Plants keep dying in the same spot.

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Hi there,
I've had 2 different young plants die in one particular spot over the last 2 months. The location is where we had a small oak tree which we had taken down 2 months ago. It was not sick but due to its location near our house it was recommended be removed before it grew bigger. I can't remember the exact spot but it is around the area where these plants (Nepeta Tuberosa catmint and another pink flowering plant of which I don't recall the name) keep dying after a few weeks before they get to establish their roots. Now the 3rd plant (Coleonema pulchelleum Sunset Gold) is also starting to show a sign of yellowing. It's puzzling because all my other plants around that spot and across the garden are thriving.

When the second plant died, I inspected the soil which was dry, and many ants were coming and going from the spot. My initial thought was that the soil was not kept moist enough and that the ants started to make their nest or passages, creating air tunnels that eventually dried up the roots. So before I planted the 3rd plant, I made sure that the irrigation system is set at the right watering frequency and that the ants were mitigated. Now when I saw the yellowing of my Sunset Gold, I dug the plant out to see the roots and the soil. Looks all good and the soil is moist but ants came back very active. I dug further and saw there was a remaining root from the oak tree. Mind you, the oak tree's stem was completely removed and separated from the roots, so the roots are not alive.

Could it be possible that these roots are the cause for the ants (maybe pests reside there?) Any insights or advice is much appreciated!

PS- we live on the coast in Norther California (zone 10a). The garden location is SW, and the spot in question is in shade until about 1pm, thereafter full sun for about 6 hours.
 
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Welcome b-garden. :) The oaks roots will have sapped the soil of all nutrients which is possibly why your plants are dying. Have you dug in any manure or compost to revive it?
 
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Thanks for your reply Sheal. That makes sense. Another gentleman responded, whose post seems to be removed (but I saw in my email) said the similar thing. That underground roots are starting to decompose and they detract nitrogen from the soil. After the oak tree was removed, we amended the soil and mixed high quality compost. Perhaps that was not enough and I need to add more nitrogen?
 
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Is your irrigation system on all the time? Have you checked how far down in the soil the water is reaching? I'm wondering whether your Coleonema being a new plant is either suffering from over watering or not enough. It's roots won't have completely established yet and will still be close to the surface. If the watering is continuous then it's likely any nutrients are being washed straight through the soil and past the roots. Can you stop the irrigation around the plant for a few weeks to see if it makes a difference, watering by hand only when it needs it?
 
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Thanks for your feedback! Our irrigation system is not on all the time but it is set on schedule based on the weather forecast. I do check on a daily basis by hand the moisture level to make sure the plants are getting the right amount of water. When I dug out the Coleonema the soil was moist up to the surface, not soggy but seemed about the right amount. So, the depletion of nutrients sounds more like the plausible cause. I will have to experiment by adding more nutrients in that spot. Thanks much!
 

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