154 THE STUDY OF PLANT COMMUNITIES * Chapter VII 



When decomposition of litter does not exceed accumulation, the 

 Ao horizon has a surface stratum of undecomposed twigs and 

 leaves, which is termed the L layer. Beneath this is a stratum of 

 decomposing but still identifiable plant remains, which is marked 

 by fungal hyphae in abundance and is called the F or fermenta- 

 tion layer. In contact with the mineral soil there may be an H or 

 humus layer if the climate is sufficiently cool and moist. The term, 

 humus, is applied to material decomposed beyond obvious recog- 

 nition. Soil animals and percolating water carry the humus into the 

 soil where, through further decomposition, its chemical constitu- 

 ents are slowly released for use by succeeding generations of or- 

 ganisms. 



When a distinct layer of humus (H layer) is present with a 

 rather abrupt transition to mineral soil, the humus type may be 

 designated as mor. If there is no distinct layer of humus but rather 

 it is mixed with the surface mineral soil, the humus type is mull. 120 



Local variations in amount, nature, and rate of decomposition 

 of humus are to be expected. Evergreen leaves do not decompose 

 as readily as deciduous ones, nor do they have the same chemical 

 composition. 264 Even the leaves of deciduous species do not all 

 yield the same decomposition products. Organisms causing decom- 

 position may be active and abundant in one habitat but quite 

 incapable of living in another because of such factors as tempera- 

 ture, moisture, and aeration. Consequently, humus may be un- 

 equally effective in different habitats, and soils of similar origin 

 may have quite different productive qualities. 



Regional Soil Variations.— Climate, which varies with latitude 

 and longitude, includes the important factors in soil formation, 

 especially temperature and rainfall. Within a climatic area, differ- 

 ences in parent material and topographic position often are re- 

 flected in soil variations, which may be chemical or physical. Such 

 variations are most pronounced where parent rock is newly ex- 

 posed or where soil materials have weathered but slightly, as below 

 a receding glacier. After longer exposure the developing soils be- 

 come much more alike, and the longer the time involved, the less 

 noticeable will be differences related to local conditions. Evidence 

 is sufficient to indicate that, within a climatic area, soil develop- 

 ment progresses toward a particular kind of soil and profile regard- 



