Improving Clay Soil

Hi,

I want to create a 25’ x 25' Permaculture mandala garden in my Philadelphia backyard (Zone 7a), but I have poor clay soil.

Since I want to improve the soil immediately, I don’t mind spending money to have a lot of new soil delivered and dumped on top of the clay.

Would this be appropriate?

If so, then what type of soil?:  mushroom soil or a Mix with top soil?
(Of course, each island of the mandala garden would have slightly different soil depending on what was planted there. For example, certain herbs prefer sandy, rocky alkaline soil.)

On the other hand, since I’m familiar with sheet-mulching & Hugelkultur, would it be best for long-term soil health & stability if I dug 12” down, put a weed barrier, then a layer of pebbles for drainage, then a layer of branches/small logs of apple & maple wood, then some old leaves and compost, then dump 6-12” of mixed soil on top, then some mulch?

Any advice would be appreciated.

If you have a source lined up and don't mind the expense, I would choose the mushroom compost.  It's advantage is that it is basically sterilized before mushrooms are grown in it, and so you are unlikely to be introducing  new weeds, insects, or other pests onto your site with it.....which is always a danger with many kinds of bulk amendments.  

Thanks

Would you bother with the Hugelkultur in addition to adding 12" of soil on top?

If you have the materials and energy, sure.  Just beware of seeds and pests in outside materials, particularly soil but also mulches, etc.

PLEASE -- don't waste your money on new soil.  Clay is wonderful, but you need carbon to make it better.

You need biomass.  Grow a cover crop.  Import wood chips for mulch by the truck load.  Compost everything you can get your hands on.  Carbon is the solution to heavy clay soils.

I would think to look for a couple yards of the nicest compost that you can find, spread it over your area a few inches deep and till it in as deep as you can. Then landscape and plant/mulch each area of the garden as desired. you just need to loosen the clay and give the biology a space, the clay is great mineral food. Just get looser structure and biology, including plant roots, in there and the soil will gorw. You can invest as much energy as you want into this garden and it will yield a corresponding glory.

Clay needs carbon and biology. dead plants and fungi. You have all the micronutrients you just need to give them the structure to lets the roots access them. The most greatestest thing I imagine is hugel trenches that you bury with your native soil mixed with a high quality compost at about 7:1. Then the various beds could be planted and mulched, innoculated wood chip for the perenial fruit beds and straw for the annuals.

You will build soil if you grow plants conscientiously, post pictures when you do.

I suggest getting a soil test done on your native clay soil. Calcium will loosen clay and if your clay soil is acidic, raising the pH with lime will add some of that calcium. Gypsum is another great way to add calcium to a soil to loosen it up and give that clay a nice crumb structure. I'll echo what others have mentioned with adding compost to this clay as well. Importing topsoil can be unpredictable like Alder mentioned with unknown weed seeds, and it's nearly impossible to know where that topsoil came from, like next to a busy highway where it's been contaminated with 70 years of automobile exhaust.

It depends what kind of plant you want to grow.  Some plants would prefer rich compost to grow in, and others would be quite happy in the clay soil enriched with a nice layer of mulch.  If you choose the more "exotic" plants for your region, than adding compost might be best.  But if your plants are those that are already adapted to your climate and clay soil then, relying on mulch is on top of your clay soil might be fine.  Or maybe a mixture of compost and mulch is the way to go for a faster boost to your garden.

Be careful where you source outside materials to add to your garden.  The more we rely on locally sourced material especially profiting from yard cutting "waste" to recycle into our gardens will ensure long term health of your garden.

There is more than one "perfect" method to gardening.  Plants often have a surprising way of adapting to it's environment, unless you really do have the wrong plant for the wrong location. ex. planting a sun loving plant in total shade.... or planting a plant that does not like wet feet in the flood zone of one's yard etc...

Hugelculture logs and sticks buried into the ground or mount will have long term benefits if you have access to them in abundance.  But not everyone has access to them in abundance.  If you have them, use them.  If gathering leaves in the fall is what you have in abundance use them as mulch.  Your garden will be more than happy with either method.  Other methods can work great too like wood chips mulch if you have them in abundance.

Please post pictures of the progress of your garden throughout the year.

On the other hand, since I’m familiar with sheet-mulching & Hugelkultur, would it be best for long-term soil health & stability if I dug 12” down, put a weed barrier, then a layer of pebbles for drainage, then a layer of branches/small logs of apple & maple wood, then some old leaves and compost, then dump 6-12” of mixed soil on top, then some mulch?

Sheet mulching is a great way too.

"If" I dug down 12 inches, I would skip the weed barrier and pebbles.  The " layer of branches/small logs of apple & maple wood, then some old leaves and compost" would give it the natural drainage I would need, then I would put back the soil I dug out.  As it makes no sense to haul it away to be replaced with imported soil.  Then I would add some form of mulch, wood chips, leaves, any plant material etc.... whatever I could get my hands on.

Now digging down 12 inches is a lot of work so you have to determine if the plants that you want to grow really need all that effort.  Carrots yes, they would be very happy to have the depth, but most perennials have strong enough roots to root down if they need to and usually their roots grow on the top layers of the soil as feeder roots.  Now if I did dig down, then I would likely want to have large quantities of logs & branches to make my effort justified if I had access to that material.  On the other hand, those who do the double dig method, just simply loosening up the clay soils has worked wonders for a lifetime of thriving plants so one does not neccessay "need" logs & branches buried to have thriving plants.  And garden beds where I did no dig but simply piled on the layers of "mulch", the plants thrived too.  So if you have the energy and time, digging down in the soil has benefits.

If I have a garden bed area that is not urgent to plant in that season, I will pile layers of plant material & leaves, small branches to prepare the harden soil for the coming season.  The mulch layer will keep the clay soil workable and moist when it is time to plant.

Note: the reason I don't recommend a weed barrier as what does that weed barrier look like in a few years when all the roots are tangled in it.  Most weed barriers do not decay and are forever stuck in your soil tangled in roots.  I have seen it, and done it and have learnt the hard way.   This is not what nature intended.  Learning about permaculture as steered me away from this technic.  Adding a layer of pebbles 12 inches deep has no advantages.  If your soil is high in moisture then plant the kind that likes moisture or raise your bed 6 inches or so or on higher mounts to get above the water table for those plants that like less moisture.  Pebbles, either you go to the river bed to find them or you buy them.  I cannot see myself  doing either unless I really wanted to create a very small decorative rock garden for fun.

Many great responses already. For your purposes, I agree with going hugel but maybe you could rent a shredder/chipper  for a day or two and make your own growing medium (hopefully you still have your leaves from last autumn).

Interesting ideas