The soil and the organisms living on and in it comprise an ecosystem. The active components of the soil ecosystem are the vegetation, fauna, including microorganisms, and man.

– Vegetation: The primary succession of plants that colonize a wethering rock culminates in the development of a climax community, the species composition of which depends on the climate and parent material, but which, in turn, has a profound influence on the soil that is formed. For example in the Mid-West of the U.S. deciduous forest seems to accelerate soil formation compared to grassland on the same parent material under similar climatic conditions. Differences in the chemical composition of leaf leachates can partly account for a divergent pattern of soil formation. For example acid litter of pines or heather favors the development of acid soils with poor soil structure, whereas litter of decidious trees favors the development of well structured soils.

– Meso-/Macrofauna: Earthworms are the most important of the soil forming fauna in temperate regions, being supported to a variable extent by the small arthropods and the larger burrowing animals (rabbits, moles). Earthworms are also important in tropical soils, but in general the activities of termites, ants, and beetles are of greater significance, particularly in the subhumid to semiarid savanna of Africa and Asia. Earthworms build up a stone-free layer at the soil surface, as well as intimately mixing the litter with fine mineral particles they have ingested. The surface area of the organic matter that is accessible to microbial attack is then much greater. Types of the mesofauna comprise arthropods (e.g. mites, collembola) and annelids (e.g earthworms, enchytraeids).

 

Table 3.2.1. Earthworm biomass in soils under different land use (White, 1987)

Earthworm biomass	[kg/ha]
Hardwood and mixed woodland:	370 – 680
Coniferous forest:	50 – 170
Pasture:	500 – 1500
Arable land:	16 – 760

– Microorganisms: The organic matter of the soil is colonized by a variety of soil organisms, most importantly the microorganisms, which derive energy for growth from the oxidative decomposition of complex organic molecules. During decomposition, essential elements are converted form organic combination to simple inorganic forms (mineralization). Most of the microorganisms are concentrated in the top 15 – 25 cm of the soil because C substrates are more plentiful there. Estimates of microbial biomass C range from 500 to 2,000 kg /ha to 15-cm depth (White, 1987). Types of microorganisms comprise bacteria, actinomycetes, fungi, algae, protozoa, and soil enzymes.

– Man: Man influences soil formation through his impact to the natural vegetation, i.e. his agricultural practices, urban and industrial development. Heavy machinery compacts soils and decreases the rate of  water infiltration into the soil, thereby increasing surface runoff and erosion. Land use and site specific management (e.g. application of fertilizer, lime) also act on soil development.

 

– from Soil Morphology, Classification and Mapping, Dept of Soil Science, U. of Wisconsin-Madison