Salt on the School Path
In winter, a school caretaker spreads rock salt on an icy path beside a flower bed. Meltwater runs into the soil. The next spring, plants near the path are stunted and some have yellowed leaves, while plants 3 m away are healthy. A student suggests the salt has poisoned the plants. A teacher suggests the salt has affected water uptake instead. The soil is sandy and drains quickly. The caretaker says salt is harmless because it is just sodium chloride, the same as table salt. The class is asked to judge between the explanations and consider a better winter strategy.
1. Compare the poisoning and water-uptake explanations, and evaluate the caretaker's claim that salt is harmless.
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Both explanations are plausible, but the water-uptake explanation fits the pattern better. Salt in soil lowers the water potential outside the roots, so water moves out of root cells by osmosis or is harder to take up. Stunted growth and yellow leaves are consistent with poor water and mineral supply. A poisoning explanation would need evidence of a specific toxic effect, which is not given. The caretaker's claim is weak: sodium chloride is harmless in small amounts in food, but high concentrations in soil harm plants. The same substance can be safe in one context and damaging in another. The sandy soil matters because it drains fast and does not hold the salt long, yet the effect is still seen, suggesting the salt concentration near the path was high.
2. Describe an investigation to test whether salt concentration affects plant growth, and suggest one alternative to rock salt.
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Use several identical pots of the same soil and the same plant species, with at least five salt concentrations, for example 0, 0.1, 0.5, 1.0 and 2.0 g per 100 g of soil. Water all pots with the same volume and keep light and temperature constant. Measure plant height and leaf colour every few days for three weeks, and record the mean of several plants per concentration. A control with no salt shows normal growth. If growth falls as salt rises, the salt explanation is supported. To separate poisoning from water uptake, measure soil water content or use a plant that is salt-tolerant. A better winter strategy is grit or sand for grip, or clearing paths before ice forms, because these do not add soluble salt to the soil.