Plants require certain mineral elements for healthy growth. Potassium is one of the most important along with nitrogen and phosphorus. For most plants adequate potassium naturally occurs in soils. In California potassium deficiency is common in palms and otherwise generally occurs only in fruit and nut trees grown in sandy soils and plants growing in containers or planter boxes.
All palm species are susceptible to potassium deficiency. Symptoms always appear first in older leaves and eventually progress into younger leaves, but symptoms otherwise vary among palm species. The most common symptom is yellow or orangish flecks or spots on older leaves, which appear translucent when discoloring is observed from the leaf underside. In some species, yellowing begins at the leaf margins or tips and leaves gradually become entirely yellow, then brown and withered. Leaf midribs may be yellow instead of their normal green.
Potassium deficiency in broadleaves causes leaves to turn yellow and then brown at the tips and margins and between veins. Older leaves are affected first and can entirely discolor, crinkle, curl, roll along edges, or die and drop prematurely. In potassium-deficient conifers, older foliage turns dark blue-green, progressing to yellow then reddish brown. Needles are often undersized with brown, dead tips.
Except for fruit and nut trees in sandy soils, palms, and plants growing in containers or planter boxes, potassium deficiency symptoms are usually not due to insufficient potassium in soil. Symptoms in other plants and situations are generally due to adverse soil conditions and anything that injures roots or restricts root growth, thereby limiting nutrient uptake. Common causes of deficiency symptoms include aeration deficit, compaction, high soil pH (especially with plants adapted to acidic soil), inappropriate irrigation, mechanical (physical) injury to roots, poor drainage (waterlogging), root decay pathogens, and root-feeding nematodes. Some sucking insects, foliar pathogens (e.g., diamond scale fungus of palms), and preemergence herbicides cause similar damage symptoms.
Diagnose potassium deficiency based on visual symptoms, plant susceptibility (e.g., container plants, fruit and nut trees in sandy soil, palms), and the presence of soil conditions that favor deficiency (high leaching, sandy soil, sparse topsoil).
To correct a deficiency, spread organic mulch beneath plants and apply potassium fertilizer, preferably slow-release forms such as potassium silicate or sulfur- or polymer-coated potassium products. Potassium sulfate may be used, and potassium will be held by organic matter and clay particles. Especially with sandy soils, avoid readily leached materials (e.g., potassium nitrate and potassium sulfate). Potassium nitrate may also cause an excess of nitrogen unless nitrogen deficiency is also a problem. Do not use potassium chloride where chlorine or salt toxicity are problems. Because a high potassium concentration reduces magnesium availability, and excess magnesium makes potassium unavailable, it may be best to add both potassium and magnesium in combination if one of these is deficient.
A slow-release fertilizer containing magnesium and a 3-1-3 ratio of nitrogen, phosphorus, and potassium is good for palms. Symptomatic palm foliage will not recover, and you must wait for new growth. To avoid aggravating potassium deficiency, do not remove symptomatic leaves until they have turned entirely brown.
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).
Yellow stippling of older fronds of queen palm due to potassium deficiency.
Yellow citrus leaves bent at tips and margins due to potassium deficiency.
Cupped edges and bronze to gray discoloration along margins of potassium-deficient pear leaves.
Yellowing between veins of avocado leaves due to potassium deficiency.