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Alkaline soils are widely distributed in Britain, and many of these will be derived from chalk or limestone and are commonly referred to as chalky. Although they include Britain's most productive agricultural soils they can be a challenge to garden. Chalky soils are often shallow, stony and free-draining, and added organic matter can decompose rapidly, making them difficult to keep fertile.
Poor growth and yellowing leaves (chlorosis) are a result of the plants not being able to absorb iron and manganese by their roots. Choosing plants that thrive in alkaline conditions is the best way forward.
Chalky soils are highly variable and range from gravelly to clay-like. The clay-like element may in fact be mainly finely divided calcium carbonate making it very poor for plant growth. However where true clay is present in the soil, nutrient levels may be higher and the water holding capacity greater.
Naturally lime-rich soils contain chalk and limestone in excess, and are often associated with Downland, herb-rich pastures and chalk and limestone woodlands.
Hidcote Manor, Chipping Camden, Gloucestershire National TrustHighdown, Nr Worthing, West Sussex (the late Sir Frederick Stern’s chalk garden)University Botanic Garden, CambridgeWaddesdon Manor, nr Aylesbury, Buckinghamshire National Trust
But on the plus side;
Note: It is not practical to reduce alkalinity in chalky soils by adding sulphur because huge quantities of sulphur would be required over many years to counter the free calcium carbonate.
It is more difficult to lower soil pH (i.e. reduce alkalinity) than it is to raise the pH (i.e. increase alkalinity). Consequently, lowering the pH is only practical in slightly alkaline soils. Where attempts are made to reduce alkalinity, the soil should be checked annually to monitor the change in pH and repeat the treatment if necessary.
To a degree, lime-induced chlorosis (typified by yellowing between the veins) can be rectified by applying sequestered iron in a liquid form, which also contains manganese and magnesium. However, if it becomes apparent this is necessary on an annual basis then serious consideration should be made to either growing the affected plant in a container with lime-free (ericaceous) compost or replacing with plants suited to chalky soils.
On very shallow soils (less than 10cm/4in) over chalk, it may be necessary to increase the growing depth by importing topsoil. For lawns the topsoil depth should be at least 10-15cm (4-6in); for borders 20-30cm (8in-1ft); and for shrub beds >45cm (18in).
Acidifying soilsBuying topsoilChalky soils: plants forClay soilsClay soils: plants forNutrient deficienciesRHS soil analysis serviceSandy soils: plants forSoil typesSoil: cultivationSoil: understanding pH and testing
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