Geology for naturalists - how to classify rocks and soil?

I’m very interested in moss, and the substrate it’s growing on is often an important factor in identification. The trouble is, I know next to nothing about how to recognise such things. I can probably tell if a rock is slate or chalk, but outside of that I’m lost.

So I’m looking for tips! I don’t need to be able to scientifically analyse every rock I come across, I just want to get better at broadly classifying a substrate in ways like acidic vs alkaline. Does anyone have advice or hints for doing this at a glance? Or resources I can refer to? What geology knowledge can you share to help inatters? :D

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I guess for alcidic/alkaine soil litmus paper should work good enough.

I’d add to the companion kit a strip for pH detection coming in pockatable rolls and a small bottle of distilled water, ready to go, cheap and effective.
Distilled water is obviously fundamental.

Check out the NRCS Web Soil Survey ( if you are in the USA. Once you have identified the local soil, google the soil’s name for more specific information.


I really appreciate and extensively use the USDA web soil survey- but at the level of specificity needed for things like mosses and small shallow-rooted plants it’s not always so useful, since the organic layer can have very different surface conditions determined by tree duff, moss beds, peat formation etc. Surface conditions can vary dramatically within areas identified on that kind of soil map as all being the same material. That’s definitely where direct testing is a big help.

I’ve sort of cobbled together some testing on occasion but if anyone posting in this thread has specific recommendations for economical testing products to buy and where to find them, that would be great. Everything I’ve found locally has had limited effective ranges that are fine for deciding whether to lime your lawn or not, but of limited value in more varied environments.

On rock, that’s harder (pun?). If it’s a bedrock outcropping then there should be geological maps that can guide you to figure it out, but if it’s something like a glacial erratic or rip-rap that’s been moved from its point of origin that may be harder to readily identify without learning some geology.


I understand that chemical testing is the best (only?) way to get a definite result, but it’s expensive and a lot of effort for something that’s only one factor in identifying one organism. I’m guessing it also isn’t possible with large rocks/boulders because the stuff needs to be dissolved in water, right?

And geological or soil maps aren’t very helpful for the reasons @er1kksen gave - it’s not accurate at a close enough ‘resolution’ for things like moss. I want the exact opposite from a map of general areas: I want a way to tell whether this exact rock is acidic or basic, regardless of where in the world it is.

I feel like there must be simpler ways of getting a rough idea, even if they aren’t 100% accurate! It seems like a lot of moss ID keys mention in passing things like “lime-rich rock” or “acidic substrate”, in a way that suggests other people can tell those things at a glance as easily as telling the difference between wood and metal. What do they know that I don’t?! :D

Here’s a couple apps you can use to access USDA-NCSS detailed soil survey data (SSURGO) using Google Earth or via an interactive Google maps app in your browser.
For the underlying geology, you can access geologic maps here (click on map view and zoom in to your location):
Let me know if you have trouble with burrowing in to get the data you need.

Basic pH testing doesn’t have to be expensive, necessarily, but I’ve yet to find a field test kit or device that suits my own use cases. But I have to believe a solution is out there!

But that’s soil and water. For rocks I’ve heard of using a dropper of some kind of acid and seeing if it bubbles in contact with the rock (which would indicate a lime-rich rock).

When it comes to your last question, though, I think that yes, some of it just does come down to having learned a bunch of individual facts about different substrates to the point of being able to visually identify them. For example I’ve learned that the shales exposed in ravines in my area are calcareous by reading about local geology, but that the sandstone and conglomerate layers in some of the upper strata are more acidic, and I can visually identify at least some kinds of granite, which are acidic, and I know from reading that the leaf litter beneath most of our hardwoods tends towards alkaline, especially under basswood, ash, and elm- while the needle duff beneath hemlock canopies is generally acidic. I’ve also learned that clays are confusing and hard to make assumptions about.

Another helpful thing to learn is common indicator organisms- plants especially. Again it’s sort of a case-by-case basis, but certain ferns will only be growing on lime-rich rocks or acidic rocks, and are easier to identify in and of themselves than the associated mosses. Likewise the presence of particular plant species in a groundwater-fed wetland can tell you whether the water discharge is more mineral-rich/alkaline or not, and low herbaceous flora is also often a good indicator on dry land.


Yes, you may be referring to using 10% hydrochloric acid to test rocks for limestone/dolomite and calcareous cements and clasts, a fundamental tool of geology. (Rinsing off the drop of acid afterwards with water is good etiquette too, as the acid will stain often the rock.)

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It’s definitely not expensive, lichen lovers use tons of chemicals for testing, not that they have to be millioners. :D Decide what you need and most probably it will be something cheap that you can buy in chemical store. Thing is their formulas are simple, they’re chep to make (and ueful for many things & ppl), so they sell cheap too.

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