Common salt, or sodium chloride (NaCl), has a powerful biological effect that is primarily lethal due to osmosis. This mechanism describes the movement of water across a semi-permeable membrane from low to high solute concentration. When salt is applied to an organism, it creates a hypertonic environment where the salt concentration outside the cells is significantly higher than inside. This imbalance forces water out of the cells to equalize the concentration, leading to rapid dehydration and the collapse of cellular function.
Salt’s Role as a Microbial Inhibitor
The ability of salt to desiccate cells is the foundation of its long-standing use in food preservation, targeting microscopic life such as bacteria, molds, and fungi. By drawing water out of microbial cells, salt effectively lowers the water activity of the food product. Bacteria require a certain level of moisture to sustain their metabolic processes and reproduce, and high salt concentrations inhibit this necessary function.
Historically, techniques like curing meat and fish or brining vegetables relied on this principle to prevent spoilage. Even in modern food production, salt is used to create a hostile environment for pathogens. For instance, in fermentation processes for foods like sauerkraut and kimchi, salt restricts the growth of undesirable spoilage bacteria while simultaneously favoring beneficial, salt-tolerant lactic acid bacteria. High salt concentrations are highly effective at stopping microbial growth and rendering cells dormant or non-functional.
Desiccation of Soft-Bodied Invertebrates
Salt’s dehydrating effect is immediate and highly visible on specific soft-bodied invertebrates, such as slugs, snails, and leeches. These terrestrial gastropods and annelids are extremely susceptible because they lack protective barriers like the thick, keratinized skin of mammals or the hard, waxy cuticle of many insects. Their skin is highly permeable and functions as a semi-permeable membrane, which is necessary for respiration and maintaining their moist environment.
When salt granules touch the moist, mucus-covered exterior of a slug, the salt dissolves and creates a powerful hypertonic solution directly on the skin’s surface. Water inside the creature’s cells rushes out through osmosis to dilute the external high-salt solution. This rapid and uncontrolled water loss causes the animal’s cells to shrivel and collapse, resulting in death by desiccation within minutes. The visible reaction, where the creature appears to “melt,” is a direct result of this catastrophic fluid loss.
Salt’s Effect as a Non-Selective Herbicide
Using salt as a weed killer causes plant death through severe dehydration and toxicity. When a high-concentration salt solution is applied to the soil, it dramatically increases the concentration of solutes surrounding the plant roots. This hypertonic environment reverses the natural process by which plants draw water up through their roots.
Instead of absorbing water, the plant roots lose water to the soil through osmotic pressure, leading to wilting, browning, and eventual death, known as osmotic stress. Furthermore, if the plant absorbs the sodium and chloride ions, the excess minerals interfere with internal cellular processes like photosynthesis, contributing to the plant’s demise. Salt is a non-selective herbicide, meaning it kills any plant it contacts.
The sodium ions are particularly damaging to soil structure and remain highly residual, meaning they do not easily break down or evaporate. This lingering salinity can prevent future plant growth in the treated area for a significant period. Consequently, salt is most effective in areas like cracks in pavement or driveways where barren ground is the desired outcome.
Responsible Application and Environmental Impact
The residual nature of salt presents considerable environmental risks that require cautious application. When salt is used excessively in gardens or lawns, it increases the overall salinity of the soil, which can degrade soil health and fertility. High concentrations of sodium can disrupt the delicate balance of beneficial soil microorganisms, such as bacteria and fungi, which are necessary for nutrient cycling.
A significant concern is the potential for runoff contamination, where rainwater washes the dissolved salt into nearby soil, surface water, or groundwater. This salt runoff can contaminate streams or affect other plant life far from the application site, potentially harming aquatic ecosystems. For these reasons, salt should only be used as a spot treatment in isolated, non-drainage areas, such as walkways or patios. Limiting salt use to areas where permanent sterility is acceptable is the only responsible way to employ it as a control agent.