Plants require certain mineral elements for healthy growth. Phosphorus is one of the most important along with nitrogen and potassium.
Phosphorus generally occurs in soil in adequate amounts for most trees and shrubs. Exceptions are where topsoil has been removed during site development and in soils derived from serpentine minerals, such as in the Sierra Nevada foothills and parts of the Coastal Range.
Symptoms vary greatly when plants are deficient in phosphorus. In broadleaf plants, young leaves may be dark green and have purplish veins, especially on the underside of leaves. Older leaves can develop an overall purplish tint and tip dieback. Leaves may be curled, distorted, smaller than normal, or drop prematurely.
In phosphorus-deficient conifers, foliage on older trees is discolored gray-green or dull blue-green. With severe deficiency, few or no new needles are produced, and needles die prematurely starting with lower needles and progressing upward. Phosphorus-deficient seedling needles can turn purple starting at their tips and progressing inward and upward through the canopy.
A plant may be phosphorus deficient if tests of current-season foliage of conifers or woody broadleaves find less than 0.1% phosphorus. Another indication is comparison testing that reveals substantially less phosphorus in foliage of symptomatic plants versus nearby healthy plants of the same species.
If phosphorus is deficient in leaves, the likely causes are soil or root problems as described for nitrogen deficiency that prevent roots from adequately absorbing phosphorus. Certain herbicides also cause leaf distortion and curling that can resemble phosphorus deficiency symptoms.
In unusual cases when soil is phosphorus deficient, add a fertilizer such as ammonium phosphate. Chicken manure also provide phosphorus but can be detrimental because manure often has a high salt content. Regardless of the form, adding phosphorus to most soils in California is rarely beneficial to landscape plants.
Fertilizers commonly contain nitrogen (N), phosphorus (P), and potassium (K) listed as NPK on the fertilizer label. Except when growing palms or where soil is highly leached or very sandy, soil around landscape trees and shrubs is rarely deficient in all three elements. Adding sufficient complete fertilizer to provide the deficient element can result in an excess of other nutrients and may contribute to salinity problems and pollute water.
Established woody plants should be fertilized in response to specific needs. Complete fertilizers are generally not recommended for woody landscape plants, except for palms and possibly other woody monocots (plants with a single seed leaf). Also avoid products containing both fertilizer and pesticide. See "Soil and Fertilizer Management" in the California Master Gardener Handbook for more information.
Adapted from the publication above, 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).
Phosphorus deficiency commonly causes older leaves to curl, distort, and remain smaller than normal.
Unusually purple leaf veins and tip dieback from phosphorus deficiency.
Purpling of leaf undersides due to phosphorus deficiency.