Wet Soil, Thirsty Roots: Why Road Salt Pulls Water Out of a Plant
On a January night before a storm, trucks spread salt and brine across Virginia’s roads so the ice cannot bond to the pavement. The salt does its job, and then it goes somewhere. Plows throw salty slush onto the shoulder, passing cars lift a fine salt spray into the shrubs, and snowmelt carries dissolved salt into the soil along the road. By spring, a hedge beside a busy road can show leaves brown at the edges and tips, while the same kind of shrub fifty feet away looks fine. The odd part is that the soil around the brown hedge may be wet the whole time.
Which way the water goes
A root takes in water by osmosis. Water crosses the cell membrane from the side where it is more concentrated to the side where it is less concentrated, which means water moves toward the side with more dissolved particles. Normally the inside of a root cell holds more dissolved sugars and salts than the soil water, so water moves into the root. Virginia Cooperative Extension describes what happens when salt builds up in the soil: the direction reverses, and water moves out of the roots even when the soil is wet. The soil water has become hypertonic to the root cells.
Nothing in the cell pushes that water out. Osmosis is passive transport. It runs down the concentration gradient and costs the cell no ATP, just as water slipped out of the red onion cells under your microscope. In a plant cell, the stiff cell wall keeps its shape while the membrane and the cytoplasm shrink away from it. That pulling away is plasmolysis. A cell in that state cannot do its job, and leaves far from the root dry out at their edges first.
Turning the arrow around
Water in a cell that sits in fresh water pushes outward against the wall. That push, turgor pressure, is what keeps a watered plant standing and a crisp stalk of celery crisp. Some plants that live in salty places spend energy to protect it: protein pumps in their membranes use ATP to move salt out of the cytoplasm or lock it away in the vacuole, against its concentration gradient. That is active transport. A roadside boxwood does not do this well, so it loses the water instead.
| Salt in the cup (% by mass) | Mass before (g) | Mass after 24 h (g) | Change in mass | How the core felt |
|---|---|---|---|---|
| 0 (tap water) | 2.20 | 2.40 | +9% | firm, snaps when bent |
| 0.5 | 2.25 | 2.30 | +2% | firm |
| 1 | 2.20 | 2.11 | −4% | slightly bendy |
| 2 | 2.30 | 2.05 | −11% | bends without snapping |
| 4 | 2.25 | 1.85 | −18% | limp, rubbery |
- Osmosis
- The movement of water across a selectively permeable membrane, toward the side with more dissolved particles.
- Hypertonic
- Describes a solution with more dissolved particles than the cell. Water leaves the cell.
- Hypotonic
- Describes a solution with fewer dissolved particles than the cell. Water enters the cell.
- Plasmolysis
- The membrane and cytoplasm of a plant cell shrinking away from the cell wall as water leaves.
- Turgor pressure
- The push of water inside a plant cell against its cell wall. It keeps plants firm.
- Passive transport
- Movement across a membrane, from high concentration to low, that costs the cell no energy.
Sources: Laurie Fox and Joel Koci, Trees and Shrubs that Tolerate Saline Soils and Salt Spray Drift, Virginia Cooperative Extension publication 430-031 (2021), for water moving out of roots in salty soil even when the soil is wet, salt spray, and leaf-edge “salt burn”; the potato-core table is a sample class data set, labeled as such. SOL codes are a judgment call — confirm against the current Curriculum Framework.
Practice: Claim, Evidence, Reasoning
Claim: one sentence that answers the question and could be shown wrong. Evidence: a number, a row of Table 1, or a line from the reading. Reasoning: the principle that ties them together.
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Claim only
A neighbor says, “Road salt dries out the soil, and that is why the hedge turned brown.” Write a claim about where the hedge’s water actually went.
Claim -
Full CER · from the data
Using Table 1, decide about how salty the inside of a potato cell is, compared with the cups. Your evidence must cite at least three rows, including the two on either side of the switch from gaining to losing mass.
ClaimEvidenceReasoning -
Full CER · from the reading
Does a roadside shrub spend energy to lose water into salty soil? Use the words hypertonic, osmosis and passive, and compare it with what a salt-tolerant plant does.
ClaimEvidenceReasoning -
Mastery · new situation
A red seaweed that grows in the salty Chesapeake Bay is rinsed and left overnight in a bowl of fresh tap water. Predict what happens to its cells, and explain why its cell walls matter.
ClaimEvidenceReasoningWhat would change my mind -
Advanced Biology only
A classmate says, “A potato is not a boxwood root, so Table 1 tells us nothing about the hedge.” Decide how far the potato data can support a claim about roadside shrubs, and describe one test with real roots that would make you more confident.
ClaimReasoning