ARCHITECTURE OF THE SOIL WORLD - The Soil - page 9

ies of these earthworms create an intimate mixture of organic matter and mineral soil
known as mull humus. This is the typical form of humus found in deciduous wood-
lands, lowland grasslands and in derived arable soils.
These terms mull and mor were coined in 1878 by the Danish forester P.E.Müller
who first recognized their significance as indicators of soil condition and forestry po-
tential. W. L. Kubiena's classic work in 1949 on The Soils of Europe recognized 16
main types of humus of which mull and mor represent two extremes. An intermedi-
ate condition, which Müller called 'insect mull' but which is now generally known
as moder, is characterized by a well developed H layer with thin L and F layers. This
condition is attributed to the feeding activity of large arthropods such as millipedes,
woodlice and fly larvae in soils with low earthworm populations; sections through
samples of the litter impregnated with agar or resin often show a granular structure
produced from the faecal pellets of these creatures. Cause and effect, however, are
difficult to disentangle in soil ecosystems. The presence of a particular soil faunal as-
semblage controlling the breakdown and incorporation of organic matter is dependent
on the nature of the overlying vegetation and on the chemical and physical proper-
ties of the soil itself. An interesting experiment illustrating the interaction between
earthworms and different mixtures of trees is described in chapter 5 . Such a positive
feed-back system is disrupted when there is a major perturbation such as woodland
clearance or liming.
The greatest differences between soils, and hence between associated soil faunal
communities, are found over the range of natural and semi-natural plant communities.
Cultivation obliterates most of the inherent natural differences in humus type and soil
chemistry. Under a regime of arable cropping, all or most of the aerial vegetation may
be harvested, so there is little or no 'litter' to be returned to the soil. There is, never-
theless, a substantial root biomass which is returned annually; more than would die
naturally under a perennial vegetation cover. Indeed, the roots of a barley crop, to-
gether with the stubble and chaff, exceed the organic matter in the straw. This explains
why clear differences in soil organic matter have seldom been found when comparing
fields where the straw has been burnt, with fields where the straw has been dug in (see
chapter 8 ) .
Whilst humus plays a relatively minor role in crop nutrition, it plays a very im-
portant role in modifying the texture and structure of arable soils. A high organic mat-
ter content in soils makes for easier cultivation and better seed beds. This is because
of its effects in promoting the stability of soil crumbs. A rich humus encourages earth-
worms which in turn promote a spongy, porous structure through the intimate mixing
of mineral particles and organic matter in their casts.
T HE MINERAL SKELETON