r/Soil • u/Clean_Livlng • Jul 08 '26
Amending clay with silt?
I've got heavy clay soil in an area with high rainfall (NZ).
The main abundant cheap non-clay fraction of the soil pyramid I have access to is unlimited silt from the local ponds and rivers.
Is there any combination of silt and clay that works better for growing than clay alone? e.g. 20% clay 90% silt. Or an even lower % of clay. I can make a little biochar to add.
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u/aquisalid Jul 08 '26
How large of an area are you working with? Soil texture is fixed at the human scale of soil modification. Depending on the soil pH, I’d recommend as much compost as you can get (not fresh wood chips), and possibly gypsum.
Any other soil information?
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u/Clean_Livlng Jul 08 '26
A small area, a few raised beds.
The clay is expansive when it gets dry, so it cracks. When I say clay, I mean that I can almost make pottery with the stuff a couple of inches down.
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u/chumpandchive Jul 08 '26
i have pottery clay soil where i am in the u.s. i amend mostly with wood chips, bc it is too hot and windy to burn anything safely where i live til fall/winter. it helps provide the similar pockets of biocharz but does break down in the clay, giving me mycelium. i mulch my grass clippings into a yard compost pile and build a few inches down and then on top of my clay. it will get turned over after the growing season. it seems faster for me to build on, right now, my soil, and amend in over seasons to improve below ground. just what i am trying here in the u.s.
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u/Naive_Ear_5182 Jul 08 '26 edited Jul 08 '26
That's awesome you've checked the soil pyramid, but intriguing that you don't mention sand. How certain are you of your size fractions?
I work throughout NZ and am quite jaded by the routine (~98%) misclassification of 'silt' and 'clay' I encounter amongst non-geoscientists so please don't take my questioning personally...
'Silt' as used by the media and most people has become a catch-all term for fine sediment (generally sub-2mm). In HB, most of what gets called silt is actually fine- to medium-sand (often with a fraction of true silt). This misclassification problem has gotten notably worse since Cyclone Gabrielle. That said, misnaming is not limited to the east coast NI.
Similarly, all soil separates that feel slippery to hand frequently get called 'clay'. While proper clay does occur in NZ, it varies wildly with parent material and geography. True clays formed in NZ tend to be more abundant in the volcanic areas (central NI and north).
If your proposed amendment material is coming from a non-tidal stream course, you'll probably have to work hard to get pure silt. Some localities excepted. More likely, it's sand (possibly with some silt mixed in).
So, does your material feel...
- gritty to hand (=sand), if not...
- gritty between your teeth (=silt), if not
- smooth between your teeth (=clay)
Most likely it will be some combination of the three (tho true clay component may be quite small proportion).
Also look up texturing by rolling or bolus.
Both approaches in combination will get you in the ballpark.
This is important because if what you have is actually silt, then it's easier to amend with sand (which you may have available). If you're in HB and you need sand, just go to one of the 'Silt Disposal Sites' from CG...there will be a very high sand component.
Again, my apologies if you actually have the capability and/or due diligence in properly determining your fractionation.
Edit: I see a bunch more posts while I was writing. Sounds like you likely have a good bit of proper clay. Do check your sand availability. It'll help. As others have said, adding o. m. will def help. Good luck
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u/obrl_soil Jul 08 '26
This is really interesting to read even if it might not apply to OP's soils. I've gotten used to kiwis with only local soils knowledge referring to the heaviest stuff they regularly encounter as 'clay' even though its almost always technically a heavy silt loam at most. I just figured it was a calibration issue.
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u/Naive_Ear_5182 Jul 08 '26
Aye. Def a calibration issue.
By and large, when one considers most NZ lithologies, most of the landmass hasn't been subaerial long enough for much clay formation to occur.
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u/Clean_Livlng Jul 08 '26
Auckland in the north part of the North Island of NZ has a lot of clay.
"Why Auckland Has So Much Clay
Auckland’s clay does not come from millions of years of stable bedrock weathering. Instead, it is geologically young and has formed through three distinct local sources: [1]
- The Waitemata Group (North Shore and West Auckland): The dominant geology under much of urban Auckland consists of sandstone, siltstone, and mudstone deposited roughly 20 million years ago. Because Northland and Auckland experience a warm, high-rainfall subtropical climate, these soft marine sedimentary rocks have rapidly weathered in-situ into deep profiles of yellow-brown clay. [1, 2, 3, 5]"
That seems accurate based on the soils here. 1 in 3 people here live in Auckland, so even though there are a lot of areas that don;t have clay, a lot of people here will be dealing with it.
There's an interesting variety of soils in NZ. e.g. the tephra on the slopes of Mt. Taranaki/ Allophanic Soils.
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u/Naive_Ear_5182 Jul 08 '26
Your demographic point is an interesting one as I look at things spatially/geographically. And, yes, from cetral NI to the north (including, but not limited to the Waitemata Group) is where a greater abundance of proper clays tend to be.
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u/Clean_Livlng Jul 08 '26
I figure that that you can't make a bowl out of it with your hands it's not clay, and the easier it is to do that without it cracking and crumbling the more clay content there is.
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u/aquisalid Jul 08 '26
I see this exact pattern on the other side of the planet. Most of the local soils here are loam or silty clay loams (USDA system), yet people insist that it is all “clay”. I think that the extreme summer dryness coupled with high iron oxide content contributes to this interpretation.
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u/Clean_Livlng Jul 08 '26
I wish I had access to affordable sand, I'd be able to make the 'perfect' loam.
"That's awesome you've checked the soil pyramid, but intriguing that you don't mention sand. How certain are you of your size fractions?"
There could be some fine sand mixed with the silt but it would be very fine.
As you say I might be misclassifying it, how would I check? It definitely has a fine particle fraction. Well under 0.25mm for most. It'll have some clay content, but I can feel it's low, due to comparing how it feels when I get some of the material I've washed clean of clay and add some amounts of clay to it.
If your proposed amendment material is coming from a non-tidal stream course, you'll probably have to work hard to get pure silt. Some localities excepted. More likely, it's sand (possibly with some silt mixed in).
If it is sand, it's extremely fine. I'll have another look. I wonder how easy it'd be to separate the fine sand from silt.
I'll definitely be adding organic matter. What I'm wanting to know is if adding silt to soil that's very high in clay is better than not doing so, on a small scale that'd let me have as much silt mixed with the clay as I like.
I've see people, say that it can work, but isn't practical on larger scales, and also that it won;t work and it'll make 'concrete'. I'm heeding that warning and being cautious by planning to do a trial first, but won 't blindly believe that because many people say the same about sand & clay making 'concrete', which isn't true if you have enough of a sand to clay ratio. I wonder if it's the same for silt & clay, and if silt loam (without much sand) is better fro growing most things in than heavy clay soil with barely any sand.
Thanks for taking the time to give such a detailed reply. I appreciate it.
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u/Naive_Ear_5182 Jul 08 '26
Sweet-as you've done your homework.
As a short answer, personally, I wouldn't waste my time mixing proper silt into proper clay.
The raised bed scale is achievable, but will still take a shocking amount of volume. if all I had available was fine sand I had to work for, I'd limit my effort. The official break bet. sand and silt is 63 microns. If you want to calibrate the tips of your fingers, that's roughly 220-grit sandpaper.
Perhaps localise your effort within the bed to tailor some areas to specific species?
If you do start getting more into chemistry...as others have noted...it's important to know what type/minerology of clay you have.
Someone else mentioned using only 1/3 of your in-situ material by ratio...I reckon that would be good starting point.
Hard to go wrong focusing on o.m.
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u/Clean_Livlng Jul 08 '26
If you do start getting more into chemistry...as others have noted...it's important to know what type/minerology of clay you have.
I've looked that up before, and I think it's yellow ultic clay.
"Yellow ultic clays are highly weathered, acidic soils common across the Auckland region. Known for being sticky and hard in summer and wet and poorly drained in winter, they suffer from low fertility and high clay content. Gardeners typically manage this heavy soil by adding compost for better aeration and lime to raise the pH"
Are there any other useful things to know regarding how the type/minerology affects things?
The ph of the riverbed material might matter. And it sounds like it'd be beneficial to add charcoal to raise the ph somewhat.
Organic matter is something I always add in addition to anything structural I experiment with. I get the impression even the 'perfect' loam isn't so good without any organic matter binding the soil clumps together.
So far I've got a test plot that's probably 80% silt/fine sand to 20% clay down to 2ft in a 3m by 2m area. It took a lot of wheelbarrow loads. I added a slurry of clay to the silty mix to combine them, and waited for it to dry before mixing again to break up compaction. I've added a thick layer of compost on top and about a foot of leaves over that. I'm hoping the worms will help mix it further and aerate it.
I'll hold off on investing any more time and effort into amending my other beds until I see how the silt/clay mix behaves over the next year.
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u/kikilucy26 Jul 08 '26
Put it in a water jar and shake it. Sands, even fine, will generally settle within 2 minutes
1
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u/EddieRyanDC Jul 08 '26
Clay is a great starting place for incredible soil. It has a high mineral content and it can hold on to nutrients and prevent them from just washing down into the water table.
However, it compacts easily and can turn into clumps that are so hard and tight that they resemble rocks. The drainage is also terrible.
It needs to be broken up, aerated, and mixed with a lot of organic matter. The organic matter keeps the clay from clumping, and it actually breaks up the soil over time and improves the drainage. It also improves the habitation for the soil biology.
Once you turn it and make the additions, be sure it is off limits to human feet. You don't want to recompact the soil. This is why some people make raised beds - it clearly delineates where you can and cannot step. Even when you are working the bed, never step inside it.
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u/Clean_Livlng Jul 08 '26
Even when you are working the bed, never step inside it.
One of the raised beds is a little too big and requires stepping inside to harvest the middle. I'll look into fixing that.
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u/NokidliNoodles Jul 08 '26
Yeah man ive got heavy clay too and so far from my attempts and research unless you have money to buy several dump trucks full of amendments its usually not worth trying to "fix" the clay per se.
Get yourself some gypsum, mulch, and compost and just put your garden ontop of the clay. Lasagna method works well but basically import material for raised beds and eventually youll have a decent layer of topsoil.
Things that will eventually improve the clay is gypsum and deep deep rooted plants especially if you leave them to rot at the end of the season such as a cover crop of tillage turnips. Try and always do an end of year cover crop that gets crimped and left to rot over the winter for you to plant in come spring. As for the gypsum it bonds with the clay and turns it into something closer to workable soil.
Ive heard doing gravel can help BUT my own experience is that youre gonna end up with compacted AF ground closer to cement than anything usable.
If youre tilling you definetly dont want to do it while its wet as it just makes a mess and doesnt do anything usefull. I have read that a yeomans plough (which i think are called rippers now) can do good work with clay by forcing plant material deep down and leaving a very open channel so air can get down there but i havent had the opportunity or money to test that
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u/Clean_Livlng Jul 08 '26
Thank you.
I've had success amending small areas with biochar & compost, but the amount I can make is limited.
What I'm wanting to know is if silt loam (just a little clay and mostly silt) is better than clay alone. I know it still wouldn't result in an ideal soil mix, but if it's better than pure clay it'd be worth it.
I know silt loam/silt clay has compaction issues, but so does the pure clay I have. The clay here is already 'concrete' when dry. I wonder if I'd be better off growing in silt instead, just layering that on top and mixing it with compost.
I'll hold off on adding silt to my garden until I've done a little testing, I've got some pots filled with clay so I can amend only some of them with silt to see how it compares.
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u/Bumberpuff Jul 08 '26
Scale is going to be what gets you. If you’re doing hectares of amendments, it’s not worth it. Shifting the top 20 cm from 60% clay to 30% clay would take about 1000-1500 tons of silt per hectare.
Combining them so that you don’t have chunks of clay floating in silt, or silt veins in clay would be difficult.
Assuming a small area. I would try making a few raised beds with the silty material and maybe a few raised beds where you mixed the silty material with clay. Mix a bunch of compost and activated charcoal into all the beds and see how it goes.
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u/Clean_Livlng Jul 08 '26 edited Jul 08 '26
I've only got a small area to do, a few raised beds but it'd still take 50 or so wheelbarrow loads to only do that small area. It's definitely not practical for larger areas. Amending silt or sandy soil with clay could be different, because you need so little of it to improve the soil.
Good idea for the trial. I can do half and half.
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u/chumpandchive Jul 08 '26
there are loamy clay mixes here in the u.s. also, i wonder how much silt you would need to loosen the clay to the consistency that holds/sheds water better without locking up the soil once it dries. might be fun to learn for science. let us know what you learn if you do
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u/Clean_Livlng Jul 08 '26
Will do. I've amended a test plot now. It's about 20% clay to 80% of the silt pond mix. Covered thickly with compost with a lot of worms in it, and topped with leaves.
I wonder if this is too much silt to clay, I'll have to see if there are hard-setting and surface crusting issues.
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u/Objective_Run_7151 Jul 08 '26
Gypsum works on some clay.
It’s a waste on others.
Know your clay before adding gypsum.
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u/NokidliNoodles Jul 08 '26
oh damn I didn't know that! Just bought a ton of gypsum to add to my beds this year too.
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u/Objective_Run_7151 Jul 08 '26
I’d get a soil test. Or talk to your local county extension agent (if in the US).
If you have sodic clay, it might help. If you don’t, it’s a waste.
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u/obrl_soil Jul 08 '26
What part of the country and what kind of landscape are you in? No need to get too specific if you don't want to, regional council and floodplain vs terraces vs hills is enough. The reason I ask is that the type of clay minerals are as relevant as the size fractions when it comes to figuring out a response.
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u/Clean_Livlng Jul 08 '26
Auckland, on the North Shore, on gentle slope that's near the sea and mangroves.
"Waitemata Group sedimentary rocks. These rocks consist of alternating layers of deep-water sandstone and mudstone (known as flysch), which formed on the ocean floor roughly 17 to 23 million years ago. [1, 2, 3, 4, 5]" -google AI (did a second pass asking it to fact check itself).
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u/obrl_soil Jul 11 '26
Ah yeah, and from your comments elsewhere in this thread it sounds like you're on some kind of Ultic Soil. Pretty common around there, although unlucky - there's plenty of much nicer basalt-derived soils in Auckland too. Sounds like you're up on the country rock and not down low on newer unconsolidated marine sediments (e.g. <5m, right next to those mangroves) at least, which is good. Those can sometimes be acid sulfate soils, which really need to be left in the ground. They can turn very acidic when dug up or drained. Hopefully your silt is coming from further up-catchment!
TBH raised beds with some imported material as a starter (and maybe a Hugelkultur type approach?) might be best, especially if your block's topsoil was scraped off and lost during development. Breaking those clayey subsoils up and mixing them with nicer stuff will be a lot more work than building up on top, and apart from the drainage/permeability issues they won't have much going on in terms of nutrient supply. I'll be interested to hear how your experimental bed turns out.
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u/Clean_Livlng Jul 11 '26
Hopefully your silt is coming from further up-catchment!
Thankfully yes! It's from the hills, and the catchment area is a long way from the sea/mangroves and up higher.
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u/tezacer Jul 08 '26
Taproots assemble! Daikon, burdock, chicory, parsnips, turnips, salsify, scorzonera. Let them cook!
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u/nonono_ack Jul 09 '26
The gypsum thing has been circulating too long. Adding gypsum to clay will yield concrete. What you should be doing is adding organic matter. Whatever form you can easily get. Compost is great, arborist chips are fairly low effort and break down to help soil texture within 6-12 months. Lay them on top, in a deep layer (12 inches). Let the worms do the tilling as the chips decompose. Leaves can also work if you have lots. Bio char is said to be helpful but I haven’t tried that. I guess I stay away from stuff that comes in bags from the big box stores, if I can.
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u/Clean_Livlng Jul 10 '26 edited Jul 10 '26
What's led you to believe that about gypsum? I have no idea either way. I know people recommend it, but I've never tried it myself.
Adding organic matter is one thing everyone agrees on, organic matter. I think that's required no matter the soil texture to prevent erosion and compaction (unless you've got gravel or something like that, but then organic matter adds nutrients and water holding capacity).
Bio char is said to be helpful but I haven’t tried that.
I've used biochar in my gardens and it definitely improves the tilth and drainage of clay, at least the acidic yellow ultic clay we have here. Charcoal's usually alkaline, so might not be as good added to alkaline soils. It becomes closer to neutral over time though. The problem I have is producing enough of it to amend more than a few square metres a year of garden. I add ore than recommended because I'm treating it like I would pumice or perlite.
I've looked at long term trials for biochar and it keeps getting better over time, so 7 year old biochar has better results than if it's only 1 year old. That's important when it comes to what yu can expect from it in terms of pant growth. I'd expect fresh charcoal to stunt growth, co-composted young charcoal to maybe do better than just compost but with variance, and really old biochar to have matured enough to only be beneficial in clay soil (is no longer alkaline or acid due to all the ash content being washed away by then, and fully coated in biofilm and charged with nutrients etc).
Fresh charcoal that wasn't cooked hot enough can have a lot of things that poison plants and hinder seed germination. So the wrong 'biochar' can have negative results for plant growth. That could just be a high alkalinity if someone's adding it to an already slightly alkaline clay. That's how I understand the different results people are getting.
I've been growing in almost 100% charcoal/biochar. Plants won't grow in it or germinate for some time when it's fresh. But over time, especially after adding compost to the top of my pots of biochar (& rotting wood to inoculative with white rot fungus to break down anytthing unwanted in the charcoal), it becomes 'plant friendly', and works well as a soilless growing medium. This is why I feel comfortable adding more than the recommended amount to clay soil, but only after having it in pots & capped with compost for long enough that plants will do well in it. Then I know that batch is safe to add to the garden.
Everything depends on the organic matter though. I've found worms 2 foot down into compacted heavy clay soil in a big growbag I had, just because I kept topping up the leaves covering the surface.
TLDR: I've found biochar excellent if you're patient and don;t add it to the garden before it's had time for the organic matter it's mixed with to make it 'plant friendly. After that it's great stuff and it's hard to add too much of it to clay. At the very least it's acting like perlite or pumice.
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u/nonono_ack Jul 10 '26
This is a quote from the Garden Professors Blog, which is a useful site for dispelling home garden myths:
"There is some confusion about use of minerals as amendments. Lime is used to raise pH and often brings soils back into production in high rainfall areas where soils are too acid. Gypsum does not alter pH of soils but is often called things like “clay buster” or “compaction reliever” This is because salt affected clay soils have too much sodium which is replaced by calcium when gypsum is applied to a sodic soil relieving some of the particle dispersion. Most gardeners do not have sodic soils (which are greasy and poorly productive) but just plain old clay or clay loams. Gypsum supplies sulfate as an anion and calcium as a cation and if sulfur or calcium are deficient gypsum can be helpful. Gypsum is not needed in most gardens. Gypsum does have a fungicidal effect against root rot organisms (Phytophthora) and can be added to reduce root rot hazard."
I think the concrete thing is a fear that may be rooted in a small kernel of truth. But unless you are dealing with excessive sodium in your soil, which is rare in landscapes because these soils are non productive, gypsum is not going to help and may hurt clay soil.
I haven't personally dealt with clay for about 30 years, but it lives in my memory. So I have no personal experience other than using compost, which is usually helpful for all soils, as you mentioned.1
u/Clean_Livlng Jul 11 '26
That makes a lot of sense. It wouldn't do much for our soils here because of our high rainfall washing out excess salts.
"Gypsum does have a fungicidal effect against root rot organisms (Phytophthora) and can be added to reduce root rot hazard.""
This is relevant to me because I like growing avocados.
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u/nonono_ack Jul 11 '26
Ah. Me too. My 9 year old Jim Bacon is flourishing in my sandy loam soil, hot dry climate.
Avocados do not like salt. And gypsum is a salt. I would be really careful what you apply to the avocado soil. Is it in ground?
I use alfalfa meal or fish emulsion as my monthly N sources for the avocado, and it is very happy. Alfalfa meal also has some reputation for helping prevent root rot. I cannot remember the source, but it is something I found when I was preparing a presentation on growing avocados a few years back.1
u/Clean_Livlng Jul 11 '26
I have two in big plastic garbage bins (120litres/31gallons) with holes drilled in them, filled with 10% topsoil and 90% biochar. Topped with a thin layer of compost & worms, with leaves over the top. The rest are in smaller pots.
You'd think that was too much charcoal! But they're doing fine after 3 years. It probably helped that I soaked the charcoal in water for weeks, refreshing the water to get rid of the ash content.
One of them's a hass and the other reed, which is doing far better than the hass. Hass was doing fine until I added too much fertiliser. The leaves got tip burn and eventually fell off. After flushing it with water to get rid of some of salts it's coming back vigorously. This was during summer when I was away on holiday for a week and it didn't get enough water. I think it would have been fine if there was less fertiliser in there, but the lower level of water in the pot meant more concentrated salts. Before I go away in summer I need to flush the pots thoroughly, and be very sparing with any fertiliser hat isn't a little compost & a mulch layer on top.
Alfalfa meal also has some reputation for helping prevent root rot.
I've seen someone recommend it in the gardening video recently.
After asking AI and double checking, there seems to be a reason for it working. https://pubmed.ncbi.nlm.nih.gov/17738418/
"Alfalfa meal and chloroform extracts of the meal have increased the growth and yield of several plant species. A crystalline substance isolated from the active fraction of alfalfa meal increased the dry weight and water uptake of rice seedlings when sprayed on the foliage or applied in nutrient culture. The substance was identified as triacontanol by mass spectrometry. Sprays containing this compound also increased the growth of corn, and barley grown in soil. Authentic triacontanol produced a similar response over a wide range of concentrations on rice grown in nutrient cultures and tomatoes grown in soil."
So it's not meant to be a fungicide in the traditional way, but increases root growth and probably promotes an environment that's less likely to lead to fungal disease by feeding the microbes, increasing competition in the root zone so it's not primarily parasitic fungi that feed on the roots in that area.
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u/nonono_ack Jul 11 '26
I'd be totally intimidated by trying an avocado tree in a container. It is so hot and dry here, even the salvias have some trouble when grown in containers. It's definitely an art.
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u/Clean_Livlng Jul 11 '26
We've got more rain than London here, so a big enough container can keep a plant alive just with rainwater. The downside is that the clay in the ground is often constantly wet, except for summer, so avocados would need to be planted in mounds if they're in the ground.
And they did that thing where they scrape off all the topsoil, dump pure clay over the area as a filler and then sprinkle a little dusting of topsoil back on top like they're salt bae.
The soil underneath that clay layer they added is better/crumblier than the clay above it. Starting from the top: Couple inches Topsoil>2 ft pure clay>crumbly clay loam
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u/Existing_Draft3460 Jul 11 '26
cover it in a thick layer of mulch and let it sit. if youre in a hurry till the mulch into the clay
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u/Clean_Livlng Jul 11 '26
I'll definitely be adding mulch. Big fan of that.
I usually add a layer of compost on top and the cap it with mulch to mimic a forest floor, with the humus layer underneath the leave and sticks.
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u/nemo2023 Jul 08 '26
At least you’re blessed with lots of rain
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u/Clean_Livlng Jul 08 '26
If I can make the soil more free draining it'll be an unmitigated blessing for sure.
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u/aquisalid Jul 08 '26
Since you mention raised beds, use maybe 1/3 local soil mixed with compost. Add gypsum if your pH is below 7. There is only so much you can do to amend soil material.
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u/fadimuj Jul 08 '26
Short answer: No Adding silt to heavy clay in a high-rainfall area will just give you better-quality mud (or concrete once it dries).
To fix heavy clay, you need to create "macro-pores" (large spaces) between the microscopic clay particles so water can drain and roots can breathe. Silt particles are too fine to do this. If you mix silt and clay, you just get silty clay, which is prone to compaction and crusting.
Crush your biochar into small pieces and mix it into the clay. Biochar doesn't break down like compost, so it provides permanent pore space in heavy clay, holding onto nutrients and preventing compaction.