"Building soil"? Or just making compost?

I am confused by claims that, through sheet mulching, some people have managed to add (x number) of feet of "topsoil" to their property in only (x number of) years. While OM is a part of topsoil, it does not, in itself, constitute a complete soil. I learned this the hard way by trying to direct sow into a lasagna bed and discovering that I only had success in the areas where there was soil placed on top rather than just fine compost. Furthermore, if these feet and feet of topsoil are made up of 100% compost, then you would start loosing topsoil as the compost breaks down into simple elements.

Let me make one thing clear: I am not calling "bullshit" on those of you who have improved your land with sheet mulching. I fully believe, 100%, that you have done just that. I am laying out my understanding, and lack thereof of the process of building topsoil with sheet mulching.

So what am I missing here? How does pure OM, laid on top of the ground and left to break down into humus constitute "soil"? How does this "soil" not disappear as the humus ultimately breaks down into simple elements?

Another question just popped into mind:

If I planted perennials or trees to get them started and then tried to build the soil by sheet mulching around them, by the time I had a few extra feet of "top soil", wouldn't the trees and perennials end up getting buried since they are originally planted at ground level?

I can only relate my own experiences.

Living upon primarily lava rock, I have had to create my own soil. I encourage lots of organic material to decompose, but I also add a lot of various amendments : home processed coral rock and bones, lava dust and gravel, cinder, wood ash, biochar. These all physically add to the composition. Without mineralization of the organic matter, the decomposing material doesn't act like soil in my situation.

I find that the decomposing organic matter rapidly decreases in volume. I've heard some folks around here say, "Mulch is a waste of money because it just disappears." What they are seeing is the mulch decomposing and thus greatly, greatly be reduced in volume. Apply a foot deep layer of mulch and I'd end up with less than an inch of organic material after a year or two. So unless I constantly add organic material, I surely wouldn't see a thick layer of new "soil" being produced.

How much mineral material do I keep adding? It all depends on what I see the "soil" looking like. If it's hydrophobic, it needs mineralization. If it gets wet and mucky, it needs mineralization. If it gets too gravelly feeling or dries out quickly, it needs organic material.

Would I expect to see new "soil" production to be measured in feet? No way. I'd have to be dumping tractor trailer loads of material for that to happen. As an example of what I would expect to see, I once had 11 cubic yards of mulch dumped on a corner of the garden. The pile was literally 6 feet high. Because of lack of time, I was never able to get to that pile, so it sat. After a bit over a year the pile was down to about 24 inches high. It was soggy, pasty, like gooey muck. I wouldn't call it soil. I broke the pile up at that stage and used it in compost piles. But if I had left it, the height of the pile would have continue to decrease as the material decomposed more. The end result would have been a few inches, I'd venture to guess. Without having added minerals, there would not have been much left.

I use sheet mulching style method to fill in what I term my biotrash pits. Giant holes or trenches in the ground, many big enough to hold a car! I have some that I've been dumping material in for years, literally, and they are not full. Every year the level goes back down as the material decomposes. By the way, these pits are great for mining "improved soil" when I need some. One of these biotrash pits I planted with banana trees four years ago. I've never had to fertilize or irrigate those trees to date. The soil level has gone down by about 50%, making the trees look like they are quite a bit shorter than they actually are. Someday someone is going to wonder about those trees growing in a deep hole. Gonna puzzle them.

i always top the sheet mulched lasagna beds at the very top with the very best of what i have as far as soil. i mix all the best ingrediants i can get for the top layer.

ideally really rich primo soil, like finished compost, manure, recycled bag soil or even (every once in a while when i dont have other materials) buying bag soil for the mix at the very top.
then i will add some soil dug from the ground. by throwing a compost screen down i will keep screening and sifting the soil i dig, and add whatever materials i can come up with, sometimes sand and moss, and also the IMO i have been learning to make.
or well i will call it a kissing cousin to IMO, my own version of how i have been figuring out how to make IMO, mostly from gathering soil and leaf mulch from under trees near rivers and adding it to these soil mixes so it can spread in them. or other places that have good local IMOs in nature, making the IMO proper with sugar and rice/wheat....getting the hang of making it still, so i am still learning and trying to not get it too hot or wet/dry in my scatterbrainedness =/

thats the "cheese" part of the lasagna to make the metaphor work. and also fitting with the metaphor theres a layer of "cheese" inside the bed too.

underneath that is ideally deep layers of leaves/straw and a proportional amount of unfinished compost, as thick as i can get it, a layer of "cheese" in the middle, and under it all is cardboard/wood/leaves/biochar.

i am not particularly precise about any of this, and theres how i would ideally build it as compared to how it actually happens, so you know...theres a lot of variation. but thats the general idea and plan for me making new beds, and theres some actual soil (and or bag soil) inside and on top. when i can, later after sprouts are happening and clearly making it, i will ideally mulch the top again with leaves/straw. actually since we are talking ideally i would keep on adding every few months a new layer of something, tho this doesnt always happen due to time involved and availability of materials to mulch.

if you are missing anything i think it might be time, it takes a long time to really gel. another thing is worms, who slowly work it all together, including the stuff under the bottom layer of cardboard.

Dan, if I understand correctly you had problems with your lasagna bed and attribute the problems to something about compost not being enough for plants to grow. Am I getting the gist right?

My experience is quite contrary to that, although I am not working with any sort of sheet mulch or lasagna beds. One of the most productive spots in my garden this year has been my compost pile. This compost pile is not even remotely finished breaking down, but volunteer squash appeared out of it and have grown with more vigor than anything else in my garden.

This makes me wonder if the problem you ran into might be attributable to something other than compost. There are quite a few "moving parts" in sheet mulching and a bunch of possible culprits that could be responsible for things not doing well.

Peter Ellis wrote:Dan, if I understand correctly you had problems with your lasagna bed and attribute the problems to something about compost not being enough for plants to grow. Am I getting the gist right?

My experience is quite contrary to that, although I am not working with any sort of sheet mulch or lasagna beds. One of the most productive spots in my garden this year has been my compost pile. This compost pile is not even remotely finished breaking down, but volunteer squash appeared out of it and have grown with more vigor than anything else in my garden.

This makes me wonder if the problem you ran into might be attributable to something other than compost. There are quite a few "moving parts" in sheet mulching and a bunch of possible culprits that could be responsible for things not doing well.

I had squash volunteering in the lasagna bed too. In fact, I had no problem with large seeds that could be sown deep enough. The problem I had was that the finished compost on top drains too quickly so the surface, where the small seeds are sown, doesn't stay wet for long enough. My father says he ran into the same problem. That being said, starts and large seeds do exceptionally well in compost.

Dan, if you finish off with a layer of dirt, I believe you will have better water retention in the compost layer. Compost, by its very nature is rather fast draining compared to dirt. It doesn't take much, between one inch and two inches should help your beds produce more variety.

So the problem is drainage, not compost per se. My sandy soil drains so fast compost absolutely holds moisture better.

My understanding is that one of the ways that sheet mulching and other soil building techniques work is that they help to encourage life in the existing dead or depleted subsoil. This might be very close to the surface. Building topsoil doesn't necessarily mean building inches up. It can also mean extending the layer of quality topsoil down.

I'm with Karen I watched a video a long time ago that was talking about the decay of organic material and it was not that new soil was being created on top, it was the organisms are taking the organics and pulling them down up to a foot into the ground and thus slowly turning plan dirt into the soil you are talking about.

Yes, I ran into some problems without some mineral dirt on top. Even tomatoes, which grew wonderfully in my sheet mulch beds, grew even better in my shallow hugelkulture, where I covered low mounds of manure and sticks with soil, and then with a layer of mulch.

And trenches of pure finished compost to plant things in only made it worse.