Boron in olive: foliar and soil application, rates and the toxicity threshold

Savvas Pastopoulos, Agronomist MSc — Pastopoulos Agronomics, Neos Mylotopos, Pella, Greece

Boron is the element with the widest gap between the amount a tree needs and the damage a wrong estimate causes. The olive removes a few grams a year, the sufficiency band in the leaf ends at 150 ppm and toxicity begins at 185 — the narrowest margin of any nutrient. At the same time, the olive is one of the few crops in which boron genuinely moves inside the tree, and that changes everything about how and when it is supplied. This article brings together what the literature shows for foliar and for soil application, with the arithmetic worked out per hectare, and with the trials that found no benefit reported alongside those that did.

1. What boron does, and why it is decided at bloom

Boron has a single function proven at the molecular level: it cross-links molecules of rhamnogalacturonan II, a pectin of the primary cell wall. Under sufficiency, more than 90% of these molecules are found joined in pairs through a borate bridge [11]. Without the bridge the wall loses its mechanical cohesion and cell elongation stops.

This explains why symptoms appear first where cells elongate fastest: in the apical meristems, in young shoots and — most critical for the olive — in the pollen tube. The pollen tube is a single cell that elongates continuously towards the ovary; without sufficient boron, pollen germination and tube growth are arrested.

In the olive the consequence is immediately economic. The tree produces an enormous number of flowers and sets very few of them; a proportion of the flowers are imperfect from the outset, with an atrophied ovary, and will never carry fruit. Foliar boron applied before bloom significantly reduced the percentage of imperfect flowers and increased fruit set [4] [5]. The element does not feed the fruit — it determines how many flowers are capable of becoming one.

References: 4 5 11

2. The olive is an exception: boron travels in the phloem

In most plants boron is treated as practically immobile: it rises with the transpiration stream, accumulates in the leaves and does not return. That is why, in most crops, a foliar boron spray feeds only the leaf that was wetted.

The olive belongs to the group of exceptions. It produces and translocates mannitol, a polyol that forms a complex with boron and carries it within the phloem. In olive plants held under low boron supply, mannitol concentration in mature leaves rose by 48% in a growth chamber and by 27% in the field relative to controls, and that rise maintained the flow of boron from mature source leaves to the young leaves demanding it [3]. The tree itself, in other words, switches on a redistribution mechanism when boron runs short.

Confirmation came with a tracer as well: using isotopically enriched ¹⁰B, boron was shown to move from sprayed leaves into developing reproductive tissues, with the conclusion that foliar application can be effective at various stages of development and not only within one narrow window [4].

References: 3 4

3. Where it is measured: leaf, soil, water

The leaf is the main instrument

Interpretation uses the established limits for olive leaves sampled in July in the northern hemisphere [1]:

Values refer to dry weight and to July sampling [1].

Sampling uses 150 to 200 leaves taken from the middle of current-season shoots that carry no fruit, from many similar trees [7]. The words "no fruit" are not a detail: fruit acts as a sink and changes the concentration in the neighbouring foliage.

The soil is a weaker indicator than it is assumed to be

In a survey of seven olive-growing regions of Greece — Lesvos, Attica, Messolonghi, Thesprotia, Preveza, Larissa and Fthiotida — deficiency symptoms appeared when leaf boron fell below 15 mg/kg of dry matter. Critical soil boron levels ranged from 0.05 to 0.41 mg/kg depending on the extraction method. The most important finding, however, was a different one: boron deficiency was not related to the soil boron concentration but to soil depth [2].

In practice this means that a soil analysis showing "normal" boron does not rule out deficiency in the shallow, eroded or stony parts of the same block. In Greece the deficiency was found locally, at sites on Lesvos and in Attica and Larissa, and not as a generalised phenomenon [2].

The irrigation water

The olive is classified among the sensitive crops, in the upper range, with a tolerated irrigation-water boron of 0.75 to 1.0 mg per litre. Water below 0.33 mg/L is rated excellent, 1.00 to 1.25 doubtful and above 1.25 unsuitable for sensitive crops. Leaching boron out of the root zone is possible but requires up to three times more water than removing chlorides [14].

Where irrigation comes from boreholes, the water analysis is the first test to be run before any boron application is even discussed. There are regions where the problem is not deficiency but excess, documented in Greek work [1] [16] [18].

References: 1 2 7 13 14 16 18

4. The symptoms of deficiency

Deficiency strikes the new organs, because that is where cell elongation happens. Under controlled boron deprivation, symptoms appeared 41 days after the start and consisted of new leaves that were small, thick and malformed, followed by death of the apical meristem of the stem, cessation of new root emission and rot of the secondary roots [12].

In the field, the picture recorded in olive is:

• Leaves yellowing from the tip with the base staying green — often with a yellow band between the brown necrotic tip and the green part.
• Misshapen fruit, irregular in outline and dimpled.
• Short shoots with very close internodes and dieback of the tips, followed by lateral bud break that gives a broom-like appearance.
• Rough, cracked bark on young shoots.
• Small leaves with tip burn and limb dieback [7].

The aim is not to recognise the full syndrome — that appears only in advanced deficiency and is rare. The aim is the subclinical deficiency, the one that shows nothing in the foliage and appears only in the fruit set.

References: 6 7 12

5. The symptoms of toxicity

Toxicity is the mirror image of deficiency, and confusing the two is the commonest diagnostic error, because both produce necrosis at the leaf tip. Three elements separate them:

The reason toxicity strikes the old leaves is the way boron arrives there: it rises with the transpiration stream and accumulates where transpiration ends, at the tips and margins of the blade, and more so in leaves that have transpired for longer [12].

Sensitivity differs markedly between cultivars and between scion-rootstock combinations, as shown in Greek work with four cultivars at six boron concentrations and with different graft combinations [16] [17]. The form of nitrogen supplied has also been shown to affect the behaviour of olives growing in soils with high boron content [18] — relevant where the excess is a given and cannot be corrected.

References: 12 14 16 17 18

6. The identification tool

The tool compares the selected symptoms with the profiles of eight causes that produce similar pictures in olive. It does not simply count matches: each symptom carries a weight according to how distinctive it is, and some count negatively. Necrosis on old leaves argues for toxicity and against deficiency, while malformation of the young leaves does the opposite.

7. When deficiency appears — five situations

Shallow or eroded soil

This is the strongest finding in the Greek data: deficiency was associated with soil depth and not with the boron concentration in the soil [2]. An olive block with uneven relief shows the problem in patches, on the ridges and wherever the topsoil has gone.

Sandy soils low in organic matter

Boron leaches readily. On sandy soils poor in boron, deficiency is the default scenario rather than the exception [9].

Liming and high pH

This is the situation the grower creates, and the most underestimated one. In an experiment on boron-poor schist and granite soils with initial pH of 4.53 and 4.86, liming at the equivalent of 4 to 5 tonnes of calcium carbonate per hectare raised the pH to 7.36 and cut the dry matter yield of young olive trees to 15.6 grams per pot against 28.6 in the control — a fall of 45% — with the plants showing severe boron deficiency symptoms. Adding boron at 1.5 kg per hectare as boric acid gave the highest yield in the trial, 50.8 grams per pot [9].

Drought

Boron reaches the root mainly by mass flow of the soil solution. When the surface layer dries out, transport stops even where the reserve exists. Rainfed groves in a dry summer show a seasonal deficiency that does not correspond to any real poverty of the soil.

Young plantings

In a comparison of soil and foliar application in two cultivars, the increase in dry matter from boron application was significant in young plants while no increase was recorded in the established, bearing orchard [8]. A young tree has a small root system and a high proportion of new, growing tissue — both make it more vulnerable.

References: 2 8 9

8. Foliar application

What the successful trials showed

In California, on 'Manzanillo', a foliar application of Solubor at 246 or 491 mg boron per litre before bloom significantly reduced the percentage of imperfect flowers and increased fruit set, with the benefit greater when fruit set was low [4]. The applications were made on trees with no visible deficiency symptoms.

In a more recent account from the same service the rate is given as 1 to 2 lb of Solubor per acre in 100 gallons of water per acre — that is, 1.12 to 2.24 kg of Solubor per hectare in 935 litres per hectare, exactly the same concentrations — applied three weeks before bloom, with a result of roughly 27% to 30% higher yield in a low-crop year. In a heavy-crop year no benefit was recorded [6].

What the unsuccessful trial showed

In southern Italy, on 'Leccino', two foliar sprays at 270 mg boron per litre, before and after anthesis, affected neither fruit set nor yield: fruit set 4.29% in the treated trees against 4.65% in the untreated, yield 32.2 against 30.6 kg per plot [10]. The authors themselves propose further work with different timing and different amounts, particularly in low-crop years.

Timing

Autumn application is established in other crops with mobile boron and is supported in olive by the mannitol mechanism, but it does not rest on a published olive trial with a measured yield response. It is reported here as plausible, not as documented.

References: 3 4 6 10

9. Soil application

Soil application follows a different logic from foliar: it does not target the bloom of the particular year but the restoration of the reserve for several years.

The rates of the American recommendation

The established recommendation for correcting a deficiency is half a pound to one pound of a 14% to 20% boron material per tree, broadcast on the soil surface within the drip line, which corresponds to 25 to 50 lb of material per acre, and a single application lasts several years [7]. The same service also gives the rate as 5 to 10 lb of actual boron per acre [6].

In metric units

Converted from 5 to 10 lb of actual boron per acre [6], that is 5.6 to 11.2 kg of actual boron per hectare. This is a single application covering several years against a documented deficiency, not an annual dressing. The values are indicative and the final rate is set after leaf analysis, soil and water analysis and on-site assessment by an agronomist.

The European rate is far lower

In the Portuguese experiment the rate that fully restored growth was 1.5 kg of actual boron per hectare as boric acid [9] — roughly one quarter to one seventh of the American rate. In Brazil, soil application of ulexite at 25, 50 and 100 grams per tree raised leaf boron within the same growing season [13].

The divergence is not an error on either side. The American rate is a one-off, multi-year correction in an orchard with a documented deficiency; the European one is annual maintenance. On a sandy soil that leaches, the small repeated dose is safer than the large single one. On a heavy soil with little leaching, the large single dose lasts longer but leaves less room for error.

References: 6 7 9 13

10. Foliar or soil — what the comparison shows

The only published work that compares the two routes directly in olive comprises four experiments — two three-year field trials and two pot experiments — on 'Arbequina' and 'Cobrançosa'. The conclusions [8]:

• Boron concentration increased significantly in all tissues and in all experiments with application.
• In the established orchard soil application did not increase yield; in young plants it significantly increased dry matter.
• Soil application proved the more powerful tool than foliar spraying for raising boron concentration in the tissues.
• Boron was confirmed mobile within the tree to some extent, with differences between the two cultivars.

References: 8

11. The forms of boron and their contents

The calculation is always made in actual boron and never in kilograms of product. The same weight of product delivers almost twice the boron depending on the form.

References: 13 15

12. Calculating the rate, and the density trap

Foliar: the rate per hectare, not the concentration

The American recommendation is given as a concentration because it assumes a spray volume of 100 gallons per acre, that is about 935 litres per hectare. Mediterranean olive groves are sprayed with several times that volume, often 3,000 to 6,000 litres per hectare on large trees. If the concentration of 246 mg boron per litre is held and the volume is multiplied by five, five times the boron is applied.

Soil: a per-tree rate does not transfer between densities

References: 6 7

13. What boron does not do

• It does not replace pollination. The olive is mainly wind-pollinated and many cultivars benefit from a pollinator. Boron improves flower fertility; it does not bring pollen.
• It does not solve alternate bearing. It limits it indirectly when it supports fruit set in a low-crop year [6], but the cause of alternate bearing is the previous season's crop load and its effect on bud differentiation.
• It does not substitute for water. Under water stress at bloom, no foliar application saves the fruit set.
• It does not correct the toxicity of other elements. Marginal necrosis from salinity or chloride resembles boron toxicity and needs a water analysis to be separated from it.
• It has no documented independent effect on oil quality. Published results showing improved oil content come from compound products that contained nitrogen, phosphorus, potassium and calcium alongside the boron [19], so the contribution of boron cannot be isolated.

References: 6 19

14. The programme through the year

The programme is indicative. No line of it is applied without the analysis that triggers it.

15. The key points

Olive nutrition — what the tree actually removes, and what fertilisation does to alternate bearing: the full article

16. References

The numbers in the text refer to the list below. Where only the abstract or the bibliographic record was used, and not the full text, this is stated explicitly.

• [1] Fernández-Escobar, R. (2019). Olive Nutritional Status and Tolerance to Biotic and Abiotic Stresses. Frontiers in Plant Science 10:1151. Source for the interpretation limits of leaf boron in July — 14 ppm deficient, 19 to 150 ppm adequate, 185 ppm toxic — and for the record of deficiency in California and Italy and of excess in areas of Greece. Full text.
• [2] Tsadilas, C.D. and Chartzoulakis, K.S. (1999). Boron deficiency in olive trees in Greece in relation to soil boron concentration. Acta Horticulturae 474:341–344. Source for the survey of seven Greek regions, for the appearance of symptoms below 15 mg/kg of leaf boron, for critical soil boron levels of 0.05 to 0.41 mg/kg depending on extraction method, and for the finding that deficiency is related to soil depth rather than to soil boron concentration. Abstract.
• [3] Liakopoulos, G. et al. (2005). Boron remobilization at low boron supply in olive (Olea europaea) in relation to leaf and phloem mannitol concentrations. Tree Physiology 25(2):157–165. Source for phloem mobility of boron in olive through mannitol and for the 48% rise in mannitol in a growth chamber and 27% in the field under boron deficiency. Abstract.
• [4] Perica, S., Brown, P.H., Connell, J.H. and Hu, H. (2002). Olive response to foliar boron application. Acta Horticulturae 586:381–383. Source for the reduction of imperfect flowers and the increase in fruit set with Solubor at 246 or 491 mg boron per litre before bloom, for the greater benefit when fruit set was low, and for the confirmation of movement using the 10B isotope. Abstract.
• [5] Perica, S., Brown, P.H., Connell, J.H., Nyomora, A.M., Dordas, C., Hu, H. and Stangoulis, J. (2001). Foliar boron application improves flower fertility and fruit set of olive. HortScience 36(4):714–716. The original publication of the California foliar boron experiments in olive. Bibliographic record only; the full text was not accessible and the figures quoted in this article come from sources [4] and [6].
• [6] Faber, B.A., Andrews, E. and Fichtner, E. (2024). Pre-bloom foliar boron application on olive may improve yield. University of California Cooperative Extension. Source for the rate of 1 to 2 lb of Solubor per acre in 100 gallons per acre, for the timing of three weeks before bloom, for the 27% to 30% yield increase in a low-crop year at a leaf boron of 16 ppm, for the absence of benefit in a heavy-crop year, and for the soil rate of 5 to 10 lb of actual boron per acre. Full text.
• [7] University of California Cooperative Extension (2006). Fertility Management for Oil Olives. First Press 1(3):1–2. Source for the leaf sampling procedure, for the soil rate of half a pound to one pound of a 14% to 20% boron material per tree or 25 to 50 lb per acre broadcast within the drip line, for the duration of the application, and for the deficiency symptoms — misshapen fruit, short branch growth, limb dieback, rough bark, small leaves with tip burn. Full text.
• [8] Ferreira, I.Q., Rodrigues, M.Â. and Arrobas, M. (2019). Soil and foliar applied boron in olive: tree crop growth and yield, and boron remobilization within plant tissues. Spanish Journal of Agricultural Research 17(1). DOI 10.5424/sjar/2019171-13796. Source for the comparison of soil and foliar application across four experiments on 'Arbequina' and 'Cobrançosa', for the conclusion that soil application is the more powerful tool, for the dry matter increase in young plants and not in the established orchard, and for the partial mobility of boron with differences between cultivars. Abstract.
• [9] Arrobas, M., Raimundo, S., Conceição, N., Moutinho-Pereira, J., Correia, C.M. and Rodrigues, M.Â. (2023). On sandy, boron-poor soils, liming induced severe boron deficiency and drastically reduced the dry matter yield of young olive trees. Plants 12(24):4161. Source for the induction of severe boron deficiency by liming, for the rise in pH from 4.53 and 4.86 to 7.36, for the fall in dry matter to 15.6 grams per pot against 28.6 in the control, for the 50.8 grams obtained with 1.5 kg of boron per hectare as boric acid, and for the recommendation to apply boron after the lime. Full text.
• [10] Stellacci, A.M., Caliandro, A., Mastro, M.A. and Guarini, D. (2010). Effect of foliar boron application on olive (Olea europaea L.) fruit set and yield. Acta Horticulturae 868. Source for the trial on 'Leccino' in southern Italy with two sprays of 270 mg boron per litre before and after anthesis, and for the absence of any effect: fruit set 4.29% against 4.65% and yield 32.2 against 30.6 kg per plot. Abstract.
• [11] Funakawa, H. and Miwa, K. (2015). Synthesis of borate cross-linked rhamnogalacturonan II. Frontiers in Plant Science 6:223. Source for borate cross-linking of rhamnogalacturonan II as the only experimentally proven molecular function of boron, for the figure of more than 90% of molecules dimerised under sufficiency, and for the NIP and BOR transporters. Full text.
• [12] Machado de Souza, C. et al. (2019). Visual symptoms and nutritional deficiencies in olive plants subjected to nutrient deprivation. Acta Scientiarum. Agronomy 41:e39582. Source for the appearance of boron deficiency symptoms 41 days after deprivation, for the small, thick and malformed new leaves, for the death of the apical meristem and the effects on the roots, and for the accumulation of boron at the tips and margins of the older leaves through transpiration. Full text.
• [13] Rosseto, M. et al. (2023). Effect of boron fertilization and phenological period on boron content in olive leaves. Research, Society and Development. Source for the change in leaf boron by phenological stage in 'Arbequina' — 52.6 to 58.6 mg/kg before flowering, 29.2 to 32.3 at flowering and 63.9 to 67.8 at early fruit development at the first site — and for soil application of ulexite at 25, 50 and 100 grams per tree. Full text.
• [14] University of California Cooperative Extension. Water Quality Technical Note — Boron (B), based on Ayers, R.S. and Westcot, D.W. (1985), Water Quality for Agriculture, FAO Irrigation and Drainage Paper 29. Source for the classification of olive among sensitive crops in the upper range with a tolerated irrigation-water boron of 0.75 to 1.0 mg/L, for the water quality limits from excellent below 0.33 to unsuitable above 1.25 mg/L, and for the need of up to three times more leaching water than for chlorides. Full text.
• [15] U.S. Borax. Solubor — Product Data Sheet. Source for the content of 20.5% boron, for the chemical formula Na2B8O13·4H2O, for the solubility of 4.5% by weight at 10 °C against 21.9% at 30 °C, and for the solution pH of 7.3 to 8.5. Full text.
• [16] Chatzissavvidis, C. and Therios, I. (2010). Response of four olive (Olea europaea L.) cultivars to six B concentrations: growth performance, nutrient status and gas exchange parameters. Scientia Horticulturae 127:29–38. Source for the differing response of olive cultivars to increasing boron concentrations. Bibliographic record taken from the author's curriculum vitae; neither the full text nor the abstract was accessible, and no numerical result is attributed to it in this article.
• [17] Chatzissavvidis, C., Therios, I., Antonopoulou, C. and Dimassi, K. (2008). Effects of high boron concentration and scion-rootstock combination on growth and nutritional status of olive plants. Journal of Plant Nutrition 31:638–658. Source for the variation in boron tolerance according to the scion-rootstock combination. Bibliographic record only; the full text was not accessible.
• [18] Chatzissavvidis, C., Therios, I. and Antonopoulou, C. (2007). Effect of nitrogen source on olives growing in soils with high boron content. Australian Journal of Experimental Agriculture 47:1491–1497. Source for the existence of Greek soils with high boron content and for the effect of the nitrogen form on olive behaviour in them. Bibliographic record only; the full text was not accessible.
• [19] Hmmam, I. et al. (2026). Foliar application of nano-NPK and calcium-boron during the off-year season enhances yield, improves oil quality, and mitigates biennial bearing in 'Picual' olive cultivar. BMC Plant Biology 26:913. Source for the results of a compound foliar application in a low-crop year — perfect flowers 68.51% against 28.25% and final fruit set 26.95% against 12.31% — with the explicit note that the product contained nitrogen, phosphorus, potassium and calcium alongside the boron and that the contribution of boron cannot be isolated. Full text.
• [20] Database of approved plant protection products, from the Register of the Greek Ministry of Rural Development and Food. Source for the plant protection approvals in force in Greece by crop, active substance, rate and pre-harvest interval. In Greek; valid in Greece only.