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Wednesday, 10 September 2014

Preseli soils

In case anybody is interested, here is the lowdown on Preseli soils.

0654a HAFREN

Detailed Description

This association consists mainly of soils with peaty surface horizons-stagnopodzols and stagnohumic gley soils on plateaux and steep valley sides. It covers some 1,300 km² of hill land mainly in Wales, with small areas in South West and Northern England. Hafren soils, loamy ferric stagnopodzols, are most widespread. Similar ironpan stagnopodzols belonging to the Hiraethog series are locally dominant. Wilcocks series, loamy stagnohumic gley soils in drift, and very stony (lithoskeletal) stagnopodzols are also common. Most of the soils are developed in rock debris from Palaeozoic mudstones, shales, siltstones and occasionally slates. Altitudes range from about 300 m O.D. near the west coast of Wales to more than 900 m O.D. on the Carneddau range in Snowdonia, although further inland at Radnor Forest in east Wales the soils are found only above about 500 m O.D. Ironpans occur frequently and are most common in north Wales and Radnor Forest where the Hiraethog series dominates, whereas in the larger areas of central and south Wales Hafren series (Lea 1975) is usually more common. Together these series cover about half the association. In the mountains of north and mid-Wales however, high ridges and plateaux have similar soils in which organic matter was mixed with or washed into very stony shattered rock debris by frost heave or leaching. These were described as a dark brown subsoil phase of Hiraethog series by Rudeforth (1970). Stone stripes and polygons are found in some of these places (Ball and Goodier 1970). Cliffs, rock fields and scree are also features of this more rugged country. Cliffs like those at Graig Goch have an overriding influence on the use and management of the land despite their limited extent. Around Machynlleth and Corris the soils are developed on slaty rocks metamorphosed by the Cader Idris igneous activity. Shallow humic rankers, Skiddaw and Revidge series , are frequent, especially in north and mid-Wales, where there are narrow rocky ridges. Basins, valleys and flushes have stagnohumic gley soils of the Wilcocks, Mynydd and Rhondda series, and also peat soils of the Crowdy and Floriston series). Winter Hill soils, fibrous peats, are included on some high plateaux. On valley sides at lower altitudes brown podzolic soils are frequent and include the Parc and Manod series. In the Preseli hills, igneous intrusions give rise to local occurrences of Bangor and Preseli series. The climate is cold and wet. Low summer temperature is the main influence on the development of stagnopodzols as distinct from brown podzolic soils on similar slopes and under similar rainfall. The accumulated temperatures range from about 800 to 1,500 day degrees centigrade and moisture deficits are generally less than 80 mm. Western districts have cooler summers and milder winters than in the east where the seasonal range of temperatures is greater.

On unreclaimed central parts of Exmoor, Hafren, and Hiraethog soils with ironpan, are accompanied by stagnohumic gley soils of the Wilcocks and shallower Mynydd series. On slopes and the drier fringes of Exmoor the peaty topsoils become thinner and some are humose rather than peaty. Here too there are fewer gley soils. On enclosed and cultivated land, particularly in Exmoor Forest, the once peaty surface horizons have become humose. In Cornwall also, on St Breock Downs and close to the St Austell granite, most topsoils are humose rather than peaty. Where repeated cultivation has ruptured the ironpan and has reduced organic matter in the topsoil, well drained Manod soils are found. Scattered Wilcocks and Mynydd soils also occur, with Cegin soils more common on long-cultivated land. Small depressions, basins and flushes have peaty soils of the Crowdy and Freni series.
The association occurs mainly in the Howgill Fells but is also at Black Combe and near Loweswater, Cumbria, and near Ravens Knowe, at the Scottish border. The climate is cold and wet. Low summer temperature is mainly responsible for the development of stagnopodzols rather than the brown podzolic soils found on similar slopes under similar rainfall elsewhere. The accumulated temperatures range from about 800 to 1500 day degrees centigrade. Western districts have cooler summers and milder winters than in the east, where the seasonal range of temperatures is greater.

Soil Water Regime

The water regime of these soils is complicated by the presence of the contrasting horizons. Water is held in the surface horizons of Hafren and Hiraethog soils, the peat acting as a sponge so that they are seasonally waterlogged even though the subsoils drain freely (Wetness Class III or IV). Where strongly formed, the ironpan also impedes water movement. Wilcocks series, in slowly permeable thick drift on the lower ground, is naturally wetter (Wetness Class V). Rainwater passes rapidly into streams and rivers when the upper horizons are already saturated, particularly in winter.

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