Soil Q&A

I know you’ve been looking forward to another in my gripping Agronomy Series, so you’ll be happy to know that I got my soil test results back from the University of Massachusetts at Amherst lab.

Now that I’m an adult, I don’t take tests very often. Although I’m tested all the time, it’s in the life-hands-you-lemons kind of way, not the please-begin-you-have-one-hour kind of way. So it’s been a while since I opened an envelope with test scores.

Naturally, I wanted to do well. Despite the fact that our property is built on a substance called Carver Coarse Sand, I felt personally invested in the results of the soil test. After all, we’ve been amending for two years now. Had we managed to turn our sand into something resembling nutritious soil?

The envelope, please!

Below are my results, in full.

The reason I’m posting them for all the world to see is that there are some things about them that I don’t understand. I have questions. My experience here at Starving has taught me that you often have answers. So here goes.

Q: Organic Matter. Ours is 11.8%, and the desirable range is 4-8%. Way high, in other words. Did it get this way because we tilled a whole lot of compost into that sand? Once you have too much organic matter, I can’t imagine there’s a way to get it out, so I assume you fix that problem by adding things that aren’t organic matter. It’s like if you oversalt the soup, and you add more stock and vegetables. Is this a problem that will go away of its own accord if we keep amending according to recommendations, and don’t add more compost? But aren’t you supposed to add compost?

Q: Buffer pH. I understand about pH, and that 5.6 is too acidic. Our hoophouse soil probably got this way because we grew potatoes in part of it last year, and mulched them with a mix that was high in pine needles. The recommendation we got was to mix 20 pounds of ground limestone per 100 square feet of garden (the hoophouse is about 150 square feet).

But the explanation included in the result says that buffer pH “is a measure of the soil’s capacity to resist pH change after lime has been added.” It goes on, “The extent to which the buffer pH is lower than 6.8 is proportional to the amount of limestone needed.”

Great. So not only do we have a garden that’s too acidic, we have one that’s resistant to change. Just what I need – pushback from my dirt.

While I understand all the words, I don’t really understand what it means. What makes soil resist having its pH changed?

Q: Cation exchange capacity and base saturation. Per the information that came with my test results, CEC is “an important measure of the soil’s ability to retain and to supply nutrients.” This one had me at a loss until I read this excellent explanation by soil scientist Michael Astera. Now I know, among other things, how to pronounce ‘cation.’ It’s not like a vacation without the va. It’s like the chipmunk who’s blissfully unaware that there’s a ‘cat eye on’ him. (Stupidly, although I know what an anion is and that it isn’t pronounced like companion, I never figured out that cation was the opposite.)

I also understand that the CEC is simply the extent to which your soil can attract positively charged particles. Specifically, those associated with calcium, magnesium, potassium, and sodium. It gets a little more complicated when we start talking about base saturation. I’m not even going to try to figure out whether “saturated” means that all those negatives looking for positives are hooked up. I’m just worrying about the percentages.

The UMass materials say that a balanced soil is 70/12/4 (calcium/magnesium/potassium), and my new favorite soil scientist (everyone should have one!), Michael Astera, says 65/15/4, but adds that these are only rough guidelines.

We’re at 47/13/3, which is clearly low on calcium. Here’s the question – if we amend with limestone, will that solve the calcium problem?

Q: Micronutrients. We’re low on almost all of them, from boron to zinc. The only one we’re anything like rich in is sulfur. While it’s nice to know that we’ve got brimstone, in case we want to add fire and simulate hell, I’m worried about the rest of the micronutrients. Do we correct for them?

Q: Chicken poop. We’ve got a barrel full of beautifully composted chicken poop. I think that should correct our moderate nitrogen deficiency (and we also have phosphate for the phosphorus). But will it help with other things as well? And, since the poop composted in pine shavings, will that make the soil more acidic?

Okay, I’ll stop now. If you have any insights, comments, or, even better, actual answers, I’d very much appreciate them. If this goes well, I’ll think about tackling other fascinating subjects, like the burning potential of various kinds of hardwoods, or the pros and cons of annual bottom painting (those are boat bottoms, for those of you who thought that might not be so dull).

Meanwhile, it’s all soil, all the time.

7 people are having a conversation about “Soil Q&A

  1. insights and comments, maybe. Probably no answers. But I have a question for you. When you filled out the form for your soil test, did it ask you what crops you planned to grow in the soil? That can affect how they grade your soil, in the following sense. Say your calcium content is X. If you want to grow blueberries in that soil, X calcium will be graded “very high.” If you said you want to grow pumpkins there, X calcium might be medium. I’ve had my answer to that question produce really weird looking bar graphs on the soil nutrients on the same soil from year to year.

    Also, if it turns out that limestone is something you want to add, consider crushed eggshell, which I presume you have a ready and free supply of. A couple of different state Extension offices have studied this and say it can work. Doesn’t work quite as quickly, pound for pound, but the effect lasts longer.

    Finally, I really doubt that excessive organic matter on that scale is a problem. I’m fairly sure there are (or maybe were) healthy ecosystems with comparable levels of organic content. I’d guess the extension office doesn’t think it’s feasible to get conventional agricultural soils much above 5% OM, so they picked 8% as an “aim for” point.

  2. I am currently reading Steve Solomon’s Gardening When It Counts: Growing Food In Hard Times, and here’s what he has to say about lime: “There are three types of lime, wihc is ground natural rock containing large amounts of calcium. ‘Agricultural lime’ is relatively pure calcium carbonate. ‘Dolomitic lime’ contains both calcium and magnesium carbonates, usually in more or less equal amounts. ‘Gypsum lime’ is calcium sulfate. (Do not use quick lime, burnt lime, hydrated lime, or other chemically active “hot” limes.) If you have to choose only one kind, it probably should be dolomite, but you’d get a far better result using a mixture of the three types. These substances are not expensive if bought in large sacks from agricultural suppliers.”

    Unfortunately, he doesn’t really say anything about improving sandy soils (clay soils, yes, but sandy soils, no) except to say that you really only need to worry about the seed bed, and to address that add compost and well-rotted manure.

    He does have a formula for complete organic fertilizer, though, because he is more concerned with the balance of nutrients more than anything else. You might want to see if your library has the book and take a look at it. I’m not one hundred percent convinced he’s one hundred percent right- the book is about gardening in hard times, after all, and then he tells you to get all this stuff. What if the hard times include not being able to get that stuff? I’ll keep reading it, and I think he has good information; it just doesn’t all make sense, though.

    To answer your question about the chicken poop/pine needle mixture- yes it will probably add more acid, but don’t forget you can ‘sweeten’ your soil with wood ash, preferably hardwood ash. Since it’s spring and nearing the end of burn season, I’d tell everybody I knew to save me their wood ash. And don’t forget that wood ash is good for keeping varmints from digging up all the fish guts that I know you’re putting in your garden. Speaking of fish; oyster shells? Wouldn’t rinsed and crushed oyster shells be a good source of slow-release calcium? It’s just an idea….

  3. I agree with Kate – usually ‘soil analysis’ is a recipe for however many tons of agribusiness crops you intend to harvest.

    Too much organic matter – I think that is subjective. Organic matter degrades over time, and over being worked. Simply tilling the soil reduces the carbon content in open fields.

    You are relying on compost for providing micronutrients as well as basic carbon and microflora substrates. The only downside I see is the pH – you don’t want the pH working against your garden plans. I would downplay the amount of compost – or at least work the compost really, really deep, until you get the pH balanced better.

    About the buffer pH. Ideally, when you add the limestone, the pH of the soil would be balanced at 6.8. Any little spot or moment when the pH dropped to 6.6 – the increased acidity would interact with a bit of adjacent limestone, incorporate a bit more calcium into the soil, reduce that bit of limestone, and restore the pH to 6.8. I think the ‘buffer’ being measured is the amount of unconsumed limestone that is ready to adjust for changes.

    Thus, you would add limestone for the difference between 6.4 and 6.8 pH values. The soil pH is apparently an interesting number that is a function of current crops and composts, but not a persistent chemistry indicator.

    Just wondering – are you planning to drop a handful of pennies or stripped copper wire snippets into the next compost batch? (for the low copper reading) Will you pick up a bag of Ironite, or just sift some iron filings into the compost?

    It looks like the pH is the big surprise from this report, along with the moderately weak calcium (probably these are related). How to use this report will depend on who you as. Take it to the local elevator, and they have the standard megafarm application rates hanging on the wall. Ask someone focused on garden produce or flowers, and they will likely answer in terms of what Wal-Mart has on the shelf. Lowe’s had a simple meter with a 10 inch peg for soil pH testing (also one for soil moistness, for houseplant watering monitoring) that works reasonably well and is immediate and long lasting. I like mine.

    Egg shells, broccoli, and oyster shells do indeed convey calcium into the soil. Growing broccoli, cabbage, etc. also removes calcium from the soil.

  4. The pH may be the reason all your micronutrients are low as well. They can be bound up in inaccessible form at low pH. Hopefully they are bound to all your great organic material so that when you get the pH fixed they will still be there in the soil for your plants. Organic matter is particularly important in sand-based soils, since they can store cations – clay bonds with cations too but likely you have little of that. So if I were you I would not try to reduce the organic matter content in your soil. I can’t really imagine why 11% organic matter could possibly be a problem – many crops are perfectly happy growing in the compost pile in 100% organic matter.

  5. Kate — They didn’t ask about specific crops, but I did specify that I wanted to grow vegetables (as opposed to flowers or other things). The eggshell suggestion is great — I’m just afraid that, since they’re suggesting 30 pounds of limestone, I’d need a whole lotta eggshells to get the equivalent. We do, however, put eggshells in the compost, so perhaps that will help ward off the problem in the future.

    Paula — I’ll add Solomon to my reading list and wood ash, which we have in abundance, to the garden when we add the chicken poop. Thanks for the suggestions

    Brad — THanks for the buffer explanation! That makes sense to me. And I didn’t realize you could actually up your copper and iron by adding actual pieces of copper and iron. I figured they’d just sit there, inert, but I suppose they must leach some of their essence into the soil. As for the local elevator, we don’t have one, but we do have garden stores with the pH gizmo. We’ll get one so we can keep track. Thanks for your help!

    Jen — Not worrying about the organic matter seems to be the consensus, so I’m going to stop worrying about the organic matter (although I’ll confess that it wasn’t keeping me up nights). And I’ll be you knew how to pronounce cation before I so usefully explained it.

  6. I just had my soil analysis back from our agronomist and it consists of the soil pH, and the P,K and Mg available. That’s it. One line, three measly boxes of basic info. Your results from UMASs as so much more detailed. Now I have analysis envy.

    N and organic matter are both transient. You can’t add too much organic matter. N is like Vitamin C – the soil just loses what it can’t hold in the water. This is bad as it can cause a build-up of nitrates that affect water supply (feed weeds, smother fish) Add it in spring and not in autumn.

    Oh, organic matter should be well rotted for a reason: CN ratio. If you add too much woody or high carbon material (e.g. woodchip bedding from chickens) the carbon molecules lock up available nitrogen so the plants can’t get it.

    I can’t really add anything. Your post, including questions, informed me. I love the soil geek stuff.

  7. Oh..i don’t think i’d like to receive one of those soil test results…too much detail which would scare and worry me no end. We grow about 8 acres of non certified organic veg here in Australia and this is what we would do for some of your issues – I hope it helps you a little.

    Calcium – burn up a pile of oyster shells, pound them and then sprinkle them on the soil. (we used to use dolomite but it is so much more fun if you can produce it yourself and who doesn’t like a good fire?)

    Calcium – blood and bone in liquid form. we spray this directly on the leaves of plants to help them with soil deficiencies. the plants thrive and as the blood and bonemakes it’s way down through the plant into it’s roots, it then helps to feed the soil.

    Mineral deficiencies – basalt rock dust. we buy cheap rock dust here and sprinkle it all over the farm. basalt dust is the best and contains many trace elements and nutrients your soil needs.

    we also burn bones for phosporous, make smelly compost tea, and try a million crazy things all because we reckon if mother nature did it without a sop for all those years, then we should be able to go without as well.

    good luck with it all.

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