Lowland Soil Catenas in Oxford
All soils originate from rock – the parent material. The type of soil that is produced depends upon a number of
factors, including changes in the parent rock, the soil’s position on a slope and drainage conditions in that location.
This case study will examine the soil types and processes around the Oxford area, and how variations in soil
sequences (catenas) produce an array of different soils.
Overview of soil types in Oxford
Figure 1 shows the range of soil types in the Oxford area and the influence of particular local conditions and
processes:
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Brasenose Wood, Shotover – the mix of sandstone, limestone and clay has produced a range of soil types,
such as brown earth (on the sandstone), gley (waterlogged soils) on the clay, and rendzina on the
limestone.
Port Meadow – on this low-lying floodplain that floods annually, groundwater gley predominates due to the
high water table.
Summertown and Wolvercote – these outwash terraces initially comprised fluvio-glacial sand and gravel,
and there is evidence of podzolisation producing podzolised brown earth in places.
Boars Hill and Wytham – the Corallian limestone parent rock has produced a mix of calcareous brown
earths and rendzinas.
Within the urban, built-up area – human impacts can be seen in the way that soils are typically lime-rich
(from mortar, cement and rubble), cultivated (ploughed in a general sense) and deficient in nutrients, as
minerals are removed through the mowing of vegetation.
Agricultural area – this region shows the effects of intensive cultivation such as ploughing, draining,
compaction and contamination from fertilisers and pesticides.
Figure 1. Soil types in the Oxford area.
Soil-forming processes
All soils originate from the parent rock. The passage of water through the soil profile and the consequent movement
of materials significantly influence the type of soil that is formed. The parent rock determines the depth, texture and
drainage of the soil. Hence, on a local scale, the processes effecting the formation of various soils are often the result
of topography and climate.
Figure 2. An example of brown earth.
Climate
The climate of the Oxford region is temperate. Average temperatures in winter are about 6–7°C and in the summer
are 17–18°C. Rainfall is approximately 650 mm and evapotranspiration about 550 mm, so there is a net downward
movement of water through the soil. This allows lessivage (the downwashing of materials in suspension), as well as
leaching or solution (materials carried in soluble form). Soils that are determined by climate are called zonal soils.
Solution or leaching is more common on the acidic sands and gravels of the terraces. Under conditions of extreme
acidity (below pH 4.5) podzolisation may occur.
Drainage
Like most lowland areas in Britain, the Oxford region shows widespread gleying due to poor drainage and gentle
relief features. There are two types of gleyed soil:
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Groundwater gley – where waterlogging occurs as a result of a high water table, such as in the areas close
to the rivers.
Surface water gley – where waterlogging occurs as a result of impermeable rock, such as those on the
superficial clay deposits of Wytham and Shotover hills.
In addition, annual deposition by the Thames and Cherwell rivers leaves fine, grained material on the surface.
Therefore, the soils in that area are still being formed and are described as ‘azonal’ or immature soils.
Figure 3. An example of clay-gleyed soil
There are three soil types that have formed organically due to the particular micro-topography of their sites:
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Eynsham area – the soil here was once covered by a layer of peat, which formed in the marshy conditions
before the land was drained for agriculture. Now used for arable farming because of the lower water table,
it has become a rich, organic ‘puff’ soil that is difficult to work once dry (a puffy, spongy surface does not
readily wet again).
Oxey and Pixey Meads between the Thames and the Oxford northern bypass road between Wolvercote
and Cassington – these are floodplain soils with a high water table, about 1 m lower than the puff soils of
Eynsham. The meads are common land belonging to the villages of Begbroke and Yarnton that appear
never to have been ploughed, only cut for hay. Organic matter has accumulated on their surface of enough
depth to form a ‘floating meadow’.
Otmoor – this is a basin site in the floodplain of the River Ray that has carried down clay, mainly from
Jurassic formations. The soil possesses a high water-holding capacity and is rich both in clay and humic
colloids, exhibiting a range of wet soil profiles. Previously it was marshy grazing land, but since the Second
World War it has been more effectively drained, partly by pumping to meet the water requirements of the
Arncott Camp.
Figure 4. An example of surface-gleyed soil.
Weathering
The rate of weathering in Oxford is likely to be slow on account of its temperate, mild conditions. On the Corallian
limestone of Wytham and Boars Hill, carbonation produces calcium bicarbonate, a soluble material that is carried
away in solution. As a result, the soils typically found on limestone are thin and organic rich, consisting of impurities
in the limestone and the remains of organic matter. When the rock type determines the soil type, the soil is said to be
intrazonal.
Human impacts
Human activities have a major impact on most of the soils in the region. Owing to widespread deforestation,
clearance of land for cultivation, draining, ploughing and the addition of fertilisers, few soils will have remained
unaltered. Coupled with the use of heavy machinery, this has led to direct changes in soil structure such as:
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Soil compaction and other damage to the physical properties of the soil.
Loss of organic material and soil biodiversity due to overuse of fertilisers and pesticides. Vegetation
removal limits the return of nutrients to the soil, and there are important differences between farmed and
un-farmed soils (Figure 5).
Figure 5. The effects of intensive agriculture on soil profiles.
Soil characteristic
Natural state
Intensive farming
Organic content
A horizon high (7%)
Uniform (3-5%) in
ploughed horizon
B horizon (0%)
Carbonates
A horizon low/zero
Uniform if limed and tilled
B/C horizon maximum
Nitrogen
Medium/low
High (nitrate fertiliser)
Biological activity
High
Medium
Ca 80%
Ca 70%
K 5%
K 10%
P 3%
P 12%
H 7%
H 4%
Exchangeable
cation balance
Catenary sequences
There is a close relationship between soils and bedrock in Oxford and Oxfordshire. The region falls naturally into
several well-defined topographic areas, such as north Oxfordshire, the Cotswolds, the Chilterns, and the great vale
between these two (Figure 6). The main geological outcrops run from northeast to southwest, dipping southeastward
and giving scarps on their northwest faces. This gives rise to typical sequences of parent material and soil from the
top of the crest to the bottom of the slope. Such sequences of soils are termed catenary sequences or soil catenas.
Where soil types vary with relief, the resultant distribution is called a catena. Figure 7 shows a generalised soil
catena.
Figure 6. Map of Oxfordshire.
Figure 7. A soil catena.
A catena is composed of a number of soil series and soil types. Three kinds of catenas are found in the Oxford
region:
The geological catena
This is where the parent material changes regularly down the slope and where denudation has created colluvial soils
of mixed origin. A good example of the geological catena is in the broken topography of the steep hills and deep
valleys to the north and northwest of Deddington (Figure 8). At Wigginton Heath, the podzolized Northampton Sand
on the top of the hills lies over the top of the Marlstone Red Loams (which is of high base status as it is derived from
the Marlstone forming the slopes). These, in turn, lie over the top of Gleyed Meadow Clays, derived from the Lower
Lias (clays) that form the valley bottoms. Each bed is contaminated by overland runoff and downwash from the beds
above, giving mixed colluvial soils.
The true catena
A true catena is where the parent material is the same all along the slope, but the soils range from well drained at the
top of the slope to poorly drained, or with a high water table, at the base of the slope. The true catena or drainage
catena is found on the gentle dip slope of the Great Oolite limestone of the Cotswolds, along a transect running
roughly between North Leach and Witney (Figure 8). This shows that when a soil is derived from one kind of rock
only, the variations found in the soil profile are controlled very closely by aspect, slope and drainage. At the same
time, the finest particles of weathered soil material tend to drift down to the lower sites, making these soils less
permeable and not so well aerated as those higher up.
The mixed catena
This is found where the catenas vary simultaneously down the slope. This third kind of catena can be seen in a
transect from the clay-with-flints over the chalk on the Chiltern Hills or Berkshire Downs, down the scarp to the
floodplain of the Thames Valley gravel between Dorchester and Abingdon (Figure 8).
Thus, a highly dissected landscape produces a geological catena, a gentle slope produces a true soil catena, and a
long slow fall over various beds produces a mixed soil catena. Soils in the region not derived from solid geological
rocks fall into three groups:
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Colluvial soils of mixed origin – these are largely dependent upon the soils higher up the slopes from which
they are derived. Mass movements, such as soil creep, have taken place and formed colluvial deposits that
are of mixed origin but, in general, are related to nearby geological outcrops.
Gravels occurring on plateaux, river terraces or floodplains – these may be classified by the nature of the
material forming the gravel, the depth of the water table and the altitude. The depth of the water table,
which has great influence on the agricultural value of the soil, depends upon the topographical position.
Alluvial soils – these soils are found in river and basin sites with high water tables and much organic
matter.
Figure 8. The three types of catena in Oxford.
Role of drainage
In any of the low-lying soils of Oxfordshire, water is one of the dominant soil-forming factors. Topographical drainage
dominates the catenas so that, while the higher slopes tend to be well drained and sometimes even eroded, the
lower slopes and valleys have impeded drainage. Soils with impeded drainage include brown earth and meadow soil,
in which a gleyed horizon may be found. The permeability of the soil is important, and this depends on its texture. If a
catena formed on clay received equal precipitation overall, a greater proportion of the rain would run off the slopes
and a smaller quantity would percolate into the soil than if the same condition of topography and rainfall existed on a
more permeable material such as sand. The clay catena would also receive a lot of fine material on to its lower
slopes by downwash, so that the lower soils would be liable to be badly drained and waterlogged and to have
strongly gleyed horizons in consequence.
Figure 9. An example of rendzina soil.
A comparison between the clay catenary sequence derived from Kimmeridge Clay at Hinksey Hill and the limestone
catena of the Cotswolds illustrates this effect. On the clay at the top of the crest slopes of Hinksey Hill, where
overland runoff greatly exceeds infiltration, the soil profile shows no characteristics of poor drainage. The lower sites,
however, give typical groundwater gleys (meadow soils) with a well-defined gleyed horizon over a permanently wet
subsoil very close to the surface. On the Cotswold catena, on the other hand, the soil texture is much lighter and the
limestone rock beneath the soil is very permeable, so that surface runoff and erosion are somewhat limited. The
crests are eroded to form shallow, stony soils of less than 20 cm in depth that form rendzinas. On the lower slopes,
the soils are much deeper, the texture is heavier and percolation is more restricted, so either 'humus-accumulating'
black calcareous rendzina soils develop, if there is enough calcium carbonate, or brown earths if there is not. Gleyed
horizons do not appear in either case, unless some further factor, such as high water table, is present.
A wide variety of soil textures can be found, ranging from an almost pure sand containing only 1 per cent of clay,
which occurs in the A horizon of the podzols on Tadmarton Heath, to heavy 60 per cent clays on the alluvial site of
Otmoor. The depth of soil naturally depends on topography. A deep soil can be developed from parent material but
only if it is on a level site.
Figure 10. An example of podzol soil.
Three classes of soil depth are recognized on the upper part of slopes: eroded, in which the soil is less than 20 cm
deep; shallow, when it is between 20 cm and 40 cm deep; and deep, when it is over 40 cm. On the lower part of the
slope the soil is usually a colluvium, exceeding 38 cm in depth. This is composed of soil material that has been
washed down from higher up the slope.
Conclusion
Soils types can vary enormously in a small area. On a local scale, where climate is relatively similar, variations in
rock type largely determine the soil type. On the same rock type, at a very small-scale, variations in drainage often
cause differences in the soils. Increasingly, human activities are also having an impact on soils and soil-forming
processes, affecting vegetation and drainage.