Soils in their environment - Fundamentals of the Physical Environment - page 434

as a kind of 'nutrient pump'. Shallow-rooting species
(e.g. coniferous trees and many heath plants) are at a
disadvantage and, other things being equal, will not be
able to counteract leaching losses as effectively. In farming
practices, liming and fertilizer application are used in
order to balance both the leaching losses and the heavy
withdrawal of nutrients by crop yields.
The main soil profile formed by leaching is the brown
earth soil (FAO: Cambisol or Phaeozem). The sequence
of horizons is Ah-Bw-C-R, with Ah replaced by Ap where
ploughed ( Figure 18.6 , Plate 18.4 ). The colour, texture and
pH of these soils vary with the type of parent material.
Sandy textures can lead to a 'slow weathering-intense
leaching' regime which produces lower pH values in
the range 4·0 to 5·5. In the past these brown earths were
called 'low base status brown earths', in order to distin-
guish them from 'high base status brown earths' on less
acid parent materials (e.g. basic igneous rocks, calcareous
deposits).

Plate 18.4 Brown Earth (FAO: Dystric Cambisol) under
sessile oak woodland on glacial till, Loch Lomond, Scotland.
Surface humus (H) above thin brown leached layer (Eb) above
weathered reddish-brown horizon (Bw).
Photo: Ken Atkinson

pH c m pH

Mull humus
or
ploughed layer

Ap

7.1

Ap 6.1

Leaching
of
bases

Bw 6.8

30

Loss of
cations and
anions

Bw 6.0

C

6.9

Leaching
plus some
precipitation
of calcium

60

Lime
CaCO 3

6.0

C

R

90

Brown Earth
(FAO : chromic cambisol)
'high base status'

Brown Earth
(FAO : humic cambisol)
'high base status'

Figure 18.6 Profiles of Cambisols (Brown Earths).