Soils of Redgrave and Lopham Fens

SOILS OF REDGRAVE AND LOPHAM FENS J. R. PRICE* Introduction A n investigation into the soil types and their distribution over R e d g r a v e and Lopham Fens was made by the Soil Survey of E n g l a n d and Wales, for the Suffolk Trust for Nature Conservation during the first three months of 1978. The area of 1.3 km 2 is seven kilometres west of Diss and approximately three kilometres south of South L o p h a m ; it is the source of both the Little Ouse and Waveney rivers. The terrain is Hat and lies beneath the 30m contour.

Previous surveys into the sub-surface composition of the Fens include a hydrological investigation carried out by the Anglian W a t e r Authority, Norfolk and Suffolk Division (1977) and a study of Lopham Little Fen by Tallantire (1953).

T h e surveys revealed the presence of shallow peat overlying calcareous marls and fine sand, beneath which are firm lake muds.

T h e Ouse-Waveney valley has been described as a glacial overfiow Channel by some geologists, while others suggest its origin as a pre-glacial or interglacial valley subsequently infilled by boulder clay (Bellamy, D. J. and Rose, F.; W. H. Burrell in Flora of Norfolk, 1914). South Lopham Fen has been described by Tallantire (1953) as the site of a late glacial lake infilled by lacustrine muds and later by fen peat.

Method A D u t c h ' E d e l m a n ' auger was used to examine the peat soils to a depth of up to 1.20m. Borings were made at approximately 600 locations at an average interval of 50m. Supplementary augerings to a depth of 2.0m were made using a Hiller auger at 24 locations along three pre-selected transects.

Sixty-four samples were taken from 50 sites for further laboratory analysis.

T h e object of a soil survey is to identify, describe and classify different soils and to record their distribution on a map.

T h e basic unit of both description and Classification is the soil profile, a vertical section through the soil in which layers, roughly parallel to the ground surface, called soil horizons, are recognised. These horizons are distinguished by their colour, particle-size class distribution, structure and organic matter * Soil Survey of England and Wales, Norwich.

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content. Colours are defined by reference to Standards in Munsell Soil Colour Charts. Further details on aspects of soil identification can be found in the Soil Survey Field Handbook (Hodgson 1976).

Soil profiles with similar horizon sequences are grouped in soil profile classes defined at four levels; major groups, groups, subgroups, soil series. The soils are grouped according to the Classification of the Soil Survey of England and Wales (Avery 1973). To describe and compare profiles, soil horizons are given a letter notation (Hodgson 1976), the same symbol being applied to analogous horizons in profiles of the same soil group or subgroup.

In this survey, laboratory analyses were made of soil samples to determine pH. (Detailed results of findings and all other analyses can be obtained from the author.)

Lithological Horizons Three lithological horizons are found in the fens to a depth of 2.0m. In general, peat with occasional calcareous lenses, overlies sand, but a calcareous marl separates these two deposits in the deeper parts of the Fens.

The peat can be separated into two distinct types. In shallower parts of the Fens, where sand occurs at a depth of less than 35cm, the overlying peat is a black amorphous humified deposit without recognisable remains. In deeper parts of the Fens, there is 40cm or more of amorphous peat underlain by a semi-fibrous reddish-brown peat which forms a horizon 10-40cm thick above the sand. Occasionally the two peats are found in the shallower parts of the Fens, the semi-fibrous peat being no more than 10-15cm thick. pFI values of peat samples show little consistent Variation, but the shallow peat is generally more acid. As the peat thickens towards the centre of the basins, the pH values rise.

Some samples were tested for salinity by measuring the electrical conductivity of the saturated soil paste, and the organic carbon content was measured.

Very thin calcareous lenses are also found in the amorphous peat at less than 40cm below the surface. These superficial calcareous layers are usually close to the Little Ouse and Waveney rivers and adjacent to the sand ridge that separates Little Fen from Middle Fen. No well preserved organisms are found in these lenses, but shell fragments are discernible. The lenses result from calcareous matter being washed into the Trans. S u f f . Nat. Vol. 18 part 1.

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Fens, possibly duringtimes of flood, by the rivers Little Ouse a n d Waveney. Where these lenses are present, the peat is very dusky red, not black.

T h e underlying sand is fine grained and well sorted except at the northern end of the Middle Great Drain, beneath the h e a t h and meadow, where it contains small stones, and constitutes a sand and gravel deposit.

T h e calcareous marl is light olive grey to olive grey and its distribution is shown in figure 1. It occurs at depths of between 90cm and 1.80cm, forming a thin layer with a maximum thickness of only 40cm between the peat and sand. In the marl are m a n y well preserved organisms and the presence of these indicates a quiet environment of deposition, fed by slow moving water. T h e fauna are as follows:-* Bithynia tentaculata (Linne); Planorbis planorbis (Linne); Lymnaea (Radix); peregra (Muller); Valvata cristate (Muller) and the stems and seed cases of Chara, a lime secreting plant.

T h e distribution pattern of these fen deposits can be seen in * I am indebted to Mr. P. G. Cambridge, Treasurer of the Geological Society of Norfolk, for faunal identification.

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FENS

Sand Outcrops Thick

Peat

Figure 2, where the depth of peat to sand is shown. The greatest depth of peat was 210cm in Middle Fen. The sand crops out in the north-east corner of the Fens and again in the south-west corner, in addition to marginal outcrops on the northern edge of Little and Middle Fens.

Soils Some soil development can be recognised in these fen deposits. Where the sand crops out, there is a humose topsoil beneath which the sand is bleached and leached of iron with a lower horizon showing diffusion and accumulation of iron in the form of grey and rusty mottling. In the peat area it is amorphous to a depth of 40cm, presumably through humification following lowering of the watertable. Where the peat is thicker, the lower layers have not been appreciably oxidised so that they are semi-fibrous and the profile has upper and lower layers.

W h e r e there is less than 40cm of amorphous peat or humose soil over sand profiles may be classed as Isleham Series, a typical humic sandy gley soil in glacio-fluvial sand.

A typical profile in this fen consists o f : Trans. S u f f . Nat. Vol. 18 part 1.

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Suffolk Natural History, Vol. 18, Part 1 Vertical

Section

Showing

Lithological

Horizons

rsddlsh btown aami -ftbrc

Adventurers'

Shallow Phase With S h e l l

Marl

FIG . 3

cm 0-35 Oh

Very dark grey (10YR 3/1) amorphous peat, no recognisable remains; common fine live roots; stoneless, structureless; diffuse boundary.

Reddish grey (10YR 5/1) stoneless loose sand with 35-50 few fine prominent yellowish brown (10YR 5/6) Eg mottles associated with few fine partially decomposed woody roots; diffuse boundary.

50_90+ Light grey (10YR 7/2) stoneless loose sand, few fine Cg prominent streng brown (7.5YR 5/8) mottles adjacent to decomposing woody roots.

The occurrence of Isleham series corresponds to those areas shown in Figure 2 that form the western, north-eastern and northern margins of the Fens and the central sand ridge separating Little and Middle Fens.

Where the peat is more than 40cm thick (Figure 2) the profiles would be regarded as Adventurers' Series (Figure 3) an earthy eutro-amorphous peat soil. The shallow phase consists of:— Trans. S u f f . Nat. Vol. 18 part 1.

SOILS OF REDGRAVE AND LOPHAM FENS

109 cm 0-40 Very dark brown (10YR 2/3) peat; stoneless; strucAop tureless, occasional narrow lenses of calcareous peat speckled with white shell fragments;finelive roots common in upper 20cm; some woody fragments; sharp boundary.

40-60 Reddish brown (5YR 3/3) semi-fibrous matted peat; II Bg common fine roots; some woody fragments; gypsum present; distinct brownish yellow (10YR 6/8) mottles below 50cm, generally around old plant remains; sharp narrow boundary.

60-90 Light grey (10YR 7/2) stoneless loose sand; few fine III BCg prominent strong brown (7.5YR 5/3) mottles.

A typical Adventurers' deep phase consists of:— 0-70 Very dusky red (2.5YR 2/2) amorphous peat; stoneAop less; structureless; commonfinelive roots in upper 30cm; woody remains and few fine roots below 30cm; sharp undulating boundary.

70-90 Reddish brown (5YR 3/3) semi-fibrous calcareous II Cg peat;fibrousroots common but mostly dead; below 80cm the matted peat on exposure changes colour to dark olive (5Y 2/3) after about 3 minutes; sharp narrow boundary.

90-120 Light olive grey (5Y 6/2) to olive grey (5Y 5/2) calIHCg careous silty clay with many small well preserved calcareous shells and fragments.

Occasional profiles in the deeper peat show pH values below 3.5. These are placed as Mendham Series and regarded as earthy sulphuric peat soils because of these very low pH. values. While no consistent pattern of distribution could be found, most occurred in the western part of the Fens. The profiles are similar to those of the Adventurers' series and can only be distinguished by analysis. The profile consists of: — cm 0-45 Very dark brown (10YR 2/3) structureless peat; fine Aop fibrous roots common in upper 25cm, less common below; occasional wood fragments, but otherwise no recognisable peat remains; narrow slightly undulating boundary.

45-80 Reddish brown (5Y 3/3) semi-fibrous matted peat; Trans. Suff. Nat. Vol. 18 part 1.

Suffolk Natural History, Vol. 18, Parti Om commonfibrousroots, but mostly dead; sharp even boundary.

80-120 Light olive grey (5Y 6/2) to olive grey (5Y 5/2) silty IIBCg clay with well preserved calcareous shells and calcareous fragments.

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Discussion The survey has revealed the presence of two shallow peat basins separated by a north-east to south-west trending sand ridge. The section (Fig. 4) is taken along transect line A - B marked on Figure 1 and shows the main sub-surface structural features.

SCHEMATIC

CROSS

S E C T I O N OF L O P H A M

FENS

FIG.4

Sand ridges form the basin margins and separate Little and Middle Fens. The calcareous marls found in deeper parts of the Fens indicate lacustrine conditions before the onset of peat formation. The black amorphous peat and semi-fibrous peat have not built up to any great depth. The maximum recorde'd depth in Middle Fen is 210cm indicating a relatively shallow peat fen.

Sulphuric and potentially sulphuric soils are found in the western part of the Fens. The evidence for this is the very low pH values of less than 3.5 measured just after sampling and

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again after three months when a considerable drop in value was noticed in some samples. Similar very acid peat soils occur downstream in the Waveney Valley near Harleston. Analyses of these (Corbett in the press) identified pyritic sulphur in the soil and pyrites in the substrata confirming them as acid sulphate soils. Such soils cannot be consistently identified in the field from profile features and before drainage they may not be acid. However, upon drainage the pyrites oxidises giving sulphuric acid and the pH drops unless there is calcium carbonate present to neutralise the acid. Below about pH 4.0 the bacteria Thiobacillus ferroxidans catalyses the reaction. Thus drainage itself precipitates deterioration of the soil through excessive acidity and formation of iron ochre. Such acid layers can lie close by calcareous layers.

References Anglian Water Authority, Norfolk and Suffolk Division (1977) Investigation of the Hydrology of Redgrave and Lopham Fens—Divisional Engineer's Report.

Avery, B. W. (1973) Soil Classification in the Soil Survey of England and Wales. J. Soil Sei. 24, 324-38.

Bellamy, D. J. and Rose, F. (Date unknown) The WaveneyOuse Valley Fens of the Suffolk-Norfolk Border. Suffolk Naturalists' Society Booklet.

Corbett, W. M. (in the press) Soils in Norfolk IV Sheet TM 28 (Harleston). SoilSurv. Record, Harpenden.

Hodgson, J. M. (Ed.) (1976) Soil Survey Field Handbook.

Soil Surv. Tech. Monogr. No. 5.

Tallantire, P. A. (1953) Studies in the Post-Glacial History of British Vegetation, VII. Lopham Little Fen, a Late-Glacial Site in Central East Anglia. J. Ecol. 41,361-373.

J. R. Price, University of East Anglia University Piain, Norwich, 88C