Garden experiments with hard soil

Josh CentersContributor - November 11, 2020

Skills

I had a pretty good garden last spring, but my hard, rocky clay required a lot of mechanical intervention to be workable. I decided to take the fall to experiment with making fixed beds with handtools like my broadfork and pointed hoe.

My goal was to make the nice borderless raised beds you see in books by Steve Solomon and John Jeavons. That just doesn’t seem to be possible with this soil. I’m experimenting with heavily amended narrow strips, which are easier to dig out. I’ve made a video showing some of what I’ve tried.

David the Good has posted a video from a friend of his with a similar soil profile to mine, who tried the lasagna gardening method. She piled on straw and woodchips and let it rot for a couple of years. In theory, this builds a new soil layer and softens the soil underneath. While she got that nice upper layer, the soil underneath is still hard as bricks and limits the size of her vegetables. She’s now trying to break that layer up with a broadfork.

I’d be curious to hear if anyone has had success gardening with hard soil. If my strip plan doesn’t work, it looks like I either need to buy a tractor or construct raised beds.

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I live in TN so I understand the rocky soil. For a few years I tried straw bale gardening which was helpful because once the season ended the bales broke down and added a layer of soil. Sounds like you have a really large garden though and straw bales won’t cut it. 

 

The soil in my garden area, on top of the hill, is mostly hard clay.  When I built some raised beds, I brought in a big dump truck load of garden mix, which is kinda a mix of hardwood mulch & potting soil.  I went down to my bottom pasture, which is sandy loam, and brought up many bucket loads of sand using the loader on my tractor.  I then broke up the soil with a sharpshooter, threw some of the sand & garden mix on top, and then tilled with a big powered hand tiller that I rented for the day.  Once that was mixed well, I continued the process several times.  Worked great for me.

In a big area like yours, I’d ask around & see if you could hire someone that has a tiller on their tractor.  Those things can chop up anything into a fine powder.  Then have him use his front bucket to bring in sand & garden mix and till it all together.  When done, you should have some mighty fine soil.  Won’t be cheap, but it will solve your problem. 

Cheaper way is as I did, and build a few raised beds, just using a single 2×6 for the sides.  I use cut off pieces of 4×4 in the inside corners to screw the boards into.  If the land isn’t level, I use a bit longer 4×4 to also act as a leg on the low side, so that the bed is perfectly level.  I use 10′ long boards, where I cut one in half for the ends and 2 for the sides.   I screw in a 5′ 2×4 across the middle, to keep the sides from spreading as you add to your soil.  If you want your bed longer than 10′, then just use 4x4s to join lengths together.  So as I said, use a sharpshooter to break up the soil inside the bed, rent a tiller & chop it up inside your bed, add garden mix & maybe sand, & till it all together.  When done the soil should be just below the level of the sides.  When doing the initial tilling, you want to go down around 12″.  That way, when finished you have around 17″ deep of great garden soil inside the bed… with most of it below ground level.  Then you can add other beds later.

 

I’ve got a few ideas for you but they’re slightly different topics so broke ’em up.

(1/2) Methods

I’ve gardened in clay soils my entire life – even once had to break up a clay soil with a jackhammer – so I understand the struggle. But I have had success with a lot of work and even more patience – it takes a lot of time.

As a basic growing medium, clay soil is not actually bad – since the particles are charged it can actually hold a lot more micronutrients much better than sandy soils. What makes it so awful is a lack of what’s called tilth – how easy is it to till, how easily can seedlings emerge, can roots penetrate or will they turn into depressing mats? Clods are the opposite of tilth, and as you well know, you’ve got clods. So what you’re really trying to do is improve the tilth. Organic matter is a way of incorporating tilth, as is aerating with a broadfork or tiller. Beyond that. . .

1) Mitigating and amending options. If you’re willing to look upon this as a long term project, you can do a lot, especially if you have the ability to use raised beds in the interim – even just a year of working on the soil can have some effect. I’ve used all of the following in various combinations and had success, but yeah, it takes time. Obviously you’ve tried/know some of this already but just adding support for the methods.

Two important aspects when amending and any tilling: 1) Do the pinch test. Clay likes to expand and shrink. That shrinkage is what creates those cement like clod chunks. To prevent clods, till, aerate, or amend your soil only when it’s at the right moisture level: roll a ball between your hands, then pinch it. If it falls apart, cracks, or crumbles, it’s good. And 2) If you add a lot of woody material, make sure you add nitrogen either as a fertilizer or nitrogen fixing plants, else you’ll create nitrogen depletion in the soil.

• Contouring or cross contouring: I’ve watched your video a few times and it looks like you might have a slight slope and maybe a hill nearby? If you do, this could work, but it has to be done before everything else. Contouring can include terracing, permaculture swales, and even your raised beds to affect water flow – the idea is partially that high areas dry out faster and lower areas collect organic matter to help break down the soil (especially if you have add organic matter too – kind of like an in-place compost bin), but mostly to control drainage so that all the clay at the bottom of the slope isn’t compacted by all the water hitting it at once. (It doesn’t take much slope at all to do this.) It’s a long term solution to consider and you need care not to affect any building foundations, but you’d be surprised at how much you can do in a small area.
  • Contouring can also be done by working with the general slope and the orientation of your beds.
• Plant deep rooted cover crops to break up the soil – here is a list that you could pick through to find something appropriate for your climate zone. Cowpeas are an excellent clay buster as are mustards and the permaculture favorite comfrey. Then chop and drop without pulling the roots out to use them as a “green manure” and mulch. There are other clay busting plants that aren’t necessarily cover crops – my favorites are Daikon radishes, yarrow, and sunflowers (sunflowers can have really deep root systems). You seem to like sweet potatoes, but some others can get much deeper – it’s worth mixing them up.
  • Watching the one video, I suspect the reason the radishes and sweet potatoes didn’t work as well is because they already had a nice comfy layer of mulch in which to grow laterally and thus had no need to punch through that clay level.
• The in-place compost “bin” – essentially just dumping loads of compostable materials with no real regard to layering or anything, dumping lots of mulch on top to protect things like kitchen scraps from becoming a pest problem, and just letting it sit for awhile. I imagine you’ve read it since you mentioned him, but for those haven’t, David the Good’s Compost Everything book goes into alllll the various ways you can do this in a much more thorough way. It looks like you’re already doing this to some degree, though.
• The trusty mattock, compost, and till method: break up the surface with your mattock/pulaski/pickaxe/hoe type tool of choice (you can use the other end to break up and crush clods, which if super bad will just be spit out by the tiller) as deep as you can. Add inches of compost and any necessary nutrients/pH amendments (possibly phosphorus, sulfur, or gypsum depending on clay type), then rototill in. It’s a lot of work and wouldn’t work on a large field, but that mattock can really break up the soil. (Broadforking before and after using the mattock helps, too. I’ve used the two in combination more than a few times.) Then, as you’ve mentioned, add more compost until you have a “raised” bed, then do the same again in the fall, and the next year. . . you’ll find that the clay soil just continually swallows all that compost. And I add worms. Lots of worms.
  • Using the right broadfork type is essential – most broadforks are not made for breaking up new ground but already broken up gardens; broadforks for new ground are not meant for aerating already prepared soil.
• Mulch heavily and water carefully. Water – especially hard driving rain – can cause compaction quite quickly. Mulch helps prevent this.
• Depending on your clay type, sunken gardens can work really well. But drainage can be an issue (they’re really great for arid areas!)- if you were to try this you might experiment with a smaller one for a year or two before risking the possibility of a large crop being flooded.
• Don’t add gypsum unless a soil test calls for calcium, and, though it seems logical, do not till in sand to improve structure or drainage – adding sand is a myth. Some people say the right kind of sand grains (angular, not rounded) will help, but that’s a myth too: the reality is that no matter the sand type over time it will create impenetrable layers for both roots and water. It’s the opposite of creating tilth. The only time this method works involves a 1:1 ratio in a pot – it’s basically impossible to implement correctly in-situ and even in raised beds.

2) Since you’re experimenting, do you have enough wood material to attempt hugelkultur? It will take time but it has an additional benefit of helping improve the soil below it. (Also I’m just a big fan of adding logs and branches to clay soils to help break them down).

3) These are the main methods that have worked for me. But frankly, at the end of the day, when I consider all the work, organic matter, and sheer amount of time invested, I’ve come to a place in life that, in hard clay, raised beds are the least amount of total expenditure in the long run, even if they cost money up front. (It’s worth considering the cost of your time in this too.) As a result my main method now is deep raised beds with a hugelkultur layer at the base. That doesn’t mean I totally give up – I usually have a few areas that I slowly work on over time in which I don’t grow anything but clay busters or cover crops, and throw extra leaves, logs and branches, kitchen scraps, and bones at. They’re “have a spare 5 minutes” areas.

 

(2/2) Really the 1st step: understanding the clay soil itself

[note: you may know this already but it’s worth throwing out for others who may not]

You mention you have a rocky clay, but do you have a detailed idea of what your soil looks like beyond the upper surficial level? This is more comprehensive and beyond a basic local extension office soil test. If you don’t, it would be worth considering getting a good look at your soil profile. It’s free! The soil maps and profiles gathered over decades by the Soil Survey provides information about what kind of soil lurks deeper than that topmost surficial layer, which can affect drainage and root accessibility. Soil maps are detailed for each county (example), and also include evaluative features of what’s referred to as “suitabilities and liabilities”, like plant productivity and irrigation.

You can investigate it online using a combination of the archived soil report PDFs (link for counties in Tennessee) and the soil survey web mapping tool, but like most land agencies the Soil Survey is severely underfunded so the mapping tool is quite antiquated. It’s still worth having a look. However, if you have a county extension (good) or USDA NRCS (even better) office in your area, an easier and probably more informative option is to simply visit them. They’ll be able to help you interpret the maps and soil profile, and what the agronomy terms mean without you needing to figure out exactly the difference between two types of loams if you may not understand what a loam is (as a hypothetical).

The end result will be a description that looks like this example, from a random soil in Clay County:

H1 – 0 to 6 inches: loam
H2 – 6 to 10 inches: loam
H3 – 10 to 16 inches: clay loam
H4 – 16 to 31 inches: clay
H5 – 31 to 84 inches: gravelly clay
[H refers to horizon and is how they classify the varying layers within each part of the different subtypes of soil beneath the surface.]

Slope: 12 to 20 percent
Depth to restrictive feature: More than 80 inches
Drainage class: Well drained
Runoff class: Medium
Capacity of the most limiting layer to transmit water (Ksat): Moderately high (0.20 to 0.57 in/hr)
Depth to water table: More than 80 inches
Frequency of flooding: None
Frequency of ponding: None
Available water capacity: Moderate (about 6.8 inches)

If this soil profile were yours it would tell you that while you’ve got some good surface loam in which to plant and then the beginnings of lots of clay about 1 foot underneath the surface that might benefit from some compost till in and some deep rooted cover crops, that you don’t have a layer of bedrock just a few feet down (important to know), and that it’s pretty well drained (that gravelly clay layer helps!). Other details in the profile would include the parent materials of the soil (a clue to the mineralogy make up and how acidic/basic it might be, and thus nutrients and fertilizer use), if the water it holds might become alkaline or have other salinity issues, and if it’s considered good for farmland or for drastically different uses. It doesn’t have to be “prime farmland” (their classification) but it’s worth paying attention to if it is better suited for some other kind of land use, which could inform the necessity of raised beds or other gardening methods. You might also discover two or more different soil types on your land, which you could then use in your planning.

It’s worth having a look at even if you think the soil has been disturbed due to development; depending on the depth of the disturbance you can still gain useful information, or even possibly discover that your soil doesn’t match the description at all so it may have been a cut or fill area. Even then there might be information (especially for a cut).

Examples on how to use it: You could discover a hardy top layer of caliche or clay that extends down ~8 inches and then a beautiful friable loam. In that case a sunken garden in which you dig down and remove the clay layer and use it to pile around the sides might be good idea even if it’s a terrific amount of work. Or you may have a perched water table (very difficult to correct) or a limestone derived clay (that’s what it looks like from your video) that extends for a considerable depth, in which breaking it up is equivalent to breaking up concrete, and raised beds are the better option to consider in both cases. It’s even possible you have a shallow clay layer with a loam layer underneath that a double dig method in which you have an opportunity to mix the two along with your organic additions could create a better medium.

Even if at the end of the day you find out that you have nothing else than the rocky clay soil as already known you’ll have a much better understanding of what kind of rocky clay soil it is and what’s occurring below the surface; understanding your soil is a key part of using it and growing it since good gardening soil is “grown”, not created. I’ve really learned it’s so much better to adapt your methods to your soil than beating yourself to death trying to adapt your soil to a specific gardening method.

Note: this is different than a soil test; there are also all kinds of DIY clay soil tests you can do if you haven’t already (ribbon tests, clump tests, drainage tests etc.) that are also helpful to understanding your clay.

 

a tiller is your best friend when getting started with any type of virgin soil – even the best black gold soil isn’t ready for the whole spectrum of crops …..

ditto on the sand for your hard clay – course as sand as you can find – tiny ground rock screenings wouldn’t hurt to get things going in the right direction ….

you can add the organics to increase the soil volume and start drawing in the worms & insects that’ll eventually complete your healthy soil complex …..

it’s a long road – get a section open to re-plant during the season >>> take the time to soil remediate first ….