Ammonia (\(\text{NH}_3\)) has a dual nature regarding plant health. It is a fundamental component of the nitrogen cycle, necessary for all life. Plants require nitrogen to build proteins, nucleic acids, and chlorophyll, and ammonia is the direct precursor for these building blocks. While ammonia is vital for growth, its ionic counterpart, ammonium (\(\text{NH}_4^+\)), and its gaseous form become highly toxic when they accumulate in high concentrations in the soil or air. The difference between a necessary nutrient and a damaging poison depends entirely on the concentration and the environment surrounding the plant roots.
Ammonia as a Critical Nitrogen Source
Nitrogen is the nutrient plants require in the largest quantity, defining plant productivity. It is incorporated into chlorophyll, which captures light energy for photosynthesis, and into amino acids that form proteins and enzymes necessary for metabolism and growth. Plants primarily absorb nitrogen from the soil solution in two inorganic forms: ammonium (\(\text{NH}_4^+\)) and nitrate (\(\text{NO}_3^-\)).
Ammonium is an immediate nitrogen source that plants can use with less energy than nitrate. Once absorbed by the roots, ammonium is quickly assimilated into organic compounds like glutamine and glutamate through the glutamine synthetase-glutamate synthase (GS-GOGAT) pathway. This rapid conversion is necessary because the accumulation of ammonium ions inside plant cells is toxic.
In most well-aerated soils, the risk of ammonium toxicity is mitigated by soil microbes through nitrification. Nitrifying bacteria first oxidize ammonium to nitrite (\(\text{NO}_2^-\)), and then rapidly convert the nitrite into the much less toxic nitrate. Nitrate is the most prevalent form of nitrogen in aerobic soils and can be stored in the plant’s vacuoles until needed.
How Excess Ammonia Causes Harm
When ammonium concentrations become too high, the assimilation process is overwhelmed, causing cellular damage. High levels of ammonium interfere with the plant’s ability to maintain a balance of essential ions, such as potassium (\(\text{K}^+\)), calcium (\(\text{Ca}^{2+}\)), and magnesium (\(\text{Mg}^{2+}\)). The excessive uptake of \(\text{NH}_4^+\) disrupts the root cell membrane’s electrical balance, leading to a shortage of these nutrients.
Direct cellular toxicity occurs because the plant must spend energy converting excess ammonium into non-toxic organic compounds. This process consumes large carbohydrate reserves, diverting energy from growth. Assimilation of \(\text{NH}_4^+\) also generates protons (\(\text{H}^+\)), which rapidly acidify the plant’s root environment (rhizosphere).
The drop in external pH inhibits root growth and damages root tips, impairing nutrient and water absorption. Gaseous ammonia (\(\text{NH}_3\)) loss (volatilization) from fertilizers or fresh manure can also rapidly increase the soil’s pH, which is damaging in alkaline conditions. This gaseous form can burn above-ground plant tissue.
“Fertilizer burn” is caused by highly concentrated ammonia salts. These salts increase the osmotic potential of the soil solution, drawing water out of the plant roots and tissues. This desiccation causes plant cells to shrivel and collapse, leading to the scorched, necrotic appearance of damaged foliage.
Identifying Symptoms and Remediation Strategies
The visual symptoms of ammonia toxicity manifest as a combination of nutrient deficiency and cellular damage. A common sign is chlorosis, or yellowing, which often appears first between the veins of the newer leaves in young plants. In mature plants, symptoms may appear on older, lower leaves.
The leaf margins may curl upward or downward, and in advanced stages, the chlorotic areas develop brown, dead spots known as necrosis. Below the surface, root growth is significantly inhibited, with root tips often dying off, which reduces the plant’s ability to recover.
Toxicity often originates from common practices, such as the over-application of nitrogen fertilizer high in ammoniacal nitrogen, or the use of fresh animal manure. Conditions that slow nitrifying bacteria activity, such as low soil temperatures (below 55–60°F), waterlogged soil, or very low growing medium pH, also increase the risk of ammonium accumulation.
The immediate remediation strategy for fertilizer burn or soil saturation is to flush the root zone thoroughly with clean water to leach out excess ammonium salts. If toxicity is due to soil buildup, switching to a fertilizer that uses only nitrate nitrogen can help restore the chemical balance. Correcting the soil pH is necessary, as an optimal range of 6.0 to 6.5 promotes the activity of nitrifying bacteria.