Woody species in landscapes, with the possible exception of young plants and fruit and nut trees, should not be routinely fertilized. As long as your trees and shrubs have normal-sized leaves and appropriate growth and color, the nutrients in your soil are probably adequate.
Learn to recognize symptoms of nutrient deficiencies in established plants.
Commonly, when woody plants are nutrient deficient, nitrogen and iron are the most likely suspects.
Yellowish older leaves or needles; undersized, slow-growing plants; sparse new growth; and premature foliage drop are all signs of nitrogen deficiency.
Iron deficiency can mean small and yellowish new foliage that is green only along the veins, dead spots between the veins, or leaves that drop and dry prematurely.
Less common nutrient deficiencies are zinc, phosphorus, and potassium.
A zinc deficiency could produce yellowish leaves, delay new growth, cause small and narrow leaves, or result in purplish foliage.
Dark green or bluish foliage, short and spindly slow growth, or spots developing on foliage can indicate a need for phosphorus.
Plants in need of some potassium can have sparse foliage growth or yellowish older foliage that may have brown tips and margins near the leaf edge or between veins.
When you have eliminated other problems as the cause of poor growth and suspect nutrient deficiency, fertilize only as needed, following the manufacturer’s directions.
Overfertilization, especially with high-nitrogen fertilizers, can promote excessive growth, which invites increased populations of pests, such as mites and aphids, to the young leaves and may also crack the bark on your plant, allowing the entry of fungi.
• The Green Thumb is a column by Tracy’s master gardeners. University of California certified master gardeners are available to answer gardening questions from 9 a.m. to noon Tuesdays, Wednesdays, and Thursdays at 953-6112 or firstname.lastname@example.org.