Many citrus trees in the southern end of the San Joaquin Valley are grown on moderately calcareous soils and frequently have high levels of boron in the leaf tissue. Citrus is sensitive to boron. Boron, when excessive, may cause defoliation and significant yield loss. At high, but nontoxic concentrations, leaf symptoms are similar to those caused by excessive salt, deficient potassium, heat stress, or biuret toxicity from urea foliar sprays. Therefore a leaf tissue analysis is important for delineating causes.
Excessive levels of boron produce a yellowing of the tip of leaves and yellow spotting of the leaf surface. Death of the leaf tissue may occur along the margins. Higher levels of boron may cause brownish, resinous gum spots on undersides of leaves but this symptom is not always present. Leaf symptoms are most severe on the “hot” south side of the tree. Boron accumulates in the leaves as they age so symptoms usually appear on older leaves first. Older leaves with high concentrations of boron are relatively short lived compared to trees that have boron at optimum concentrations. Often excessive boron and sodium appear together in leaf tissue analyses. Boron is associated with a decreased distance between leaf nodes. Trees with high leaf tissue boron concentrations appear to be less vigorous with shorter branches, probably as a result of the association of boron with decreased distance between leaf nodes.
Discussion of levels of boron which would be considered excessive in September-sampled spring-flush leaf tissue may be misleading because the particular leaves that are selected for the sample can greatly influence results. If only leaves with the most severe symptoms are sampled, such as leaves that are mostly yellow with dead margins, concentrations of boron can reach into the thousands of parts per million (ppm). A truer picture of the boron status of the grove can be gained by pulling leaves with ‘average’ symptoms. Using this sampling technique, the highest level of boron in orange leaves seen in this office over the past eight years has been 600 ppm from an isolated and particular calcareous part of an orchard located near the town of Edison in Kern County.
Standards from citrus in Florida for the concentration of boron in leaf tissue (4-6 month old leaves on nonfruiting terminals) correlate well with observations made in the San Joaquin Valley as follows:
Deficient <20
Low 21-35
Optimum 36 - 100
High 100 - 200
Excess > 250
Leaf boron concentrations greater than 250 ppm are excessive, but in older orange, lemon and grapefruit trees visible leaf symptoms are not usually manifested until leaf-tissue boron concentrations exceed 300 ppm. A range of 300 to 400 ppm show little outward sign of boron toxicity except for some slight tip yellowing and some reduction in vigor. Excessive defoliation does not usually begin in most citrus until concentrations of approximately 450 ppm are reached. Trees at 450 ppm and greater will, generally, exhibit a thin-canopied, unthrifty, somewhat stunted appearance. The yield of the tree does not appear to be affected nearly as rapidly as the appearance of the canopy. At least one large lemon grove in Kern County, that characteristically produces excellent yields of early-maturing, good quality fruit, has elevated leaf-boron levels. Moderate levels of leaf boron, in the 300 to 400 ppm range in this orchard appear to reduce tree growth, reducing the need to prune, while yield remains relatively unaffected.
Leaf boron concentrations greater than 300 ppm probably warrant further investigation as to the source of the boron. Orange leaf tissue samples taken from trees planted in the 1960’s or early 1970’s in Kern County routinely show levels of 300 to 400 ppm. Young trees appear to increase in boron concentration rapidly but at about 300 to 400 ppm the concentration tends to plateau. Why boron levels tend to plateau is not known. Chandler pummelos appear to be the most sensitive to excess boron, followed by lemons, grapefruits and oranges. Leaf boron concentrations of 400 ppm in Chandler pummelos appear to have caused severe stunting of the trees in several orchards in Kern County, while similar levels in Melogold (a pummelo x grapefruit hybrid) resulted in only some tip burn.
There are actions the grower can take to reduce the amount of boron in the tree. First the source of the boron should be determined if possible. If boron levels are increasing in the leaf tissue, analyze both surface water and well water. Avoid using water with greater than 0.5 ppm of boron for irrigation of citrus. Levels of boron that are beneficial to cotton or pistachio can cause severe problems with citrus. Surface water comes from diverse sources in Kern County. Surface delivered water may have started out as well water, or in some instances as diluted oil-field waste water which may contain relatively high concentrations of boron. Water districts will know if oil-field waste water is being diluted in irrigation water. Use of oil-field waste water can be seasonal and irrigation derived in part from oilfields may fluctuate in boron concentration. If boron is in the water even at slightly elevated levels, avoid spraying it directly on the trees when treating for insect pests or when applying foliar fertilizers. Fertilizers are foliarly applied because of the quick uptake of dissolved minerals through the leaves. If boron is in the spray solution, it will be absorbed quickly by the tree along with the potassium, zinc, manganese, nitrogen and other foliar nutrients. Organic matter, manure, composted materials, and mulches on the ground have been shown to reduce boron uptake by the plant from irrigation water with high concentrations of this element.
In the southern San Joaquin Valley, soils should be tested before citrus is planted. Areas of soil with high boron are found in the most unexpected places. Boron may have accumulated on some properties when high-boron well water was used before the advent of easier access to water from Sierra snow melt.
If leaf-tissue boron is high and the water or soil is not, check the foliar fertilizer blends being used. Often, boron is included in many micronutrient mixes because boron can be deficient in acid soils. Determine how much boron soil amendments may contain. Pit gypsum can have varying quantities of boron in it. A ton of this gypsum may contain as much as 20 pounds of boron.
Discovering the cause of high boron in citrus leaves may require an extra soil test in addition to the typical saturated pest extract. Soil tests for ‘available’ boron using a saturated pest extract can be deceiving. In many instances where the concentration of boron in a ‘typical’ leaf averaged greater than 300 ppm, plant-available boron in the soil and water frequently averaged less than 0.25 ppm. However, total soil boron in these same orchards was at very high levels. Total soil boron estimates both available and unavailable boron. To help determine where the boron in the trees originates, both readily available and total soil boron should be sampled. This disparity between plant-available and total boron suggests that boron moves between the relatively small plant-available pool in the soil and the much larger ‘unavailable’ pool tied up in these calcareous soils. Soil acidifying agents and acid-forming fertilizers probably increase the availability of boron to citrus trees by making boron that is relatively unavailable to the trees at high pH, more available at lower pH. At any given time, plant-available boron may be relatively low but its constant replacement from the unavailable pool keeps the boron concentration in trees relatively high. In orchards where total soil boron is elevated; soil pH should probably be kept as high as tree health permits. Where the total amount of soil boron is moderate and soils are relatively well-drained and topography is flat, acidifying and leaching is probably the preferred option for reducing boron levels. Acidifying the soil and not supplying sufficient water to leach the boron from the root zone can compound the problem by making more boron readily available to the tree.
If boron is not found in the upper soil profile, but is found or suspected to exist deeper, irrigations could be scheduled that are more frequent but of shorter duration so that most of the citrus roots remain in the upper, lower-boron portion of the soil profile.
Actively growing, vigorous trees may dilute the concentration of boron in the leaf tissue through the production of a thick canopy. Old leaves tend to accumulate boron and drop. Adequate nitrogen ensures that enough nitrogen is present for production of new leaves. Increasing the nitrogen fertilization rate can encourage vegetative production and enhance this effect, but too much nitrogen may be associated with adverse fruit quality characteristics like regreening of Valencias, later maturity of early navels or higher yields of smaller fruit. Keeping other nutrients in the leaf in balance is important if boron is present at excessive concentrations. Maintaining high concentrations of phosphorous and calcium in the leaves through an appropriate fertilization program should be beneficial as these nutrients have been shown to reduce absorption of boron.
