Nutrition

Weak Root System: The Main Constraint

The root system of the onion is its "Achilles' heel" and simultaneously the key to successful cultivation. The onion has a fibrous, poorly developed, and shallow root system (Autko et al., 2012; Swiader & Ware, 1992).

What this means for the gardener:

The bulk of onion roots are located in the topsoil, only 5–20 cm deep (Autko et al., 2012). Onion roots are relatively thick, rarely branch, and practically lack root hairs, which in other plants greatly increase the absorption area (Brewster, 2008).

Imagine: the surface area through which an onion can absorb water and nutrients is significantly smaller than that of most other vegetable crops.

This is why the onion is called a "plant with low absorptive capacity" (Autko et al., 2012). It simply cannot "reach" nutrients located deeper than 40 cm in the soil. Even if the overall soil fertility is good, nutrients deep in the soil are inaccessible to the onion.

Practical takeaway: Fertilizers must be located in the zone of active root growth — in the topsoil, close to the plants. Applying fertilizers deep during digging or into the subsoil layer simply makes no sense for onions.

High Sensitivity to Salt Concentration

The second key feature of onion nutrition is its exceptional sensitivity to high salt concentrations in the soil solution (Brewster, 2008; Hochmuth & Sideman, 2023).

What this means in practice:

'Salt' here does not mean table salt (sodium chloride) but any soluble mineral compounds, including fertilizers. When you apply mineral fertilizers, they dissolve in the soil moisture. The concentration of this solution must not be too high, otherwise, it acts on the roots like a "toxic broth."

Onion is one of the most salt-sensitive vegetable crops (Brewster, 2008). Research shows that onions start losing yield already at soil electrical conductivity (a salinity indicator) of 1.2 dS/m, with a 16% yield loss for each additional unit (Allen et al., 1998, cited in Brewster, 2008). For comparison, many other crops tolerate salinity much better.

Especially vulnerable are:

• Seedlings and sprouts — their young roots are most sensitive to "salt shock." A high concentration of fertilizers near the seed can literally "burn" them. Therefore, the initial fertilizer dose when sowing seeds should be minimal.
• Plants during drought — when soil moisture is low, the salt concentration naturally increases, and the risk of root damage rises.

This is precisely why the recommendation is to apply nitrogen fertilizers in split doses rather than one large dose. This helps "keep" salt concentration at a safe level while providing the plant with nutrients throughout the growth period (Brewster, 2008).

Root System and the Need for "Starter" Nutrition

Despite its weak root system, the onion has a short but extremely important period when it requires a high concentration of nutrients for maximum growth.

Studies have shown that in young onion seedlings, the rate of phosphorus uptake per unit root length is nearly three times higher than in older plants (Brewster et al., 1975, cited in Brewster, 2008). The young root must work at maximum intensity to supply the rapidly growing sprout with everything it needs.

Practical takeaway: In the first few weeks after emergence or after planting sets, it is crucial to ensure the availability of readily available forms of nutrients (especially phosphorus) directly in the root zone. This is like "startup capital" for the plant: if it has it, growth is fast and vigorous; if not, the plant may lag permanently, and this delay cannot be compensated for later (Costigan et al., 1983, cited in Brewster, 2008).

What We've Learned About Onion Nutrition:

1. Onions have a weak, shallow, and compact root system — roots are mainly in the top 20 cm of soil. They cannot obtain nutrients from deep layers.

2. High sensitivity to fertilizer salts — requires careful, split application, especially at the start.

3. A short, critical period at the beginning of growth where nutrient availability (especially phosphorus) is critically important.

4. Onion roots cannot absorb sufficient nutrients from poor soils, so topsoil fertility and regular fertilization are essential for a good harvest.

Understanding these features will help you competently approach the selection of fertilizers, their amounts, and application timing. This means achieving larger, higher-quality onions with better storage life.

2.1. Nitrogen (N) – The Engine of Growth

Nitrogen is the main element for building green mass. It is responsible for the formation of proteins and chlorophyll, and thus for vigorous leaf growth.

This is especially important for onions because:

• The size of the future bulb directly depends on how powerful a leaf apparatus has been formed by the time bulb initiation begins. More leaves mean a larger bulb (Brewster, 2008).
• Up to 70% of all nitrogen consumed by onions during the season is absorbed in the second half of the growing season – during active leaf growth and the beginning of bulb filling (Zink, 1966, cited in Brewster, 2008).

How to tell if the onion lacks nitrogen?

• Leaves become pale green or yellowish, especially the older (lower) leaves.
• Growth slows down, plants look stunted.
• Bulbs form small, with a thin neck.

Nitrogen – an element with a "double character":

Excess nitrogen is as dangerous as deficiency:

• Delays bulb maturation – they "fatten," growing leaves at the expense of bulb formation.
• The bulb neck becomes thick and dries poorly, drastically reducing storage life and opening the door to diseases, especially neck rot (Botrytis) (Swiader & Ware, 1992; Autko et al., 2012).
• Taste deteriorates and nitrate content increases.

Practical recommendations for nitrogen:

1. Apply in split doses. Due to the onion's high sensitivity to salt concentration and the risk of nitrate leaching from the topsoil, nitrogen should not be given in one large dose. The optimal scheme is: part before sowing/planting, part as top dressings during active growth (Brewster, 2008; Autko et al., 2012).

2. Align with the development stage. The main need for nitrogen occurs from the appearance of 3–4 true leaves until the start of bulb formation. After the bulb begins to fill, nitrogen top dressings should be reduced or stopped entirely to avoid delaying maturation.

3. Consider soil type. On light sandy soils, nitrogen leaches faster, making split application especially important. On loams and peat soils, the need for nitrogen may be lower (Swiader & Ware, 1992; Autko et al., 2012).

Approximate rates: Depending on expected yield and soil fertility, onions need 80–120 kg/ha of active nitrogen (approximately 8–12 g/m²). In the starter dose – before sowing or in holes when planting sets – give no more than 30–40% of the total amount; the rest is applied in 1–2 top dressings (Autko et al., 2012; Brewster, 2008).

2.2. Phosphorus (P) – Energy and Roots

Phosphorus is the element responsible for energy exchange in cells, root system development, and the initiation of generative organs. For onions, phosphorus is critically important at the earliest stage of life.

Why is phosphorus so important for onions?

• Onions have a very weak root system, and it is phosphorus that stimulates its growth and branching (Swiader & Ware, 1992). Well-developed roots are the key to the plant's ability to "extract" water and other nutrients from the soil.
• Phosphorus participates in cell division, thus influencing the number of primordia (buds) in the bulb, which determines its multi-seededness (important for varieties grown for greens and for seed production).
• In the first weeks after emergence, the rate of phosphorus absorption by roots is several times higher than in adult plants. This is a "critical window" that must not be missed (Brewster, 2008).

Signs of phosphorus deficiency:

• Slowed growth, plants appear dwarfed.
• Leaves take on a dark green or purplish-reddish hue (especially on older leaves).
• Root system is weak, bulbs form late and poorly.

Peculiarities of phosphorus nutrition in onions:

Phosphorus is an immobile element in the soil. It does not leach out, but it also does not "travel" to the roots. For phosphorus to become available, it must be located directly in the root growth zone. Therefore, starter (seed-placed) doses of phosphorus, applied in the furrow or hole, are especially important for onions (Brewster, 2008; Swiader & Ware, 1992).

Research shows that local application of a small amount of readily available phosphorus into the future root zone gives a powerful boost to onion growth and allows for higher yields even with relatively high soil phosphorus levels (Brewster, 2008). This is the so-called "starter fertilizer" effect.

Practical recommendations:

• Apply the main dose of phosphorus (70–80% of total needs) during digging or in furrows before sowing/planting. This creates a "reservoir" of phosphorus in the root zone.
• The remaining part can be given as a liquid top dressing at the beginning of active growth.
• On soils low in phosphorus, especially light sandy ones, be sure to use a starter fertilizer – for example, 10–20 kg/ha P₂O₅ in rows or holes (Brewster, 2008; Autko et al., 2012).

2.3. Potassium (K) – Quality and Storage Life

Potassium is the element that determines not so much the yield size but its quality. It participates in carbohydrate synthesis, regulates water balance, and increases plant resistance to diseases and adverse conditions.

For onions, potassium performs several key functions:

• Increases bulb density and improves their taste (promotes sugar accumulation) (Brewster, 2008).
• Strengthens bulb tissues, increasing resistance to mechanical damage during harvest and improving storage life.
• Regulates stomatal opening, helping the plant use water more efficiently during dry periods.
• Reduces the risk of fungal diseases, especially during storage.

Signs of potassium deficiency:

• Edges and tips of older leaves turn yellow, then brown and die (marginal burn).
• Bulbs become soft, loose, and store poorly.
• Plants are more susceptible to diseases.

Potassium – antagonist and helper:

Excess potassium can interfere with the uptake of magnesium and calcium, so it's important to maintain balance. At the same time, with adequate potassium supply, the onion makes better use of nitrogen: the N:K ratio influences whether growth energy goes into leaves or into the bulb.

Practical recommendations for potassium:

• Potassium, like phosphorus, is mainly applied during primary tillage (in autumn or spring before planting). On light soils, potassium can leach out, so split application is possible (part – as a top dressing at the beginning of bulb formation).
• Onions respond well to potassium in chloride form (potassium chloride), but on sensitive soils or when moisture is lacking, it's better to use sulfate forms (potassium sulfate) to avoid excess chlorine (Brewster, 2008; Swiader & Ware, 1992).
• The rate of potassium varies depending on soil fertility but averages 100–150 kg/ha K₂O. On peat soils, the need for potassium may be higher (Autko et al., 2012; Swiader & Ware, 1992).

2.4. Element Ratios and Balance

It is important not only the quantity of each element but also their ratio to each other. This allows you to manage onion growth and direct its energy in the right direction.

Note: Ratios are given in parts of active ingredient (N, P₂O₅, K₂O).

Why this is important:

• Early in the season, the dominant role of nitrogen helps rapidly build leaf mass – the "factory" for the future harvest.
• Closer to mid-season, when the bulb begins to form, the proportion of potassium is increased to stimulate sugar accumulation and the formation of a dense head.
• At the end, when the onion is preparing for maturation and harvest, nitrogen is practically eliminated, while potassium and phosphorus are maintained in moderate amounts – this speeds up leaf lodging and improves storage life.

Starter Fertilizers – A Separate Topic

As mentioned, the use of liquid starter fertilizers, applied directly into the seed or root zone during sowing/planting, is extremely effective for onions. Such fertilizers typically have a higher phosphorus content and a small amount of nitrogen. They give a quick boost to development, especially in cold weather when the activity of soil microorganisms is reduced and phosphorus from the soil is poorly absorbed (Brewster, 2008).

In commercial vegetable growing, liquid starter mixtures (e.g., 10-34-0 or 7-21-7) with added micronutrients are widely used. For amateur gardeners, ready-made granular starter fertilizers (e.g., 5-10-10 or 8-16-16) are available, which are applied into furrows during sowing.

Key takeaways from this chapter:

1. Nitrogen is the main engine of leaf growth; apply it in split doses, focusing on the first half of the growing season; excess is dangerous – it delays maturation and reduces storage life.

2. Phosphorus is critical for root development and early growth; localized application in the root zone is essential, preferably as a starter fertilizer.

3. Potassium is responsible for bulb quality, density, and storage life; its proportion increases in the second half of the growing season.

4. The N:P:K balance changes depending on the growth phase – this allows for effective management of crop development.

Understanding the role of the major nutrients is the first step toward a correct fertilization strategy. In the next chapter, we will analyze why onions need calcium, magnesium, and sulfur, and why sulfur is a special case for onions.

3.1. Calcium (Ca) – Structural Material and Disease Protection

Calcium is the "cement" of cell walls. It is responsible for tissue strength, proper root growth, and disease resistance.

Why onions need calcium:

• Strengthens cell walls, making bulb tissues dense and resistant to rot pathogens (Brewster, 2008).
• Participates in root growth and cell division at the growing point. Calcium deficiency is especially dangerous for young, actively growing tissues.
• Reduces the risk of physiological disorders such as "wet" or "glassy" onions (softening of inner scales), which often occur with unbalanced nutrition.

Signs of calcium deficiency in onions:

• Young leaves become deformed, their tips die off.
• Roots grow poorly and become brittle.
• Bulbs lose density, become loose, and store poorly.

Peculiarities of calcium uptake:

Calcium is an immobile element in the plant: it practically does not move from old leaves to young ones or into the bulb. Therefore, even if there is enough calcium in the soil, but its uptake is disrupted (e.g., due to drought or excess potassium), young tissues may suffer from deficiency (Hochmuth & Sideman, 2023). This is one reason why onions sometimes exhibit internal necrosis even when soil calcium tests are normal.

Practical recommendations for calcium:

1. Maintain optimal soil pH (6.0–7.0). In acidic soils, calcium becomes less available, and its deficiency is common (Autko et al., 2012; Swiader & Ware, 1992). Liming acidic soils solves the problem comprehensively: pH is normalized, and calcium is added.

2. Avoid excess potassium and magnesium – they compete with calcium for root uptake. If you have high potassium levels in the soil, this can provoke calcium starvation even with adequate calcium levels (Brewster, 2008).

3. Apply calcium top dressings during critical periods – during active leaf growth and at the beginning of bulb formation. Calcium nitrate (Ca(NO₃)₂) can be used – it provides both calcium and nitrogen, but should only be applied in the first half of the growing season to avoid delaying maturation.

4. Foliar calcium applications (1% solution of calcium chloride or nitrate) can help with acute deficiency, especially if the problem is in uptake, not in soil content (Brewster, 2008).

3.2. Magnesium (Mg) – The Core of Chlorophyll

Magnesium is the central atom in the chlorophyll molecule. Without it, photosynthesis is impossible, meaning the plant cannot accumulate energy and build tissues.

Role of magnesium in onions:

• Ensures normal green leaf color and high photosynthetic activity.
• Participates in phosphorus transport within the plant.
• Increases resistance to stress (drought, temperature fluctuations).

Signs of magnesium deficiency:

• Light green or yellow patches between veins on older leaves – so-called interveinal chlorosis (Swiader & Ware, 1992).
• Leaves gradually yellow from edges to center, edges may curl upward.
• Plants appear weakened, bulbs become smaller.

Peculiarities of magnesium nutrition:

Magnesium is readily mobile within the plant, so its deficiency appears first on older leaves, from where it migrates to younger ones. On light sandy soils, magnesium leaches easily, and deficiency is more common than on loamy or peat soils (Swiader & Ware, 1992; Autko et al., 2012).

Practical recommendations for magnesium:

1. Use dolomite lime for liming. This not only raises pH but also provides magnesium. Dolomite is ideal for onions if the soil is acidic and simultaneously low in magnesium (Autko et al., 2012).

2. Apply magnesium in top dressings. On light soils, where magnesium leaches easily, it is beneficial to apply 30–50 kg/ha MgO as magnesium sulfate (Epsom salts) or potassium magnesia (also contains potassium) (Brewster, 2008).

3. Consider antagonism with potassium. Excess potassium hinders magnesium uptake. If you apply high doses of potassium, especially on soils low in magnesium, you may need to increase the magnesium dose to maintain balance.

3.3. Sulfur (S) – The Key to Onion Taste and Aroma

Sulfur is a special element for all allium crops. Sulfur-containing compounds are responsible for the characteristic pungent taste, smell, and tearing effect for which we value onions (Brewster, 2008; Swiader & Ware, 1992). This is not just "additional" nutrition – it is the formation of the commercial quality of the harvest.

Why onions need sulfur:

• It is part of amino acids (cysteine, methionine), from which alliin and other sulfur-containing flavor precursors are synthesized (the very substances that, when cutting onions, turn into volatile compounds that irritate the eyes and give dishes a piquant flavor).
• Sulfur is necessary for chlorophyll synthesis (to a lesser extent than magnesium, but still important).
• Participates in protein and enzyme formation, increases resistance to stress and some diseases.

How sulfur affects onion taste:

Sulfur is a "regulator" of pungency. Research shows that:

• With adequate sulfur supply, onions accumulate more 1-propenylcysteine sulfoxide – the main precursor of the pungent taste (Randle & Lancaster, 2002, cited in Brewster, 2008).
• With sulfur deficiency, pungency levels drop, and sweetness may seem more pronounced, but the onion becomes "flat" in taste.
• If sulfur is very low, the ratio of different precursors changes: instead of pungent compounds, less pungent ones (e.g., methylcysteine sulfoxide) accumulate. The taste becomes less rich (Brewster, 2008).

This matters for different onion types:

• If you are growing pungent, storage varieties – sulfur is essential for a bright, "oniony" flavor.
• If you are aiming for sweet, salad onions (like Spanish or Grano), then sulfur is limited – it reduces sweetness and increases pungency (Brewster, 2008). This is why sweet varieties do better on light sandy soils poor in sulfur.

Signs of sulfur deficiency in onions:

• Leaves become pale green or light yellow, resembling nitrogen deficiency (but without yellowing of lower leaves).
• Growth slows down, plants appear weakened.
• The taste of the bulbs becomes bland and unremarkable.

Practical recommendations for sulfur:

1. Consider the sulfur content in the soil. On most mineral soils, sulfur is sufficient, especially if sulfate forms of fertilizers (ammonium sulfate, potassium sulfate) are used. On light sandy and some peat soils, sulfur can be a limiting factor (Brewster, 2008; Swiader & Ware, 1992).

2. Use sulfur-containing fertilizers. The easiest way to provide onions with sulfur is to use ammonium sulfate ((NH₄)₂SO₄) as a nitrogen source, or potassium sulfate (K₂SO₄) for potassium top dressing. Gypsum (CaSO₄) is also used – it provides both calcium and sulfur. The sulfur rate is approximately 20–40 kg/ha S (equivalent to 60–120 kg/ha sulfate) (Autko et al., 2012).

3. Dose sulfur according to the target product. For pungent onions – give enough sulfur. For extra-sweet ones – limit sulfate fertilizers and choose sites with sulfur-poor soils (Brewster, 2008).

4. Sulfur competes with chlorine and selenium. Excess chloride fertilizers can interfere with sulfur uptake. If onions taste bitter or unpleasant, you may have overdone it with potassium chloride.

3.4. Comparison of Signs: How to Distinguish Deficiencies of Different Elements

Key takeaways from this chapter:

1. Calcium – ensures tissue density and disease resistance. Deficiency is often caused not by a lack in the soil but by uptake disruption (excess potassium, drought). Maintain pH and balance with potassium.

2. Magnesium – the basis of photosynthesis. On light soils, regular monitoring and top dressings are necessary, especially if you use high doses of potassium.

3. Sulfur – is the "taste" of onions. Without it, onions are bland, losing pungency and aroma. Use sulfate fertilizers, but adjust the dose depending on the goal: more for pungent onions, less for sweet ones.

Understanding the role of secondary nutrients allows not only to "do no harm" but also to purposefully shape the quality of the harvest. In the next chapter, we will analyze micronutrients – those substances needed in small doses but without which onions cannot develop normally.

4.1. Boron (B) – The Most Important Micronutrient for Onions

Boron is perhaps the most critical micronutrient for onions. It participates in carbohydrate metabolism, cell wall synthesis, pollination, and seed formation. For onions, boron is important at all stages: from root formation to bulb maturation and seed set (Brewster, 2008; Swiader & Ware, 1992).

Why onions need boron:

• Participates in sugar transport from leaves to the bulb – directly affects sweetness and head density.
• Stimulates root growth and development of growing points.
• Necessary for proper flower stalk formation and seed setting in seed plants.

Signs of boron deficiency in onions:

• Leaves become brittle, may curl or become deformed.
• Internal necrosis and cracks appear on bulbs – tissues soften, become watery, then brown. This is especially noticeable on a cross-section.
• In seed plants, "blossom-end rot" – flowers form, but seeds do not set.
• Growth of growing points slows down, plants look stunted.

Why boron deficiency occurs:

• On sandy and calcareous soils (alkaline), boron easily leaches out or binds into inaccessible forms.
• During dry periods, boron supply to roots decreases.
• Excess potassium or nitrogen can exacerbate boron deficiency.

Practical recommendations for boron:

1. Pre-sowing application. On soils low in boron (less than 1 mg/kg by some data), it is recommended to apply 1–2 kg/ha of boron as borax or boric acid (Na₂B₄O₇) as part of the base fertilizer (Autko et al., 2012; Swiader & Ware, 1992). This is about 10–20 kg/ha of borax, corresponding to 2–4 g/m².

2. Foliar applications. When deficiency symptoms appear or during critical periods (early growth, bulb formation), spraying with a 0.1–0.2% solution of boric acid (1–2 g per liter of water) is effective (Brewster, 2008). Repeat 2–3 times at 7–10 day intervals.

3. Caution with the dose. Excess boron is toxic – leads to "marginal burn" of leaves and growth retardation. Therefore, exceeding the recommended doses should be avoided. On acidic peat soils, the need for boron may be higher than on alkaline ones.

4.2. Manganese (Mn) – For Photosynthesis and Resistance

Manganese activates many enzymes, participates in photosynthesis (as part of photosystem II proteins) and respiration. Manganese deficiency is more common on peat soils at pH above 6.5 or on calcareous soils (Swiader & Ware, 1992; Autko et al., 2012).

Signs of manganese deficiency:

• Characteristic interveinal chlorosis on young leaves: veins remain green, but the tissue between them turns yellow. Brown spots may appear later.
• Growth slows down, bulbs become smaller.
• On peat soils, manganese deficiency can be a serious problem, especially in cold spring periods.

Practical recommendations:

• Foliar applications of 0.05–0.1% manganese sulfate solution (MnSO₄) – the most effective way to correct deficiency, especially at soil pH above 6.5, when root application is less effective (Brewster, 2008).
• On peat soils with pH > 6.5, it is recommended to apply manganese into furrows before sowing as manganese sulfate at a rate of 15–20 kg/ha (Swiader & Ware, 1992; Autko et al., 2012).
• When applying to the root, consider that manganese is better absorbed in acidic conditions; on alkaline soils (pH > 7), its availability drops sharply.

4.3. Zinc (Zn) – Growth Regulator

Zinc participates in the synthesis of auxins (growth hormones), proteins, and nucleic acids. Zinc deficiency is most common on alkaline and calcareous soils, as well as on soils with high phosphorus content (excess phosphorus "blocks" zinc) (Swiader & Ware, 1992).

Signs of zinc deficiency:

• Young leaves become small, narrow, may curl.
• Chlorotic spots (yellowish or bronze) appear on leaves, often as stripes between veins.
• Growth slows down, bulbs form small and uneven.

Practical recommendations:

• Foliar applications of 0.05% zinc sulfate solution (ZnSO₄) – during active leaf growth, especially if symptoms are noticed.
• For prevention, zinc can be applied to the soil as part of complex micronutrient fertilizers, but on alkaline soils, foliar applications are more effective.
• Application rates – about 2–5 kg/ha of zinc as sulfate or chelates.

4.4. Copper (Cu) – Tissue Strength

Copper participates in oxidative enzymatic reactions, affects protein synthesis and disease resistance. Copper deficiency is more common on peat (organic) soils at pH > 6.5 (Swiader & Ware, 1992; Brewster, 2008).

Signs of copper deficiency:

• Young leaves lose turgor, become pale green, then wilt.
• Leaf tips die off ("dieback").
• Bulbs lose density, show cracks and dents.
• Reduced resistance to fungal diseases.

Practical recommendations:

• On peat soils with pH above 6.5, application of 5–10 kg/ha copper sulfate (CuSO₄) during primary tillage is recommended (Swiader & Ware, 1992; Autko et al., 2012).
• In case of acute deficiency symptoms – foliar applications of 0.05% copper sulfate solution with lime (Bordeaux mixture), but more often complex micronutrient fertilizers with copper in chelated form are used.

4.5. Molybdenum (Mo) – Nitrogen Assimilation

Molybdenum is a crucial element for nitrogen metabolism, being part of the enzyme nitrate reductase, which reduces nitrates to ammonium. Molybdenum deficiency is often found on acidic soils (pH < 5.5), as molybdenum is strongly bound and becomes less available in acidic conditions (Swiader & Ware, 1992; Autko et al., 2012).

Signs of molybdenum deficiency:

• General yellowing and weakening of plants, resembling nitrogen starvation (because nitrogen stops being absorbed).
• Leaves become narrow, bushiness decreases.
• In severe cases – death of growing points.

Practical recommendations:

• On acidic soils (pH < 5.5), liming to pH 6.0–6.5 is usually enough to make molybdenum available (Brewster, 2008).
• For rapid correction of deficiency – foliar applications of 0.01–0.02% solution of ammonium molybdate or sodium molybdate (0.2–0.5 g/l water) (Swiader & Ware, 1992).
• Soil application rate – 0.5–1 kg/ha molybdenum, but foliar application is more commonly used.

4.6. Iron (Fe) and Chlorine (Cl) – "Auxiliary" Micronutrients

Although iron and chlorine rarely become limiting factors for onions, they are worth mentioning:

• Iron is part of enzymes related to photosynthesis. Iron deficiency manifests as interveinal chlorosis on young leaves – very similar to manganese deficiency, but in iron deficiency, the veins may also lighten. Occurs on calcareous soils at pH > 7. Treatment – iron chelates in foliar applications (0.1% solution).
• Chlorine (Cl) is not deficient for onions on most soils, as chlorides come with rain and fertilizers. Conversely, excess chlorine can interfere with sulfur uptake. Use sulfate forms of potassium fertilizers to maintain balance (Brewster, 2008).

4.7. How to Apply Micronutrients: General Principles

1. Determine the need. There are three ways:

• Visual diagnosis. Observe young leaves – they react to micronutrient deficiency faster than old ones.
• Soil analysis. Especially important for boron, copper, manganese, and zinc on problem soil types.
• Leaf analysis (tissue testing). The most accurate method for practitioners, especially for boron and manganese.

2. Choose the form. Micronutrients in chelated form (EDTA, DTPA) are better absorbed in foliar applications and are more effective on alkaline soils. Sulfate forms – for soil application on acidic soils.

3. Apply in split doses. Micronutrients are applied either with the base fertilizer (boron, copper, zinc) or as foliar applications during critical periods. Excess micronutrients can be toxic, so do not apply all micronutrient fertilizers in one dose.

4. Combine with major elements. For example, boron is better absorbed against a background of adequate potassium, and molybdenum on neutral soils. Complex micronutrient fertilizers often already contain a balanced set of elements and are recommended for regular use.

4.8. Table: Micronutrients for Onions – Quick Reference

Key takeaways from this chapter:

1. Boron is an absolute priority for onions. Its deficiency leads to bulb deformation and quality loss. Apply preventively on light and calcareous soils.

2. Manganese, zinc, and copper are critical on specific soil types (peat, sand, carbonates). Visual inspection of young leaves is your main tool.

3. Molybdenum is a problem only on acidic soils; liming solves it best.

4. Micronutrients are applied in split doses and carefully – excess is toxic. Use chelated forms for foliar applications and sulfate forms for soil applications.

5. Tissue testing (leaf analysis) is the most reliable way to assess micronutrient sufficiency if you are unsure about your site's needs.

5.1. Stage 1. Rooting and Initial Growth (from sowing/planting to the appearance of 2–3 true leaves)

This is a critical but often overlooked period. At this time, it is decided whether the plant will have a powerful root system and a healthy start for further growth.

What happens to the plant:

• The seed germinates or the set begins to grow.
• The primary root system forms, which must quickly explore a small volume of soil.
• The first true leaves appear, and photosynthesis begins.
• The plant cannot yet effectively use nutrients from deep soil layers; it "lives" on the reserves of the seed or set and what is immediately around the roots.

Which nutrients are important:

• Phosphorus (P) – absolute priority. In this phase, the rate of phosphorus absorption per unit root length in onions is 3 times higher than in adult plants (Brewster, 2008). Phosphorus stimulates root development and the formation of future growing points.
• Nitrogen (N) – starter dose. A small amount of nitrogen (especially in nitrate form) is needed to start leaf growth. But excess nitrogen at this time can be harmful – it stimulates too rapid top growth at the expense of roots and increases disease susceptibility.
• Potassium (K) and magnesium (Mg) – important for forming healthy cell walls and activating enzymes, but their role at this stage is secondary.

Practical recommendations:

• Starter fertilizer. Apply a small amount of phosphorus fertilizer directly into the furrow or hole when sowing/planting. For example, 5–10 g/m² of superphosphate or a special starter fertilizer with high phosphorus content (10-34-0 or 7-21-7) (Brewster, 2008; Swiader & Ware, 1992). This provides roots with readily available phosphorus at the most crucial moment.
• Minimal nitrogen. Nitrogen at this stage is better given in nitrate form (calcium nitrate, potassium nitrate), which is available even at low soil temperatures. The dose should not exceed 20–30% of the total planned nitrogen rate (Autko et al., 2012).
• Do not overwater. Overwatering at this stage can lead to seed and root rot. But do not let the soil dry out either – phosphorus availability drops sharply with moisture deficiency.
• Temperature regime. For good phosphorus uptake, soil temperature of at least 10–12 °C is desirable. In cold soil, phosphorus is practically not absorbed (Swiader & Ware, 1992).

5.2. Stage 2. Active Leaf Growth (from appearance of 3–4 true leaves to the start of bulb formation)

This is the most intensive phase of vegetation. The plant builds leaf mass – the "factory" of photosynthesis that will "feed" the future bulb. The size and quality of the harvest directly depend on how powerful a leaf apparatus can be created.

What happens to the plant:

• Leaves grow very quickly, their number increases.
• The root system continues to develop but still remains shallow (most roots are in the top 20 cm layer).
• The formation of vegetative organs is completed, and preparation for the transition to the reproductive phase (bulb formation) begins.

Which nutrients are most important:

• Nitrogen (N) – the main engine. Nitrogen ensures rapid leaf growth. Nitrogen demand peaks in this phase – up to 70% of all nitrogen absorbed during the season is taken up during this period (Zink, 1966, cited in Brewster, 2008).
• Potassium (K) – becomes increasingly important. Potassium helps the plant use water and carbohydrates efficiently, strengthens leaf tissues, and promotes sugar accumulation, which will later be transferred to the bulb. The N:K ratio in this phase should be approximately 1:1 or with a slight excess of potassium (Brewster, 2008).
• Magnesium (Mg) and iron (Fe) – needed to maintain high photosynthesis levels, as they are part of chlorophyll.
• Calcium (Ca) – continues to ensure cell wall strength, important for disease resistance and mechanical damage.

Practical recommendations:

• Split nitrogen application. Apply nitrogen in 2–3 top dressings at 10–14 day intervals. It is better to use nitrate forms (calcium nitrate, ammonium nitrate) or urea if the soil is not too cold (Brewster, 2008). The total nitrogen dose during this phase should be 40–60% of the total seasonal norm.
• Potassium top dressing. Potassium can be given together with nitrogen or slightly later, but before intensive bulb formation begins. Potassium sulfate works well (it also provides sulfur), but potassium chloride is also acceptable if the soil is not saline (Swiader & Ware, 1992).
• Magnesium. If you notice signs of magnesium deficiency, add it to the top dressing (e.g., Epsom salts – magnesium sulfate) at a rate of 5–10 g/m².
• Watering. In this phase, onions are very sensitive to moisture deficiency. Do not allow the topsoil to dry out, otherwise leaf growth will slow down, and nitrogen will not be absorbed.
• Avoid excess nitrogen. If you overfeed onions with nitrogen in this phase, they will continue to "fatten," growing leaves, and the transition to bulb formation will be delayed. This can lead to the formation of a thick neck and poor storage life (Swiader & Ware, 1992).

5.3. Stage 3. Start of Bulb Initiation (setting the bulb)

At this point, the plant "switches" its program: instead of growing leaves, it begins to direct carbohydrates and nutrients towards forming a storage organ – the bulb. This is a transitional stage requiring nutritional adjustments.

What happens to the plant:

• The diameter of the leaf bases increases, fleshy scales begin to thicken.
• The growth of new leaves gradually slows down.
• The transport of sugars and amino acids from aging leaves to the bulb is activated.

Which nutrients are most important:

• Potassium (K) – comes to the forefront. Potassium regulates carbohydrate transport and participates in the synthesis of starch and sugars. Increasing the proportion of potassium during this period promotes the formation of a dense, sweet bulb (Brewster, 2008; Swiader & Ware, 1992).
• Phosphorus (P) – still needed. It is necessary for energy metabolism and cell division in the growth zone.
• Sulfur (S) – its role increases, as sulfur-containing compounds (alliin, etc.) begin to be actively synthesized in the bulb itself. This determines the future taste.
• Nitrogen (N) – its share begins to be reduced. Excess nitrogen at this stage stimulates leaf growth and inhibits bulb formation, leading to "fattening" and weakening of the bulb.
• Boron (B) – critical for sugar transport into the bulb. Boron deficiency in this phase can cause cracks and internal necrosis (Brewster, 2008).

Practical recommendations:

• Reduce nitrogen. If you were applying nitrogen in the previous phase, its share should be reduced by the start of bulb formation. The last nitrogen application should be no later than 3–4 weeks before the intended harvest (Swiader & Ware, 1992).
• Increase potassium. Include a potassium fertilizer in the top dressing, preferably potassium sulfate (which also provides sulfur). Dose – 20–30 g/m² K₂O (about 40–50 g/m² potassium sulfate) (Autko et al., 2012).
• Boron. If you haven't applied boron earlier, a foliar application of 0.1–0.2% boric acid solution is very useful in this phase. This will help avoid internal defects (Brewster, 2008).
• Maintain moisture. In this phase, onions are very sensitive to water stress; moisture fluctuations can cause bulb cracking.

5.4. Stage 4. Bulb Filling (intensive bulb growth)

This is the most critical phase for forming the commercial yield. The bulb rapidly gains weight, and nutrition during this moment determines not only the size but also the density, sweetness, and storage life.

What happens to the plant:

• The bulb actively increases in size and weight.
• Leaves gradually turn yellow and die, starting from the lower ones.
• Mass transport of nutrients from leaves to the bulb occurs.
• The formation of covering scales is completed.

Which nutrients are most important:

• Potassium (K) – the main element for filling. It regulates cell turgor, promotes sugar accumulation, and increases resistance to storage diseases.
• Calcium (Ca) – important for forming dense covering scales and resistance to mechanical damage.
• Sulfur (S) – continues to participate in the synthesis of flavor and aroma compounds.
• Nitrogen (N) – almost excluded. Any nitrogen application in this phase delays maturation, reduces storage life, and promotes the development of neck rot (Brewster, 2008; Swiader & Ware, 1992).

Practical recommendations:

• Nitrogen – absolutely not. Stop all nitrogen applications 3–4 weeks before harvest. If you notice signs of nitrogen starvation on old leaves in this phase – it's normal, that's how it should be. The bulb uses nitrogen from dying leaves.
• Potassium – maintenance dose. If the soil is poor in potassium, you can give one more potassium top dressing, but no later than 2–3 weeks before harvest.
• Micronutrients. Boron, copper, and zinc can help form a dense bulb structure and prevent cracking. Best done as foliar applications at the beginning of this phase.
• Gradually reduce watering. During the bulb filling period, soil moisture should be stable but not excessive. When the bulb reaches about 70% of its final size, watering is reduced, and by the time leaves lodge, it is stopped completely. This accelerates maturation and improves storage life (Swiader & Ware, 1992).

5.5. Stage 5. Maturation (preparation for harvest)

This is the final stage, when the onion stops growing, leaves lodge, and covering scales dry. The gardener's task is not to interfere with the plant's proper completion of the cycle.

What happens to the plant:

• Complete cessation of growth.
• Leaves turn yellow, dry, and lodge.
• Final formation of covering scales, which become dry and dense.
• The bulb enters dormancy.

Which nutrients are most important:

• Practically none. The plant has already completed vegetation, and additional fertilizing is not needed. Moreover, they can be harmful, delaying maturation and reducing storage life.

Practical recommendations:

• Complete cessation of all fertilizing. Applying fertilizers in this phase is pointless, and applying nitrogen or potassium can stimulate regrowth or damage covering scales.
• Do not water. Stop watering completely 2–3 weeks before harvest. This accelerates leaf lodging and improves scale peeling.
• If necessary – desiccation. In some cases (e.g., in rainy weather when onions don't want to lodge), desiccants (e.g., diquat) are used to speed up drying. But that's a topic for another article. In amateur gardening, it's better to wait for natural maturation.

5.6. Summary: Nutrition by Stages in Table Form

Key takeaways from this chapter:

1. Development stages are the key to success. The nutrient needs of onions change radically from phase to phase. What is good in one phase can be harmful in another.

2. Main rule: nitrogen for leaves, potassium for bulbs. Nitrogen is needed until bulb initiation begins, potassium during and after. Sulfur and boron are critical for quality and taste at all stages.

3. Timeliness and split application. Apply fertilizers exactly in the right phases, preferably in split doses rather than one large dose. This reduces the risk of root burn and fertilizer loss.

4. Monitor plant condition. External signs (leaf color, size, bulb shape) are the best indicators of whether you are feeding the onions correctly.

6.1. Organic Fertilizers: Pros, Cons, and Application Features

Organic fertilizers include manure, compost, humus, poultry manure, green manures, bone meal, and other natural materials. They improve soil structure, increase water-holding capacity, and serve as a source of nutrients in slowly released forms.

Advantages of organics for onions:

• Improvement of soil structure. Organics make light sandy soils more water-holding and heavy clay soils more loose and air-permeable. This is especially important for the weak root system of onions (Autko et al., 2012; Swiader & Ware, 1992).
• Long-lasting action. Nutrients are released gradually as organic matter mineralizes, reducing the risk of "salt shock" to the roots.
• Complexity. Organics contain not only major elements (N, P, K) but also micronutrients and organic substances that stimulate the development of beneficial soil microflora.
• Increased soil buffering. Organic matter mitigates fluctuations in pH and salt concentration, which is very important for sensitive onions (Brewster, 2008).

Disadvantages and limitations:

• Slow action. In spring, when onions need a quick start, organics cannot always provide sufficient amounts of readily available phosphorus and nitrogen.
• Risk of weeds and diseases. Fresh manure may contain weed seeds and pathogens dangerous to onions (especially if the manure comes from animals fed on plant residues) (Swiader & Ware, 1992).
• Delayed maturation. Fresh manure applied directly under onions stimulates rapid leaf growth and delays bulb formation, reducing storage life (Autko et al., 2012).
• Imbalanced composition. The NPK content in organics varies greatly and depends on the type of animal, bedding, and storage conditions. It is difficult to accurately calculate the dose.

Practical recommendations for organics:

1. Apply organics to the predecessor crop, not directly under onions. The best option is to apply well-rotted manure or compost in autumn for fall plowing or in spring 1–2 months before planting onions (Swiader & Ware, 1992; Autko et al., 2012). This allows the organics to partially mineralize, and the onions will receive available forms of nutrients without suffering from excess fresh organics.

2. Use only well-rotted manure or compost (at least 1–2 years old). Fresh manure is categorically not recommended for onions – it causes root burns, provokes diseases, and delays vegetation (Autko et al., 2012).

3. Green manures are an excellent alternative. Sowing green manure crops (mustard, phacelia, lupine, winter rye) in winter with subsequent incorporation into the soil in spring is an environmentally friendly way to increase fertility and improve soil structure. The risk of disease introduction is minimal, and nutrients become available at the right time (Brewster, 2008).

4. Combine organics with mineral fertilizers. In practice, a combined scheme is often used: organics as the basis of fertility (main application) and mineral fertilizers for "precision" fertilizing according to development stages. This gives better results than using only organics or only minerals.

6.2. Mineral Fertilizers: Precision and Speed

Mineral fertilizers are concentrated sources of individual elements or their combinations. They allow for precise dosage of nutrition and rapid correction of deficiencies.

Advantages of mineral fertilizers for onions:

• High concentration of active ingredient. You can apply the required dose in a small volume.
• Fast action. Nutrients become immediately available to the plant (especially nitrogen in nitrate form and potassium).
• Precision of dosage. You know exactly how much and which element you are applying.
• Possibility of adjustment. You can quickly correct a deficiency with a foliar application.

Main types of mineral fertilizers for onions:

Disadvantages of mineral fertilizers:

• Risk of salt burn. High concentration or uneven application can damage roots (Brewster, 2008).
• Leaching. Nitrogen (especially nitrates) can easily leach from the root zone, especially on light soils.
• Soil acidification or salinization. Long-term use of some forms (e.g., ammonium nitrate) can acidify the soil, which is undesirable for onions.
• Lack of organic matter. Mineral fertilizers do not improve soil structure, so their use should be combined with organics.

Practical recommendations for mineral fertilizers:

1. Use sulfate forms of potassium fertilizers instead of chlorides – onions are very responsive to sulfur, and excess chlorine can impair taste and cause root burn (Brewster, 2008).

2. Apply phosphorus fertilizers locally (in furrows when sowing or in holes when planting sets). This increases phosphorus use efficiency 2–3 times compared to broadcast application (Brewster, 2008).

3. Apply nitrogen fertilizers in split doses, aligning with development stages. Use nitrate forms in cold weather (available at low temperatures), and ammonium and amide forms when the soil is warm.

4. Complex fertilizers with micronutrients are a good choice for main application if there are no specific deficiencies on your site.

6.3. Example Fertilization Schemes for Different Scenarios

Below are approximate fertilization schemes for onions. All doses are given in grams of active ingredient per square meter (g/m²) or in grams of a specific fertilizer per square meter. Recalculate proportionally for your plot area.

Important note: These schemes are not dogma. They need adjustment depending on your soil fertility, variety, weather conditions, and visual plant condition. The best approach is to start with these recommendations and "fine-tune" them during the season, observing leaves and bulbs.

6.4. How to Adjust Schemes Depending on Conditions

All the schemes provided are a starting point. In reality, you will have to adjust them:

By soil type:

• Light sandy soils: increase the frequency of fertilizing (but reduce the single dose), as elements leach faster. Be sure to use sulfate forms of potassium and magnesium (Brewster, 2008).
• Peat soils: pay attention to micronutrients – copper, manganese, zinc. Peatlands often lack potassium (Swiader & Ware, 1992). Apply nitrogen in smaller doses.
• Loamy and chernozem soils: you can reduce the frequency of fertilizing but increase the single dose. Starter fertilizers work well.

By weather conditions:

• Cold and wet spring: use nitrate forms of nitrogen (calcium nitrate) – they are available at low temperatures. Delay ammonium forms until the soil warms up (Swiader & Ware, 1992).
• Hot and dry weather: reduce fertilizer doses (especially nitrogen), increase watering frequency, use foliar applications of micronutrients – they help the plant cope with stress.
• Rainy summer: increase the number of nitrogen fertilizing applications (nitrogen leaches out), but reduce the single dose.

By plant condition:

• Pale green leaves, slow growth – likely needs nitrogen (top dressing 15–20 g/m² urea or ammonium nitrate).
• Reddish-purple leaf tinge – phosphorus deficiency (apply superphosphate or do a foliar application of 1% potassium monophosphate solution).
• Yellow edges of older leaves – potassium deficiency (top dressing with potassium sulfate 15–20 g/m²).
• Internal necrosis of bulbs – boron deficiency (urgent foliar application of 0.1–0.2% boric acid) (Brewster, 2008).
• Small, narrow leaves – possible zinc deficiency (spraying with 0.05% zinc sulfate).

6.5. How to Apply Fertilizers: Root and Foliar Methods

Root application (watering under the root with fertilizer solution or dry incorporation) is the main method for onions. Dry fertilizers are best incorporated to a depth of 5–7 cm in the root zone (no deeper!) and always watered in after application to dissolve the granules and avoid burning. Liquid fertilizing (solution) is the most effective method, as nutrition reaches the roots immediately. The solution concentration should not exceed 0.5–1% (5–10 g fertilizer per 1 L water) for young plants and 1–2% for adults to avoid root burn (Brewster, 2008).

Foliar (leaf) application – spraying leaves with a fertilizer solution – is not the main method for major elements (NPK), but is very effective for micronutrients and for rapid correction of deficiencies (Brewster, 2008). Foliar applications do not replace root applications but can be an excellent supplement:

Important rule for foliar applications: Apply them in cloudy weather or in the evening to avoid leaf burn. Adding a surfactant (soap, special adjuvants) improves leaf wetting and application efficiency.

6.6. Brief Summary: Golden Rules for Onion Fertilizing

1. Apply the base fertilizer (NPK + organics) before sowing/planting, creating a "reservoir" of nutrients in the root zone.

2. Starter fertilizer (phosphorus + small nitrogen) – mandatory in the furrow or hole. This gives a powerful boost to root and shoot growth (Brewster, 2008; Swiader & Ware, 1992).

3. Nitrogen – in split doses, in the first half of the growing season. Apply the last nitrogen top dressing no later than 3–4 weeks before harvest. Excess nitrogen is more dangerous than deficiency – it delays maturation and reduces storage life (Swiader & Ware, 1992; Autko et al., 2012).

4. Potassium – in increasing doses, with emphasis on the second half of the growing season. Potassium is the key element for bulb quality and storage life. Use sulfate forms (Brewster, 2008).

5. Boron – mandatory foliar application at the beginning of bulb initiation. This prevents internal bulb defects and improves their density (Brewster, 2008).

6. Monitor plant condition. External signs of deficiency or excess of elements are your main "indicators." Adjust the scheme during the season.

7. Consider soil type, weather, and variety. There are no universal schemes; adapt recommendations to your conditions.

8. Foliar applications – for micronutrients and rapid correction of deficiencies; for NPK – mainly root application. But foliar applications complement root applications well, especially under unfavorable conditions (weather, cold, stress).

In the next chapter, we will take a closer look at foliar (leaf) applications: when they are needed, which elements are best absorbed through leaves, and how to properly perform spraying to get maximum effect.

7.1. Why Does Onion Need Foliar Applications?

Onions need foliar applications for several reasons:

1. Rapid correction of micronutrient deficiencies. Boron, manganese, zinc, copper, and other micronutrients in chelated form or as sulfates easily penetrate the leaf surface and are quickly absorbed by the plant (Brewster, 2008; Swiader & Ware, 1992).

2. Bypassing root problems. In cold, waterlogged, or dry weather, onion roots work poorly, but foliar applications remain effective. This is especially relevant for onions, which are very sensitive to root zone conditions (Autko et al., 2012).

3. Possibility of targeted action during critical stages. For example, at the start of bulb formation, when you need to quickly "feed" the plant with boron and potassium without overloading it with nitrogen, foliar application is ideal (Brewster, 2008).

4. Fertilizer savings. When applied to leaves, micronutrients are absorbed almost 100%, while with root application, a significant part can be bound by the soil or leached (especially on light and alkaline soils).

5. Stress reduction. Foliar applications of micronutrients help the plant better tolerate adverse weather conditions (drought, frost, temperature fluctuations).

7.2. Which Elements Are Best Applied Through Leaves?

Not all elements are equally effective when applied to leaves. Some easily penetrate the cuticle, others with difficulty, and some are not absorbed through leaves at all.

7.3. When to Apply Foliar Applications to Onions

Foliar applications should be timed to specific development stages and when deficiency signs appear. Here are the optimal times:

7.4. How to Properly Conduct Foliar Applications

The technique of applying foliar applications is as important as choosing the fertilizer. Violating the rules can render efforts futile or even harm the plant.

7.5. Example Foliar Application Schedule for Onions (Basic Option)

7.6. How to Recognize Deficiencies "By Leaves" – Quick Reference

7.7. Important Reminders and Cautions

1. Foliar applications do NOT replace root applications. They are only a supplement to the main nutrition through the soil (Brewster, 2008; Swiader & Ware, 1992). Major elements (N, P, K) are mainly applied to the root, while foliar applications are "emergency aid" and "supplement" for micronutrients during critical stages.

2. Do not exceed the concentration. Especially for boron and copper. Overdosing can cause leaf burns and toxicity (Brewster, 2008). Always check the recommendations on the product label.

3. Boron is the leader for onions. If you can do only one foliar application per season, let it be boron at the start of bulb initiation. This gives the greatest effect on quality and storage life.

4. Use quality products. For micronutrients, chelated forms are preferred (e.g., "Boron chelate," "Zinc chelate," "Manganese chelate") – they penetrate the leaf better and do not cause burns. For boron, inexpensive boric acid can also be used (Brewster, 2008).

5. Do not fertilize through leaves in heat or bright sun. High temperature and sunlight can cause evaporation and crystallization of salts on leaves, leading to burns (Hochmuth & Sideman, 2023).

6. Maintain intervals between treatments. Usually 7–10 days is enough time to assess the effect.

7. Regularly inspect plants. Visual inspection is your main tool for timely detection of deficiencies and adjustment of the fertilization scheme.

Key takeaways from this chapter:

1. Foliar applications are a quick and effective way to correct micronutrient deficiencies, especially boron, magnesium, zinc, and manganese. For onions, they are critically important at the start of bulb initiation (Brewster, 2008).

2. Boron is the most important element for foliar application in onions. It prevents internal bulb defects and improves their density and storage life.

3. Technique: fine spray, treatment of both sides of leaves, in cloudy weather or evening, with a surfactant. This increases effectiveness and reduces the risk of burns (Hochmuth & Sideman, 2023).

4. Don't overdo it – 1–2 treatments per season is enough. Foliar applications do not replace root nutrition but complement it.

5. Visual diagnosis is your best helper. Regularly inspect leaves and bulbs to notice deficiency signs in time and adjust the fertilization scheme.

8.1. How to Distinguish Deficiency from Excess: General Principles

Before considering specific symptoms, remember a few general rules:

1. Deficiency of mobile elements (N, P, K, Mg) appears on old (lower) leaves. The plant "translocates" these elements from old tissues to young ones, so old leaves suffer first (Brewster, 2008; Swiader & Ware, 1992).

2. Deficiency of immobile elements (Ca, Fe, Zn, Mn, B, Cu) appears on young (upper) leaves. These elements cannot move from old tissues, so new leaves are the first to lack them (Swiader & Ware, 1992).

3. Excess of elements often manifests as marginal leaf burn (necrosis) or chlorosis (yellowing) between veins, sometimes as curling or deformation. Excess nitrogen – "fattening" (dark green, succulent leaves) and delayed bulb formation (Brewster, 2008).

4. Symptoms of different deficiencies can be similar. For example, nitrogen and sulfur deficiencies both cause general yellowing, but in sulfur deficiency, young leaves turn yellow, while in nitrogen deficiency, old leaves do. Look carefully at which leaves show symptoms (Brewster, 2008).

8.2. Nitrogen (N) Deficiency

How it manifests:

• General yellowing of leaves, starting from the lower (old) ones.
• Growth slows down, plants appear dwarfed, stunted.
• Leaves become narrow, pale green, then yellow.
• Bulbs form small with a thin neck.

What to do:

• Apply root nitrogen top dressing: 10–15 g/m² urea or ammonium nitrate (depending on soil temperature) (Autko et al., 2012).
• On light soils, nitrogen may leach – split application may be needed.
• Important: nitrogen deficiency is more common at the beginning of the season. By the bulb filling stage, slight yellowing of lower leaves is normal, a natural aging process.

8.3. Phosphorus (P) Deficiency

How it manifests:

• Dark green, sometimes with a purple or purplish tinge on lower leaves (Swiader & Ware, 1992).
• Growth is slow, plants are stunted.
• Root system is weak, bulbs form late and poorly.

What to do:

• Locally apply phosphorus fertilizer to the root zone (e.g., superphosphate 10–15 g/m²) (Brewster, 2008).
• On acidic soils, phosphorus is bound – lime first (pH 6.0–6.5).
• Important: phosphorus is immobile in the soil, and symptoms often appear not from its absence but from it being outside the root zone. Therefore, starter application in holes is very important (Brewster, 2008).

8.4. Potassium (K) Deficiency

How it manifests:

• Yellowing and browning of edges and tips of lower (old) leaves – marginal burn (Swiader & Ware, 1992; Autko et al., 2012).
• Leaves may curl, become wrinkled.
• Bulbs are loose, soft, store poorly.
• Increased susceptibility to diseases.

What to do:

• Apply root potassium top dressing: 15–20 g/m² potassium sulfate (preferable to chloride) (Brewster, 2008).
• On light sandy soils, potassium may leach – apply in split doses.

8.5. Calcium (Ca) Deficiency

How it manifests:

• Deformation and dieback of young leaf tips.
• Bulbs lose density, may soften and rot at the neck.
• Watery areas may be visible on a bulb cross-section.

What to do:

• Check soil pH – at pH < 5.5, calcium is less available. Apply liming (dolomite lime) (Swiader & Ware, 1992).
• Avoid excess potassium, which competes with calcium.
• In acute deficiency – foliar application of 0.5–1% calcium nitrate solution (Brewster, 2008).

8.6. Magnesium (Mg) Deficiency

How it manifests:

• Interveinal chlorosis on lower (old) leaves: veins remain green, but the tissue between them turns yellow (Swiader & Ware, 1992).
• Leaf edges may curl upward.
• Leaves become brittle, prematurely die.

What to do:

• Foliar application of 1–2% magnesium sulfate solution (Epsom salts) – the fastest way (Brewster, 2008).
• On light soils, apply 10–15 g/m² magnesium sulfate to the soil.
• When liming, use dolomite lime (contains magnesium).

8.7. Sulfur (S) Deficiency

How it manifests:

• General yellowing of young (upper) leaves, similar to nitrogen starvation, but unlike nitrogen – young, not old leaves are affected (Brewster, 2008).
• Growth slows down, plants are weakened.
• Onion taste becomes bland, unremarkable (due to lack of sulfur-containing compounds) (Brewster, 2008).

What to do:

• Use sulfate forms of fertilizers (ammonium sulfate, potassium sulfate) in top dressings (Brewster, 2008).
• Apply gypsum (calcium sulfate) – 20–30 g/m² (provides both calcium and sulfur).
• The problem is more common on light soils.

8.8. Boron (B) Deficiency – The Most Critical for Onions

How it manifests:

• Internal necrosis and cracks in bulbs – when cut, watery then browning areas are visible (Brewster, 2008; Swiader & Ware, 1992).
• Leaves become brittle, may curl.
• In seed plants – poor seed set ("blossom-end rot").

What to do:

• Urgent foliar application of 0.1–0.2% boric acid solution (1–2 g per 1 L water) – 1–2 times at 7–10 day intervals (Brewster, 2008).
• On acidic or sandy soils – pre-sowing application of boron (borax 10–20 g/m²).
• Important: excess boron is toxic, do not exceed the concentration!

8.9. Zinc (Zn) Deficiency

How it manifests:

• Small, narrow, chlorotic (yellowish) young leaves (Swiader & Ware, 1992).
• Interveinal chlorosis in stripes or spots.
• Growth slows down, bulbs are small.

What to do:

• Foliar application of 0.05% zinc sulfate solution (0.5 g/L) (Brewster, 2008).
• On alkaline soils – use chelated forms of zinc.

8.10. Manganese (Mn) Deficiency

How it manifests:

• Interveinal chlorosis on young leaves – veins green, tissue between them yellow (Swiader & Ware, 1992).
• Often appears on peat soils at pH > 6.5.

What to do:

• Foliar application of 0.05–0.1% manganese sulfate solution (Brewster, 2008).
• On peat soils – apply manganese to the soil (15–20 kg/ha).

8.11. Copper (Cu) Deficiency

How it manifests:

• Pale, wilting young leaves, "dieback" – leaf tips die off (Swiader & Ware, 1992).
• Bulbs lose density, develop dents.

What to do:

• On peat soils – apply 5–10 kg/ha copper sulfate.
• Foliar application of 0.02–0.05% copper sulfate solution.

8.12. Molybdenum (Mo) Deficiency

How it manifests:

• General yellowing and weakening of plants, similar to nitrogen starvation (Swiader & Ware, 1992).
• Leaves are narrow, bushiness reduced.
• More common on acidic soils (pH < 5.5).

What to do:

• Apply liming to pH 6.0–6.5 – this is the best way (Brewster, 2008).
• In acute deficiency – foliar application of 0.01–0.02% ammonium or sodium molybdate solution.

8.13. Excess of Elements: How to Recognize and What to Do

Excess of nutrients is less common than deficiency, but it is no less dangerous. Most often, plants suffer from overfeeding with nitrogen, boron, and copper.

8.14. Differential Diagnosis: How Not to Get Confused

Sometimes symptoms of different problems are very similar. Here are some "clues" to help distinguish one from another:

8.15. Quick Reference Table: Deficiency and Excess Symptoms

Key takeaways from this chapter:

1. Observation is your main tool. Regularly inspect onions, paying attention to leaf color and shape, and during harvest, bulb condition. The sooner you notice a problem, the easier it is to fix.

2. Deficiency of mobile elements (N, P, K, Mg) – on old leaves; immobile (Ca, Fe, Zn, Mn, B, Cu) – on young. This is the main rule of diagnosis (Swiader & Ware, 1992; Brewster, 2008).

3. Boron is the most common deficiency in onions. Its symptoms (cracks and necrosis in bulbs) are a critical signal requiring immediate foliar application (Brewster, 2008).

4. Excess is as dangerous as deficiency. Especially nitrogen, boron, and copper. Always follow recommended doses.

5. Don't confuse deficiencies with diseases or pests. If symptoms appear on individual plants or as spots – it could be a disease (e.g., downy mildew). If symptoms are uniform across the field – it's more likely a nutrition problem.

6. For an accurate diagnosis, use soil and tissue testing. Visual diagnosis is only the first step. If in doubt, take samples and send them to a laboratory.

9.1. Mistake #1: Overfeeding with Nitrogen (Especially in the Second Half of the Growing Season)

What the mistake is: Many gardeners believe that the more nitrogen, the better the onion grows. They apply high doses of nitrogen throughout the season, including the bulb formation and filling period.

Consequences:

• Onions "fatten": leaves become dark green, succulent, powerful, but the bulb does not set or sets late and poorly (Swiader & Ware, 1992).
• A thick, succulent neck forms, which does not dry well at harvest. This opens the door to neck rot pathogens (Botrytis) during storage (Autko et al., 2012; Brewster, 2008).
• Bulbs become loose, watery, store poorly, lose density and taste.
• Maturation is delayed – the onion does not have time to form dense covering scales before rain or cold weather.

How to avoid:

• Apply nitrogen in split doses and only in the first half of the growing season – until active bulb formation begins (Swiader & Ware, 1992).
• Apply the last nitrogen application no later than 3–4 weeks before the intended harvest (Brewster, 2008).
• Align with the development stage: as soon as the leaf base thickening begins – exclude nitrogen.
• If you have overfed the onions with nitrogen, try to balance the excess with an increased dose of potassium (e.g., potassium sulfate top dressing). Potassium promotes carbohydrate flow to the bulb and partially compensates for the imbalance (Brewster, 2008).

9.2. Mistake #2: Potassium Deficiency and Chlorine Excess

What the mistake is: Using only nitrogen fertilizers, ignoring potassium, or using cheap potassium chloride as the main potassium fertilizer on sensitive soils.

Consequences:

• Potassium deficiency manifests as marginal leaf burn, bulbs become soft, loose, store poorly (Swiader & Ware, 1992).
• Chloride ion from potassium chloride can cause root burn and impair onion taste, especially in dry conditions (Brewster, 2008).
• Excess chlorine interferes with sulfur uptake, reducing onion aroma and pungency.

How to avoid:

• For potassium fertilizing of onions, potassium sulfate (K₂SO₄) is preferable – it contains no chlorine and provides sulfur (Brewster, 2008).
• Potassium chloride (KCl) can be used only on non-saline soils, with adequate moisture, and preferably for main application (in autumn or a month before planting) so that chlorine leaches out.
• Apply potassium in split doses – main part for primary preparation, and part as a top dressing at the start of bulb initiation (Autko et al., 2012).

9.3. Mistake #3: Incorrect Phosphorus Application – Broadcast, Not Localized

What the mistake is: Broadcasting phosphorus fertilizers over the soil surface without incorporating them into the root zone.

Consequences:

• Phosphorus is immobile in the soil. When broadcast, it remains in the top layer and does not reach onion roots, especially at early stages (Brewster, 2008).
• Young seedlings experience acute phosphorus deficiency, which delays root development and stunts growth (Costigan et al., 1983, cited in Brewster, 2008).
• As a result, onions lag in development, and it is very difficult to catch up.

How to avoid:

• Apply phosphorus locally – in the furrow during sowing, in the hole when planting sets, or in a narrow strip at a depth of 5–7 cm (Brewster, 2008).
• Use starter fertilizers with high phosphorus content (e.g., 10-34-0 or 7-21-7) directly in the root zone.
• For main application, be sure to incorporate phosphorus fertilizers into the soil to a depth of at least 10–15 cm so they are in the root zone.

9.4. Mistake #4: Ignoring Boron

What the mistake is: Many gardeners believe that micronutrients are not needed if the soil is "good" and do not apply boron, unaware of its critical role for onions.

Consequences:

• Boron deficiency is one of the main causes of internal necrosis and cracks in bulbs, making onions unsuitable for storage and sale (Brewster, 2008).
• Bulbs lose density, become soft, may rot at the neck.
• In seed plants – "blossom-end rot" – poor seed set.

How to avoid:

• Be sure to do 1–2 foliar applications of boron (0.1–0.2% boric acid solution) at the start of bulb initiation (Brewster, 2008).
• On light sandy or peat soils – apply boron to the soil before sowing (borax 10–20 g/m²) (Swiader & Ware, 1992).
• If in doubt, it's better to do a foliar boron application than to skip it – the risk of overdose is minimal if concentration is observed, but the benefit is enormous.

9.5. Mistake #5: Applying Fertilizers to Dry Soil Without Incorporation and Watering

What the mistake is: Spreading granular fertilizers on dry soil surface hoping for rain or simply without subsequent watering.

Consequences:

• Fertilizers remain on the surface and do not penetrate the root zone. Plants do not receive nutrition, and fertilizers decompose, releasing ammonia (especially urea) or simply remain unavailable (Swiader & Ware, 1992).
• With moisture deficiency, high salt concentration on the surface can cause root and seedling burn.
• Nitrogen fertilizers (especially urea) can partially volatilize as ammonia, losing effectiveness (Brewster, 2008).

How to avoid:

• Always incorporate fertilizers into the soil to a depth of 5–7 cm (into the root zone) or water immediately after application (at least 10–15 L/m²) (Swiader & Ware, 1992).
• It's better to apply fertilizers in solution – this ensures uniform distribution and quick access to roots.
• When using urea, be sure to incorporate it into the soil or water – otherwise, nitrogen losses can reach 30–50%.

9.6. Mistake #6: Applying Top Dressings Too Early or Too Late

What the mistake is: Applying nitrogen at the end of the season (after bulb initiation) or, conversely, stopping fertilizing too early when onions are still actively growing.

Consequences:

• Stopping fertilizing too early (especially nitrogen) – onions do not have time to build sufficient leaf mass, bulbs become smaller, yield decreases (Brewster, 2008).
• Too late nitrogen – delays maturation, reduces storage life, promotes neck rot development (Swiader & Ware, 1992).
• Too early potassium (before bulb initiation) – has no effect, as potassium doesn't "work" without sufficient leaf mass. Too late potassium (2–3 weeks before harvest) – may not have time to show its effect, and combined with wet weather, may delay maturation.

How to avoid:

• Strictly adhere to the growth-stage fertilization schedule (see Chapter 6). Nitrogen – only until bulb initiation. Potassium – from the start of bulb initiation and during filling. Boron – at the start of bulb initiation (Brewster, 2008; Swiader & Ware, 1992).
• Stop all fertilizing (especially nitrogen) 3–4 weeks before harvest, when onions begin natural maturation.

9.7. Mistake #7: Ignoring Soil pH

What the mistake is: Gardeners apply fertilizers without caring about soil reaction. At acidic or, conversely, alkaline pH, many elements become unavailable to plants.

Consequences:

• On acidic soils (pH < 5.5), phosphorus, molybdenum, calcium, and magnesium are poorly absorbed. On alkaline soils (pH > 7.5) – iron, manganese, zinc, copper, and boron (Swiader & Ware, 1992; Brewster, 2008).
• Applied fertilizers "don't work," plants suffer from deficiencies, yields drop, and you waste time and money.

How to avoid:

• Be sure to check soil pH before planting (and periodically during the process). Maintain the optimal range for onions – pH 6.0–7.0 (Autko et al., 2012; Swiader & Ware, 1992).
• On acidic soils – lime (dolomite lime) 2–3 months before planting.
• On alkaline soils – use physiologically acidic fertilizers (ammonium sulfate, potassium magnesia) and maintain organic matter.

9.8. Mistake #8: Overfeeding with Micronutrients (Especially Boron and Copper)

What the mistake is: The desire to "feed more" leads to exceeding the recommended doses of boric acid, copper sulfate, or other micronutrient fertilizers.

Consequences:

• Excess boron causes marginal leaf burn, growth retardation, and can be toxic to plants (Brewster, 2008).
• Excess copper also causes leaf chlorosis and necrosis, damages roots (Swiader & Ware, 1992).
• The difference between deficiency and toxicity for micronutrients is very small – only 2–3 times. Easy to overdo.

How to avoid:

1. Strictly follow the recommended concentrations:

• Boron – 0.1–0.2% (1–2 g boric acid per 1 L water) (Brewster, 2008).
• Copper – 0.02–0.05% (0.2–0.5 g per 1 L water).
• Manganese, zinc – 0.05–0.1% (0.5–1 g per 1 L water).

2. It's better to use complex micronutrient fertilizers with an already balanced composition and follow the package instructions.

3. Apply micronutrients only when deficiency symptoms are present or for preventive purposes at the recommended times (e.g., boron – mandatory at the start of bulb initiation).

9.9. Mistake #9: Incorrect Watering After Fertilizing

What the mistake is: After applying dry fertilizers – insufficient or, conversely, too abundant watering. Or watering immediately after foliar application.

Consequences:

• Insufficient watering after dry application – fertilizers do not dissolve and do not reach roots, and at high concentration can cause burns (Swiader & Ware, 1992).
• Too abundant watering – leaches nutrients (especially nitrogen) from the root zone, fertilizers are lost.
• Watering after foliar application washes the solution off leaves, reducing effectiveness.

How to avoid:

• After applying dry fertilizers, be sure to water at a rate of 10–15 L/m² to dissolve and deliver to roots (Swiader & Ware, 1992).
• Apply liquid fertilizers to moist soil to avoid burning and improve distribution.
• Apply foliar applications in dry, calm weather, preferably in the evening, and do not water over leaves for 4–6 hours after treatment (Hochmuth & Sideman, 2023).

9.10. Mistake #10: Mixing Incompatible Preparations

What the mistake is: Adding fertilizers to the same tank with pesticides (herbicides, insecticides, fungicides) without checking compatibility.

Consequences:

• Some fertilizers (especially boron, copper, zinc, and alkaline solutions) can degrade the active ingredient of pesticides, reducing their effectiveness or causing leaf burns (Brewster, 2008).
• An insoluble precipitate may form, clogging sprayer nozzles.
• Plants may suffer chemical burns from reactions between components.

How to avoid:

• Check compatibility of preparations: do a "jar test" in a small container, mixing preparations in the same proportions as in the sprayer (Hochmuth & Sideman, 2023).
• If in doubt, apply fertilizer and pesticide treatments separately, with a 3–5 day interval.
• Use specialized adjuvants (surfactants, wetting agents) that are compatible with both fertilizers and pesticides.

9.11. Mistake #11: Lack of pH Control of the Working Solution (for Foliar Applications)

What the mistake is: Not considering the acidity of the water used to prepare the fertilizer solution. For example, hard alkaline water can reduce the effectiveness of phosphorus and micronutrient applications.

Consequences:

• In an alkaline environment (pH > 7), iron, manganese, zinc, and copper form insoluble compounds and are not absorbed by leaves (Brewster, 2008).
• Boron becomes less available in an alkaline environment.
• Some pesticides also lose effectiveness at high pH.

How to avoid:

• For foliar applications, use water with pH 5.5–6.5. If water is hard, add a small amount of citric or acetic acid to acidify (Swiader & Ware, 1992).
• Many ready-made chelated micronutrient fertilizers already contain buffering additives, but it's better to check the solution pH before spraying.

9.12. Mistake #12: Lack of a Systematic Approach and Planning

What the mistake is: Fertilizing is done sporadically, without considering the development stage, soil condition, or weather forecast.

Consequences:

• Onions receive uneven nutrition: sometimes starving, sometimes overfed. Yield and quality become unpredictable.
• Fertilizers are applied at the wrong time, their effectiveness decreases, and costs rise.

How to avoid:

• Develop a fertilization plan for the season, based on the recommendations in Chapter 6 (fertilization schemes) and considering your soil type.
• Keep a fertilization log (date, type of fertilizer, dose, development stage). This will help analyze mistakes and improve results in the next season.
• Regularly inspect plants (Chapter 8) and adjust the plan when deficiency or excess symptoms appear.
• Consider weather conditions: in cold weather, use nitrate forms of nitrogen; in hot weather, reduce doses and use foliar applications of micronutrients to reduce stress (Swiader & Ware, 1992).

9.13. Quick Checklist: How to Avoid Fertilizing Mistakes

Conclusion: Main Principles of Onion Nutrition

Summarizing the entire article, we can highlight several key principles that will help you grow healthy, productive, and high-quality onions:

1. Weak root system – the main feature. All fertilizing should be directed to the upper, well-aerated soil layer. Local application of starter fertilizers is critically important (Brewster, 2008).

2. Nitrogen for leaves, potassium for bulbs. Strictly observe the growth stage: nitrogen in the first half of the growing season, potassium in the second. Boron is a mandatory element at the start of bulb initiation (Brewster, 2008; Swiader & Ware, 1992).

3. Split application and timeliness. Apply fertilizers in small doses, at the right time. This reduces the risk of burns and losses, increases effectiveness.

4. Regular observation of plants is your best helper. External signs of deficiency or excess allow timely correction of the fertilization scheme.

5. Excess is more dangerous than deficiency. Especially regarding nitrogen, boron, and copper. Always follow recommended doses and concentrations.

6. Foliar applications are a great supplement to root applications, but not a substitute. Use them for micronutrients and rapid correction of deficiencies.

7. Consider soil type, weather, and variety. There are no universal schemes – adapt recommendations to your specific conditions.

8. A systematic approach is the key to success. Develop a fertilization plan for the season, keep records, and analyze results. This will allow you to achieve increasingly higher yields of quality onions suitable for long-term storage each year.

Remember that proper onion nutrition is not just "feeding" but creating conditions in which the plant can fully realize its genetic potential. Good luck and bountiful harvests!