Greetings everyone on this beautiful day here on the Gold Coast.

A question regarding clay soil. Our house was built on clay dredged out of the moat behind the Turf Club and Water Treatment plant. Obviously, this makes for a fabulous building base, however, it is proving shocking as far as gardening is concerned (I’ve already been through one mattock handle)…

Due to the drainage problems (water sits around for days following rain) planting things in the ground is proving problematic for us. We have solved this by removing some 6” of top clay and replacing with sandy soil in a couple of low lying areas, however, we aren’t really in a position to do this for the entire block. Indeed, where we are keeping lawn it isn’t really a serious issue as the lawn seems to manage.

We were given a lovely magnolia which we want to put in the ground, however, the poor thing keeps getting flooded out. We were watching Gardening Australia the other night and there was a segment where (can’t remember his name) the presenter was digging in pine bark to clay soil to assist in airating it following flood.

Would it be appropriate to dig the 3-4 metres around where the magnolia will be replanted (currently residing in a pot until we work this out) and dig in pine bark and organic matter, or would it simply be easier/better to remove all the clay in the area and start over? Indeed, how deeply do we need to perform this, or shall the magnolia simply sort itself out once it establishes itself and we really only need to put in soil to a depth of perhaps 1 metre immediately surrounding the tree?

Long winded I know, but that you in advance for your thoughts.

I’m thinking it might be better to create raised mounds to plant in. Any holes dug in the clay no matter what you use will just fill up with water when it rains, like a bucket.

How far down does the clay go?

I know that at some spots in Brisbane, the clay (right near the seaboard) goes down 10 metres or more…

You didn’t say what the original soil was made of…

Also, can you obtain the engineering report re soil structure, that the architects and/or builders get, before they build the houses?

My grandparents owned a property where the homestead was built on alluvial, flooding gidyea-type clay…grandfather dug a humungous hole, some 2-3 metres all round I think, then he just kept flinging the cowpats (plenty of those!) in and when the hole was built up, planted a fig tree that was famous in it’s maturity…

…but first you need to know what’s under the building clay…

I’ve been working on clay for years and it does get better. I wouldn’t dig in pine bark,compost is what you want, and lots of it. gypsum might help. You just have to be patient and keep adding organic matter and mulch, mulch, mulch. Also don’t try and work the soil while it’s wet, you’ll only make it worse.

composted cow manure will certainly help the Magnolia… and try to raise the tree in a mound.

As a young lad barely able to see over the car windowsill, I asked my mother “why don’t native plants regrow on the canal banks after they have been dug?” (irrigation canals).

She replied “because they dig up subsoil and tip it upside down.”

There are two issues here.. The seed which was in the soil seed bank was redistributed to the bottom of the pile of dirt. The subsoil which was at the bottom of the hole is now at the top of the heap.

Now you can attempt to tip it the other way up but you will likely make a bigger mess.

Just mulch it down into a proper tilth. This may take a little longer than it would normally however it all depends on your knowledge of mulch and mulching.
If you feed the worms and add some to feed.they’ll work out the soil structure for you.

In one instance I was given a salt pan in the middle of a rice farm.. I walked over to his old stables and scraped up the old straw and manure which had rotted down well. Tossed it on top of the soil and planted .. People came from all around to see the magic that occurred.

Magnolia was probably not the best plant to start with but it should still work out fine.

There isn’t a lot wrong with clay apart from the lack of friability. Friability is rather easily fixed with worms and compost. I don’t believe gypsum will assist.

I was given a block of land which had calcerous clay and lumps of calcrete all over the surface. Within five years a local orchardist stopped off to pick up his son who had stoopped to play on his way home from the school bus.. He stood there looking at my yard and said.“how long have you been here?” I said “almost five years why?” ..He stood and looked for a while longer then said.. “because your trees look like you have been here for more than twenty years”.

Mulch and worms.. you don’t have to bring the worms .. just put the mulch there.

Mixing sand into clay soil isn’t going to help because by the time you get it right, your yard will be higher than your house.
If you use sand as a mulch you will have to be prepared to watch the topsoil dry out and your clay below to be soggy.

In an agricultural community, I see semi-trailer loads of gypsum go on paddocks with negligible results worth talking about.

Semi-trailer loads of grape marc or bagass or hay.. etc.. this is the way to improve clay soil.

Thank you all for your considered opinions.

I’ve dug a couple of holes in different areas at least 1m deep and the clay seems to be only about 500cm deep in most places. The soil under the clay is a bit stinky, like a dead fish… I will try and get a soil test done it in the next while.

Reference the mounding, I am wondering whether we would be able to make a mound big enough for the magnolia, would we did a wide and deep hole (1 metre by 1 metre), fill it with good soil and then mound it up say 20-30 centimetres and the tree will sort itself out, as long as water doesn’t sit around the trunk?

Firey