Boron toxicity or other specific ions in excess
Tiny amounts of boron are essential for plant growth. But there is a narrow acceptable range between sufficient and excess concentrations. Boron and other specific ions (most commonly chloride and sodium) can be toxic to plants even when overall salt measures, such as electrical conductivity (EC) and total dissolved solids (TDS) as discussed in Nutrient and Mineral Excesses, Salinity, and Salt Toxicity, are within acceptable limits. Toxicity symptoms of specific ions are difficult to distinguish from each other and from symptoms of overall high salinity or water deficit.
Typical symptoms of toxicity from exposure to excess boron or other ions are yellowing of leaf margins and tips and sometimes between veins. Symptoms progress to necrosis (browning) and premature drop of foliage.
In many plants, boron accumulates in older foliage, causing the margins or tips of leaves and needles to turn yellow, then brown or blackish. In certain plants (e.g., cotoneaster, gardenia, privet, pyracantha, rhaphiolepis, and syringa) boron accumulates mostly in new terminal growth and symptoms of toxicity appear in young, expanding foliage. In apple, pear, and stone fruits and other Prunus spp., boron toxicity symptoms appear in young, expanding foliage. Symptoms can include misshapen fruit and cankering and dieback of petioles and young twigs. In any plant, excess boron can cause bark to crack or become corky. Severely affected plants can die.
High concentrations of boron, chloride, and sodium occur naturally in soils and water in the situations described in Nutrient and Mineral Excesses, Salinity, and Salt Toxicity and this table. Ammonium toxicity can occur after excess application of ammonium fertilizers or incorporation of soil amendments high in ammonium, such as certain manures.
Diagnose specific ion toxicity based on the visual symptoms in plants, knowledge of locations where those ion concentrations are high, and ion-specific laboratory tests of soil, water, or both. For most species of landscape plants, boron toxicity occurs when soil contains about 1 ppm or more of boron or irrigation water exceeds about 0.5 ppm boron.
General salinity tests (measures of total EC and TDS) do not tell you the concentrations of specific ions. More than one test may be needed depending on the suspected causes of problems, such as measure of EC, sodium adsorption ratio (SAR), and each specific ion. You may need to separately collect and submit for laboratory analyses more than one type of sample (e.g., irrigation water, older plant leaves, and soil). When testing foliage, collect separate samples from nearby plants with apparently healthy foliage for comparison testing.
Manage specific ion toxicity with the "Solutions" described in Nutrient and Mineral Excesses, Salinity, and Salt Toxicity. Modify irrigation to prevent wetting of foliage. Where soil is high in boron or other salts, apply only high-quality (low salt) irrigation water and improve drainage so that ions are gradually leached below the root zone. Grow boron-tolerant plant species, such as those listed in Abiotic Disorders of Landscape Plants.
For more information see the publication above, Boron is High in Many Southern San Joaquin Valley Citrus Trees, Redefining Boron Toxicity Symptoms in Some Ornamentals , and Water Quality: Its Effects on Ornamental Plants .
Adapted from the publications above and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Boron toxicity in citrus, highest concentration (top left) to lowest (bottom right).
Marginal necrosis in chrysanthemum due to boron toxicity (right) compared with healthy leaves.
Marginal necrosis of walnut leaves due to excess boron and chloride.
Brown, dead needles on coast redwood due to root exposure to irrigation water high in boron.