ARCHITECTURE OF THE SOIL WORLD - The Soil - page 5

amorphous, finely divided organic matter. These three layers have been given a vari-
ety of names, but it is convenient to refer to them as the L (litter), F (fermentation)
and H (humus) layers. The fermentation layer is where most of the litter decompos-
ition takes place. This is the home of several kinds of millipedes, woodlice and fly
larvae, some small earthworms, many mites and shorter-springed springtails. These
are described in more detail in chapters 4 and 5 .
Grassland litter differs from woodland litter in that the dead grass does not fall
to the ground in the same way but remains arched over the surface for some months,
and only gradually sinks down and disintegrates. Highly siliceous grasses, such as
tor grass Brachypodium pinnatum , form a distinctive and persistent litter mat which
few invertebrates appear able to digest. Even palatable meadow grasses, however, can
give rise to a peaty mat on the surface of the ground if earthworms are absent; this
was seen in New Zealand, for example, when settlers first converted the native veget-
ation into pasture land with introduced grasses from Europe.
We can move one step nearer to appreciating the structural diversity of litter if
we take vertical sections and view them through a microscope. This has been done
in both woodland and grassland by cutting small cores or blocks, impregnating them
with gelatine, and slicing them up ( Fig. 3 and Plate 1 ). In a study of woodland soils,
J.M.Anderson recognized up to seven main classes of structures in a section through
the litter, fermentation and humus layers of a sweet chestnut stand. These included
not only various leafy and woody items and the cavities between them, but also plant
roots with or without their fungal associations, faecal pellets of various invertebrates,
and animal remains. He subdivided several of these classes to produce a total of 24
microhabitat categories which were thought to be significant for soil mites ( Table 1 ) .
By examining many sections in a standard way, it then became possible to relate the
number of kinds of mites identified in a section with an index of diversity computed
from the number of microhabitats present. The richest zone tended to be the fermenta-
tion layer which attracted about 21 species of oribatid mites, whereas the humus layer
below only supported some 10 species.