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Olive Tree QuestionMy olive tree is planted in the ground as shown

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Olive Tree Question

My olive tree is planted in the ground as shown. However, the gardener planted it in a very sandy potting mix beneath which (about a meter down) is the native soil. The native soil is very hard and stony (probably compacted by heavy machinery during the construction of my house). The first year the leaves turned yellow and most of them died. This year, after trimming all the old foliage off, the tree has come back to life and now the roots spread throughout the brick flower bed it is planted in. However, now some of the leaves at the tips of the branches are starting to lose their color again. I suspect this may be due to a drainage problem caused by the hard compacted soil beneath the sandy potting mix. It might also be a problem with too little or too much water. Obviously, it would be better if the tree had been planted in better conditions, but given the current conditions does anyone have any ideas on how to keep it healthy? For example, how often should I water it over the winter? Also, is there any way to improve drainage without digging the tree up?

Olive P1030986 P1030989



Gosh! Its looking great isnt it! Well, compared to mine it looks great anyway! Japan has a big range of climates...what is yours like? I would have thought it a little early yet for the tree roots to be battling with the soil if its a metre down...but they could be. But honestly..its looking good to me.they lose a lot of leaves every do all evergreens. Thats natural and to be expected. Im certainly no expert, but if you are in a cooler climate it will probably like to be drier during the winter months. Sorry I can’t be of more help. :)

10 Nov, 2017


It looks as though there's just about room for a thin layer of mulch which might help as the worms take it down.

11 Nov, 2017


On the other hand, it looks like a layer of fresh potting soil has been added at the top, and the root flair has been covered. That will cause problems with the bark,and can eventually kill the roots and top of the tree, especially in poor drainage conditions.
As for the drainage, can you pound a breaker bar through the soil, into the compacted layer beneath? If so, pound the bar as deep into the layer as you can, and pull back hard on the bar to try to crack the layer. Do that several times around the edge of the watering basin. Compost tea will also break up compacted soil, both through flocculation and by attracting earthworms. It is slow acting, however, requiring several applications a year, for several years to make a noticeable difference.

11 Nov, 2017


Thanks for the comments. I've added two more photos above. One showing how the leaves at the tips of the branches are starting to lose their dark green color. The other shows the base of the tree with a couple of centimeters of top soil removed to give an idea of the depth the tree is planted at. What looks like fresh potting soil on the top is actually the remains of some mulch applied in the Spring. Looking at the leaves, do you think the discoloration is caused by poor drainage, over/under watering, planting depth or something else?

15 Nov, 2017


It looks like removing 2 cm of soil is just about right, Outernational. Ideally the soil level should be lowered over the entire root area, to simplify watering. It can be replaced with a coarse mulch, feathered down near the trunk, to make life harder for the weeds. If this were Phoenix, I would say that those leaves were showing salt poisoning--too much sodium in the soil or water. That seems improbable in Japan, though, unless you garden very close to the ocean.

15 Nov, 2017


Thanks Tugbrethil. It's interesting that you should mention the possibility of salt poisoning. The gardener filled the flowerbed with a very sandy soil so it's quite possible it contained too much sodium. I have another olive tree, planted in native soil, that is positively thriving, so I've always suspected the sandy soil in the flower bed could be the problem. Is there any way to test the salinity of the soil?

16 Nov, 2017


I don't know of any way other than to send samples to a soil lab. Salt in the soil should leach out over a period of months, with good rainfall, but I never heard of anywhere in Japan being considered very dry--maybe just my ignorance talking, there, though! :}

16 Nov, 2017


Yes, we have plenty of rainfall here, so maybe it's not the salt! It did get me thinking about the soil though. I did a bit of research and found the soil used by our gardener is actually "decomposed granite soil". Apparently it's very fine grained with poor drainage and breathability. In addition, it has almost no nutrition. If you're going to use it for growing things you need to mix in a lot of compost material and give the soil time to develop, which didn't happen in this case. Now I can see why I've had so much trouble with this olive tree! Unfortunately, all the landscape gardeners in my area seem to use this soil by default (probably because it's cheap and easy to transport). Now I don't know what to do. I'd like to have a really big olive tree in the center of the garden one day, but wonder if that is achievable in the current conditions. If I was to start again, what would I need to do? I guess I would need to remove all the granite top soil, break up the hard compacted layer of native soil beneath, and then mix the native soil with a more suitable potting mix?

22 Nov, 2017


What is the nature of the native soil, that it is so hard?
If it is clay, Dig it out at least 30 cm deep, break it up, and mix it 1 part clean sand, 2 parts compost, and 4 parts native soil. Before you fill in the area put a layer of conditioned soil about 2 cm deep in the bottom, and scratch that in with a cultivator, to create a transition layer. Any sudden change of soil texture will stop drainage. Gypsum, if the soil is alkaline to neutral, or limestone, if it is acid, will also help soften and open clay soils, over time.
If it is compacted sand, break up the compacted layer, and cover it with a 5-8 cm layer of compost. It may take a few years of twice yearly applications of compost to truly transform the soil. Breaking the compacted layer should restore drainage. If that doesn't happen you will need to dig deeper to find out what the true problem is.

23 Nov, 2017


I'm not 100% sure, but I think the "native soil" may have been compacted by heavy machinery during the construction of my house 18 years ago. To be honest, I don't know if it is truly native soil. It could have been transported here to provide a firm foundation for the house. I will have to ask the builder to see if he remembers.

Just a couple more questions:

At the moment there is about 80 cm of the "decomposed granite soil" on top of the compacted soil below. How much of the compacted soil would need to be loosened up to support a large olive tree? Digging up 30 cm of compacted soil would give a total depth of about 110 cm. Would that be enough, or would it need to be deeper?

If I decide not to dig up the olive tree, is there any chance the roots could grow into the compacted soil and the tree thrive in the future? I don't think using a breaker bar is an option due to the hardness of the compacted soil and the possibility of damaging the roots, but I'm caught in two minds about digging up the tree.

Any work would be done in the Spring so I have a while to think about it yet.

23 Nov, 2017


80 cm?! Waugh! It looks like the decomposed granite now IS your soil. 95% of any trees' roots will be in the top 70 cm of the soil. The compacted layer will still cause problems with drainage, but the most practical thing to do with it now is to drill holes in it every couple of meters, to give the water somewhere to go. To rehabilitate the granite, I would lay down 10 cm of composted crop waste (or whatever is similar available in your area) over an area about 7 meters across where you want the Olive tree. To that, I would add about 3-4 kg of bone meal, and about 2 kg of fish meal, then rent a good rototiller and till it all in about 30 cm deep. Afterward, I would put a sprinkler in the center, and run it long enough to apply a good 15 cm of water--3 hours for a saltshaker or owl-eye sprinkler, up to 12 hours for an impact or oscillating sprinkler. It's a good idea to keep about 5 cm of compost over the area to continuously replenish the organic content of the new soil, and thereby encourage the action of earthworms.

23 Nov, 2017


The area is covered with pebbles - if there's a membrane underneath doing all that will be rather challenging?

23 Nov, 2017


Yep, the first job would be to remove all of the pebbles and membrane (if any), though I thought that that was pretty obvious.
If you are able to wait, Outernational, you could just spread out 5-10 cm of compost, maintaining it twice a year, and after a few years that granite will turn into soil.

24 Nov, 2017


Thanks Steragram and Tugbrethil.

Just to clarify - there is an 80 cm deep layer of granite soil inside the bed. Beneath and to the sides is compacted soil. The inner diameter of the raised bed is 1 meter across. There's an anti-weed sheet beneath the pebbles so I'd rather not disturb the soil outside of the raised bed.

Would spreading out 5 to 10 cm of compost inside the bed work? Also, would a layer of decomposed leaves (mulch?) have the same effect? What exactly would you recommend? I don't have a compost patch so would need to buy something from the local garden center. There's a wide variety of products so I want to make sure I choose one with the right ingredients!

Also, would it be possible to drill holes in the compacted layer beneath the bed without damaging the roots?

25 Nov, 2017


Ah, I'm getting a better picture, now! You wanted to plant an olive tree in the yard, so the "gardener" dug a deep hole in the ground, filled it with decomposed granite, and planted the tree in that. He can't have apprenticed with an authentic Japanese master gardener!
If you want the tree to grow large, now, you will need to coax the tree to grow its roots outside the present bed. That means removing the membrane and conditioning the soil around the bed, ideally out to a 3 meter radius.
Afterward, you can use the improved soil to grow herbaceous plants or low shrubs, or you can cover it again with 5-8 cm of pebbles, which should work as well as membrane for suppressing weeds. The drawback to that is having to remove the pebbles every 6-12 months to apply more compost, and putting them back, after sifting all of the stray soil out of them.
As for the conditioning material, I don't know what products are available in Japan. In the U.S., what I am thinking of is called "garden compost" or "planting mix".
Don't worry about drilling holes in the clay under the present bed. The compacted layer is only in the top 15-40 cm of soil, and your "gardener" has already dug through that. Any present difficulties in drainage there are due to the sudden transition from granite to clay, and there is nothing that you can do about that now.

25 Nov, 2017


I'd rather not disturb the pebbles and membrane so any soil improvements will have to be done within the bed.

I'm trying to weigh up the pros and cons of the following two options.

Option 1: Improving the condition of the granite soil.

Option 2: Removing the granite soil and starting again with new soil.

Regarding option 2:

Would digging a deeper and wider hole, and then filling it with better soil work?

Could adding a layer of drainage material at the bottom of the hole improve drainage?

Could the current olive tree be replanted, or would it be better to plant a new olive tree?

3 Dec, 2017


OK, just so you know, not conditioning the soil outside the bed means--best case--the tree's roots will be very slow to spread outside the bed, forever stunting and stressing it. It will be much as if it is planted in a small island in a parking lot. If the membrane is the porous type--sold as "weed barrier" in the States--twice yearly applications of compost tea will gradually improve the soil, but may stain the pebbles, especially if they are marble or limestone.
Improving the granite can help some, and could start with compost tea, as above, and include a 5 cm layer of good compost on top. Digging compost in, the same way as improving the clay soil, will involve digging out and replacing the olive tree, so you might as well remove the granite entirely. If you do, I would replace it with more of the local clay soil, plus amendments, as in my fourth post back. If you use a commercial "top soil", it will be like growing the tree in a pot, and it will have to be watered and fed accordingly, and be just as stunted as if it was in a pot.
Thirty years ago, we master gardeners used to recommend leaving a layer of gravel at the bottom of the planting bed "for drainage", but we found out that that actually has the opposite effect. Any sudden change in soil texture stops the drainage. Instead, put a cm or two of the improved clay soil in the bottom of the hole, and scratch it in with a cultivator, to make a more gradual change to the unimproved soil.
As for keeping the present tree, you will need to dig up nearly the whole bed as a root ball, and then nearly bare-root the tree to remove the granite. That will be a serious shock to the tree, but olives are tough, and is likely to survive if around 1/2 to 2/3 of the foliage is pruned off, and you use a good root stimulator.

3 Dec, 2017


Thanks, that's given me quite a lot to think about! I really wish the landscape company had explained these issues to me when we were planning the garden! One thing I noticed about the granite soil is that there are no earthworms. Do you think introducing earthworms to the bed could be beneficial? Or would they have a hard time surviving in that environment? There's a field nearby with loads of earth worms but I wouldn't want to move them to the bed unless they had a very good chance of surviving.

4 Dec, 2017


Until there is a lot more organic matter in the granite, it will be an unfriendly environment for earthworms, because of all the small gravel and coarse sand in the mix. Other, smaller soil organisms will help prepare the way, so that the worms can get between the sharp particles without trying to swallow them. Putting a few handfuls of that good soil at the edge of the granite will help build up the populations of those smaller ones.

4 Dec, 2017

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