Soil to cover a hugel bed

My parents have lots of wood debris on their property form recent storms and so I have offered to build them a hugelkultur bed instead of hauling away the branches to the dump. They are nearly convinced, however my step-father is concerned I will not have enough soil to sufficiently cover hugel bed, even if I dig a trench for it first and use the sod on top. Could someone with experience building hugel mounds please share details on how they got their wood buried? How deep must the logs be buried? Did you have to haul in top soil, or excavate another part of your property? Are you using exclusively quality garden soil, or do lower quality soils work at the beginning?  Please let me know. Thanks.

It's a tricky question, because it really depends on at least these factors (if not more factors):
(1) How deep you need your soil to be (i.e. what plants you're growing in it)
(2) How well packed your woody material is. If the wood has a lot of gaps in it, soil or other materials will be needed to fill those gaps. If you don't, the soil will sink in there over the years making your bed rather uneven.
(3) How steep you plan on making it. If you manage to get a steep slope on your hugel bed, you'll need less soil than if it's less steep, as the soil will happily sloff off the sides. You could, I guess, start out with your wood being a less steep angle, so that soil covers it more naturally.

Here's what tends to happen if you want a steep hugel:

Though, there are some things you can do to combat the sloffing that are mentioned in this thead: https://permies.com/t/50120/Tricks-Dirt-Sliding-Hugel


But, I sucessfully made a small hugel by just digging down about twice the height I wanted, putting that soil on a tarp, with the sod in it's own pile on the tarp. Then, I filled the hole with sticks/logs and continued adding sticks to about 1/2 the depth of the hole. Then, I put the sod on the sticks, followed by the dirt, and then grassclippings/other mulch on top of that. This worked out pretty well, but it did require a lot of digging! So, you can make a hugel without any soil trucked in, but it takes a lot of excavating!

As for what quality of dirt you need, I'd say use topsoil, or at least topsoil on top. Or, add in lots of manure/compost when you build the mound. Otherwise, you won't be getting any real growth on your mound for a while, except for weeds. At least, that's been my experience: I have one hugel bed that I planted cover crops on, but they didn't do very well. Creeping buttercup, on the other hand, is doing great and taking over the bed!

I hope that helps!

You've got a few questions in this quote:

How deep must the logs be buried? Did you have to haul in top soil, or excavate another part of your property? Are you using exclusively quality garden soil, or do lower quality soils work at the beginning?

As with most things permacultural, the general answers are almost invariably... it depends.  All of these questions will depend on other people's preferences and circumstances.  There are no set answers, though Sepp might disagree.  

My answers:  

Question 1.) The logs do not have to be buried deeply, but typically they are buried deeply, at least a bit deeper than the roots of your plantings, and sometimes a lot deeper!  So the question of how deep will, as Nicole explained, be tricky to answer.  The best thing to do, is have more than enough, but that isn't so helpful for your circumstance, so I would think that to be helpful I should say something like a foot of soil over your wood would be very adequate, but six inches might be enough if you were using shallow rooted plants with the addition of a lot of nitrogen against your wood and compost in your topsoil/mulch layer, and two feet or more would be exceedingly beneficial.  The way I understand it, the more Earth/Soil on the wood, the more the wood is kept damp, and thus the better it gets fungi, and the faster it turns to soils/or integrates fungi and soil as a single matrix.

Question 2.) I hauled in soil.  I was digging fruit tree holes, so some of that material ended up on the hugul, and I was bringing in a few truckloads of composted manure.  I only dug down two feet below the mound.  As Nicole explained, if you dig down, you will have the soil you need, but it should be noted that some of this is aggregate and much of it is not fertile/living material, and in my mind this would be inferior, at least in the first few years.  My wood was covered with about 1.5 feet of material.

Question 3.) I used all the soil I could get without discriminating, but I tried to be methodical about how it was layered into the mound.  I had  a lot of quackgrass in the hugul area, and this has become a problem with invasions onto the mound, but I also took the quack sod and jammed it into the matrix/holes of the wood so that it was utilized.  This might have contributed to the quackgrass problem, but the problem might have been inevitable considering that I did not deal effectively with the quack surrounding the mound.  At any rate, I wanted the sod to help keep the wood wet and to give lots of bacteria and other soil life right by the wood.  Next went the subsoils and aggregate, and then the topsoil, and manures (sheep and horse).  Now I think that if you were to pile any old subsoil on the wood and packed into the wood matrix cracks, then you would still get the same effect over time, but it would not be as productive, until microbiological fertility (living soil) is built in your surface soil.  I planted a dense crop of field peas on my mound for the first year in order to help develop a living fertility.  Of course some of the great benefit of building hugulkultur is that the wood/fungi matrix provides a deep fertility and nutrient bank, which will provide for years into the future, but you can not really count on this as a nutrient source in the beginning.      

Hi Starr;

My two cents:

Go ahead and use whatever organic matter you have. You do not have to use a lot of soil. You don’t have to put soil throughout the bed as you build it. You can use some other form of organic matter. We built buried wood beds (basically an underground hugelkultur bed). We don’t have any soil, just sand, so we used alder chips, grass mowings, year-old Scotch broom wood chips and year-old grass/leaves/weeds that were breaking down nicely, and sand, to fill in between the logs and branches. The beds were then topped off with about two feet of the year-old mix, with a little sand, a little clay, and compost. And it grew an amazing garden the first year. The only soil that went into the bed was what went in with the transplants. Basically it was 2 feet of mulchy compost type stuff. It needed to be watered, as the bed was built in May and planted in June, and didn’t have the winter to absorb water. But I expect that after about 3 years, it will hit its stride and will not need much supplemental irrigation. Hopefully eventually it won’t need any. That’s the plan. It's out there soaking up water as we speak!

Depending on where you live and what your climate is like, you can have varying degrees of success. Arid climates need rather large hugelkultur beds as they need to hold much more water, and are generally best built below ground, or partially below ground, if you have long hot dry summers. Bigger is better in those cases. If you are in a more humid or wet area, and your bed can soak up lots of water during the winter, then it will be more successful. I think digging down at least a foot is a good idea, as that will help stop and collect water for your hugelbed to soak up. If the bed dries out, not only will your plants not grow, but the materials won't break down very fast, and it will lose all of it's good hugelkultur attributes. I think they are very difficult to get rehydrated again after they've dried out a lot.

So, that's my experience will growing using wood filled beds. It works! But you need to adapt it to fit your climate, and what you want to grow. Can you give us some more information on what you might be growing in the bed, how much sun it gets, and your rainfall and summer climate? Then we can perhaps give you some more specific information.

Good luck!
Tracy

Thank you all for your feedback! I was thinking I would try to supplement limited soil I have with fallen leaves that are available in abundance, or at least use those to fill in between logs and branches. Has anyone built a hugel the fall before planting in the spring? I was thinking that would allow for some decomposition before planting, but realistically it'll probably just be frozen all winter.

Leaves are great to use. And anything else that is/was green - grass, veggie scraps, manure. Freezing will slow decomposition, but the act of freezing will help the process by breaking things down into smaller pieces, and it will still absorb a lot of water when things start to melt in the spring. Should work fine. Cover it up with leaves, or some other mulch for the winter if you don't have time to plant a cover crop, and then pull the mulch back in the spring so the soil can warm up faster and get things working.

Has anyone built a hugel the fall before planting in the spring?

 

From my understanding, this is the preferred method.  Although decomposition is slowed in the winter, so is evaporation, thus the hugel is charged with fall rains and snow melt water, which is necessary for the decomposition process to get kick started when the spring warmth blooms.  Also, if you have snow, and the time to shovel it, then pile it deep on the mound, this will insulate the mound from a deep freeze, and provide it with more water.  

I was thinking that would allow for some decomposition before planting, but realistically it'll probably just be frozen all winter.  

 You might be surprised that the bed may keep from freezing longer than you think, since there will be decomposition happening, and it is a massive structure that is exposed to the sun during winter warm spells and in the spring.  To enhance this effect with my first and only hugul (built this spring), I shaped my hugul as a Sun Scoop with a South/South East facing curve to it.

I was thinking I would try to supplement limited soil I have with fallen leaves that are available in abundance, or at least use those to fill in between logs and branches.

 

Leaves might work against you within the hugul log/branch structure, as they tend to have a shingle effect, shedding water from one to the next, leaving lots of dry spaces...  ...unless the leaves are broken down (by running them through a shredder, running them over repeatedly with a lawn mower at which point the shingling effect is minimized and the leaves can accept more water/biological activity.

There is a discussion going on about leaves here

Instead of putting the leaves within the woody structure, I think that they would be best served as mulch over top of the thinnly soiled mound.  If you have some access to topsoil, or compost, in the spring, you can open up channels in the leaves, applying your fertile material, and then plant in the channels, thus minimizing the amount of topsoil/compost, that you utilize to begin your spring planting.  

 

In the final stages of my first big hugelbed now, taking queues from Tracy and her experiences because I have similar conditions here.  My bed is going into a sand/silt soil with very little biological activity, so I'm focusing on getting as much organic matter as possible in there.  I have to agree with the comment about shredding the leaves some first, as I've done it both ways where I lived before and giving them one or two passes with the mower has greatly improved the result.  Much less compaction, more air penetration, a better environment for the worms (which I'll probably have to import here), and a generally fluffier result.  Intact leaves do take much longer to break down when buried in place, I think because you're basically insulating them from a lot of the forces that help the breakdown in nature - wind/rain/temperature fluctuations, critters, etc.  

Since I'm in such a poor soil here I've set aside money to have at least 18 yards of good topsoil/compost mix trucked in to go on top, but I'll start with lots of small branches, twigs, shredded leaves, and whatever animal bedding and other compost materials I can get packed in around all the bigger wood.   I'm also taking advantage of the abundance of dead and rotting eastern white pine we are surrounded by (the predominant species in the area), as it is not much good for anything so most folks let if sit on and rot when it comes down and there's piles of it everywhere.  It gets buried in a couple feet of snow each winter, breaks down quickly, and gets colonized by a variety of mushrooms and ferns.  The result is after a year or two you can literally pull apart the outer few inches in big wet chunks and this is the stuff I'll be doing most of my filling with.   I want to make it so when the good (and expensive) topsoil arrives it doesn't all sink into the pile, since that'll be what supports the beans and peas and other builder crops I'll grow there the first year.  

I'm still not sure of the amount of topsoil needed for a large bed, my original estimates are way off since the dig has grown to over 40' by 20' and gotten about 4' deep, but I figure if needed I can add another truckload next year if things settle down too much.  Since I'm going for maximum water retention my end result will be a bed mostly level with the surface, which makes things a lot easier here and makes the addition of future compost/soil less of a problem - it won't slide off