Common salt (sodium chloride) can kill algae, but its practical use is complicated by the necessary conditions for effectiveness. Algae are diverse, photosynthetic organisms found in nearly all aquatic and damp environments. While using salt to control these organisms is often considered, the method relies entirely on concentration and application. This introduces significant risks to surrounding life and materials. Understanding the biological mechanism is key to evaluating this method of algae control.
How Salt Affects Algae Cells
Salt works against algae by exploiting the biological process of osmosis. Algae cells, like most plant cells, are surrounded by a semipermeable membrane that regulates water flow. In a natural freshwater environment, the solute concentration inside the cell is higher than the surrounding water, causing water to constantly move into the cell.
When algae are exposed to a high concentration of sodium chloride, the surrounding environment becomes hypertonic. This means the water outside the cell has a significantly higher salt content than the water inside the algae cell. Due to osmosis, water rushes out of the algae cell to balance the concentration on both sides of the membrane.
This rapid water loss causes the cell’s internal contents to shrink and pull away from the rigid cell wall, a process known as plasmolysis. Severe dehydration and the resulting cellular collapse irreversibly damage the cell’s structures, including the chloroplasts responsible for photosynthesis. If the salt concentration is high enough and sustained, this cellular damage prevents the algae from growing and eventually leads to cell death.
Practical Effectiveness in Different Settings
The mechanism of cellular dehydration means salt can be an effective algaecide, but only at concentrations that are often impractical or dangerous in real-world settings. On hard, non-porous surfaces like patios, walkways, or empty fountains, high concentrations of salt can be applied directly. This is a viable, temporary solution since there is no concern for aquatic life. The salt can be left to dry and dehydrate the surface algae before being rinsed away.
In swimming pools, a common misconception exists regarding salt use and algae control. Salt chlorine generators maintain a low salinity level, typically between 3,000 and 4,000 parts per million (ppm). This level is far below the concentration required to induce plasmolysis in algae. The salt itself does not kill the algae; rather, the generator uses the salt to produce free chlorine, which is the actual sanitizing agent.
Using salt to combat algae in ponds, aquariums, or other living water systems is generally ineffective and strongly discouraged. The concentration of salt needed to be toxic to algae is often lethal to freshwater fish, beneficial bacteria, and aquatic plants. For instance, sensitive fish species can be harmed by concentrations above 1,000 ppm. To kill string algae, a concentration of approximately one pound of rock salt per 100 gallons of water is necessary, a level typically too high for a mixed-species ecosystem.
Environmental Impact and Safety Concerns
The primary concern with using high concentrations of sodium chloride is the extensive collateral damage it inflicts on the surrounding environment and infrastructure. Salt build-up in soil, caused by runoff, is highly detrimental to terrestrial plants, including lawns and garden beds. High salt levels create “physiological drought,” where salinity prevents plant roots from absorbing water, even if the soil is moist.
Impact on Soil and Plants
The sodium ions in the salt can destroy the structure of clay-containing soils, leading to compaction and poor drainage. This effect inhibits root growth and can render fertile land unproductive. Symptoms like scorched leaves, stunted growth, and premature plant death appear gradually over time.
In aquatic environments, adding excessive salt disrupts the delicate ecological balance. Aside from direct toxicity to fish and amphibians, the influx of chloride ions can affect oxygen levels and the natural mixing of waterways. The resulting mass of dead algae rapidly decomposes, consuming oxygen and degrading water quality. This often creates more problems than the initial algae bloom.
Damage to Infrastructure
High salt concentrations also pose a physical threat to human-built structures and equipment. Sodium chloride is corrosive to metal components, potentially damaging expensive pool pumps, filters, and heaters. Over time, salt can also degrade porous materials such as concrete, stone, and tile, leading to cracking and spalling.