Sounds fine to me. But a lot of work. Do you really need to dig down deeper? What are you planting?

A few random thoughts / considerations:

-not entirely clear why you've decided to do a trench - but so be it.

-how's the drainage in context of what you're planting? MIght get a bathtub effect where water doesn't drain out. Probably not if top 12 inches is silty clay, but if drainage is a problem, trench may retain water.

-You might consider broadening the area you're amending beyond the trench and less digging down - spread your compost and all that further around to a wider area. Plants with roots and compost will slowly- very slowly - spread some of that benefit down. (Might choose some plants known for deeper roots, like tiller radishes)

-Don't know that the barrier layer of pine bark is needed. You may be just as well off adding more woody materials in a quasi hugelkultur at the bottom level - branches, logs, woody waste, whatever. "Regular" compost like kitchen waste will decompose fine underground anyway and can just go over this layer; mix and till it all in.

-Take into account that you'll get settling as the soil itself settles and stuff decomposes. Another potential advantage to doing a wider area - build it all up higher than the area around and let it settle in time.

-Mulch. Depending on what else is going on, if you have pine bark, leave on top as a mulch. You may have to do year to year, it'll decompose over time up there. Other compost good too.

I'd also be concerned about the bathtub effect. A trench dug in a slow draining clay soil and filled with moisture retentive organic matter is typically a recipe for disaster but the depth may be to your advantage in this case, depending on what you intend to grow.

As an aside, I wonder about the need to go to all that work when a raised bed in the same position would work equally as well, if not better, and require about a tenth of the effort :-)

Thanks for the comments, Armoured.

Yes, there was bathtub effect observed, once i hit the hard clay layer at about 12 to 14 inches depth. I found out, when i saturated this hard clay layer in order to make it easier to dig down further beyond the 13". About 1 inch of water took 24 hours to disappear into the hard clay. And then, i could dig for another 2 inches, with my garden fork (and stepping on it, standing on it).

Currently, the trench is 32" at top and 25" at the depth of 14". The length is about 7.5 feet, but i am planning to use only a small center part of the length for planting. Mostly this is being dug up as soil amendment routine. It was easy digging until this point (when i stand on the garden fork). It is all silty clay, until the slate like hard clay structure was found at bottom. The digging is not really necessary beyond this, except for the purpose of burying my composting material. Cornell or Texas A&M had an article i found that recommends atleast 18" of soil above the buried compost. I have racoons, squirrels, rats, and skunks that come around at night. The depth also ensures that i will have a better draining depth for plants in subsequent years.

I will follow your advice and build up a small raised bed type structure by putting in wooden barriers that raise above yard level, for another 12". I can bury this wooden planks to a depth of 12" or so and let them come out of ground for another 12" (filled with soil to 10" high). This way, i would have a V-shape from bottom to top of ground level, at the edges (because the top is 32" and bottom only 24"). This V-shape can be filled with easier draining mulch. I am following Al's method and principles for creating a high porosity soil and avoiding PWT. It seems like Hugelkultur principles would go against Al's principles regarding PWT. The water would simply hang onto the top layer, and does not drain much into the bottom logs/wood-sticks. Not sure if Tapala has commented on this elsewhere. The mulch barrier can be replaced with cardboard layers (or newspaper layers) to cut down on cost, since i intend build on this principle for a bigger area later this year.

@Gardengal,

The digging effort is mainly for composting under the required depth of about 15 to 18". I can bury the compost below 15" instead of the suggested 18". This way i can cut on the digging but not totally avoid it. The depth makes sure animals won't smell the decaying compost and dig it up. The compost materials would consume atleast 4" in this trench, because the suggested volume of composting materials is a minimum of 1.5 cubic feet, more volume recommended (only if someone wants to finish the composting process faster; doesn't matter in my case).

Thanks for the explanation, Sam. Is it fair to say that the soil mostly drains okay in normal weather, even heavy rains, it's only down at the lower level of clay that you're seeing the slow draining? Wherever the water is going to horizontally, if it drains out, that's a different issue.

My suggestion would still basically be to avoid the hard work and put more effort into adding organic matter, even up above your normal ground level. Bury the composting stuff (sounds like kitchen waste) if you want. I don't know if the wood barriers are needed, but up to you.

I'm not sure I know what Al/Tapala methods you're talking about - it sounds though like more about container planting. Different set of issues in regular ground, I'm not sure if the same cautions apply.

As for the hard clay underneath - my best guess is work from the other side. Build up instead of digging down, and let the compst/organic matter/worms/roots of plants do any work on the clay below, and leave it alone unless you really know you need to do something about it. Not that there's anything much wrong with trying to do it, but excess effort for results.

But then, I'm a bit lazy.

I would add that all the wonderful info provided by Al (and that's Tapla, btw) pertain ONLY to container gardening. You cannot extrapolate one methodology to the other!! Two very different set of circumstances apply - you cannot improve garden soil by the same means as you would create a good potting soil. And you will always have a perched water table when attempting to surface amend a clay soil....that's where the bathtub effect comes in! And conversely, you cannot use inground gardening practices or even the same materials for successful container gardening.

I agree with gardengal and armoured. Animals aren't attracted unless meat and dairy are in compost.

Look up "sheet composting".

For the sake of completion of this thread, this is the university of california (at Davis) article i am following, for this compost + planting project.

If I need to dig a deeper hole and the soil is hard, I just add a good amount of water to the hole and let it drain and soften up for several days, sometimes a week, depending on weather and drainage. Then digging deeper is easy.

Oh, that's what you're trying to do! I do that all the time. In the fall I dig a trench, fill it with yard clean up stuff, and cover it with dirt. In the spring I plant tomatoes directly into the trench. It works great. One year I just piled up a bunch of sod that I had stripped off the front yard. I just planted into the sod. I always add plenty of extra nitrogen as well.

Trench composting worked really well for me when I lived in Chicago for a couple of years too. The trench was warmer than the surrounding soil so I got good early growth in the short season.

I just want to say that while adding a bit of nitrogen when growing some crops like tomatoes can make sense, or to compost materials that are really heavy in browns (sawdust or dry wood chips), "plenty of extra nitrogen" can be outright bad - both for the plants you're growing, and also for the environment. Any 'excess' will just wash out and enter the water table and waterways and is nothing good. Go easy. (Want to say, I'm not a fanatic, add nitrogen if you need it, but too much is both unnecessary and unequivocally bad.)

Jxbrown: sounds like you're using nitrogen appropriately, and not extra.

JxBrown - thank you. I did not know that Nitrogen is volatile in soil (but not K or P). Is this true just for container soil - or even in my yard soil ? I was under the assumption that in my silty clay soil backyard, all essential nutrients would be bound up in some form of salt within the soil, in essential levels (for at least the growth of a 2 foot plant). I have several trees and shrubs in my backyard that have done well (even when i neglected them for years, without watering or fertilizing).

Most nutrients are locked up in salts in the soil, but nitrogen is both water soluble and will off-gas into the atmosphere. Nitrogen is a basic building block of protein and plants can't grow, if they can't synthesize protein. Some plants, like lettuce, have very modest nitrogen needs. Some plants, the legumes, can symbiotically with certain bacteria produce their own nitrogen. Heavy feeders like tomatoes and the cucurbits won't grow at all well without rich (i.e. nitrogen-containing) soil which means you will need to add nitrogen yearly for most soils. Soil testing labs normal don't report nitrogen levels since it is such a volatile element.

Ideally, you have trees and shrubs that are well adapted to your particular soil and climate and don't need fertilizer. Often, pushing ornamental woody plants along with nitrogen will result in excessive, weak growth and will make them more susceptible to insects or disease. OTOH, you may get a better fruit crop with appropriate fertilizers. So it depends, but most likely, your trees, if healthy, are fine.

I never amend anymore. Years of doing it all the time as told...well,I noticed that plants I put in? When I dug them out to move them?..all I saw was native soils..all my compost had gassed off I guess. So why bother?..dig your hole and backfill with what you have.

NOW,using compost on top is fine..right off it adds nutrients and reduces evaporation. But as you probably know..in 6 months THAT'S gone..and then you need to re do it.

Amazing to me that a small ground up pine tree I hauled in 90 gallon containers for a small side yard ..was not even a speck 2 years later!..as a mulch. 6" deep..gone.

Ok..Peat or potting soils mixed in for ferns..that's different.

Except for vegetables, I don't amend either. I spread compost on top of the soil and keep everything mulched with wood chips. I actually started trench composting (I thought that I invented it!) just to keep the vegetable garden looking tidy since I didn't have room for a compost pile.

Stanofh - We live in nearby towns.....Quite possibly our soil is similar and i wonder about your comment....

"Amazing to me that a small ground up pine tree I hauled in 90 gallon containers for a small side yard ..was not even a speck 2 years later!..as a mulch. 6" deep..gone."

You don't think that this pine mulch has blended into your soil, thanks to earthworms ? 6" depth gone in 2 years is surprising to me, as a newbie.

It was raw,Sam. With that nice pine smell,some sticky sap,green needles. I could almost watch it go down by the week. From nice and soft to walk on..to nothing but a few twigs.

Bacteria digest it and..turn it to methane I guess.

Also-- I water that area. So the pine mulch stayed moist. It wasn't a mulch that never saw water.

Vegetable garden is another exception..as long as you constantly dig in fresh compost per growing season. If you've done that for 20 years and switch to ornamental plants for that spot..its better than when you started. But for your ficus hedge starting?..plain dirt. A waste of time ,effort and money to amend for a straight one shot.

I have dug this trench for about 13" at one end and about 15" deep in opposite end. These two ends are the width of this rectangle and the length of rectangle is 6ft. Ground slopes from one end to the opposite end of the widths, and that explains the difference in depths. The slope of ground is about 15 degrees. The soil is hard clay after 14" deep, and i stopped digging, as it was breaking into concrete type chunks when i hit on them. Instead of going deeper, i decided to build this up into a raised bed garden, adding about 8" of topsoil above the ground level.

Upto the ground level, i plan to mix 50% compost with 50% of native soil (which is 50% clay and 50% silt). So, total height of the raised bed would be about 14" + 8". I have read elsewhere that digging trenches in heavy clay like mine would result in a bathtub during winter rains, with water getting trapped in the trench. To avoid this, my plan is to cover the entire raised bed + 2 ft perimeter with tarp during winter. Also, i would dig water drains (about 5" deep) around the perimeter of this trench, so that incoming water will be channeled off and away from the trench.

Is this a feasible plan ? I would like to plant something here, for all the hard work i did in past few weeks. How much time should i wait after mixing in the compost and filling up the trench and top filling raised bed with top soil from HD ? Thanks for all suggestions.

Oh, BTW the original idea of composting in the same trench has been abondoned. Now the plan is to buy compost and mix it up with yard soil to fill the trench. This would make for the soil amendment, but not sure how long i have to wait before planting.

sam, if you have hard clay you will have to plant a winter cover crop to punch through. composting on top will not help. Once you have the root channels, drainage will improve and over time other roots will move down past 13".

The usual suspects will do, daikon being the most prominent (in SF, plant in October). But you can also consider alfalfa, chicory, cardoon, the latter two being vegetables.well, daikon is a vegetable too, but if you plant the cover crop varieties you will be unable to eat them, they are way too hot. I can eat cover crop chicory, and I juice alfalfa in a greens mix.

Fill your bed with soil. They sell "raised bed mix" now. Use the soil from the trench as well. Plant and then top with some sort of wood mulch. Don't worry about a bathtub effect. We'll be super lucky if we get enough rain to cause you any kind of problem next winter. Some plants hate having wet feet for days, but you're unlikely to have that problem in your winter vegetable garden.

If you don't stop analyzing and get started putting your summer vegetables into the ground pretty soon, you aren't going to get any crop!

I was considering filling in my trench last week. Thank goodness i postponed and the rain happened yesterday. I was able to first hand see the effect that gardener was talking about. Yesterday there was about 1/2" of rain. The trench got filled with water. Mostly because the trench is in the low area of nearby ground, and probably all the water seeped in or came running into from nearby ground. Ground slopes at about 5 degrees from the trench into the road ditch outside property. Considering the volume of water in it, this will take about 1 week to drain or evaporate. We are expecting another storm middle of next week. So, if i had started a composting process in this trench, it would take months if not a year, to finish composting process. So, the answer to my original question is a resounding NO, if the ground is sloping and soil is 50% or more clay in nature.

It seems to me, that a Hugelkultur raised bed on top of this might work out (if i understand the youtube videos on that process). First fill up the trench tree branches and dry tree stems ( between 2" to 1/4" in diameter + tree leaves that are fresh cut to supply the nitrogen ). This will fill up the volume of the trench and hence displace any incoming water into it during winter time. Then, build a raised bed on top of it to a height of 2 feet, filled with yard soil + compost + top soil from Home depot. Even if the raised bed sinks into the underlying trench over winter, i can refill it next year. I just need this to last 1 season. (Meanwhile, my plants are happy in their grow bags for another month or so. I can transplant some of them at end of June). Comments or enhancements to this idea are welcome.

That trench may have filled up with water from surface runoff, but there is another mechanism at work with clay.

Clay is so impermeable that it can give up water from the trench walls (albeit slowly) because the water much prefers coming out than trying to push through the clay. Also the topsoil layer is often a lot more permeable, so even water infiltrating at some distance away can run toward the hole and seep out from the topsoil layer. If it were sand, there wouldn't be pressure pushing it out the walls of the excavation as hard. I'm not sure I'm explaining it well but it has to do with pressure and flow in the soil matrix. I once dug an outhouse pit in clay at the top of a rise and protected it from runoff, but when it rained it filled up with water anyway.

And of course none of that water wants to seep out of the bottom into the clay very badly, so it sits there. FIll it with very porous organic soil and the soil will sit there waterlogged.

IF i understand you correctly, you are saying that the water would rather push out (of trench) through the side walls, but not through the bottom (which is heavy clay). This is what i would expect too, since bottom is pretty dense, even more than the side walls.

There was no surface runoff, since the rain was not heavy, but a drizzle that continued for most of 36 hours, on and off. So, the accumulated water is from the top soil nearby, running towards the road, since the ground is sloped.

So, the good thing about Hugelkultur is that the wooden blocks sitting inside the trench would displace the water, hence the amount of water inside the trench would be reduced (but not completely eliminated). I am not planning on gardening at this trench during winter rains. There is nothing else that i can think of, as the best use of the trench that has been dug up. Basically the trench is not going to be useful soil, i agree. Hence the plan to build raised bed of 2 feet above the trench. Decomposition of the Hugelkultur bottom would take atleast couple of years. From what i am reading, most plants root heavily only at the top 12 to 20 inches. Comments are welcome.

You might want to do something to improve the drainage all around, not just in this area. Some people swear by tillage radishes - don't know if would suit your situation, do some internetting.

The trench bathtub filled with underground water, seeping in from higher ground. When the rain stopped, it drained by itself in 7 days, going into lower ground. Trench has been filled now with 60% tree branches / twigs, 40% compost + yard soil. Branches are fresh and 1 to 3" wide, and NOT all will rot/compost within 1 year.

During rainy season i plan to cover up that area with tarp and protect from rain, but that won't help. Gardner is suggesting that i fit a pipe (at 6" depth) at low point of trench (the one nearer to you in the above picture) and run the pipe to lower ground, to divert some of the water. (Note: Trench is 1 ft deep, at low point). I would have to dig a small trench again that is 10 ft long, 8" deep and about 3" wide in order to fit the pipe underground. Then this smaller piping trench will be covered with yard soil. Hard clay starts only below 1 feet depth.

Will this plan work ? I am inclined to simply leave the trench as it is, and build my raised bed structure on top of it. Idea is that during rain season, i will not be gardening, and hence it does not matter if the trench is filled with water for 5 months. If i do fit the pipe, i am afraid that my composted goodies will leak away from the trench area (through pipe water).