Boric Acid for Tomatoes

Why Do Tomatoes Need Boron?

Imagine the plant as a city, and boron as the courier service that ensures communication between districts and delivers building materials to construction sites. Without it, the city comes to a standstill.

Here are boron's main tasks in the tomato plant:

• Cell construction and strength. Boron participates in the formation of cell walls. It makes them strong and elastic, which is especially important for rapidly growing tissues – young leaves, roots, and most importantly, fruits (Jones, 2008). Adequate boron ensures that fruits are firm and do not crack.
• Flowering and pollination. Boron is crucial for pollen formation and germination. It stimulates flower development and increases their lifespan. With boron deficiency, pollen becomes sterile, flowers drop without setting fruit. This is one of the most common causes of poor fruit set during hot or, conversely, cloudy weather.
• Sugar transport. Boron is responsible for the transport of carbohydrates (sugars) from the leaves, where they are produced during photosynthesis, to the fruits and roots (Jones, 2008). This is why with good boron nutrition, fruits become sweeter and more flavorful. If transport is disrupted, sugars accumulate in the leaves, while fruits remain watery and tasteless.
• Calcium helper. Boron helps the plant absorb and utilize calcium. Calcium, in turn, is responsible for the health of growing points and prevents the common problem of blossom-end rot (Bradfield & Guttridge, 1984; Jones, 2008).
• Root growth. Boron stimulates root system development. Healthy and powerful roots are better at extracting water and nutrients, making the plant more resistant to drought and stress (MacInnes & Albert, 1969).

How to Tell If Tomatoes Are Lacking Boron?

The symptoms of deficiency are very characteristic and noticeable:

• Growing point dies off. The most alarming sign – when the main top of the bush stops growing and begins to die. This is an SOS signal.
• Poor flowering and empty flowers. The plant blooms, but flowers dry up and drop without setting fruit.
• Fruit deformation. Fruits may be small, have cracks, or brown, corky areas at the base (on the fruit "shoulders") (Jones, 2008; Atherton & Rudich, 1986).
• Shoot brittleness. Stems and leaf petioles become brittle and break easily.
• Changes in leaf color and shape. Young leaves may curl, become small and pale green.

It's important to remember that these symptoms can also be caused by other factors, so it's best to rely on a comprehensive approach: observing the plants and timely preventive feeding.

Bottom line: For tomatoes, boron is the element responsible for yield (number of fruit sets) and fruit quality (taste, firmness, shelf life). By providing the plant with boron at the right time, you lay the foundation for a bountiful and delicious harvest.

Stage 1. Budding (Start of Flowering)

This is the most important and mandatory stage. Treatment at the stage when the first buds appear lays the foundation for the future harvest. At this moment, reproductive organs are forming, and boron directly affects:

• Pollen quality: Boron makes pollen viable, which is critically important for pollination (Jones, 2008).
• Ovary development: It stimulates flower formation and prevents their drop. This is your main chance to increase the number of fruit sets per cluster.

When exactly: As soon as you notice the first buds on the plant. This usually corresponds to the start of flowering of the first cluster.

Stage 2. Mass Flowering and Fruit Set (Second–Third Clusters)

This stage consolidates the success of the first. While the plant is actively flowering and setting fruit, the need for boron remains high. A second treatment ensures that all subsequent clusters also form full-fledged ovaries and fruits.

When exactly: Approximately 10–14 days after the first treatment, when the plant enters the mass flowering phase.

Stage 3. Fruit Filling (Optional, for Quality Improvement)

This treatment doesn't affect fruit quantity as much as it affects quality. Boron continues to participate in transporting sugars from leaves to fruits, improving their taste, firmness, and increasing shelf life (Jones, 2008). Additionally, it helps prevent issues like fruit cracking and deformation. Many experienced gardeners carry out this treatment to obtain sweeter and more marketable produce.

When exactly: At the stage when fruits begin to actively fill (increase in size) but have not yet started to ripen.

Two Important Points:

1. Prevention is better than cure. Boron deficiency is easier and more effective to prevent than to correct. By the time bright symptoms appear (growing point die-off, mass drop of fruit sets), part of the harvest will already be lost irretrievably. Therefore, it's better to stick to scheduled treatments at the indicated stages.

2. Foliar correction. Plants may experience temporary boron deficiency due to bad weather (cold, drought) or high soil pH, which blocks its root uptake. Foliar feeding (spraying on leaves) during these periods is the fastest and most reliable way to deliver the element directly to the "workshop," bypassing problematic roots (Atherton & Rudich, 1986).

Bottom line: Carry out at least two scheduled treatments: the first – in the budding phase, the second – during mass flowering. A third one, during fruit filling, is recommended to improve taste and marketable appearance.

Standard Recipe for Foliar Feeding (Spraying)

This is the main and most reliable method of boron delivery. The solution concentration is 1 g of boric acid per 1 liter of water (this gives a 0.1% solution). This concentration is recommended by most agronomic reference books (Jones, 2008; Atherton & Rudich, 1986) and is safe for tomatoes when application timing is observed.

Step-by-step instructions:

1. Measure 1 gram of boric acid. (If you don't have precise scales, use a measuring spoon: roughly ¼ of a level teaspoon is 1 gram).

2. Pour 1 liter of very warm water (about 50–60 °C, the water should be hot but not boiling) into a small glass or plastic container.

3. Slowly sprinkle the boric acid into the water while stirring constantly until completely dissolved.

4. Cool the obtained stock solution and pour it into the sprayer, which has been pre-filled with 9 liters of settled water at room temperature. This way, you get 10 liters of ready-to-use working solution.

5. For better leaf wetting, add a surfactant (sticker) to the ready solution (for example, a few drops of liquid soap or a special product like "Liposam"). This significantly increases treatment effectiveness.

Alternative Recipes (for Different Tasks)

1. For pre-sowing seed treatment. Soaking seeds in a boric acid solution (0.2 g per 1 liter of water) for 12–24 hours improves germination and sprouting energy. This is especially useful for old or questionable seeds.

2. For emergency root feeding. In extreme cases where spraying is not possible, you can water plants at the root with a solution of lower concentration – 0.2 g per 1 liter of water (this is 2 g per 10 L). However, this method is less effective because boron can bind in the soil and become unavailable to the plant (especially on alkaline soils). Root feeding is more of a forced measure, not the primary method.

Important Preparation Nuances

• Use clean utensils. Residues of chemicals or fertilizers in the container can react with boric acid.
• Do not exceed the concentration. 1 g/L is a proven and safe dose for spraying tomatoes. More concentrated solutions can cause leaf burn.
• Do not store the solution. The prepared solution should be used within a few hours. When stored, it loses its properties, especially in light and warm places.
• Water temperature. Dissolve boric acid only in hot water. In cold water, the process will take hours and complete dissolution may not occur.

Bottom line: For spraying tomatoes, use 1 g of boric acid per 1 L of water, dissolving it beforehand in a small amount of hot water. Always prepare a fresh solution immediately before application.

Optimal Air Temperature for Treatment

The best time for spraying tomatoes with boric acid is morning or evening hours, when the air temperature is in the range of +18 °C to +25 °C.

Why this is important:

1. Stomata are open. Nutrient uptake through leaves occurs mainly through stomata. At temperatures above +25–28 °C, many plants, including tomatoes, close their stomata to reduce moisture loss. Treatment in heat will be ineffective because the solution simply cannot penetrate inside the leaf (Atherton & Rudich, 1986).

2. No burns. In hot weather, drops of solution under direct sunlight act like lenses, causing burns on leaves. Even if you use the correct concentration, heat can cause damage.

3. Absorption rate. At optimal temperatures, metabolic processes in the plant are most active. The solution is quickly absorbed and incorporated into metabolism.

What to avoid:

• Treatment in heat (above +28 °C) – is ineffective and risks burns.
• Treatment at low temperature (below +12–14 °C) – is ineffective because the plant slows down all life processes, and boron uptake will be minimal (Jones, 2008).
• Treatment before rain or immediately after – if temperature drops sharply, effectiveness decreases. Additionally, rain will wash off the solution before it has time to be absorbed.

Optimal Temperature for Solution Preparation

As mentioned in the previous chapter, boric acid dissolves poorly in cold water. For initial dissolution, warm or hot water is required.

Recommendation: Use water at +50–60 °C to dissolve the powder portion. This ensures quick and complete dissolution of crystals. After dissolving, be sure to cool the stock concentrate to room temperature by mixing with cold water. Do not use a hot solution for spraying – it will cause burns to leaves and flowers.

Bottom line: Plan treatments for morning or evening when the air temperature is comfortable for plants (optimally +20–22 °C). Dissolve boric acid in hot water, but bring the solution to room temperature before application.

Which Plant Parts to Treat

Boron is a mobile element, but its movement within the plant is limited. Therefore, it's important to treat the organs that need it most at a given moment.

• Young leaves. They are the primary "consumers" of boron since boron participates in building new cells. Spray the upper, actively growing part of the bush (Jones, 2008).
• Flower clusters and buds. This is the main target of treatment during budding and flowering. Thoroughly wet all flowers and buds in the clusters – this directly affects fruit set.
• Lower side of leaves. Stomata, through which nutrients are absorbed, are located mainly on the underside of the leaf blade. Therefore, spraying should be done so that the solution runs down onto the lower side of the leaves.
• Upper side of leaves is also treated, but its role in absorbing elements through stomata is significantly less.

Important: The treatment should be abundant, but without forming large drops that run off to the ground. Aim for a "fine mist" that covers the plant with a thin film.

How to Properly Carry Out Spraying

1. Choose the right time. As discussed in Chapter 4, carry out treatment early in the morning or late in the evening, when the air temperature does not exceed +25 °C and there is no bright sun.

2. Weather. Spraying should be done in calm and dry weather. Wind disperses the solution, making the treatment uneven. Rain within 4–6 hours after treatment washes away the solution, and it will need to be repeated.

3. Sprayer adjustment. Use a fine-droplet sprayer. Large drops run off leaves without being absorbed, while too fine a "mist" can be blown away by wind or evaporate before contact with the leaf. The optimal drop size is one where the solution evenly wets the leaf without forming running streams.

4. Solution consumption. For an adult tomato bush in open ground, 0.5–1 liter of working solution is sufficient. For greenhouse plants, this volume may be slightly less, as they are smaller in size.

5. Application technique. Move the sprayer evenly, trying to treat all parts of the plant, especially shoot tips and flower clusters. Start spraying from the lower part of the bush, gradually moving upward.

Surfactants (Stickers)

Adding a surfactant (sticker) to the solution significantly increases treatment effectiveness. This can be:

• A special agricultural product (e.g., "Liposam," "Trend 90," etc.).
• Regular laundry or liquid soap (a few drops per 10 liters). Soap reduces the surface tension of water, allowing the solution to spread better over the leaf, cover it with a thinner and more even layer, and take longer to dry.

Important: If using soap, choose a neutral one without fragrances or antibacterial additives. And don't overdo it – too much soap can leave a sticky residue on the leaves.

Compatibility with Other Treatments

Spraying boric acid is often combined with treatments for diseases or pests. However, caution is needed here. We will discuss in detail what boric acid can and cannot be mixed with in the next chapter.

Bottom line: For successful spraying, use a fine-droplet sprayer, thoroughly treat flower clusters and young leaves (especially the underside), carry out the procedure in dry, calm weather, and always add a surfactant for better wettability.

General Rules for Tank Mixtures

1. Mix only compatible products. Always check compatibility on a small amount (test mix) before pouring everything into the sprayer tank.

2. Follow the mixing order. This is critically important for obtaining a stable and effective mixture.

• Water is poured into the tank first (about 1/3 of the volume).
• Then pH adjusters are added if required.
• Next – products in order: fertilizers → surfactants (wetters) → fungicides → insecticides.
• Boric acid, as a fertilizer, should be added at the "fertilizer" stage, before adding pesticides.
• Do not mix more than 3–4 products at a time. The more components, the higher the risk of incompatibility and reduced effectiveness.
• Use freshly prepared mixture. Tank mixes cannot be stored; they should be used within a few hours.

What Can Be Mixed with Boric Acid

Boric acid is well compatible with most common pesticides and fertilizers, provided general rules are followed:

• With most fungicides (e.g., copper-containing products, "Ridomil," "Quadris," etc.). However, when mixing with copper-containing products (e.g., Bordeaux mixture), be careful – Bordeaux pH is alkaline, which can cause boron precipitation (see below).
• With most insecticides (e.g., "Aktara," "Confidor," etc.).
• With other foliar fertilizers (e.g., urea, magnesium sulfate, humates) if they are compatible. Boron is often included in complex micronutrient supplements (Jones, 2008).
• With growth regulators (e.g., "Epin," "Zircon"). However, they are usually added last.

Important: If you are unsure about compatibility, it's better to carry out treatments separately with a 2–3 day interval.

What Should Not Be Mixed with Boric Acid

Boric acid is incompatible with substances that have a strongly alkaline reaction (pH above 8.5). When mixed with them, boron converts into insoluble compounds that plants cannot absorb.

• With alkaline fertilizers – for example, with calcium nitrate or potassium nitrate (these fertilizers have an alkaline reaction). Mixing them with boric acid in the same tank is not recommended, as this will lead to boron fixation.
• With Bordeaux mixture – it has an alkaline reaction (due to the presence of lime). Mixing will lead to the formation of insoluble calcium borate. Therefore, treatments with boric acid and Bordeaux mixture should be separated by at least 7–10 days.
• With oils and oil emulsions – they can form a film that prevents boron penetration through leaves (although there is no direct chemical interaction). It's better to avoid such mixtures.
• With strongly acidic products (e.g., with some growth regulators or acid-based pesticides) – precipitation may occur. Check the pH of the finished mixture: the optimal pH for foliar feeding is 6.0–7.0.

Practical Tip: pH Check

Before adding boric acid, measure the pH of the water in the sprayer tank. If pH is above 7.0 (alkaline water), boron effectiveness decreases. In this case, first lower the pH to 5.5–6.5 by adding a little citric acid or a specialized acidifier, and only then add boric acid and other products (Jones, 2008).

Bottom line: Boric acid is well compatible with most pesticides, but incompatible with alkaline fertilizers and Bordeaux mixture. Always check product compatibility, follow mixing order, and monitor solution pH. It's better to carry out two separate treatments than to risk the effectiveness of the entire mixture.

How to Tell If You've Overdone It with Boron

Symptoms of boron excess (toxicity) differ from deficiency symptoms and are quite characteristic:

1. Chlorosis and necrosis of leaf margins. The first sign is yellowing (chlorosis) and subsequent death (necrosis) of tissues along the edges and tips of old, lower leaves (Jones, 2008). Gradually, the damage spreads from the edges to the center of the leaf blade. This differs from potassium deficiency, where marginal burn also occurs but is more uniform.

2. Yellowing between veins. Interveinal chlorosis may appear on leaves, which eventually turns into necrosis.

3. Leaves yellow and fall off. With severe toxicity, lower leaves turn completely yellow, curl, and drop off, exposing the stem from below.

4. Growth retardation. The plant top stops growing, internodes shorten, and the bush becomes stunted.

5. Yellowing and drop of flowers and fruit sets. Paradoxically, with boron excess, as with deficiency, flower and fruit set drop may occur.

Important difference: While boron deficiency primarily affects young tissues (growing points, young leaves), boron toxicity manifests on old, lower leaves, where boron accumulates (Jones, 2008; Atherton & Rudich, 1986).

Where Does Overdose Come From?

1. Concentration violation. The most common cause – exceeding the dosage when preparing the solution. Instead of 1 g per liter (for spraying), 2–3 g or more was added.

2. Too frequent treatments. More than 2–3 treatments per season with intervals of less than 7–10 days can lead to boron accumulation in the plant to toxic levels.

3. Using solution for root watering. With root feeding, boron is absorbed significantly more effectively than with spraying, so the concentration for watering should be 3–5 times lower (0.2 g/L). Using the "spraying" concentration for watering is a guaranteed overdose.

4. High boron content in soil. In some regions, soil may be naturally rich in boron. Before using boric acid, it's worth knowing the agrochemical characteristics of the soil to avoid exacerbating the situation.

5. Using "dirty" water. Some water sources (especially artesian wells) may contain elevated amounts of boron.

What to Do in Case of Overdose

Unfortunately, removing excess boron from the plant is practically impossible. However, you can try to reduce its concentration in the soil and stop further uptake:

1. Immediately stop all boron feeding. This is the first and most important action.

2. Abundant watering with clean water. If the cause of overdose is root feeding or soil application, water the soil abundantly with clean water (without fertilizers) to wash excess boron into the lower layers of the soil. This is especially effective on light, sandy soils.

3. Liming. Adding lime (CaCO₃) to the soil binds boron into insoluble compounds, reducing its availability to plants. However, this is a radical measure that changes soil pH, so it should be used only in extreme cases and after consultation with a specialist.

4. Zinc application. Research shows that zinc can partially antagonize boron, reducing its toxic effect (Jones, 2008). Foliar feeding with zinc (zinc sulfate) can help alleviate plant condition.

Bottom line: Boron overdose is a serious problem that manifests as burns and death of old leaves, growth retardation, and fruit set drop. The main cause is exceeding the concentration or frequency of treatments. Treatment consists of stopping feeding and abundant watering with clean water. The best solution is to strictly adhere to dosages and treatment schedules.

Mistake #1: Preparing Solution in Cold Water

As mentioned in Chapter 3, boric acid dissolves poorly in cold water. If you just pour powder with tap water, most crystals will remain undissolved. This leads to two problems:

• The solution will be weakly concentrated and ineffective.
• Undissolved crystals, landing on leaves, can cause burns.

How to do it right: Always dissolve the portion first in a small amount of hot water (+50–60 °C), then bring to the desired volume with cold water.

Mistake #2: Exceeding the Dosage

The desire to "intensify the effect" by increasing the concentration of the solution is the most dangerous and common path to overdose. As we discussed in Chapter 7, the difference between normal and toxic is very small. Symptoms of overdose (burn of old leaf edges, growth stop) can appear within a few days, and it will be practically impossible to correct (Jones, 2008).

How to do it right: Strictly adhere to the recommended concentration: 1 g of boric acid per 1 liter of water for spraying. Use precise scales or measuring spoons.

Mistake #3: Treatment in Heat or Under Direct Sun

In hot weather (above +28 °C), stomata on leaves close, and the plant practically does not absorb nutrients through leaves. Additionally, drops of solution in the sun act like lenses, causing burns on leaf blades (Atherton & Rudich, 1986). The effectiveness of such treatment will be close to zero.

How to do it right: Carry out spraying in the morning (before 9–10 AM) or evening (after 5–6 PM) hours, when the sun is less active and the air temperature drops to +20–22 °C.

Mistake #4: Mixing with Alkaline Fertilizers

Many gardeners mix all fertilizers in one tank to save time. However, boric acid reacts chemically with alkaline fertilizers (e.g., calcium nitrate), forming insoluble salts. As a result, boron becomes unavailable to the plant, and you waste time, money, and fertilizer (Jones, 2008).

How to do it right: Do not mix boric acid in one tank with calcium and potassium nitrates. Carry out treatments separately with an interval of 3–5 days. If such an option is unavailable, first add an acidifier to the water, lowering the pH to 5.5–6.5, and only then add nitrates.

Mistake #5: Spraying Only the Upper Side of Leaves

Stomata, through which the plant absorbs nutrients, are located mainly on the lower side of leaves. Spraying only the upper part does not achieve the desired effect. The solution runs off the smooth surface, hardly being absorbed.

How to do it right: Thoroughly treat both sides of the leaves, especially the lower side. To do this, use a fine-droplet sprayer and direct the stream from below or from the side so that the solution covers the leaf from all sides.

Mistake #6: Watering at the Root Instead of Spraying

Boron applied to the soil easily binds to soil particles and becomes difficult for plants to access, especially on alkaline soils (Atherton & Rudich, 1986). Root feeding is less effective and more dangerous in terms of overdose. Foliar treatment (spraying) allows boron to be delivered directly to the "workshop" (leaves and flowers), bypassing the soil.

How to do it right: Use foliar feeding (spraying) as the primary method. Use root watering only in extreme cases and at a concentration 5 times weaker (0.2 g/L) than for spraying (Jones, 2008).

Mistake #7: Ignoring the Surfactant

Without a surfactant (wetter), most of the solution simply runs off the glossy surface of tomato leaves without being absorbed. This reduces treatment effectiveness many times over.

How to do it right: Always add a surfactant to the solution – a special product (e.g., "Liposam") or a few drops of liquid soap. This improves wettability and increases the contact time of the solution with the leaf.

Mistake #8: Treatment Before Rain or Strong Wind

Rain will wash the solution off the leaves within the first hour after spraying. Wind makes the treatment uneven: part of the solution is blown away, part hits some plants in excess, while others remain without feeding.

How to do it right: Check the weather forecast. Spray in dry, calm weather. If rain passes within 4–6 hours after treatment, the spraying should be repeated.

Summary: Your Checklist for Successful Treatment

To avoid forgetting all the important points, here is a quick checklist:

• [ ] Time: early morning or evening (temperature +18–25 °C).
• [ ] Solution: 1 g boric acid per 1 L water. First dissolve in hot water, then add cold.
• [ ] Surfactant: add обязательно (soap or special product).
• [ ] Compatibility: do not mix with calcium and potassium nitrates. First test on a small volume.
• [ ] Spraying: thoroughly wet both sides of leaves and flower clusters.
• [ ] Weather: dry, calm, no rain in the next 4–6 hours.
• [ ] Schedule: no more than 2–3 treatments per season with 10–14 day intervals.

Bottom line: Boric acid is a reliable assistant in achieving a high and quality tomato harvest, but only when used correctly. Avoid the listed mistakes, and your plants will thank you with abundant flowering, excellent fruit set, and tasty, firm fruits.