground for planting young trees is due to the 

 presence of a thick layer of raw humus. 



Forest or mild hunius is favourable to a mixed ground-herbage 

 of deep-rooting plants, if there is sufficient light available. In 

 the damper woods its presence is indicated by red campion 

 {Lychnis dioica), wood hyacinth {Scilla), wood garlic {A/IiU7>i), 



^ See also \V. G. Smith, "Origin and Development of Heather Moorland,"' 

 Trans., vol. xvii. p. 117, 1903. 



