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The soil profile of chalk downland is a thin soil overlaying the parent chalk. This white rock was formed under the sea some 130 - 60 million years ago from calcareous parts or coccoliths of minute marine algae and fragments of seashells, although larger animals like sea urchins can sometimes be found as fossils. Weathering of the chalk produced the necessary conditions in which plants could gain a foothold and the addition and decay of this organic material (humus) over the past 10,000 years or so, has created a characteristic soil known as rendzina. Unlike many soils in which there are easily distinguished layers or horizons, a chalk rendzina soil consists of only a shallow dark humus rich surface layer which grades through a lighter brown hillwash containing small pellets of chalk, to the white of the chalk itself. This is largely because of the purity of the chalk which is here about 98% calcium carbonate and the consequent absence of soil-building clay minerals which are abundant, for example, in the valley floor. A combination of sheep grazing over several hundred years and a soil deficient in most plant nutrients has allowed the development of the short, springy grassland. This may have up to 45 different species of flowering plants and mosses per square metre, but in order to maintain that rich diversity and prevent more vigorous grasses from dominating the low growing plants, it is necessary to continue grazing. Wherever possible, sheep are being used on these old grasslands, since their method of grazing across a field, gently and gradually reducing the height of the grass, maintains a close sward. Cattle produce a rougher sward of variable height, because they curl their tongues around tufts of the grass and tear them out. Changes in agricultural economics have led to an increase in the area of arable cropping and improved pasture which, as a result of the application of fertilisers, has become less rich in the number of plant species. The conservation of the remaining unimproved downland pasture is therefore important. Most of the grasses on the chalk downland areas of the Park are the fine-leaved fescues (Festuca spp.) among which will be found such characteristic prostrate herbs as thyme (Thymus drucei), salad burnet (Poterium sanguisorba), squinancy wort (Asperula cynanchica), the carline and stemless thistles (Carlina vulgaris and Cirsium acaulon) and later in the year the common centaury (Centaurium erythaea) and the autumn felwort (Gentianella amarella). Elsewhere on the downland pasture there are small colonies of orchids including the early purple orchid (Orchis mascula), the common spotted orchid (Dactylorhiza fuchsii) and the pyramidal orchid (Anacamptis pyramidalis). Short grassland is generally a rather poor breeding habitat for birds, although it may be important as a source of food for those which nest elsewhere. The common nesting birds are meadow pipit, skylark and often perched on a fence post, the corn bunting with a song supposedly like a bunch of rattling keys; during winter you may see fieldfare, lapwing and redwing. Steep slopes on chalk downland develop a ribbed pattern of grass covered horizontal steps a foot or two high. Although subsequently emphasised by cattle and sheep walking along them, these terracettes (commonly known as sheep tracks) were formed by the movement of soil downhill, a process known as soil creep. This still active movement probably results in alternate heating and cooling, and wetting and drying of the soil, together with the steepness of the slope and the binding action of the roots. It can be simulated by gently jogging flour down a piece of coarse sandpaper. |
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