Mineral soils and pH

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mdpillet
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Mineral soils and pH

Post by mdpillet »

Hi all,

I'm currently trying to "perfect" my generalized soil mix. I have decomposed granite, sandy topsoil and fine river chat available. Surprisingly, all these turned out to be quite alkaline (especially the chat), around 7.5-7.8 for the DG, but 8.7-8.9 for the chat! I'm fairly certain my pH meter is working correctly. I already water with added vinegar to a pH of 5.5-6, but I'd like the soil around this level as well, not using an organic component. Any ideas on materials? I'm in Tucson, Arizona.

EDIT: the overall soil pH with distilled water hovers around 8.0-8.1, same as our tap water. I acidify our water to a pH of 5.3. Using a runoff test of the soil with the vinegar water, I get a pH of about 6.1-6.2. Should be enough to resolve the issue altogether, no?
iann
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Re: Mineral soils and pH

Post by iann »

Granite doesn't have a pH because it is essentially insoluble. Only solutions have a pH. I don't know how you're taking a pH reading from it. Sticking a probe in the dry DG is meaningless. Sticking it in wet DG will measure the water. Granite and other insoluble rocks are often referred to as "acidic" since soils formed in/on them tend to be on the acid side because of the organic material in the soil and the acidic rainfall.

The sandy topsoil and river chat could be anything. If they have limestone (or marble, or similar carbonate rock) then they will be alkaline. The carbonate rocks are barely soluble but they are reactive with anything even mildly acidic, including soil acids and rainfall. Any soil formed from them will be alkaline until the carbonates completely disappear, and many plants will not do well in it no matter what you water with.

You might experiment with vinegar directly on the various soil components. If any of them fizz then that's a sure sign that they contain carbonates and are alkaline. If they don't fizz, they might still contain carbonates but probably not. For most plants, and certainly for making a soil mix on the acid side, you don't want any carbonates at all.

A "soil" contains either clay or organics (humus, etc.), probably both, although perhaps only a few percent by volume. Without clay or organics you just have rock and you're doing hydroponics. Also, without clay or organics, your "soil" doesn't really have a pH (unless it is limestone, in which case it is alkaline). Non-soluble grits and sands just have the pH of the water that falls on them, which can mean big swings of pH and nutrient levels.
--ian
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mdpillet
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Re: Mineral soils and pH

Post by mdpillet »

I was hoping you'd respond Ian - thank you! I'll make an effort to fit in a pedology course soon.

I performed the vinegar testing, and it seems the sandy topsoil is the alkaline culprit - makes sense given this is locally sourced from Arizona. I'll cut this from the substrate. I've decided to add some leaf compost to the mixture instead. I'll probably replace this with the sandy topsoil for an alkaline mixture for Ariocarpus and other limestone plants.
iann
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Re: Mineral soils and pH

Post by iann »

Leaf compost would be slightly acidic. Make sure it is very well decomposed, then it is a nice fine black texture and a good base for a soil mix. Fresher leaf mould will decompose in the pot and that can lead to all sorts of problems.

Shame the topsoil is alkaline, probably has a good texture for a cactus soil, with maybe some (sieved) DG added.
--ian
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