Lower the pH of my garden soil

crabbygardenerMarch 9, 2014

Question: I had my raised-bed garden soil tested last year, and the pH was around 7. Most of the plants I want to grow (tomatoes, lettuce, cucumbers, broccoli) like the soil a little acidic, so I was wondering what's the best way to reduce the soil pH. I suspect that the neutral pH affected the flavor of my tomatoes, as the ones I grew in pots tasted quite a bit better than those in the garden beds. Of course, that's anecdotal evidence, but...

I've heard of adding sulfur to the soil to acidify--are there any ill effects of doing so? I've also heard of adding coffee grounds, but I don't drink coffee. (I've also also heard that pine needles--which are abundant here--are acidic; if I used them as a mulch, would that effectively lower the pH, or is that not a good idea?)


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7 is close enough to most plants' needs that I would avoid messing with it. You can use sulfur but you run a high risk of overacidifying pockets of the soil. If you really want to lower the pH, I would go with small applications of iron sulfate, monitoring the pH regularly between applications, until you're at 6.5. I don't think the tomatoes were bland because of the soil pH. Personally, I would just go with plenty of additions of organic matter such as compost and avoid any mineral acidifiers for such a small change.

By the way, brassicas and legumes prefer it around 7.
My soil pH is 8.3, high enough to warrant the use of sulfur.

This post was edited by Slimy_Okra on Sun, Mar 9, 14 at 14:02

    Bookmark   March 9, 2014 at 1:59PM
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digdirt2(6b-7a No.Cent. AR HZ8 Sun-35)

Agree with Slimy for the most part. My native soil is also very alkaline @ 8.5 and although I do use sulfur since its effects are far more permanent than most other options, it must be done carefully and well tilled and monitored.

I also regularly use oak leaf mold and a great deal of homemade compost (that includes pine straw) throughout the season. But my goal is always just to get it down to the area of 7 and try to hold it there +/- a tad.

Pine straw, compost, and leaf mold will always return to the neutral pH as they decompose over the season so while great soil amendments they will also have a short term acidifying benefit if regularly applied.


    Bookmark   March 9, 2014 at 2:23PM
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seysonn(8a WA/HZ 1)

I've also heard of adding coffee grounds, but I don't drink coffee. (I've also also heard that pine needles--which are abundant here--are acidic; if I used them as a mulch, would that effectively lower the pH, or is that not a good idea?)

You've got it: Pine needle mulch can perform a double duty. For coffee ground go to a Starbucks. Leaf mold is another. Generally most organic matter are on the acid side. PLUS most garden veggies can grow fine in pH near 7. Though slightly acidic (6.8) is almost near ideal. Most herbs prefer 7++.

    Bookmark   March 11, 2014 at 1:21AM
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tishtoshnm Zone 6/NM

I do not have it saved but at some point, somebody at gardenweb did an analysis of coffee grounds and found that the used grounds were not acidic. The acid tends to go into the coffee. Coffee grounds are an excellent component of compost, etc, though.

Will pine straw help? Probably but it will take a while. Likewise, sulfur takes a while to acidify the soil. My soil tends towards alkaline (around 7) and I have had some wonderful tomatoes and some dull tomatoes which I suspect had to do more with amount of water and variety than the soil.

We do keep liquid ironite in the garage and if plants are showing signs that they are having trouble, we use it but we wait for the signs of iron chlorosis but we rarely have that problem with vegetables, even with our alkaline, very hard water that we irrigate with.

    Bookmark   March 11, 2014 at 1:22PM
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digdirt2(6b-7a No.Cent. AR HZ8 Sun-35)

You are correct Tish - the pH of most coffee grounds is 6.7-6.9 depending on the type of coffee. This per testing done by several different university labs.


Here is a link that might be useful: Science Daily - the pH of coffee grounds

    Bookmark   March 11, 2014 at 2:21PM
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I'm working with a native soil similar to Slimy_Okra's and have also been applying iron sulfate for the last few years. I've been wondering... at some point are we adding too much iron by doing this?

Results were evident by the time I tested again a couple of months after application that first year. The second year I applied less and by the third year, I refrained from adding any at all. Last season was the fourth year and we applied at the same rate we did the second year. As far as lowering pH, it's working fine. It just feels rather strange to be adding heavy metals to my soil.

    Bookmark   March 15, 2014 at 12:50PM
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Hi Macky,
Looks like you and I garden not too far from each other. I'm in the Watrous area. What about you?

Iron (ferrous) sulfate oxidizes quickly to ferric sulfate after application. This reacts with calcium carbonate in the soil to form calcium sulfate and ferric carbonate, which then decomposes to ferric oxide. Ferric iron is unavailable to plants and also insoluble at a pH above 7. The potential for iron toxicity starts to develop if the pH is shifted below 6.0, especially if also compounded by flooding, since this enables the reduction of ferric iron back to plant-available ferrous iron.

If it makes you feel any better, clay is naturally high in aluminum. Like the iron, it is completely locked up in neutral or alkaline soil.

This post was edited by Slimy_Okra on Sat, Mar 15, 14 at 22:04

    Bookmark   March 15, 2014 at 9:59PM
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daninthedirt(Cent TX; HZ10, Sunset z30, USDA z8a)

You might check the pH of the tap water, if that's what you use to irrigate. Around here, the soil is mildly alkaline, but so is the tap water (hey, it's from lakes on the same soil!). Hundred degree summers don't come with much rain around here, so irrigation is all from the tap, and lots of it. So I've never been too impressed with strategies to chemically acidify the soil. I wanted to grow blueberries, and I asked a local botanist what to do about making the soil acid. She said -- don't bother, it's a losing proposition. Now, if you have *highly* alkaline soil, then I guess you just fight the good fight.

    Bookmark   March 15, 2014 at 10:32PM
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Excellent... thank you, SO. I think chemistry was my worst subject in high school, so having you summarize that simply and plainly is much appreciated! No danger of getting our soils anywhere near 6, eh? lol

We must have excellent buffering qualities (hope I'm using that term correctly), though, because the efforts to lower pH here seem to be sticking better than areas such as daninthedirt is speaking of (great username, by the way, Dan). I also have a rather easy time lowering water pH for our aquarium fish; half distilled and half dechlorinated tap water keeps it in a comfortable range for them without having to purchase adjusting chemicals. (We're now on the river water pipeline that goes into town; well water was much, much worse and I could only use a small amount mixed with the distilled.)

We're rural in the Humboldt area, but we have family who is mixed farming the Young/Allan area. :) Prairie Garden Seeds is in Humboldt here and I met Jim and his family briefly when I picked up a seed order last year. Nice folks. I should ask for a tour of the seed gardens someday.

One more question. Our garden has flooded before. How long does it take for the iron to then get back to an unavailable state after such an incident?

So as not to hijack crabbygardener's thread any further (sorry about that), I'm going to spin off with a new post over in the soil forum regarding my question about zinc and copper.

This post was edited by macky77 on Sun, Mar 16, 14 at 11:29

    Bookmark   March 16, 2014 at 11:28AM
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Not very long once the soil is re-aerated (a few weeks). Plus, if the pH is anywhere near or above 7, most of it would be locked up, regardless of which form it was in.
Small world indeed!

    Bookmark   March 16, 2014 at 2:36PM
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Thanks for your replies, everyone!

    Bookmark   June 14, 2014 at 10:21PM
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balloonflower(5b Denver CO, HZ 5-6, Sunset 2b)

My soil in my community garden plot tested at 7.8. We still grow tomatoes, tomatillos, corn, beans, peas, and other random small plants tucked in with no problem. Add as much compost as you can and you should be fine. We're on our third year with the community plot and notice a change for the better each year with the continual composting.

    Bookmark   June 15, 2014 at 12:40AM
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