Epsom salt, known chemically as magnesium sulfate, is a mineral compound with a dual nature when applied to a lawn. It is often touted as a natural fertilizer, providing nutrients that support robust grass growth. However, the answer to whether it can kill grass is a clear yes: it is an ionic salt, and like any salt, when applied incorrectly or in excess, it will damage or destroy turf. The difference between a beneficial supplement and a harmful application depends entirely on concentration and method. High concentrations can rapidly cause severe decline in a lawn.
How Excessive Epsom Salt Damages Grass
The primary mechanism by which an overuse of magnesium sulfate harms grass is known as osmotic stress, often described as “salt burn.” Grass roots absorb water through a process called osmosis, where water naturally moves from an area of low salt concentration to an area of high salt concentration. When too much Epsom salt dissolves in the soil water, the external salt concentration becomes extremely high.
This high concentration reverses the natural flow of water, pulling moisture out of the grass roots and into the surrounding soil. The grass cells become dehydrated because they cannot absorb water, even if the soil is visibly wet. This cellular dehydration effectively mimics a severe drought, leading to wilting and browning of the grass blades.
Beyond dehydration, the excessive mineral concentration can cause a chemical burn, particularly on the foliage if a concentrated liquid solution is sprayed directly onto the blades. This immediate damage is not due to a specific toxic chemical but rather the overwhelming presence of the minerals. The harm from excessive Epsom salt is a non-selective, physical reaction to the high salinity.
The Nutritional Role of Magnesium and Sulfur in Lawns
Despite the potential for harm, magnesium sulfate is a source of two components that are integral to healthy grass growth. Magnesium (Mg) is classified as a secondary macronutrient, and it plays an indispensable part in the plant’s biological machinery. Its most recognized function is as the central atom in the chlorophyll molecule, which is the pigment responsible for capturing sunlight during photosynthesis.
Without sufficient magnesium, the grass cannot produce chlorophyll efficiently, leading to a condition called chlorosis. This deficiency is visible as a distinct yellowing between the veins of the grass blades. Magnesium also acts as an enzyme activator, helping to regulate the uptake of other nutrients, such as phosphorus, which is needed for energy transfer within the plant.
Sulfur (S), the other component of Epsom salt, is also a secondary macronutrient required in large quantities by turfgrass. It is a necessary building block for the synthesis of amino acids and proteins. Sulfur also supports the production of enzymes and vitamins necessary for metabolism.
A sufficient supply of sulfur is directly related to the grass’s ability to utilize nitrogen, which is the nutrient most responsible for deep green color and rapid growth. Adding magnesium sulfate to soil that is deficient in these two elements can promote deeper green color, more vigorous growth, and a stronger root structure. This beneficial effect is the reason the compound is widely used as a soil amendment.
Determining Safe and Harmful Application Rates
The difference between using Epsom salt as a supplement and using it in a concentration high enough to cause death is a matter of precise measurement. For safe application as a nutrient supplement, the compound must be highly diluted to avoid reaching the concentration threshold that causes osmotic stress.
For liquid application, a safe dilution rate is typically about 2 tablespoons of Epsom salt dissolved in one gallon of water. This solution should be applied to the lawn once a month, usually in the spring and fall when the grass is actively growing.
For dry application, a general recommendation is to use approximately one-half pound of Epsom salt for every 100 square feet of lawn. The dry granules must be evenly spread with a broadcast spreader and immediately watered into the soil with at least an inch of water. This ensures they dissolve and distribute throughout the root zone.
Harmful concentrations are created when dry granules are applied thickly or left undissolved on the grass surface, or when a highly concentrated liquid is used. Applying dry salt directly to specific areas, such as patches of weeds or stubborn spots, guarantees severe damage to the grass due to the extreme local salinity. The high concentration of magnesium sulfate in these small areas quickly desiccates the plant tissues.
If accidental overuse occurs, the best course of action is to flush the area immediately with a large volume of plain water. Applying water heavily helps to dilute the excess salt and push it deeper into the soil profile, moving it away from the shallow grass roots. This remediation step can often mitigate the damage and prevent the grass from succumbing to salt burn.