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I am a newbie gardener, and would like to plant around 15 polyalthia longifolia trees in my garden.

The main problem here is that my garden's reddish-clay-soil seems to be compacted.

  • Clay sticks under my sandals.
  • No worms in the compacted soil.
  • Rainwater doesnt drain easily, and on some spots he rainwater doesnt drain eventhough i have digged a hole at 50cm depth, and when i digged deeper i found out that the house-developer had buried their building debris around 50cm under the surface. Digging further than to pass through the debris and the water can then drain normally. The device i used to dig is this.

My main concerns are that:

  • The trees will have their growth stunted because they cannot break through the compacted clay soil, especially there's building debris underneath.
  • Combined with bad drainage, the root can also rot.
  • Also compaction doesnt provide a lot of pores that are good for root growth.

So i did some online searching on how to make the soil porous, enabling good water drainage, mixing compacted clay soil with organic matters to create a good environment for the trees and i came to this conclusion:

  • That i should dig the soil,
  • Remixing the soil with rich top soil, small-sized pumice stones, rice husks + rice husk biochars, composts
  • And water the soil with EM4.

What i am unsure is that how deep should i dig and remixing the soils ? Should i go to 1 meter depth ? Because the tree can grow very tall, so i assume the root would go deep also.

From my further reading online articles, some advised to avoid digging to deep because the soil will sink deeper with time, causing the tree's root collar to be lower than the soil surface, and in the end will help fungus grow and make the trees sick. Maybe i just need to raise the soil height on the plantation spots to address this issue ?

Another concern: As per my experience with polybags containing the mixture above, the soil becomes porous indeed with good water drainage, but it seems that the plants do not have stable soil to attach to, and i had to repositioned the plants from time to time to prevent them falling sideways.

Or maybe all of these are just too much for tree planting and that i should just skip soil amendment ?

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    My main concern is that the polyalthia longifolia can survive in a situation with such poor drainage. I can find no information on the cultural needs of these trees, so cannot help with this. Another concern is rainy/dry seasonality. Is this common where you live? When I gardened in similar clay soils (we made pottery from it), a drought would turn the "soil" into a cement-like surface. Is it feasible to call someone in to physically remove the clay from the topsoil with machinery? If so, and if this doesn't affeect drainage into buildings, then that would be my recommendation.
    – Jurp
    Feb 10 at 14:16
  • Hi @Jurp I'm living in Java island, Indonesia, Banten to be exact. I'm currently experimenting mixing the 'fresh' reddish clay soil with existing clay soil (it was red, but now just a bit brownish, most probably due to erosion caused by compaction) from manual drilling .. And i intend to add wood chips as mulch. Thanks for the recommendation.
    – Albert Gan
    Feb 11 at 15:32

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The roots of the polyalthia longifolia (false ashoka tree) grow deep. However, the primary concern here is not that the roots will be obstructed by the buried debris. Once the plant grows to that size, it will have enough power for its roots to dig through the broken debris. The main concern is the clayey top soil, which will prevent both water and sufficient air to pass through when the plant is young.

For optimum growth of your plant, I think that you should dig up to 1 meter of your clayey soil, as you have contemplated. However, there is a better way to augment your soil. I would suggest that you avoid any organic matter including rich top soil from elsewhere, and just mix your dug up clayey soil with coarse sand in a three to one ratio (three part clayey soil to one part sand). Sand with a coarse grain is needed, similar to the yellow or brownish sand used in construction works. Sands with finer grain would not work here. The main reason to avoid organic matter and to go with coarse sand is that organic matter decays over time, so the porosity as well as the volume of the mixture would decrease, causing the soil to sink. Using coarse sand would alleviate these problems. Depending on the desired porosity, you may increase the sand proportion up to a one to one ratio with the clayey soil. Check that water now drains satisfactorily.

After preparing this soil mixture and filling up the dug hole with it apart from the new plant, mix some organic fertilizers on the top soil. It is enough to mix the organic fertilizers only 6-10 cm deep. Over time, it will seep downwards with water. Water your young plant regularly but do not over-water, and in time, it will grow into a majestic tree.

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    I disagree with mixing the sand and the clay, because you are essentially creating a clay flowerpot around the tree. It's far better to just dig a whole the depth of the rootball and twice its width, break up the soil you remove as much as possible, and use it to backfill around the tree. The pore spaces around the tree's roots will be the same so there will be no hesitation for the roots to move into the clay surrounding the planting hole. Putting any amendments (especially organic, so I'm with @joy here) is a bad idea, as the roots are likely to stay in the hole and not move into the soil.
    – Jurp
    Feb 9 at 14:40
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    I wrote from my practical experience. Only breaking the compacted clayey soil and use only it to fill up the hole will increase porosity initially, but with continued watering, the broken up clayey soil will again get compacted due to its very nature, unless something with coarse grain is mixed with it, which would not decompose over time.
    – joy
    Feb 9 at 15:55
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    I, too, wrote from practical experience. Compaction is caused by vehicle parking, vehicle travel, and lots of foot traffic (i.e. a footpath) . With none of those present, the settling that occurs is not considered to be "compaction" because it doesn't affect the pore sizes in the same way as traffic does. The tree roots won't care. If you're going to use sand, you'll need at least a 2:1 ratio. Covering the site with woodchips is the best way to improve the soil structure. For more information: s3.wp.wsu.edu/uploads/sites/403/2015/03/soil-amendments-2.pdf
    – Jurp
    Feb 9 at 20:20
  • Thank you for sharing your insights @joy, I really feel enlightened.
    – Albert Gan
    Feb 10 at 8:29
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    Roots of many plants are extremely good at travelling long distances both shallowly and deeply. For example, a perennial called Liatris can have roots that travel 3 meters deep, and oak and hickory trees can have very deep tap roots. They've been doing this for many years without human help :)
    – Jurp
    Feb 10 at 14:13

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