If the ‘grass is greener’ on the other side for you, then in the UK that soil is undoubtedly acidic. For some reason there’s an unwritten law that states that if you garden on a chalky soil you will want to grow ericaceous plants like azaleas and rhododendrons. The truth of the matter is that making an alkaline soil acidic is nigh on impossible and quite frankly not worth the bother. I’ve known people to move house on account of it, although if you really are fussed then a raised bed is quite a practical solution.
If however, you only want to tip the balance a little then you could regularly add composted pine needles, pine bark or sulphur chips and fork it in. In truth the only practical solution is to work with the soil you’ve got and plant the right plants.
The soil matters because although most plant nutrients are available in the ground, bad structure, water shortage and incorrect pH can lock them up and make them unavailable to certain plants. The pH scale runs from 1, which is very acid, up to 14 which is very alkaline with 7 being neutral. Most soils though are around 4 to 8 and most plants grow between 5.5 and 7.5.
Ornamental plants tolerate a wide range, but vegetables are fussier, preferring a pH in the range 6.5-7 and fruit from 6-6.5. The level of calcium in the soil basically governs the pH and in a free draining soil this can get washed or leached out, making the soil more acidic. Many acid loving plants are of woodland origin and for that reason you may need to pick a shady spot, perhaps under some trees.
Acid Loving Plants
Rhododendrons and azaleas are probably the best known acid loving plants, but all are shallow rooted so shouldn’t be planted too deeply. You can still find nurseries selling root wrapped plants rather than container grown specimens and this can work out as a cheaper way to buy them. The roots have a tendency to become dry, so give them a good soaking before planting as long as hard frosts aren’t imminent. If you’re planting now, frost can still can loosen the plants in the ground and they will need firming back in periodically before growth really gets going as spring warms up. I find this to be particularly important when planting and nurturing rhodos.
Rhododendrons are generally fairly undemanding but yellowing leaves is a common problem. This chlorosis is caused by a lack of certain trace elements, usually iron. The lime in alkaline soils locks up a lot of essential nutrients and feeding with sequestered iron or liquid seaweed makes them available again. Rhododendron williamsianum, along with all other azaleas is now correctly classified as a rhododendron. This one has distinctly rounded leaves and is smothered in pink or white flowers in mid to late spring, but unusually for a rhododendron it will also thrive in the sun.
The forest flame, Pieris formosa var. forrestii is aptly named. In the spring bright red new leaves burst out above the mature green foliage, followed by white lily of the valley-like flowers. These new leaves and flower buds are incredibly vulnerable to late frost damage, especially following our recent mild winters where they tend to open early, so plant under a canopy of evergreen trees like Quercus ilex for protection.
Failing that you have to keep an eagle eye on the weather forecast, hope that it is vaguely accurate and be prepared to trot out and cover your plants with horticultural fleece.
Peaty soils are always acidic but are generally very low in nutrients so you will have to feed generously and add bulky organic matter. It is also quite common for them to be boggy and you can take advantage of this. You could install French drains to dry out parts of the garden, take the majority of water into the lowest part of the garden and then build a bog garden there. Any cultivated cranberries and blueberries would be eternally grateful.
The legendary Himalayan blue poppy is often monocarpic which means it dies after it flowers. However, this is generally less likely if grown in moist soil especially if you pinch of all flowering tips until several crowns have formed. The problem is that if the ground is too wet and stagnant, the crowns will rot. It’s a fine line.
The Difficulties of Acid Soil
The pH dictates the availability of nutrients to plants. In very acid soils plant foods are washed out or dissolve in the water at toxic levels. Bring the pH up to around 7 with an annual treatment of ground limestone or dolomitic lime sprinkled evenly over the lawn and raked in.
Brassicas such as cabbages and Brussels sprouts won’t grow in acid soils and need lime added in winter. Earthworms dislike acid soils. If you have a wormery or compost heap and find lots of tiny flies in it this is an indicator that it is too acid but it can be remedied by a scattering of garden lime. Check the pH first though because excessive lime will lock up nutrients.
Certain diseases such as potato scab are more troublesome on acid soils while clubroot, which affects brassicas is much less so. Leatherjackets and wireworms are also more common in acid soils.
Acid soils encourage the build up of thatch in lawns, cause poor grass growth and favour the growth of moss and other lawn weeds like sheep’s sorrel. The majority of acid loving plants are spring and early summer flowering which can make it harder to provide good year round colour.
Testing the pH of your Soil
Generally speaking it isn’t necessary to know the precise pH of your soil as most people don’t actually test it and you certainly don’t have to. In fact one of the best ways to find out what you’ve got is simply to stick your nose over your neighbour’s fence and see what grows there. To get a good overall picture you should look for ‘indicator’ plants in both wild and cultivated areas.
Indicator plants of acid soils are azaleas, rhododendrons, flame creeper, blueberry, corydalis, meconopsis, autumn flowering gentian, Gentiana sino-ornata, and blue hydrangeas which only retain their colour on acid soils.
Indicator plants of alkaline soils are ceanothus, hibiscus, some clematis particularly the wild ‘old man’s beard’, scabious, and thriving grapevines and figs which do particularly well on chalk.
Taking an accurate soil sample isn’t as easy as it may seem, although it can be carried out at any time of year. Take soil from a depth of up to 15cm and take samples from different areas of the garden which have been cultivated in different ways. Collect soil from at least 10 places spread over the whole garden, mix it in a clean plastic bucket with a clean trowel and take the final sample from this. Don’t take it from areas near compost heaps, bonfires or recently manured and limed areas and don’t touch it with your hands because this could effect the results.
For really accurate results you can send the sample off to a specialist laboratory but there are cheap kits available from garden centres which are reasonably accurate. These involve mixing the soil with water and a capsule of powder. The coloured liquid can then be compared to a colour chart. You could also use a pH meter which is simpler, but not quite as accurate. A probe is pushed into the soil and a readout is displayed.
The Royal Horticultural Society offers a service to measure soil texture, organic matter, soil pH and available phosphorus, potassium and magnesium. They provide sampling bags and full instructions and will include fertiliser recommendations.
To Start Any Acidic Garden
Enkianthus campanulatus (no common name listed)
This large deciduous shrub masquerades as an elegant, graceful tree and has an RHS Award of Merit which they don’t just give to any old plant. The matt green leaves clustered at the ends of the branches give it a light, open appearance allowing you to plant other things beneath.
Although they will grow in sun, a woodland setting in a little shade is far more appropriate. The soil should be rich in humus, well-drained and preferably acid although they will just about tolerate neutral.
In late spring and early summer they produce hanging racemes of creamy yellow flowers suffused with veins of pinky red. In autumn the leaves turn orangey yellow and then red.
Enkianthus comes from the Greek meaning pregnant flower and refers to the species Enkianthus quinqueflorus which looks like a flower within a flower. Enkianthus campanulatus itself hails from Japan. The ideal time to plant is during their winter dormancy. Pruning isn’t needed although damaged and crossing branches can be removed in late winter or early spring.
Enkianthus f. albiflorus has white flowers and Enkianthus ’Red Bells’ is a deep pinkish red
Why not try
Late spring to early summer
Embothrium coccineum lanceolatum, Chilean fire bush
Leptospermum scoparium ‘Nicholsi’, Tea tree
Callistemon salignus, Bottle brush
Kalmia latifolia, Calico bush
Summer to mid autumn
Daboecia cantabrica, St.Dabeocs Heath
Tricyrtis stolonifera, Toad lily
Late summer to early autumn
Lilium lancifolium ‘Splendens’ (syn Lilium tigridium), Tiger lily
Dierrama pulcherrimum, Wand flower
Eucryphia x nymansensis ‘Nymansay’
Calluna vulgaris, Heather
Gentiana sino-ornata, Gentian
Winter to early spring
Narcissus cyclamineus, Daffodil
Hamamelis mollis, Chinese witch hazel
[This post is provided by Andy’s friend and colleague, Mike James. Mike is a freelance writer based in the UK and a content representative for multiple award-winning garden designer Andy Sturgeon]