The Clay Beneath My Feet...
April 17, 2010 10:24 AM   Subscribe

Why would new, store-bought soil turn to clay in just over a year?

We have a plot of soil we use for vegetables in the backyard. When we first started on it, we noticed that an inch down was almost pure orange-yellow clay. Instead of doing raised beds, we decided to dig out the clay and replace it with store bought soil and organic matter, and one bag of gypsum. Lots of it. About 10 bags worth.

We grew tomatoes and melons and all was well until we check back in this spring and the soil has reverted back to orange-yellow clay.

My previous understanding was that clay was soil after all the organic matter had washed out and it left to be pure mineral matter. But I doubt all of the organic matter (10 bags worth) could have been washed out in a year. And then it happens to be what looks like the same type of clay (orange-yellow)?

Could there be something else happening? Maybe where clay seeps up from beneath? Or if there's not enough drainage does something happen to soil to expedite the process? Something to do with acidity?
posted by destro to Home & Garden (15 answers total)
 
That's very strange. Do you mean that there is clay one inch down again? Where are you located? What led you to use gypsum as an amendment? When you say one bag, are you saying one bag of gypsum for the whole plot?

My previous understanding was that clay was soil after all the organic matter had washed out and it left to be pure mineral matter.


Clay is merely one soil type and is the result of the geologic process in your area. It's a long-term process, not as simple as "washing out" so I don't think your added materials could have washed out in one year.

Are you sure it's clay? You might be on to something with the drainage idea. If the clay layer is greasy and non-homogenous, maybe it's actually a layer of muck soil? I find it much easier to believe your added organic materials could have, under anaerobic conditions, contributed to a layer of muck if the area is poorly drained.

That's a really weird one; I'm grasping at straws.
posted by werkzeuger at 10:55 AM on April 17, 2010


How much was "10 bags worth?" How large an area are you talking about, or, more pertinently, for however large the plot is, how far down did you dig it out?
posted by rhizome at 10:59 AM on April 17, 2010


Did you mulch around the plants within the course of a year? If not, that may have helped.

Obviously, your yard has very poor drainage. Your plan to add all that soil and gypsum was good, but it probably would have been better to do that in addition to building raised beds. (Kinda expensive, I know. I hope you compost!) Gypsum, as you may know, dissolves in water over time. You can't count on it becoming sand. It is conceivable that it would all be gone within a year, leaving the existing quality soil to "wash out" into clay. Sand and pebbles may have been a good addition to your plot, and that's another reason I think raised beds may be worth the effort in your situation. I think it actually might be possible for 10 bags of quality soil (approx. 40 lbs each, I'm guessing) to lose its nutrients / organic matter within a year- if there were enough tomatoes and melons planted in it and you live somewhere below sea level or close to it. Also, how many years you've been planting in this particular plot makes a big difference. And some vegetables should not be planted where certain other vegetables have been planted the previous year. (If you're interested in that, I can send you a list of groups of veggies that don't belong together in consecutive years.) But your geographic location is probably key to the answer. If you haven't already, I would ask your local extension office.

If you don't get a good answer here (or maybe even if you do), I would Google [your zip code] + agricultural extension office, and look for info there. If that info isn't in a FAQ list, there is probably a toll-free number you can use to connect to someone who can answer this with confidence. I would think it is a fairly common problem, especially in some areas.
(Please excuse my rotten grammar.)
posted by Hdog at 11:20 AM on April 17, 2010


Oh, and I didn't read werkzeuger's answer before I added mine. I think werkzeuger makes a great point by asking how much gypsum you added versus the amount of soil. Definitely makes a difference.
I'm not as surprised at your finding yellowish-orangish clay only an inch under the dirt because I used to live in South Carolina. Not just that, but in a region that was aptly named "The Lowcountry". What you described is not uncommon there. Even if it is uncommon for where you live, I would start searching your local extension office for answers.
posted by Hdog at 11:27 AM on April 17, 2010


My folks had bad clay soil in their garden and they would put till in a fresh layer of peat moss and manure every spring. It took about a decade before it seemed to make a real difference.
posted by bonobothegreat at 11:35 AM on April 17, 2010


The space I'm referring to is about 8'x2' and we dug down about two feet. I might be off about the ten bags of dirt, but it was a lot. Somewhere in that range.

werkzeuger: Maybe it's muck soil from poor drainage. I don't know how to test the difference.

Hdog: we did mulch around plants over the course of the year using cocoa shell mulch (love the smell). We only used a small bit of gypsum we had, so it probably wasn't worth mentioning. Maybe we needed more?

I like the idea of contacting the extension office. I feel like this is something a specialist in our area (DC) would readily know about. DC is swampy and humid so maybe it's even lower country.
posted by destro at 12:40 PM on April 17, 2010


Remember that clay is essentially just extremely fine particles. I'm surprised that you see no difference, but if your yard has a lot of moisture movement through it, it is possible that the clay fines have permeated back into the soil you replaced from below and surrounding areas. Geotextiles can be a solution for that, but then you'd need to add enough soil on top for the root systems of anything you want to grow = back into raised beds. Or, as noted previously, just keep amending with better materials and the area should improve over time. The organics might still be there, but harder to see. Imagine a clear tube with large brown black marbles that represent the good stuff you added. Now imagine pouring in a bunch of yellow-orange bee-bees and what it would look like. This could be what is happening on a much smaller scale.
posted by meinvt at 1:10 PM on April 17, 2010


Yep, extension agent is the way to go. Maybe local garden clubs too?
posted by werkzeuger at 1:28 PM on April 17, 2010


You can test your soil composition using a jam jar - I'm not sure that this would distinguish between real clay and fine particles of soil, but it might be worth a try.

I think perseverance is key, whatever. We're in an area with heavy heavy clay and in the 3 years we've had our garden we've dug in loads of composted kitchen and garden waste. We've also added quite a lot of horse manure. It really makes a difference. We're not on perfect soil yet, but we're growing more veggies each year. On the horse poo front, there are a few riding stables near us where we can drive out and fill the boot of the car with bags of fertiliser for free - you might want to look into this. Much cheaper than going to a garden centre.
posted by handee at 2:29 PM on April 17, 2010


Hdog: we did mulch around plants over the course of the year using cocoa shell mulch (love the smell). We only used a small bit of gypsum we had, so it probably wasn't worth mentioning. Maybe we needed more?

Gypsum doesn't do a darn thing to break up clay soil unless it the soil is sodic. In that case, gypsum works because the Calcium sulfate molecule will displace the NaCl molecule, and then the sodium molecule can be leached (if the soil actually drains). Yeah, this is contrary to what any vendor of gypsum will say, and that's because they would like to sell their gypsum to unsuspecting gardeners. The calcium is a good thing, but most clay soils are usually well supplied with calcium.

Gypsum, as you may know, dissolves in water over time. You can't count on it becoming sand.


I'm not sure I understand this comment- gypsum will never be sand. It's an entirely different mineral.


What happened with your sunken bed is that all the clay surrounding it is easily dispersed in water, and it filled up your sunken bed. As mentioned above, better to make raised beds filled with good soil (not just compost), or make raised beds mixed with your native clay and compost 50/50, plus that amount again good topsoil. Don't add sand unless you want to make cement. Compost helps clay particles aggregate due to humic acid produced as the compost is broken down by soil microorganisms, and that eventually allows for greater porosity plus really excellent nutrient holding capabilities. However, it's not good to grow plants in compost alone- soil is a mineral component plus organic matter plus organisms. Compost can't give plants everything they need, and it can easily be too acidic for all nutrients to be available.
posted by oneirodynia at 6:45 PM on April 17, 2010 [1 favorite]


I've read that sand improves heavy clay soil. Definitely get the Ext. Service to do dome soil testing; and check for lead while you're at it. They're really knowledgeable and helpful.
posted by theora55 at 7:19 PM on April 17, 2010


Sand + clay =cement. What happens is all the clay particles (which are charged, so they all align and stick together in ways sand and silt particles don't) continue to stick together, but now there are sand particles among all the clay particles, making it much more structurally sound.

The clay particles themselves need to be encouraged to form aggregates in the soil, rather than a plastic mass. You do that by adding organic material. This is how people who work with these soils professionally remediate them.

Sure, if you add lots and lots of sand, you can end up with sandy clay soil that is crumbly. You would have to add as much or more sand as you have clay, and what is likely to happen is that the colloidal clay particles will eventually wash through the sand, leaving a layer of sand on top of a layer of non-draining clay.
posted by oneirodynia at 8:35 AM on April 18, 2010


all the clay surrounding it is easily dispersed in water, and it filled up your sunken bed

This doesn't sound right. Maybe I'm wrong, but the clay doesn't seem to easily disperse in water at all. If anything, it seems to block water from draining. Plus our bed sunk by a good 2 inches (possibly from the new soil compressing or from growing plants that we eventually removed and ate), while the surrounding area hasn't sunk by any noticeable degree.
posted by destro at 8:56 AM on April 18, 2010


We only used a small bit of gypsum... Maybe we needed more?
Well, I think that you are right in that you need some substrate addition. I'm not sure exactly what that addition should be, though. I think anything that allows for more drainage and looser dirt should make it easier to produce vegetables. (Of course, too "sandy" and you're plants won't be able to hold onto moisture. That may be something you never have to worry about at your current location.)

Tilling in organic matter can't hurt. I don't know that I would rent a self-propelled tiller until I looked at drainage remedies first, though. (And that is the only way I would attempt to till that ground- with a self-propelled, and not just a cultivator.)
Also, it's my understanding that manure has to be "aged" or "well-rotted" before adding it to the ground. Otherwise I think you run the risk of nitro-burning your plants. Rabbit manure is supposed to be the exception- good to go from the start.
Good luck.
posted by Hdog at 12:46 PM on April 18, 2010


This doesn't sound right. Maybe I'm wrong, but the clay doesn't seem to easily disperse in water at all.

Well, this is what clay particles do- one of the properties that makes clay "clay" is that it forms colloidal suspensions in water. Try stirring a handful of your clay into a jar of water and you'll see what I mean. The water will become cloudy- those are clay particles suspended in it. The action of water moving through your soil whenever it rains is going to move those same clay particles with it. Since you dug a low bed, that's where all the water and whatever else it carries is going to collect. (and by "whatever else" I mean nutrients that are carried by water, and for anyone living in areas with older, lead painted houses, any water dispersed toxins. It's always best to grow vegetables at or above existing soil levels for this reason.)

If anything, it seems to block water from draining.

Well, yeah, it does this too. That's because of the polarization I mentioned in my earlier comment. Clay particles are flat and align the same way, creating surfaces that can be quite impermeable. That doesn't mean they are not dispersed in water as well. It means that when they are deposited, they form a clay pan layer.

Plus our bed sunk by a good 2 inches (possibly from the new soil compressing or from growing plants that we eventually removed and ate), while the surrounding area hasn't sunk by any noticeable degree.

Because your surrounding soil also has structural particles in it, such as silt and sand. It's unlikely that you'd notice a difference, but if your bed filled with clay as you've described, the only place it could come from is the rest of your yard. Store bought soil (or any other) will not turn to clay; clay particles are created by erosion over thousands of years and have specific properties, as I mentioned. Your organic material broke down (as several people pointed out above), and colloidal clay particles washed into your dug out bed. This happens all the time in areas with clay soils.

By the way, soil remediation is part of my profession. However you don't have to take my word for the properties of clay soil- look up your local university's agricultural extension and ask them for advice.
posted by oneirodynia at 5:53 PM on April 18, 2010


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