Radial trenching, what to replace with the soil?


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I need to radial trench in an attempt to save a large old southern red oak that is just starting to show some signs of decline.
Heavy equipment 14 years ago, plus grade cut away on one side, plus slight grade increase on other side (6inches?).

Anyway, I want to increase airspace in the ground to make the soil less attractive for root decay fungi.

It's been a while since I've checked on best amendments to add to the soil for a large job like this.

What is the current best material to add to the soil to fill the trenches back in?

I always wondered about "lava" rock. http://www.homedepot.com/p/VIGORO-0-5-cu-ft-Red-Lava-R3RL/205137359

If you do any soil amending to mature trees, please share your techniques.
Thank you,


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David it depends on what the native soil is like. My standard mix is 50% native soil, 25% compost, 25% expanded aggregate 'Perma-Till'. The ratios would be tweaked if the soil is far from normal. Holes >12" deep, >2" diameter, 1'-2' apart.

I've used lava rock as a subbase for paths in conjunction with this work, but it seems kinda coarse as an amendment. If compaction is real bad the holes could be a lot bigger like trenches and I see no reason why lava rock couldn't be used.


Active Member
Airspade out one foot per inch dbh ,,,,,, roughly incorperate with air spade 3-5 bags topsoil per 10x10,,,cheap stuff from lowes.2 dollars a bag. One year topical application davey arbor pro green , blend all with spade ...mulch nice two inches dark stuff . Pray.
If it has bl scorch its a waste


Active Member
David- we remove all the grass as far out as we can and try to mix in straight compost as we airspade. I'm a big believer in compost. Then we lightly mulch. Every situation is different though. My avatar has a beech we turned around. Good luck!

Paul Biester

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we buy a pallet of dehydrated compost from home depot and amend w that....top dress w composted wood chips!!


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We use a blend of fine screened mulch, this bagged product (Recycled forest products, arbor fines, composted chicken manure, gypsum, oyster shell & dolomite limes , worm casting, bat guano, kelp meal) then I add perlite for aeration.

I keep toying with the recipe. The two ingredients I've been considering lately are volcanic rock and biochar. Haven't made a decision on it yet.


i hear really good things about biochar, but no experience with it.
I agree with the herd, compost, mulch, compost, mulch.
Get the soil environment to mimic a natural forest system, not a compacted urban lawn from Mars...


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I agree with Guy's ratios and material blends but remember that some organic matter will continue to decompose and shrink. I had to go back to a few jobs to reapply and reseed the lawn because the trenches dropped. Not a huge deal but let the home owner know that it can happen. Some folks get nutty about their lawns even though we just spent a lot of their money to save the tree...
Using thoroughly composted material helps with this.


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The question you need to ask is what are you trying to accomplish by radial trenching? De-compaction, mixing of inorganic matter, addition of materials to hold water, increase in macro-pore space? Each objective requires different amendments.

Secondly you must also know your soil and what an ideal soil looks like. Again this will guide you.

In this specific case the Xman wants to increase airspace in the soil, so add something like perlite or vermiculite that that is longer term than compost or bark which is slow decay. Porous lava rock is another option.

In the longer term creating a more active soil food web, removal of grass and mulching with wood chips is an excellent strategy to make a more sustainable strategy.