Saltwater Intrusion and Slow Tree Decline

Salt in the air burns leaf margins and everybody notices. Salt in the soil is slower, more damaging, and almost never diagnosed correctly.

How it gets there

Storm surge pushes seawater inland and it soaks into soil. High water tables near the lagoon and the Halifax can carry saline groundwater into the root zone. And irrigation with brackish water from a shallow well does it continuously.

Once salt is in the soil it stays until enough fresh rainfall leaches it out, and in sandy coastal soils that can take a long time.

What it does to a tree

Salt in soil interferes with water uptake. Osmotically, high salinity outside the root makes it harder for the root to draw water in — so a tree in salty soil can be genuinely drought-stressed while standing in moisture. Specific ions are also directly toxic to root tissue.

The result is a tree that declines slowly over several years with no obvious cause.

The symptoms

• Marginal leaf burn — browning from the leaf edges inward.
• Smaller leaves than the species should produce.
• Reduced annual growth.
• Progressive crown dieback, usually starting at the tips.
• Poor recovery from other stresses.

The pattern that points at soil salinity rather than aerosol salt is that it affects the whole crown rather than only the windward side, and it does not improve after a wet season the way aerosol damage does.

What can be done

Leaching. Deep, prolonged fresh watering can move salt down below the root zone in well-drained soil. It does not work where drainage is poor.

Gypsum can help displace sodium in some soil conditions. It is worth a soil test before spending money on it.

Stop making it worse. Check irrigation water quality. A shallow coastal well can be well above the salinity a landscape tolerates.

Species selection. Where salt is a permanent condition, the answer is a tree that tolerates it rather than one that is fighting it.

When it is too late

A tree in advanced decline from soil salinity does not usually recover. That is worth knowing before spending on treatment. We will tell you honestly which side of that line a tree is on.