Boron deficiency shows in the young organs of the plant (shoots, flowers and newly set fruit), because boron moves poorly within the plant and does not arrive in time from the old leaves; it appears above all in calcareous, dry soils. It is corrected with a boron corrector by foliar application or by irrigation, in small doses and after confirming it by leaf analysis, because the margin between deficiency and toxicity is narrow. This guide sets out the symptoms by crop, says when and by which route it is applied, and publishes the doses from our data sheet.
What boron does in the plant
Boron is a micronutrient: the plant needs it in very small quantities, but it needs it where it is growing. It forms part of the cell wall, which gives each new cell its shape; it is involved in pollen germination and in the growth of the pollen tube, that is, in fertilisation and in fruit set; and it takes part in the transport of sugars from the leaf to the fruit and the root. A plant without boron keeps its old leaves healthy: what it stops doing well is the new growth.
The fact that explains almost everything else is its low mobility: once in a leaf, boron hardly moves out again towards the shoots or the flowers, and the supply has to arrive at the moment of demand. Our BORO data sheet places that moment at “vegetative stage, post-flowering and stages of highest vegetative activity (fruit set, ripening)”: the stages in which the plant is making new tissue and fruit.
How to recognise boron deficiency, crop by crop
The diagnostic rule follows from mobility: if the damage is in the young growth (terminal bud, shoot, flower, newly set fruit) and the old growth is healthy, boron is the suspect; if the first thing to turn yellow is the old leaf, the suspicion points to another nutrient. The table sets out what is seen in the crops we are asked about most; confirmation is always leaf analysis.
| Crop | Where to look | What is seen | What it is confused with | Is it in our data sheet? |
|---|---|---|---|---|
| Olive | Current-year shoots and newly set olives | Small, deformed terminal leaves, with a yellow tip that dries out; short internodes; shoots that die back from the tip and resprout in a broom-like cluster; poor flowering and deformed olives, or olives that drop | Drought, cold on the shoot, other tip chloroses | Yes: olive, with its own treatment in the olive grove programme |
| Sugar beet | Heart (central leaves) and root | Young central leaves deformed and darkened, which end up rotting; cracked petioles; root corky and hollow inside | Mechanical damage, rots of other origin, lack of water | Yes: sugar beet (arable crops) |
| Vine | Bunch and young shoots | Poor fruit set, with small seedless berries among normal berries; young leaves with interveinal chlorosis; short internodes; death of the terminal bud | Poor set (coulure) caused by adverse weather at flowering; zinc deficiency | Yes: vine (fruit trees) |
| Citrus | Fruit and young leaves | Deformed fruit with gum pockets in the rind and pulp, thick rind, fruit drop; young leaves with thickened, cracked veins | Salt in the irrigation water, other micronutrient imbalances | Yes: citrus (fruit trees) |
| Tomato | Terminal bud, young leaf and fruit | The terminal bud stops growing and darkens; young leaves brittle and curled; stem and fruit with corky areas; cracked fruit | Calcium deficiency, which damages the fruit at the end opposite the stalk; excess salt | Yes: tomato (vegetable crops) |
| Avocado | Fruit and shoot | Small, deformed fruit, with indentations and early drop; shoots with short internodes and small leaves | Water stress, natural fruit drop after fruit set | Yes: avocado (fruit trees) |
| Apple and pear | Fruit and shoot tip | Fruit corky inside, deformed and cracked; shoots that die back from the tip; poor flowering | Calcium deficiency in the fruit; cold | Yes: apple and pear (fruit trees) |
| Others (strawberry, for example) | Young organs | The same pattern: deformation and arrest of the new growth, with the old growth healthy | The same as in the crops above | We do not publish a dose: technical enquiry |
Two crops with the same symptom may have different causes, and the boron remedy is one of those that does not allow for error on the side of excess.
Why it appears: calcareous, dry soil with little organic matter
The boron the plant takes up is dissolved in the soil water and reaches the root with it. From that follow the three typical situations: calcareous soil, with a high pH, where boron is held back and the plant cannot reach it even though it is there; drought, because without water there is no flow towards the root, which is why the deficiency worsens in dry summers and on rain-fed land and sometimes disappears with the rains without applying anything; and sandy soil with little organic matter, which holds little boron and loses it by leaching.
There is a fourth case, the opposite one, which matters just as much: excess. Some irrigation waters contain boron, and a plot that receives it with every irrigation does not need more through the leaf. Our data sheet puts it among the precautions: “check the boron content of the irrigation water”. The water analysis is as necessary as the leaf analysis.
The margin between deficiency and toxicity is narrow
With most nutrients, slightly overdoing it is costly but does no harm. With boron, the distance between the quantity that corrects and the quantity that harms is short, and that is the warning our data sheet places ahead of any dose:
The margin between the adequate and the toxic level of boron is very narrow; confirm the deficiency by leaf analysis, with technical supervision, and check the boron content of the irrigation water.
And the one that goes with it: “do not change the dose without authorisation from the technical department”. In practice these are three rules. Do not apply boron just in case, without a symptom and without an analysis: an error on the side of excess is not put right with an irrigation. Do not repeat an application because the symptom is slow to disappear, without analysing again. And do not stack boron inputs without counting them: the irrigation water, a micronutrient mix and a corrector are three inputs.
The second of those three inputs calls for a clarification. Our MICROELEMENTOS data sheet declares “Boron (B) — 0.020% w/w” alongside nitrogen, magnesium, sulphur, copper, iron, manganese and zinc: it is a micronutrient mix that supplies boron, not a boron corrector, which declares “Total boron oxide (B2O3) — 20.93% w/w” (element and oxide are two ways of expressing it: how to read them). A micronutrient mix is not the tool for a confirmed deficiency and, applied at the same time as the corrector, its boron counts too.
When to apply boron: timing and route
The timings the data sheet publishes
Three lines set the timing. The phenological timing: “vegetative stage, post-flowering and stages of highest vegetative activity (fruit set, ripening)”. The frequency by the foliar route: “generally weekly applications, between 2 and 4 depending on the crop”. And by irrigation: “fertigation at 10-15 day intervals”. Post-flowering and fruit set are the most sought-after moment, because that is when the plant is making the tissue that boron builds.
Boron in the olive: when and with what
The olive grove is the only crop for which we publish a whole calendar, and boron has its place in it: “olive grove in production: 2nd treatment of the annual programme, approximately one month after the first (the 1st is late February/early March)”. It is a 1,000 L foliar tank that the data sheet writes out in full: “2 litres per 1,000 L tank, mixed with UNIVERSAL 3 L + PK 20-20 1 L + NITRÓGENO 16 2 kg”. The tank companions have their own data sheets: UNIVERSAL, PK 20-20 and NITRÓGENO 16; the whole programme, with the four tanks and the irrigations, is in the olive grove fertilisation programme.
Foliar boron or by irrigation
Foliar boron is the maintenance and quick-response route: it reaches the young organ that needs it, in a small quantity, and the data sheet doses it by volume of spray mix (“3-6 L/1,000 L per application” on fruit trees). Fertigation is the route the data sheet reserves for severe deficiency, with a limit and a frequency: “split into a maximum of 3 L/ha, at 10-15 day intervals; pre-dilute before pouring into the tank”. When each route is advisable, and at what time of day, humidity and temperature, is covered in foliar fertiliser or fertigation.
Doses of our boron corrector for organic farming, as the data sheet publishes them
FENORGANIC BORO has two composition lines: “Total boron oxide (B2O3) — 20.93% w/w · 25.53% w/v” and “Single-cell freshwater algae solution — 25.00% w/w · 30.50% w/v”. There are six published doses, each with its route and its unit in front of the number: by foliar application, litres of product per thousand litres of spray mix; by irrigation, litres per hectare.
| Use | Dose (verbatim from the data sheet) |
|---|---|
| Foliar, fruit trees (maintenance) | “3-6 L/1,000 L per application” |
| Foliar, vegetable and arable crops (maintenance) | “3-4 L/1,000 L per application” |
| Fertigation, severe deficiency, fruit trees | “up to 4 applications (6 to 8 L/ha)” |
| Fertigation, severe deficiency, vegetable and arable crops | “up to 4 applications (1-2 L/ha)” |
| Fertigation (general note) | “split into a maximum of 3 L/ha, at 10-15 day intervals; pre-dilute before pouring into the tank” |
| Olive grove in production, 2nd treatment of the annual programme | “2 litres per 1,000 L tank, mixed with UNIVERSAL 3 L + PK 20-20 1 L + NITRÓGENO 16 2 kg” |
The data sheet publishes these doses for olive, fruit trees (almond, pistachio, vine, citrus, apple, pear, blueberry, avocado, mango), vegetable crops (tomato, potato, cucumber, melon, courgette, cauliflower, onion, carrot, pepper) and arable crops (cereals, lucerne, cotton, sugar beet, maize, sunflower). For a crop that is not on that list there is no published dose, and the way forward is a technical enquiry. The complete data sheet, with its six blocks, is the FENORGANIC BORO data sheet.
What not to mix and how to prepare the tank
The data sheet publishes one incompatibility, “incompatible products: strong acids and powerful oxidisers”, and one written compatibility: “tank mix with FENORGANIC UNIVERSAL, PK 20-20 and NITRÓGENO 16 (olive grove, 2nd treatment)”. Outside that tank, any other mix is tested first on a small plot.
Preparation is in the precautions: “pre-dilute before pouring into the tank; shake vigorously before use; use under technical advice” and “dissolve beforehand, before adding to the tank; do not apply at high temperatures”; on appearance, “physical characteristics may vary owing to the organic nature of the product”. On safety, “not classified under Regulation (EC) No 1272/2008”, with precautionary statements P102 and P270, and “store at 5-35 °C”.
Is it a boron corrector suitable for use in organic farming?
Yes, with the document in front of you. FENORGANIC BORO is listed in the annex of the ECOCERT S.A.S input attestation No. 245895/29/290920251037 (“EU - Inputs Attestation” programme, Regulations (EU) 2018/848 and 2021/1165), valid until 31 March 2027, and its data sheet states it with the correct wording: “product suitable for use in organic farming”. It is not listed in our second attestation, the one under the USDA NOP programme. The document, with its annex, is in certifications, and the procedure for checking any input is in how to know whether a fertiliser is suitable for organic farming.
Frequently asked questions
What does boron deficiency look like in the olive?
In the current-year shoots: small, deformed terminal leaves with a dry tip, short internodes, shoots that die back from the tip and resprout in a broom-like cluster, poor flowering and deformed olives, with the old growth healthy. It is confirmed by leaf analysis.
When is boron applied in the olive and at what dose?
In the second foliar treatment of the programme, “approximately one month after the first”, which is from late February to early March: “2 litres per 1,000 L tank, mixed with UNIVERSAL 3 L + PK 20-20 1 L + NITRÓGENO 16 2 kg”. Outside the programme, the foliar dose for fruit trees is “3-6 L/1,000 L per application”.
Foliar boron: what is it for and when is it applied?
To correct the deficiency in the young organ that shows it, with a small quantity, in the “vegetative stage, post-flowering and stages of highest vegetative activity (fruit set, ripening)” and with “weekly applications, between 2 and 4 depending on the crop”. By irrigation, the data sheet reserves it for severe deficiency.
Can you overdo boron?
Yes, and it is the risk specific to this nutrient: “the margin between the adequate and the toxic level of boron is very narrow”. That is why the data sheet asks for the deficiency to be confirmed by leaf analysis, for the boron content of the irrigation water to be checked and for the dose not to be changed without the technical department.
Is there a calcium and boron corrector?
In our range they are two data sheets: CALCIO 880 and BORO. In the olive grove programme they go in different treatments: boron in the second, one month after the first, and calcium in the third, once the olives have set, at “CALCIO 2 L per 1,000 L”.
The BORO data sheet is in the correctors and nutrients catalogue, and the rest of the range in the full catalogue. If the symptom does not fit the table or the analysis says otherwise, the technical enquiry is the place: we reply with the data sheet and the analysis in front of us.