Plants & soil

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Plants & soil

Plants are like people – some prefer to live in rocky mountainous regions, others moist waterside areas, others in the countryside and still others in city centres or conurbations. Although to my mind plants are a little more intelligent than people in this as the majority of plants prefer to live in the countryside whereas the majority of people seem to prefer to live in cities.

The plants that first come to mind when thinking about special soil requirements must be Rhododendrons, but, as you will see, there are many others that have specific soil preferences.

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Which plants grow best where?

Plants preferring neutral to acid soil

Yellow rhododendron © Ita McCobb

Neutral to acid soils have pH levels of between 1pH (high acid level) and 7pH (neutral).

Many evergreen plants have evolved to lay their roots in neutral to acid soil. It is also not by accident that the many plants that prefer neutral to acid soil generally grow in the cooler climatic conditions found in rocky areas, woodland or exposed moorlands, because these are precisely the environments where one is most likely to find neutral to acid soil naturally occurring and where their generally shallow roots can supply these plants with sufficient nutrition.

Among the most exotic looking of this group are Camellias, Rhododendrons and Azaleas with their colourful, vibrant blooms that prefer pH levels of between 4.5 and 5.5 pH. (Azalea is the common name given to deciduous varieties of Rhododendron as well as the many small-leaved, evergreen Rhododendron varieties.)

But not all of this group of plants are large or have luxuriant, leathery leaves, the more spiky-leaved Heathers/Heaths Erica and small-leaved Azaleas also belong to this group, (which should not be surprising since Heathers/Heaths, Azaleas and Rhododendrons all belong to the plant family known as Ericaceae).

Then there are conifers such as Fir trees Abies, Spruce Picea and the many Pine Pinus varieties that grow in profusion in pre-alp regions – all of which prefer to make their homes in neutral to acid soil.

This special preference for soil type is the reason why you will often find these varieties of plants being grown in pots, since this is the only way that we can ensure these plants the correct soil type in areas where the soil is not naturally either neutral or acid.

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Plants preferring chalk and limestone soils

Crab apple © Ita McCobb

Chalk and limestone soils are alkaline soils and have low pH levels. They are often used to mix in with high pH-level soils in order to increase their alkalinity and decrease their acidity.

Chalk and limestone soils are, not surprisingly, the choice of many rock plants as well as several bulbs, tubers and the occasional climber. Of the bulbs and tubers the most notable are Daffodils Narcissus, Tulips Tulipa, Gladioli Gladiolus and Irises Iris that, between them, can provide wonderful “splashes” of colour throughout the year..

More surprisingly, Ivies Hedera, Clematis, several Climbing Roses (such as Rosa banksai) and Wisteria sinensis are happy in chalk and limestone soils.

Many trees also like to be planted in chalk and limestone soils – trees such as the ubiquitous hedging, Leylandii Cupressocyparis leylandii, the Maple Acer negundo ‘Variegatum’, most Crab apple trees Malus and the rich-berried Holly Ilex aquifolium.

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Plants preferring clay soils

Hard fern Blechnum spicant © Ita McCobb

Clay soils are generally alkaline soils with a pH greater than 8.

The problem for plants that find themselves growing in clay soil is that it generally doesn’t drain well and so can easily become waterlogged and hence rot their roots. So as a rule there are no plant varieties that prefer to grow in clay soil. Even so, there are many plants that can tolerate clay soil – especially if someone is kind enough to fork it regularly to ensure adequate and consistent drainage!

In the bush category, the Red chokeberry Aronia arbutifolium, which offers year-round interest, the colourful Red-barked dogwood Cornus alba, the Labrador tea Ledum groenlandicum, several low-growing Willows Salix and Viburnums such as Viburnum lentago and Viburnum opulus.

Since clay soil is often found alongside the banks of rivers and lakes it is not surprising that plants that frequent these areas will also grow in clay-soil environments – plants such as most Ferns Filicopsida, Irises Iris and Primula along with many water plants such as Flowering rush Butomus umbellatus, the pretty blue Pickerel weed Pontederia cordata and the Great Spearwort Ranunculus lingua of the Buttercup family.

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Plants preferring sandy soil

Judas tree Cercis siliquastrum © Ita McCobb

Sandy soil environments are at the exact opposite end of the scale to clay soils. Sandy soil tends to drain too quickly to hold water and nutrients long enough for most plants to survive in it. Here again we need to give most of these plants a helping hand by ensuring they have sufficient water and food for their needs.

The obvious groups of plants that are happy to grow unaided in sandy soil are those that grow naturally in deserts such as Cacti and other Succulents but, other than the 1,000s of varieties of Geraniums Pelargoniums, Bulbous Irises such as the Bulbous reticulata Iris, Iris bakeriana and most Lavender Lavendula bushes. The majority of sand-growing plants need a little extra care and attention to help them get established, but if chosen carefully, once established they will do well.

Many bulbs and rock plants can also handle the rigours of sandy soil. Bulbs such as Crocus, Fressia and the pretty little Scilla, and rock plants such as Biting stonecrop Sedum acre and the Rockrose Helianthemum.

Tree varieties can also cope with the deprivations of sandy soil, trees such as Mimosa Acacia dealbata, Sweet chestnut Castanea sativa, the wonderful Judas tree Cercis siliquastrum, most Junipers Juniperus and the elegant Monterey pine Pinus radiata (to name just a few) will all do well in sandy soil once established.

Bushes and shrubs that work well in this soil type range from the oft-seen Berberis Berberis empetrifolia, and the attractive Ceanothus Ceanothus thyrsiflorus repens to the Gorse Ulex europaeus and the dramatic Yucca Yucca gloriosa – all providing you with a highly varied and interesting choice of planting.

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The exceptions

In the plant world, as in all things, there are exceptions to every rule. As well as all the above, many varieties of plants have “rogue” members that, contrary to the majority of their plant group, like to grow in soil types different to those preferred by their close relations – so it’s always a good thing to check before you plant or take on the responsibility of caring for any plant – particularly check on a specific variety’s planting preferences.

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References: Oxford English Dictionary; Collins Tree Guide, Owen Johnson & David More, Collins; Royal Horticultural Society; plant-ark.com.