Ammonia (\(\text{NH}_3\)) is often recognized as a harsh cleaning solution, but it is also a crucial nutrient for plant life, serving as the precursor to nitrogen necessary for grass growth. However, concentrated forms are highly toxic to turf. The answer to whether ammonia kills grass is definitively yes, especially when applied as a common household cleaner (typically 5% to 10% ammonia by weight). This high concentration quickly overwhelms the grass’s biological systems, causing rapid chemical damage that results in dead, brown patches across the lawn.
The Lethal Threshold: Concentration and Toxicity
The difference between a fertilizer that promotes robust growth and a chemical that destroys a lawn is entirely a matter of concentration. Grass plants absorb nitrogen in controlled, small doses, typically in the form of ammonium ions (\(\text{NH}_4^+\)) or nitrate (\(\text{NO}_3^-\)). Lawn fertilizers provide these nutrients in measured amounts, often using chemically bonded or slow-release forms like urea or ammonium sulfate to prevent an overwhelming influx. This regulated delivery allows the turf to safely metabolize the nitrogen.
A concentrated ammonia spill floods the soil with far more nitrogen than the grass can process. The toxic threshold for common grass species, such as ryegrass, is low, with negative effects visible at soil ammonium concentrations around 200 milligrams per kilogram of soil. When the concentration exceeds this point, the plant is exposed to a massive chemical overdose. High levels of free ammonia molecules and ammonium ions instantly cross the line from nutrient to poison, causing rapid cellular failure instead of healthy growth.
How Ammonia Chemically Destroys Plant Cells
Concentrated ammonia destroys plant tissue through a two-pronged chemical attack involving pH shock and osmotic stress. When the aqueous ammonia solution hits the soil, it reacts with water to form ammonium ions and hydroxide ions (\(\text{OH}^-\)), which drastically increases the soil’s alkalinity. This rapid spike in pH, which can temporarily push the soil far above its healthy range of 6.0 to 7.0, is known as a pH shock that immediately breaks down the grass’s cell membranes.
The second destructive mechanism is a severe nitrogen overdose that acts like a chemical burn, often called “salt burn.” The massive influx of ammonium ions creates a hyper-saline environment around the grass roots and leaves. This high concentration of solutes draws water out of the grass cells in a process called osmosis, leading to rapid dehydration. This disruption causes the leaves and roots to chemically dry out, resulting in the immediate browning and death of the affected turf.
Immediate Steps After an Accidental Spill
If a spill of concentrated household ammonia occurs on the lawn, the immediate action is massive dilution with water. Thoroughly soak the area with a garden hose for several minutes to wash the ammonia deep into the soil and reduce its concentration below the toxic threshold. The goal is to dilute the solution enough so the ammonia converts to less harmful ammonium and disperses before causing irreversible cellular damage.
After diluting the initial spill, homeowners should consider applying a mild acid to help neutralize the temporary alkalinity caused by the ammonia. A solution of white vinegar or a weak solution of citric acid can be lightly applied to the affected area to help bring the soil pH back down to a neutral range. Following these immediate steps, the area must be monitored, and any turf that has turned completely brown will require raking and reseeding once the immediate chemical threat has passed.