THE VARIETY OF SOILS IN BRITAIN - The Soil - page 33

years of stability, these horizons were 2 centimetres in depth; after 30 years 5 centi-
metres; and after 70 years 9 centimetres.
The oldest inland dune had been planted some 90 years before with Austrian
pine Pinus nigra. This had become closed canopy woodland, and self-sown pines
from here have colonized the adjacent dune ridges. The effect of acidifying needle
litter on the planted dune has redirected soil development. The grassland mull humus
horizon on these oldest ridges has been replaced by a mor horizon some 7 centimetres
deep. Leachates from the pine needle litter have accelerated the rate of destruction of
the original shell particles in the sand, removing the dissolved calcium in drainage
water. Together with leaching of the limited initial iron content, the result is a thin, but
well defined, bleached horizon below the organic layer. As noted in chapter 1 , sandy
soils lack the capacity to retain elements like calcium, and can therefore become acid-
ic quite quickly. After 30 years under dune grassland, the pH of the sand in the top 15
centimetres was around 6.8, whereas under the oldest pines it had decreased to 5.0.
Trends in soil development and associated faunal changes over quite short peri-
ods have also been observed in colliery spoil and other mineral workings. These are
considered in chapter 10 .

S OIL CLASSIFICATION

We said earlier in this chapter that horizon sequences in profiles provide a 'bar-code'
that can be used to distinguish soil classes. In this code, definitive symbols are al-
located to particular types of horizon. These types are identified by position within
the profile and by observable properties, supplemented in more detailed systems by
laboratory analysis. Soil classes at the most broadly defined level - the 'major soil
group' - have specific horizon sequences. Widely used conventions for horizons in-
clude variations on the following symbols, which we apply later in an outline of some
widespread British soil groups. In comprehensive schemes, a much longer list of such
symbols is used.

O horizons dominated by organic matter; subdivisions include -

L recently deposited plant litter,

partly decomposed organic matter that retains some original plant
structures,

F

decomposed organic matter, no longer retaining original plant struc-
tures. In other usages, L, F and H symbols are used for organic hori-

H