Plants require certain mineral elements for healthy growth. Nitrogen along with phosphorus and potassium are the most important. Most plants are supplied adequate nitrogen from decomposing organic matter, irrigation water, and soil. The atmosphere supplies nitrogen that soil microbes convert into forms usable by plant roots. Conversely, applying excess nitrogen can damage plants and pollute the environment.
Slow growth and uniform yellowing of older leaves are usually the first symptoms of nitrogen (N) deficiency. Nitrogen-deficient plants produce smaller than normal fruit, leaves, and shoots and these can develop later than normal. Broadleaf foliage in fall may be more reddish than normal and drop prematurely. Nitrogen deficiency in stone fruits causes yellowing and reddening of leaves or red spotting at top of tree prior to when fall color normally develops.
Nitrogen-deficient conifers may develop few or no side branches. Lower canopy needles may be abnormally close together, short, and yellowish, while the upper canopy appears normal. Nitrogen-deficient palms develop a color gradation, with the oldest leaves being most chlorotic and completely yellow or whitish in severe cases.
Most garden and landscape plants do not require nitrogen application for good growth. Exceptions include fruit and nut trees, palms, roses, and certain other profusely blossoming shrubs. Nitrogen fertilization might be needed for plants growing in soils amended with large amounts of undecomposed organic matter or soil that is highly leached, very sandy, or in containers or planter boxes.
Visual symptoms (e.g., uniform yellowing of older foliage) strongly suggest nitrogen deficiency. To help confirm nitrogen deficiency, laboratory analysis of properly collected soil samples can be useful but difficult to interpret. Trees have a relatively large root system and nitrogen availability varies with soil conditions, which can change over short time periods. Leaf analysis can be obtained, but there are no guidelines for appropriate nitrogen levels in most ornamentals, although conifer and broadleaf foliage typically has 1 to 3% total nitrogen.
If testing foliage you may want to obtain separate leaf analyses on current-season foliage of unhealthy and healthy plants of same species growing nearby and compare the tests’ results. If testing soil you may want to take separate samples for comparison from areas with plants that appear healthy and unhealthy. First contact the plant diagnostic laboratory to learn the proper methods of sample collection and submission.
When established woody plants exhibit nitrogen deficiency symptoms, the cause is often poor soil conditions or unhealthy roots, which prevent plants from taking up nitrogen from soil. The causes of symptoms can include aeration deficit, cool soil temperature, mechanical injury to roots, poor drainage, root-feeding insects or nematodes, root decay pathogens, soil compaction, and too little or too much irrigation. For example, plants may exhibit nitrogen deficiency in early spring when soils are too cold or wet to allow roots to absorb sufficient nutrients for new growth. Once topsoil warms and drains after the rainy season ends, foliage develops its healthy appearance. Adding nitrogen will not remedy these causes of insufficient nitrogen uptake by roots.
If you determine that nitrogen application is needed, use the correct type, rate, and method of fertilization for that situation. Inappropriate fertilization can damage plants and cause other problems discussed in nitrogen excess.
Nitrogen is commonly provided to plants as organic matter (e.g., organic mulch that slowly decomposes) or as inorganic compounds (e.g., ammonium nitrate or ammonium sulfate). Commercial slow-release fertilizers (e.g., sulfur- or polymer-coated urea) provide the easy handling of synthetic fertilizers but also slow-release characteristics. Although more expensive than other preparations, these can be a good choice for adding nitrogen to deficient soils.
Fertilizers commonly contain nitrogen (N), phosphorus (P), and potassium (K), listed as NPK on the fertilizer label. Except when growing palms and possibly other woody monocots (plants with a single seed leaf) or when soil is highly leached or very sandy, soil around landscape trees and shrubs is rarely deficient in all three of these elements. Adding sufficient complete fertilizer to provide the deficient element can result in an excess of the other nutrients and may contribute to salinity problems and pollute water. In most situations avoid the application of complete fertilizers and also avoid products containing both fertilizer and pesticide.
Fertilize established woody plants according to their specific needs. See "Soil and Fertilizer Management" in the California Master Gardener Handbook for more information.
Adapted from the publication above and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Uniformly yellowish older leaves and greener newer foliage due to nitrogen deficiency.
Yellow leaves especially in older foliage due to nitrogen deficiency.
Nitrogen deficiency in stone fruits causes yellowing and reddening of leaves at top of tree with some red spotting.