What the pH…?
“Spent coffee grains can increase the acidity of soil. So can a layer of mulch from pine trees.”
Oh really?
Nine months ago I planted a blueberry bush in our shady back yard, tucked into a fleeting pocket of sunshine. Given how unlikely it was to receive direct sunlight for more than a couple of hours a day I thought it best to do more than ‘stick it in the ground and see if it grew’ so I checked what conditions blueberries need to thrive. They like acidic soil.
I’m only just getting to grips with the idea that soil can be alkaline, neutral or acidic so the exact pH value of soil is a bit of a mystery. Gardening Australia confirmed what I’d read, that blueberries like a low (acidic) pH of around 4 to 5.5.
(c) Dorling Kindersley 2011
‘To lower the pH I’m going to use spent coffee grounds and pine needles,’ said the reporter on the ‘growing blueberries’ fact sheet. Organic gardening confirmed that blueberries would benefit from ‘peat moss, pine needles and coffee grounds.’
Last year’s Christmas tree provided the pine needles and since then, on an almost daily basis, I’ve tipped the remains of a coffee pot onto the base of the blueberry bush. That’s almost nine months worth of coffee grains. Some days it got a double dose. Visitors were offered coffee as soon as they walked through the door.
‘Wow, that’s strong!’
‘Drink up.’
The brews intensified. Arabica! Colombian! I’d be lying if I said I didn’t occasionally make an extra pot just for the blueberry bush.
Last month I was forced to admit that my efforts had failed. The blueberry bush looked weak and weedy. Did it need more sun? Had I overdone the coffee? Was the ground now too acidic?
The only way to find out was to test the soil. Scary-looking pH testing kids reminded me of school science projects so I bought a cheaper, less messy probe then sent it back to the shop when it suggested a pH level of around seven. Seven is neutral and that was impossible, not after all that coffee.
The pH testing kit wasn’t as messy or as scary as I’d feared and it promised a more accurate result. I watched the colour change until it settled on a pH level of eight. That wasn’t even neutral, it was alkaline!
I tested other parts of the garden and found similar levels. If anything, places where I hadn’t poured spent coffee were more acidic. Why?
This time I did more than read the first website I came to. I discovered that our brick-built raised beds have been leaching lime into the soil. Mortar contains lime, and that has raised the pH levels, making the soil more alkaline and less acidic. All the water I was adding to the coffee pot only increased that leaching process.
In the past nine months I have made conditions far worse for that poor miserable blueberry bush, which has struggled on regardless. The alkaline soil also explains why leaves on the lime, lemon and grapefruit trees are turning yellow. Citrus trees, as their name suggests, like acid soil.
A single application of liquid sulfur has, I hope, corrected the problem and the blueberry bush has been moved into the sunny front garden, which is where I should have planted it in the first place.
As for coffee, it turns out the whole thing might be a myth. “It is widely held that placing coffee grounds under acid loving plants is beneficial. But as the evidence has shown, this may just be a myth since the spent grounds are neutral,” says gardenweb.com, which claims to be the internet’s largest home and garden community.
I’m quietly relieved. All that coffee was doing me no good at all.
9 comments on “What the pH…?”
Leave a Reply
I'm a writer based in Australia with a passion for gardening, remote places and people with a story to tell.
Most interesting Deb! I have lost 2 blueberry bushes…. so am interested to hear how you are going! I’m struggling with gooseberries now! I hope they will do better than last years 3 gooseberries!!
LikeLike
Drat, what a shame about your blueberries, the man in the market told me they were impossible to kill, he even offered me a free replacement if the one I bought died, so goodness knows what’s happened to yours. Hope the gooseberries do better!
LikeLike
Hope the liquid sulfur makes your blueberries happy. They are such a tasty fruit. I often see recommendations to add coffee grounds to acidic-loving plants, so have tried to give my hydrangeas a dose once in a while. I’ve never been as focused and committed as you have been though.
LikeLike
I love the idea of the liquid sulfur, you only have to do it once every four weeks and it changes the ph of the soil, did the coffee change the colour of your hydrangeas?
LikeLike
The past severe winter killed the buds, so had no flowers this year, but I don’t think I stuck with the coffee program long enough it would make a difference either way. Last year they were blue anyway, which is what I wanted.
LikeLike
I used to work coffee grounds into the soil around my roses and clematis. And I have to admit, they liked it! But I think that had more to do with organic matter decomposing than acidity:).
LikeLike
Aha, a good reason for me to go back to drinking a morning pot of coffee, thanks Kay!
LikeLike
Love, love, love blueberries, but they are a comparatively expensive fruit here. You make gardening in Sydney sound challenging but rewarding.
LikeLike
Me too, one of my favourite fruits. And I can get an inordinate amount of pleasure from seeing a single blueberry!
LikeLike