t|iiate for efficient leaching, lundpan may form in tlie 

 B Iiorizon as the result of deposition here of ferric 

 oxide, alumina, colloidal clay, or calcium salts. This 

 layer becomes so compact and hard that it is im- 

 pervious to root penetration and the burrowing of 

 animals, although during periods of wet weather it 

 disappears temporarily. In arid regions, the hardpan 

 may l>e at or close to the surface ; but in more humid 

 climates it occurs at progressively greater depths 

 until it disappears altogether. 



Mull 



even in warm climates, and under ericaceous vegeta- 

 tion. It is also found in very wet or very dry habitats 

 where there is accumulation of poorly decomposed 

 or sharply delimited humus layers on top of the min- 

 eral soil, sand, or rock underlying. Mull humus com- 

 monly develops in warm, humid climates and is 

 found es|)ecially under hardwood or deciduous for- 

 ests. Patterns of animal and plant distribution cor- 

 relate closely with these two humus types ; indeed, 

 the formation of each is a result, in the main, of 

 uniciue combinations of biota reactions, climate, and 

 mineral soil characteristics (Romell 1935, Fenton 

 1947). 



Distinction between mull and mor humus is 

 made primarily for forest soils. Both humus types of 

 soil may be subdivided but these subdivisions need 

 not concern us here (Romell and Heiberg 1931, Lutz 

 and Chandler 1946). 



Mull is a porous, friable humus layer of crumbly 

 or granular structure and only slightly matted, if at 

 all. The Ai horizon is well developed ; bacteria are 

 abundant, annelids numerous, and nitrification oc- 

 curs. 



Mor is a strongly matted or compacted humus 

 layer. There is no Ai horizon ; the transition from 

 humus layer to mineral soil is abrupt. The underlying 

 soil is more acid, bacteria are much less numer- 

 ous, and nitrification is usually reduced if not ab- 

 sent. Abundant fungi reduce the raw humus to punky 

 material : thereafter it is worked on by the small 

 arthropods. Decomposition of mor is much less rapid 

 than of mull. Annelids are few or absent, and moles 

 are less common than in mull soils. Snails are scarce 

 in acid soils. In general, the biomass of organisms 

 inhabiting a mor soil is smaller, species are less di- 

 verse, and individuals are smaller than in mull soils. 



Mor humus is common in cold regions and at 

 high elevations, but is not limited to such climatic 

 zones. It occurs especially under coniferous forests, 



Depth distribution of orjianisms 



The small animals in the soil, including proto- 

 zoans and nematodes, are most abundant in the L, 

 F, and H horizons, becoming rapidly less abundant 

 in the mineral soil (Table 11-1). Bacteria, actino- 

 mycetes, and fungi are also most abundant in these 

 top layers, especially in F and H, although they occur 

 well down into the B and C horizons. 



The depth distribution of soil animals depends on 

 temperature and varies with season. When the top 

 layers freeze during the winter months, much of the 

 fauna keeps well below the frost line, although many 

 species are tolerant of freezing. During the cold 

 months the depth at which most soil insects, mol- 

 lusks, and annelids occur varies from about 9 cm in 

 silty clay-loam to 38 cm in gravelly clay-soil (Dowdy 

 1944). With the return of warm weather, the fauna 

 ascends to the top horizons. 



SOIL TYPES 



The interrelations between the basic min- 

 eral content of the parent substrate, biotic reactions. 



TABLE III Depth of disfribution of soil arthropods 

 Adirondack Mountains. The figures Indicate approxima 



ber of individuals per 



the total thickness 



Soil horizon 



Depth, or 



I- (computed from data given by Eaton and Cha 



Mites Springtails All others 



1942). 



Reactions, soil formation, and cycles 171 



