Nitrogen is an essential element for plant growth, but most landscape plants do not require nitrogen fertilization to grow well. Most plants are supplied adequate nitrogen from decomposing organic matter, irrigation water, and soil. The atmosphere also supplies nitrogen that soil microbes convert into forms usable by plant roots.
Nitrogen fertilization increases plants' need for irrigation and pruning and can cause plants to outgrow the available space and die prematurely. Applying nitrogen can undesirably alter soil pH, contribute to excess soil salinity, and pollute water. When applied late in the growing season, nitrogen application can increase plants’ susceptibility to freeze damage.
Symptoms of excess nitrogen include thickened and sometimes cupped leaves with atypically deep green color. Overfertilization can cause leaves to turn brown, gray, dark green, or yellow at margins and tips or overall. Affected foliage may wilt temporarily or die and drop prematurely.
Plants can be directly injured by the presence of excess nitrogen. Excess nitrogen can cause plants to grow excessively and develop overly succulent leaves and shoots, which promotes outbreaks of certain sucking insects and mites. Excessive nitrogen causes fruiting plants to produce relatively more foliage, reducing their fruit production and delaying fruit maturity. Fruit quality and yield can be reduced by the application of excess nitrogen. Excess nitrogen can kill small roots and increase plants’ susceptibility to damage by root-feeding nematodes and root decay pathogens.
Most established woody species do not need nitrogen application to grow well. Nitrogen fertilization is commonly needed only for fruit and nut trees, palms, roses and certain other profusely blossoming shrubs. Nitrogen fertilization may also be warranted for plants growing in soils amended with large amounts of undecomposed organic matter, highly leached or very sandy soil, or that in containers or planter boxes. For more information, see nitrogen deficiency.
Adapted from Abiotic Disorders of Landscape Plants, UC Agriculture and Natural Resources, and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Abnormally dark green foliage from excess nitrogen (left) compared with healthy foliage (center) and nitrogen-deficient, chlorotic leaves.
Yellow at tips and between the veins due to excess nitrogen compared with a healthy leaf (right).
Chlorotic and necrotic leaf tips and margins due to nitrogen toxicity.
Pruning and irrigation need increase when excess nitrogen is present in soil.