Tuesday 25 June 2019

Diarmuid Gavin: Soil science... how to pick the perfect plants for your plot

Knowing the pH of your soil will help you pick the perfect plants for your plot

Strawberry plant in pot
Strawberry plant in pot

Diarmuid Gavin

A very important consideration when choosing new plants - including vegetables and fruit trees - is to know the pH of your soil.

It's an area of gardening that many find confusing as it implies you need to know more about chemistry than plants, but it's worth understanding the basics so you can make informed decisions and grow plants that will be happy.

Some of our favourite edge of woodland planting such as rhododendrons, magnolias and camellia will only really flourish in acidic conditions, so this week I'd like to take a look at what that means and how to find out if your soil is suitable.

pH is a measure of acidity and alkaline on a scale of 1 to 14. The mid-point 7 is neutral - anything below that number is acidic and above it alkaline. Most garden plants do best in a slightly acidic soil which is between 6 and 7. To grow rhodos and camellias, however, you want a level of 5 to 6.

If you're not sure what your soil pH is, there are a number of ways to find out. If you have hydrangeas, the flowers will be blue on acid soil, pink on neutral to alkaline soil.

You can test with litmus paper which you can buy cheaply in art and craft shops. Mix your soil sample with some distilled water and dip the litmus paper in for a few seconds. The paper will turn red for acidic, and blue for alkalinity. This will just give you an approximate idea - to get a more accurate reading, use a soil testing kit available from garden centres. You can also use a pH meter which is like a probe you stick in the soil.

Companies such as IAS Laboratories of Bagenalstown Business Park, Co Carlow (iaslabs.ie) carry out a full range of testing on soil including, pH, phosphorus, potassium and lime requirement. In addition, a range of specific analyses are available for trace elements, primarily for farmers but also for keen gardeners.

If you find you have acid soil, then there are some really choice specimens to choose from. If your soil is alkaline you can still grow them in pots using an ericaceous compost which is acid.

If you try growing these plants in ordinary soil, you will notice the leaves going yellow - this is due to iron in the soil being unavailable to plant roots in alkaline conditions.

In years gone by, passionate gardeners would create peat beds so they would be able to grow these lovelies. However, using a lot of peat isn't brilliant for the environment. In the end, it's best to know what soil type you have and to garden in harmony with it. Here are some of my favourite acid-loving plants (also known as lime-haters!):

Kalmia latifolia, the mountain laurel, is a superb evergreen shrub that is smothered in delightfully pretty pink flowers in late spring. 'Minuet' is a dwarf version, the flowers of which are white with deep red band.

Crinodendron hookerianum is a most beautiful sight in bloom - the entire tree appears covered in small deep pink lanterns. Known as the Chilean lantern tree, this evergreen tree will appreciate a sheltered position in colder areas, protected from frost.

Kirengeshoma palmata is an unusual woodland perennial with sycamore type leaves and wonderful waxy yellow flowers in late summer, making this a valuable plant for shady areas.

Andromeda polifolia, also known as bog rosemary due to its linear rosemary-like leaves, is a small evergreen shrub with bell-shaped white and pink flowers in late spring.

Vaccinium, aka the blueberry, needs acid soil to perform and I'd also recommend planting a few bushes which helps with pollination and therefore better fruiting.

Magnolia 'Heaven Scent' is a good choice for the smaller garden and won't disappoint with its glorious pink scented goblet-shaped blooms.

Rhododendron 'Dreamland' - there are so many varieties to choose from but this is a winning one - tonnes of trusses of pale pink funnel-shaped flowers with a deeper pink edge in late spring. Just gorgeous! Its compact nature makes it suitable for pots and small gardens.

Top Tip

Ericaceous plants tend to be shallow rooting and can easily dry. A good mulching every year will help with water retention. Leaf mould is ideal to help keep soil acid. Use ericaceous feed and top up with sequestrene (plant iron) to condition the soil.

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