THE VARIETY OF SOILS IN BRITAIN - The Soil - page 38

F IG. 13
Brown Earth soil on volcanic ash, Llydaw, Snowdonia. (Photograph D.F.B.)
Rendzinas are also shallow soils, but formed over calcareous parent materials
such as chalk or limestone. By tradition, lime-rich soils have been conventionally
separated as a distinct group because of the conspicuous ecological effects of their
soil chemistry, even though their profiles are of similar character to the acidic Brown
Rankers or Brown Earths. Chapter 7 describes the influence of time on soil properties
in Rendzinas on chalk grasslands where cultivation had ceased some 30 years ( Plate
2 ) and 130 years previously. Over this period, organic matter content increased from
8 to 22 per cent in the top 10 centimetres of the soil, giving a darker-coloured A h ho-
rizon in the older soil.
Brown Earths ( Fig. 13 ) are freely drained soils characterized by a deep, well
mixed mull humus surface horizon. Weathering in these soils involves a relatively
slow, low-intensity, chemical alteration of the primary mineral constituents. The iron
and aluminium hydrous oxides that are released through this process remain in silu ,
to accumulate as stable coatings on mineral grains and in soil structural units. With
moderately acid pH values in uncultivated conditions, the horizons of Brown Earths
merge as they change character gradually down the profile. The soil illustrated in Fig-
ure 13 was formed over volcanic ash in Snowdonia. Soils of this group, on a wide
range of parent materials, are important in the lower rainfall regions of Britain. Their