Alkalinity to acidity and everything in between

…or, why soil pH is so important

Alkaline or acid soils: a quick guide to understanding soil pH levels

Before I really started to get into gardening, I have to admit, I thought the whole soil pH thing was a bit of a nonsense.

Sure, I was aware that such things as soil tests existed and that every intro to every gardening book I looked at recommended you go and test your soil but, honestly, I thought, what’s the point? It seemed a little bit earnest and overly-diligent to head out there, digging up a sample, and checking to see if the liquid turned red or green. (This much I could remember from chemistry lessons at school red = acid, green = alkaline.)

I had the soil I had, there was not much I could do about it, and I just went ahead and planted whatever plants I wanted in whatever spot in the garden I thought would look nice. Of course, with this approach, quite a few of them died really rather quickly indeed.

I also thought (very vaguely, if I thought about it at all, which wasn’t really much) that acid soil would surely be bad for plants (who wants acid, right?) and alkaline probably better and neutral best of all.

Well, needless to say, everything I had assumed turned out to be wrong when I was forced to actually pay attention to soil pH in one of my horticulture classes last year.

The pH scale

As I – very vaguely – remembered from Chemistry GCSE, the pH scale goes from 1-14. 1 is the most acidic and 14 the most alkaline. Neutral is right there in the middle at 7.

Most soils, though, tend to be slightly on the acidic side, around 6.5 in pH if you’re interested in the numbers. And, in fact, a broader range of plants prefer to be in slightly acidic soil.

When it comes to plants, the pH scale to look at is between (approximately) 4.5 and 7.5, very few plants able to survive outside those two extremes. The majority of plant species tend to thrive in soils between 6 and 7.

Interestingly, soil also tends to get more acidic over time. (Due to a number of reasons, but broadly because the alkaline nutrients are leached from the soil and because rainfall itself is slightly acidic…)

So, if you’re gardening a plot for a long time, you’ll probably want to re-check the soil pH after a few years and see if it’s altered at all.

Different plants for different places

But the most important thing I learnt was that some plants really do need to be in the right acidity / alkalinity of soil in order to survive.

Certain plants really dislike being planted in soil that’s too alkaline as it means they’re unable to take up the nutrients they need to survive. These are called calcifuges (which translates as lime haters. Lime = alkaline soil.) The most common examples are rhododendrons. If they’re planted in soil conditions that aren’t right, they will grow more slowly, perhaps become stunted, may not flower well, and often will have yellowing leaves, and ultimately will just give up the ghost and die.

There’s a whole lot to say about calcifuges, actually, so I’m going to be coming back to them in their own article in a few weeks, once we get to letter C.

Other plants may grow okay in alkaline soil but become more likely to get diseases, such as potatoes which tend to get potato scab if the soil they’re in isn’t acidic enough.

At the other end of the scale, some plants fare badly if the soil is too acidic. Plants such as saxifrages are a great example. Clematis and viburnums are other commonly-grown plants that prefer alkaline soils.

And then there are certain veg that are more prone to get diseases in heavily acid soils. Brassicas (that’s your cabbages, Brussel sprouts and so on) tend to get a disease called club root if they’re in soil that’s too acidic.

 What to do

So, bearing all this in mind, what can you do?

Firstly, go out and do a soil test. Honestly. I finally did one and found that my soil is basically slightly on the acidic side, which is great.

Once you know what your soil type is (and do check in different places as it might vary across your garden) then you can assess plants before you buy them to see if they’re likely to flourish.

The RHS plant finder online is a great resource for finding out info on plants and I always check here before buying anything.

There is such a huge variety of plants out there, that you should be able to find ones that you love which will thrive in the soil conditions you have. And this, by far, is the easiest way to go about things, rather than spending lots of money and time trying to alter the soil that you’ve got…

Altering soil pH

That said, what if your soil is really at an extreme end of the scale and you long for a greater choice of plants? Or you’re growing veg and you don’t want your cabbages to succumb to club root and your potatoes to scab.

Fret not! It is possible to alter soil pH, but is something of a faff. (That’s the technical gardening term for it, of course…)

My first option would be to grow plants in containers if I wanted a plant that needed a different soil type. For veg, you could add top soil into raised beds, or grow fruit bushes in pots to cater to their diverse but exacting needs.

Blueberries are a classic example of a plant that needs specific soil (very acidic in this case), and therefore grows perfectly well in pots if your soil isn’t suitable.

Just buy the right compost, fill up the containers and grow in your new (perfect!) conditions.

If you desperately want to change whole flowerbeds, though, or your soil is so very acidic / alkaline that it’s not feasible to grow many plants in it, you can change the pH by adding either lime or sulphur to make it more alkaline / acidic respectively.

This takes time, so you’ll need to work on the soil over a period of months, adding whichever you’re using at the rate recommended on the packet, and then keep on carrying out a soil test until you get the desired result.

But, even then, it’s not possible to sit back and enjoy it forever – soil will naturally try and return to the state it wants to be in: its make up determined by the bedrock underneath it, which will continue to break down and deposit the same minerals back into the soil. So if you live above a huge section of chalk, any attempt to make your soil more acidic will be an ongoing one.

So, an overall recap on soil pH would be: life is so very much easier if you learn to love what you’ve got. Perfect advice for almost everything really.

Phew, this post ended up far longer than I expected and I feel as if I’ve only scratched the surface of everything I could say on soil pH! If you’ve any questions, do drop me a comment below and I’ll do what I can to answer it.

Otherwise, do come back again in a fortnight when I’ll be wittering at length talking intelligently about biological control (and why ladybirds are so awesome)…

Related articles:

  • This is the first post in my new fortnightly series, a gardening jargon buster, where I’ll be going through an A to Z of gardening terms.
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9 thoughts on “Alkalinity to acidity and everything in between

  1. I absolutely love how you wrote this topic – being a hort student and working with plants myself, I’ve read countless descriptions of pH descriptions and the how to’s of altering soil – and I’m a Container Gardening designer and fanatic – thus your comment of growing blueberries in pots – well, that is one of the solutions! I’ve shared your article on Twitter and Facebook. Thank U, Cathy T

    1. Thanks Cathy, what a very lovely comment to read. And thanks so much for sharing the article too, I really appreciate it.

      I guess I am fundamentally a lazy gardener — if the soil is a certain way, it seems much easier to me to work with that, rather than put in a lot of elbow grease to change it! And container gardening is great, letting you grow just about whatever you want wherever you want… Actually, that’s reminding me that I’ve not really made the most of my pots this year. I must plant up soon for the summer!

  2. Thanks for giving us a run down on ph..I live in the San Francisco Bay Area (Marin County)…the land of adobe clay. Plenty of native plants do very well…but sometimes I need a hammer and chisel just to make a hole for a one gallon transplant. But we persevere none-the-less trying to make the perfect landscape.

    1. Ah it’s typically a real clay soil here in London too, though we seem to have got quite lucky in my house as it’s not too bad. V hard to work with, but at least lots of nutrients in there!

  3. You are my gardening go to. I am totally going to do it this year and make a garden, I shall be following your advise with interest, then probably completely ignoring it because the plant i have chosen is pretty and of course it will survive if i put it in the ground and ignore it. P.s. my boss waters my plants at work because he takes pity on them.

  4. Great post Sabrina! It’s made me wonder if a few of my plant issues could be down to soil ph. A couple just don’t seem happy no matter what I do. I’ve got a soil tester gathering dust under the coffee table, I will dig it out and investigate.

    1. It’s quite interesting actually (to me, ha ha) — but the reason that plants only do well in certain pHs is because they are actually unable to draw the nutrients they need up from the soil if the pH isn’t right. I’d always assumed it was just a preference, but they can be literally unable to get what they need to grow / photosynthesise if they’re in the wrong pH. Poor little poppets!

      Hope you get to the bottom of it, anyway.

      xx

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