What Is Soil pH and Why Is It Important?

AlanGardenMaster
Published on December 5th 2019
8
a scientist checking soil pH
Soil pH may not sound the most exciting thing to a gardener, but it has a significant impact on what we can and can't grow in our gardens. Getting it right can also give us better yields and better blooms.
Winter is the perfect time of year to delve into this more technical side of gardening and to put things right if needed. And what better time than on World Soil Day itself!
  • You’ve probably heard of ‘lime hating plants’. But do you know that many plants grow better when the soil has lots of lime, creating a high pH? I'll cite some examples later!
A pink and purple Rhododendron flowers on a plants
Lime hating Rhododendrons

What is Soil pH

  • Before we test our soils, we need to understand soil pH. I’m sure that you will know that pH is the symbol used to indicate whether a solution is acidic or alkaline.
  • A neutral pH is index seven, but gardeners tend to regard 6.5 pH as neutral. At this pH, the majority of plants will be happiest and will be able to access the nutrients they need.
  • If you're interested in the technical side, pH is a measure of free hydrogen and hydroxyl ions in a water solution. The pH scale runs from 0 to 14, but most garden soils will be in the range of 5 to 8.
  • Anything below 6.5 is regarded as acidic, and above is alkaline.
A green electronic pH tester in soil
An electronic pH tester gives an instant soil pH reading

Why is pH important?

  • pH is crucial to plants as nutrients are more readily available at some pH levels than at others.
  • In addition to this, some diseases are more prevalent in soils at some pHs than in others.

Let’s look at so-called lime hating plants

  • Perhaps these plants would be better described as 'acid-loving', as they grow best when the pH is below neutral.
  • Most of these are in the heather family Ericaceae. Think of Erica, Calluna, Rhododendron, azalea and Pieris. Those are the most frequent ones.
  • Camellias are often lumped with lime hating plants, but in my experience, they are far more tolerant of soils closer to neutral than others. Nevertheless, they will thrive in acid soils.
  • This also applies to most Magnolia trees. However, some magnolias such as Magnolia loebneri varieties are lime tolerant. This is why you'll see Magnolias so widely planted in Britain in spring.
  • A few edible plants prefer acid soil, including blueberries and cranberries (Vaccinium), also in the heather family. They need a very low (acidic) pH.

Cranberry

  • Potatoes prefer below neutral soil pH, as do aubergine and sweet potatoes.
  • Raspberries, blackberries and strawberries prefer slightly acidic soil.
  • You should be aware of your soil pH if you want to grow these types of plants; otherwise, you may end up wasting a lot of money!

Which plants like alkaline soils?

  • There are masses of magnificent lime-loving plants that tend to be taken for granted.
  • Just think of Clematis, hellebores, lilac, Euonymus, honeysuckles, passionflower and lavender. Most of the plants we grow like alkaline soil, and I don't have room to list them all here.
A close up of a pink Clematis flower
All Clematis like limey soils including this lovely variety 'Abeline'
  • Fig, grapes, cherries, nectarines and plums all prefer alkaline soil.
  • The cabbage family are all alkaline-loving. This includes sprouts, cabbage, swedes, rocket, pak choi, broccoli, cauliflower and even wallflowers.
A person preparing Brussel sprouts at a table
Brussel sprouts need a high pH to do well
  • Maintaining soil pH above neutral is especially important in combating clubroot.
  • Many lime-loving plants will still grow in acid soils but will perform better if the pH is at or above neutral.

Check it out

  • With this in mind and as I'm currently planning my new garden, I’m checking my soil pH this week.
  • I’m using a readily available soil pH testing kit. It’s easy to use and gives a result in a few minutes.
Alan Down checking a sample of garden soil for pH
@AlanGardenMaster checking a sample of garden soil for its pH
  • In the kit, there's a simple test tube and chemical tester. You can also get an electronic but more expensive hand one with a probe.
  • It's important to remember that the pH can vary from place to place in a garden. Take ten samples over the border that you're checking and get an average.

WorldSoilDay

  • Today is #WorldSoilDay! There is no better time to take a closer look at soils, especially when soil is in a state of global crisis.
  • This time of year is the perfect time to look into your soil pH, as we need to understand whether or not to apply lime to get the soil pH right for planting and sowing.
  • Just how much lime to add to your soil will depend on your current soil pH and which plants you plan on growing.
  • If you want to grow plants that prefer acidic soils, then you should be able to lower the pH a little by adding flowers of sulphur to the soil or by adding some acid organic matter such as leaf mould.

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