How to select the soil type for your garden

Of prime consideration when selecting plants is the soil type in your garden, particularly whether it is acid – below pH 6.5 or alkaline or limy/chalky – above pH 7.0.

 

At around pH 7.0, usually known as neutral, certain shrubs or trees which are usually associated with acid soil will perform reason­ably well, although perhaps not growing as large or living as long as they would on soil of lower pH. Here we might include Hamamelis (witch hazel), Cytisus (brooms), winter-flowering heathers (Erica carnea cultivars), certain magnolias, Castanea (sweet chestnut) and Eucryphia, among a few others fre­quently planted.

Most trees, shrubs and plants which grow satisfactorily on an alkaline or chalky soil will perform equally well on a neutral or acid soil.

 

It makes good sense to avoid plants un-suited for your soil – particularly the larger trees and shrubs which are important to the structure or design of the garden. The plant lists state whether plants are lime-hating (calcifuge). Principal examples of lime-hating plants are members of the Ericaceae (with the notable exception of Arbutus) such as rhododen­drons, and azaleas, heathers (Calluna, Erica, Daboecia), Kalmia, Leucothoe, Pernettya and Vaccinium, or the plants such as Camellias, which in nature grow on peaty or sandy soils. Lime-hating trees include, unfortunately for those on chalky soils, such beauties as Halesia (snowdrop tree), Liquidambar and Nyssa (tupelo).

 

Also, there are several lime-tolerant trees and shrubs normally grown for their autumn leaf colour which are not recommended on chalk and limestone soils because their autumn leaves often shrivel or turn brown instead of displaying vivid hues of orange or scarlet as they would on acid soil. Particularly guilty are Acer davidii and A. rutinerve (snakebark maples) and Cercidipbyllum japonicum, the kadsura tree, but this ten­dency can vary from garden to garden within the same district. However, it is a comfort for those who garden on limy soil that Sorbus sargentiana, most of the sumachs (Rhus), many deciduous berberis, and Euonymus alatus and E. europaeus colour reliably and brilliantly in the autumn on the poorest of shallow soil over chalk.

 

Herbaceous and alpine plants for display and for groundcover are generally less troublesome over soil, but among the calci­fuge plants commonly found in gardens are certain autumn-flowering gentians, lupins and meconopsis. On the other hand, white or grey foliaged plants which secrete chalk from glands on their leaves, such as dianthus (pinks and carnations), encrusted saxifrages and gypsophila seem happier on an alkaline soil; so do lavenders, artemisias santolinas, osteospermums (Dimorphotheca) and helianthemums, perhaps because of the sharp drainage the chalk soil affords.

 

Raised or retained beds (or islands) for lime-hating plants

Although it is frequently stressed that plants unsuited to the soil of your garden should be avoided, people who have grown up with rhododendrons and azaleas since childhood are sometimes reluctant to relinquish them in their new gardens in chalk or limestone districts. An old nurseryman had a jingle: A rhododendron set in lime is like a curate doing time’, and it has to be accepted that rhododendron bushes as large, vigorous and happy as is usual on acid land are not pos­sible or practical on alkaline soil, however much one may dose them with expensive chemicals, such as Sequestrine. However, if you really must grow such lime-haters in a chalk garden, you can build a properly sealed-off raised and retained bed, fill it with a few cubic metres of special peaty lime-free compost, and grow a few of the smaller more compact rhododendrons – such little gems as R. calostrotum ‘Gigha’, yakushimanum, ‘Blue­bird’, ‘Jenny’ and ‘Princess Anne’ to name but a few of the many suitable ones now available.

 

It will dry out less in summer if it faces north or west, where often it can form a focal point of colour and interest from the house. Heathers for flower and foliage at all seasons can be added as groundcover, with other small calcifuge plants – for instance the delicious Gentiana sino-ornata or one of its hybrids would extend the season with spec­tacular autumn flowers.

 

Usually in chalk or limestone districts the public water supply is alkaline too; a white deposit on leaves is proof of this. In such localities it is worth installing one or more water-butts to catch rainwater for use on the lime-haters in your raised bed.

 

A semi-shaded site is preferred for ever­greens, rhododendrons and pieris. Heathers (Erica and Calluna) should be well clipped after flowering to maintain a compact con­dition.

Retaining walls should be 30-60cm (1—2ft) high of natural walling stone (sandstone or granite is best). Peat blocks, if available, are also very suitable. Seal off alkaline soil with 5cm (2in) of sand then a sheet of polythene, perforated for drainage. Infill with 15cm (6in) of coarse lime-free drainage material before adding at least 30cm (1ft) of lime-free peaty compost. Small rocks may be positioned between plants if desired. Draping alpine plants such as phlox, aubretia, helianthe-mum, thymes, etc may be built into the wall during construction. Low growing ground covers should be sited about 30cm (1ft) apart each way between the specimen shrubs.

 

This arrangement is also adaptable without sealing off for lime-tolerant dwarf conifers and shrubs with low growing cover plants (thyme, campanula, phlox, etc) and dwarf bulbs. Any well drained fertile soil.

 

Free-draining soil

Before we leave the question of soils and plants suitable for them, mention should be made of light sandy, free-draining soil en­countered in many districts, and heavy, badly drained, often clay, soil, hopelessly wet in winter and hard baked and cracking in summer.

 

While a large number of trees, shrubs and plants will adapt to an amazingly wide spectrum of soil conditions, some shallow-rooted subjects will suffer in severe summer droughts on light sandy soil, but survive on shallow well-drained soil over chalk – this has been well illustrated by the performance of birch and beech on these two types of soil during the record hot summer and drought of 1976. In spite of its limitations in other respects, the moisture stored in the porous chalk proved a life-saver for these trees, among others.

 

In the milder districts and in many coastal areas, shrubs and trees with grey foliage do well in very porous soils. Many of them are of Mediterranean origin – Cistus, Elaeagnus, Halimium, Phlomis, rosemary, lavender, Artemisia, etc.

 

Moist situations

Few plants, trees or shrubs are tolerant of permanently badly drained or waterlogged situations. It is worthwhile to organise some form of drainage and to then dig in copious quantities of humus, farmyard manure or other bulky organic material in an effort to improve texture and aeration. River or water­side situations are different, however – suit­able for several trees and shrubs that one normally finds in such circumstances, such as willows (Saiix) in both shrubby and tree form, a large diverse and fascinating genus, alder (Alnus), dogwood (Cornus), sea buck­thorn (Hippophae) and tamarisk (Tamarix). Soil Tests Unless you wish to grow veget­ables or fruit extensively, expert compre­hensive soil testing and analysis seems superfluous. The presence of sand, peat, chalk or limestone is usually fairly obvious, but in borderline cases, and there are many, use an inexpensive soil-testing kit to deter­mine the pH of your soil – the degree of acidity or alkalinity. Also note the plants in nearby countryside: if heather or bracken, or in wooded areas the ‘wild’ rhododendron, R. ponticum, are growing there, the soil will be acid. The wild clematis (old man’s beard) in the hedges usually indicates an alkaline soil, as does the presence of native shrubs of chalk downland, particularly yew, box, wayfaring tree and spindleberry.