Any soil experts? Or plant experts?

I've had two trees die in my backyard in the past few years.  The only thing that I can come up with is that there's something buried in the ground that gets into the roots when it pours down rain.

So here's the situation, about two years ago I had a mature Calamondin die.  It died after a severe rainstorm left 3-4" of standing water in my backyard.  The Calamondin was right on the edge of this monstrous puddle.  I thought that it just drowned, but now I'm not quite as sure.

In a different area of the yard, but still on the edge of where the low-spot of the puddle is I planted a Magnolia sapling, six feet when it went in the ground.  It was in the ground for a year and a half, about 12 feet tall now (guestimate) and then the rain happened.  Another 3-4" of standing water, with this not under water at all, now its dying.

Now, I know that Magnolias live in the swamp, this one was pulled from a swamp as a baby sapling so it drowning is incredibly unlikely.  Now, like I said, I'm thinking that there is something in the ground that is poisoning them when the rain soaks in.  

If there is something it would have to be something that would kill a Calamondin and a Magnolia, but not a Hope Philodendron or a Chinese Tallow (as they are equidistant from, and inbetween, where both were planted)

Anyone think of anything?

A few random musings:

Any sign of an attack by insects?  I have one maple in particular that always seems to have issues with damage to the bark near the ground and seems always to be infested with bugs.

The standing water could certainly be an issue for some species.

I had a couple of large Euonymus shrubs die back severely this Spring, but I attributed it to the very hot, very dry spell we had at about that time.

Consider getting a soil test.  Plants will usually tolerate a pH range, but your soil could be near one extreme for certain species.

The "Sweetbay" magnolia is the only magnolia that I see frequently suggested for very wet soil in a quick googling.

What is your soil like? Loamy or clay? Rich in organic matter or poor? Well aerated or prone to compaction? Your problem sounds a lot like the roots dying from lack of aeration. Plants don't drown from excess water, they drown from lack of oxygen to the roots when the water displaces all the air in the soil. Many swamp soils are extremely rich in organic matter, which imparts a lovely loose texture to the soil, which is good for aeration. Even if the magnolia is adapted to wet swampy ground, it won't do well with wet, compacted, poorly aerated ground. Many yards are covered in whatever the contractor dug up from building the foundation, which is often very poor subsoil, and very poorly aerated.

Call up your local ag extension office?

I do have a background in soils and plants but can't really tell you much based on the OP.

Did you apply weedkiller to get rid of the nutsedge? If so that MAY have something to do with the magnolias demise.

mtn said:

Call up your local ag extension office?

Take a couple of baggies of soil to them, put soil in their sample bags (fill out info on bag before putting soil in). They charge a small fee. Results usually take a couple of weeks.

Just came here to say that I killed a Pothos in 3 weeks.

Don't ask me for plant advice.