Nutrition

Nitrogen (N) – the engine of growth

Role. Nitrogen is the main “building block” for proteins, enzymes, and chlorophyll. It ensures rapid leaf and shoot growth and the formation of a strong bush. Without sufficient nitrogen, zucchini cannot develop the large leaf mass that is so important for fruiting.

Deficiency signs. With nitrogen deficiency, plant growth slows down. Leaves become pale green, turn yellow (especially older lower ones), and become smaller. Fruiting is delayed or becomes sparse.

Excess signs. An excess of nitrogen is no less dangerous. It causes “luxuriant growth” – rapid green growth at the expense of flowering and fruit set. The zucchini becomes “thick‑necked,” leaves are dark green and large, but there are few ovaries or they drop off. Such plants are more susceptible to diseases and store less well.

Practice. Apply nitrogen fertilizers in split doses, during the first half of the growing season. The main dose is applied in spring when preparing the soil or in planting holes. Then nitrogen is given as top‑dressing during active leaf growth and before flowering. For example, ammonium nitrate or urea are good sources of fast‑acting nitrogen. For organic lovers, an excellent choice is cow manure or poultry manure infusion (diluted 1:10 and 1:20 respectively) – they contain nitrogen in available form and at the same time improve soil structure. However, remember: fresh manure is applied to zucchini only in autumn, under digging, while in spring only well‑rotted manure is used (Karataev and Sovetkina, 1984).

Phosphorus (P) – energy and roots

Role. Phosphorus is the accumulator and carrier of energy in the plant. It is critically important for the development of a strong root system, flower bud formation, flowering, and fruit ripening. A phosphorus deficiency at the start of the growing season can permanently delay bush development.

Deficiency signs. The main symptom is stunted growth and a purple or blue‑green tint on the leaves, especially older ones. The root system develops poorly, affecting overall plant nutrition.

Practice. Phosphorus moves slowly in the soil, so it is best applied in advance, in autumn or spring during the main soil preparation. Superphosphate is a classic phosphorus fertilizer. It works well in combination with organic matter. Phosphorus is also found in bone meal. Add it to the planting hole to stimulate root growth and help the plant settle quickly in its new location.

Potassium (K) – health and fruit quality

Role. Potassium is the element of quality. It regulates water balance, increases drought tolerance and resistance to diseases. It is responsible for carbohydrate transport and sugar accumulation in fruits. Potassium is especially important during fruiting: it ensures the formation of firm, tasty, and storable fruits.

Deficiency signs. Potassium deficiency appears first on older leaves: their edges turn yellow, dry out, and curl upward (“marginal burn”). The plant looks wilted even with sufficient watering. Fruits become misshapen, pear‑shaped, and lose marketable quality.

Practice. Zucchini is a crop that is demanding of potassium. It actively consumes it throughout the growing season. The most accessible sources are potassium sulfate (chlorine‑free potassium fertilizers, as zucchini does not like chlorine) and wood ash (about 1 cup per square metre), which, in addition to potassium, contains many micronutrients. Potassium top‑dressing is especially effective during mass flowering and fruiting (Wehner et al., 2020).

Calcium (Ca) – support and protection

Role. Calcium is necessary for the formation and strengthening of cell walls. It ensures the strength of stems and leaves and protects fruits from diseases such as blossom‑end rot. Calcium improves the uptake of other elements, especially nitrogen.

Deficiency signs. With calcium deficiency, leaves become chlorotic (yellowish), and their edges curl upward. Young leaves and growing points become deformed. The most characteristic symptom is the appearance of dark, water‑soaked spots on the tips of young fruits, which later rot. This is the classic blossom‑end rot, often associated with uneven watering and calcium deficiency (Wien and Stützel, 2020).

Practice. Calcium is usually applied in the form of dolomite flour or lime if your soil is acidic. This is done in autumn or a few weeks before planting. During the growth period, a good preventive measure is a foliar spray with calcium nitrate solution (0.5–1%) or regular watering with wood ash infusion.

Magnesium (Mg) – the heart of chlorophyll

Role. Magnesium is the central element of the chlorophyll molecule – that is, without it, photosynthesis is impossible. It also activates many enzymes. Magnesium ensures the intense green colour of leaves and the normal course of growth processes.

Deficiency signs. Magnesium deficiency shows as chlorosis (yellowing) between the veins on older leaves. The veins remain green, creating a characteristic “mosaic” pattern. Yellowing begins at the edges and spreads toward the centre.

Practice. A good way to replenish magnesium is to use dolomite flour (which contains both calcium and magnesium) and magnesium sulfate for main soil preparation. When deficiency symptoms appear, foliar sprays (leaf spraying) with a 1–2% magnesium sulfate solution are effective.

Sulfur (S) – the protein helper

Role. Sulfur is a component of amino acids (cysteine, methionine) and many enzymes. It promotes protein synthesis and chlorophyll formation. Although the requirement for sulfur is small, its role in plant growth and development is very important.

Deficiency signs. These are rare, as sulfur is often present in sufficient amounts in soil and precipitation. Symptoms resemble nitrogen deficiency: general yellowing, stunted growth, but they appear more often on young leaves.

Practice. If you use potassium sulfate or magnesium sulfate as fertilizers, you automatically supply the plant with sulfur. Organic fertilizers such as manure and compost also contain sulfur in adequate amounts.

Bottom line: Balanced nutrition means that all elements are in harmony. You cannot “treat” a deficiency of one element by uncontrollably increasing the dose of another. The major nutrients are the foundation, and their ratio changes depending on the stage of zucchini development. Read the next chapter to learn how the plant’s needs change from seedlings to harvest.

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Boron (B) – the key to pollination and fruit set

Role. Boron is critically important for flowering and fruiting. It stimulates pollen germination, pollen tube growth, and consequently, ovary fertilisation. Boron also participates in carbohydrate metabolism, protein synthesis, and cell wall strengthening.

Deficiency signs. Boron deficiency appears first on growing points and young leaves: they become deformed, brittle, and internodes shorten. Flowering is weak, ovaries are few, and many drop off. Hard corky areas may appear on fruits, and the flesh becomes bitter.

Practice. Boron is the most “capricious” micronutrient. Its excess is toxic, so doses must be minimal. The best method is foliar feeding (spraying on leaves) with boric acid solution (1 g per 10 L of water) during budding and at the very beginning of flowering. Repeat after 7–10 days. Boron is rarely applied to the soil because it leaches quickly.

Iron (Fe) – respiration and chlorophyll

Role. Iron participates in chlorophyll synthesis, although it is not part of its molecule (unlike magnesium). It is necessary for the functioning of enzymes involved in respiration and photosynthesis. Iron “switches on” the processes that convert light into energy.

Deficiency signs. The most characteristic symptom is interveinal chlorosis on young leaves: the leaf blade turns yellow while the veins remain green. This happens because iron is immobile in the plant and cannot move from old leaves to new ones. Deficiency often occurs on carbonate (alkaline) soils, where iron is in an unavailable form.

Practice. For quick “first aid,” use foliar sprays with iron chelate (0.1% solution). Spray 2–3 times at 7‑day intervals. Chelates (special complex compounds) are much more effective than simple salts (e.g., ferrous sulfate), which can burn leaves.

Manganese (Mn) – health and photosynthesis

Role. Manganese activates enzymes involved in photosynthesis, respiration, and nitrogen metabolism. It also increases plant resistance to diseases, especially root rots and powdery mildew.

Deficiency signs. Symptoms are similar to iron deficiency: interveinal chlorosis, but it usually appears on older leaves. Veins remain green, while the tissue between them becomes light green or yellow. Leaves may take on a speckled appearance.

Practice. Manganese deficiency often occurs on acidic soils, where it is in toxic forms, or on over‑limed soils, where it becomes unavailable. It can be replenished by foliar spraying with manganese sulfate (0.05–0.1% solution) or with complex micronutrient fertilizers containing manganese in chelated form.

Zinc (Zn) – growth and fruit formation

Role. Zinc participates in the synthesis of growth hormones (auxins), proteins, and enzymes. It is especially important for cell division, internode growth, and seed formation. Zinc helps the plant withstand stress (drought, high temperatures).

Deficiency signs. It appears on young leaves: they become small, narrow, and chlorotic. Internodes shorten, and the bush looks dwarfed and rosette‑shaped. Light spots may appear on fruits, and ripening is delayed.

Practice. Zinc works well in combination with phosphorus. It is best applied as a foliar spray with zinc sulfate (0.01–0.05% solution) at budding and 10 days later. Remember: zinc is immobile in the plant, so spray thoroughly, wetting all young leaves.

Molybdenum (Mo) – the nitrogen “regulator”

Role. Molybdenum is a key element for nitrogen assimilation. It is part of the enzyme nitrate reductase, which converts nitrate nitrogen (from fertilisers) into ammonia form, suitable for protein synthesis. Without molybdenum, even with abundant nitrogen, the plant will starve.

Deficiency signs. Symptoms closely resemble nitrogen starvation: general paleness, chlorosis of lower leaves, stunted growth. But unlike nitrogen deficiency, molybdenum deficiency starts on young leaves, which may become deformed, curl up, and die. Cauliflower and broccoli are especially sensitive to molybdenum, but zucchini also reacts.

Practice. Molybdenum is required in the smallest doses. Its deficiency often occurs on acidic soils. The best prevention is seed treatment before sowing (soaking in 0.05% ammonium molybdate solution) or a foliar spray at the 3–4 true‑leaf stage (0.02–0.05% solution).

Remember: Micronutrients work only in combination. Using complex fertilisers with micronutrients or specialised “cocktails” for vegetable crops is the easiest and safest way to avoid deficiencies. Do not forget that excess of any micronutrient is just as dangerous as its deficiency. “More is not better” – this rule applies here as well.

In the next chapter, we will look at how zucchini’s nutritional needs change at different stages of its life: from a tiny seedling to a bush covered with fruits.

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Stage 1. Growth start (from seedlings to 4–6 true leaves)

What happens in the plant. During this period, the zucchini develops its root system and first large leaves. The roots are still weak, actively branching in search of water and food, while the leaves are just beginning to photosynthesise.

Nutrient needs. First of all, the plant needs phosphorus (P) to stimulate root growth and the formation of future flower buds. Nitrogen (N) is also important for rapid green mass production, but in moderate doses.

Practical recommendations.

• Basic application (before planting or in the hole) is your main tool. Add a handful of well‑rotted compost or humus and 30–40 g of a complex mineral fertiliser, e.g., nitroammophoska (with a predominance of phosphorus), to the planting hole. This will provide the plant with a reserve of nutrition for the first month.
• If the zucchini looks weak, 10–14 days after emergence you can give the first root feeding with cow manure infusion (1:10) or urea (1 tablespoon per 10 L of water). But do not overdo it: excess nitrogen at the start promotes stretching and delays flowering.
• Potassium (K) at this stage is needed in minimal amounts, as there are no fruits yet.

Stage 2. Beginning of flowering (budding, first flowers)

What happens in the plant. The plant switches from purely vegetative growth to generative – bud setting and flowering begin. This is a turning point: the number of ovaries depends on how well it has been prepared.

Nutrient needs. Priorities shift: phosphorus and potassium become critical. They are responsible for abundant flowering, good pollination, and fruit set. Nitrogen is now needed only to maintain current leaf growth, and its share is reduced. Boron is very important – it promotes pollen germination and ovary formation (Wien and Stützel, 2020).

Practical recommendations.

• Second feeding is done at the very beginning of budding. Use a complex fertiliser with reduced nitrogen and increased phosphorus and potassium (e.g., monopotassium phosphate – 1 tablespoon per 10 L of water). You can also use ash infusion (1 cup per 10 L of water) – this provides potassium and micronutrients.
• At the same stage, foliar spraying with boron (1 g boric acid per 10 L of water) on buds and flowers is extremely beneficial. This simple operation can significantly increase the number of ovaries (Tarakanov and Mukhin, 2003).
• The N:P:K ratio in the feed should be approximately 1:2:2 or 1:2:3.

Stage 3. Mass fruiting (peak harvest)

What happens in the plant. This is the most energy‑consuming stage. On the bush, 3–5 fruits ripen simultaneously, and new ovaries appear at the same time. A “conveyor” works – we harvest some zucchinis, others are filling out, and still others are just setting. The load on the plant is colossal.

Nutrient needs. Now the main element becomes potassium (K). It is responsible for transporting carbohydrates from leaves to fruits, their sugar content, density, and storage life. Nitrogen is also needed, but in moderate doses to support leaf growth and new ovaries, but not to stimulate “luxuriant growth” at the expense of fruits. Phosphorus continues to support energy. Micronutrients (boron, zinc, manganese) continue their work.

Practical recommendations.

• Fertilising becomes regular: once every 7–10 days. Alternate them: one feed – organic (cow manure or poultry manure infusion – a source of nitrogen and micronutrients), the next – mineral with an emphasis on potassium (potassium sulfate, 1–1.5 tablespoons per 10 L of water).
• You can use ready‑made complex fertilisers for cucurbits labelled “for fruiting” – they already have the right ratio balanced.
• Fertigation (feeding through irrigation, especially drip) works excellently – it is the most effective way to deliver nutrition directly to the roots, because in midsummer moisture and food availability are critically important (Wehner et al., 2020).
• Do not forget about foliar feeding with complex micronutrients (once every 10–14 days) – they help the plant cope with stress and maintain leaf health, which work in an intensified mode.

Stage 4. End of the season (end of fruiting)

What happens in the plant. Growth slows down, many leaves age, fruiting declines. The plant prepares to complete its life cycle. At this stage, the main task is to allow the last fruits to ripen and not to weaken the plant with excessive loads.

Nutrient needs. Nitrogen is reduced to a minimum or eliminated altogether. Potassium and phosphorus continue to support fruit quality. Feeding stops 2–3 weeks before the expected end of harvest to avoid nitrate accumulation.

Practical recommendations.

• Reduce the number of feedings to once every 14–20 days.
• Use only potassium‑phosphorus fertilisers (ash, potassium sulfate, monopotassium phosphate) at half the summer dose.
• Nitrogen fertilising at this stage is excluded! Otherwise, instead of completing fruiting, you will provoke new green growth, which will delay ripening and reduce fruit quality.
• Remove old diseased leaves – this improves ventilation and helps the plant direct remaining energy to the fruits.

Bottom line: The easiest rule to remember is – “nitrogen for leaves, potassium for fruits.” At the beginning of the season, we help the plant “grow”; in the middle, we help it “set and fill”; at the end, we help it “ripen without haste.” By following this simple principle, you will get a stable and high‑quality harvest throughout the season.

In the next chapter, we will look at the various methods of applying fertilisers and the advantages of each.

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1. Basic application (soil preparation)

This is the foundation – the laying of a “strategic reserve” of nutrition for the whole season. Its goal is to create a fertile environment in which the zucchini can start and develop quickly. This is the most important fertilising, done before planting or directly in the planting hole.

How and when. In autumn, under digging, apply organic matter (well‑rotted manure, compost – 5–8 kg per 1 m²) and phosphorus‑potassium fertilisers. In spring, 1–2 weeks before planting, loosen the soil and add nitrogen fertilisers. At planting, place a handful of humus and 30–40 g of complex fertiliser (e.g., nitroammophoska) into the hole.

Why it works. Nutrients are evenly distributed in the future root zone. Organic matter improves soil structure, increases its moisture‑holding capacity and “buffers” excess salts. The zucchini’s root system, which is still weak at the beginning of development, immediately finds a “storehouse” of nutrition (Karataev and Sovetkina, 1984).

Limitations. The reserve applied in autumn may be depleted by the time of mass fruiting. Moreover, nitrogen from organic matter mineralises gradually, and during the “peak” period the plant may lack it. Therefore, basic application is not the only one, but the first and most important step.

2. Root feeding (liquid fertilisers)

These are “meals” for your zucchini on a schedule. The goal is to quickly compensate for a current deficiency or support the plant during a critical period (flowering, fruit growth). The liquid form ensures the fastest absorption. Root feeding is used throughout the growing season, from the time the plant has taken root until the end of the season.

How and when. Dissolve the fertiliser in water and pour it under the root, strictly onto moist soil (after watering or rain). It is better to apply it in grooves around the perimeter of the bush, rather than directly under the stem. Frequency: during active growth – once every 7–10 days. Use both mineral and organic solutions (cow manure infusion 1:10, poultry manure 1:20, herbal infusion).

Why it works. Zucchini roots (especially with drip irrigation) develop in the topsoil layer. The liquid solution enters directly into this active zone and is absorbed immediately. This way we know for sure that this particular plant received the dose, not the weeds nearby.

Limitations. Careless watering can burn roots or leaves. A high concentration of solution (more than 1–2%) is harmful. Frequent root feeding can lead to soil salinisation if not alternated with plain watering. Also, liquid feeding is ineffective in cool weather, when root activity is slow (Wien and Stützel, 2020).

3. Fertigation (feeding through drip irrigation)

This is the pinnacle of precision farming. Fertigation is the supply of dissolved fertilisers directly into the drip line, i.e., into the root zone of each plant. The method is ideal for zucchinis, which like regular but moderate watering.

How and when. Fertilisers (usually fully water‑soluble complexes) are added to the water tank that feeds the drip tape. The dosage is very small, but feeding goes on almost continuously, with every drip irrigation. Applied throughout the growing season, starting from the 3–4 true‑leaf stage.

Why it works. Uniform, constant supply of moisture and nutrients to the root zone. No stress from drying out or over‑watering. The plant receives nutrition exactly when it needs it – during active fruiting this is critically important (Wehner et al., 2020). Fertiliser savings – with fertigation you can use 30–50% less nitrogen and potassium without losing yield.

Limitations. Requires a drip irrigation system, which is not always available. Fully water‑soluble fertilisers are needed (not all complex fertilisers are suitable). It is important to prevent clogging of drippers, so filters are often installed at the outlet.

4. Foliar feeding (spraying on leaves)

This is “first aid” and a “vitamin cocktail” for the plant. The goal is to quickly deliver micronutrients and, to a lesser extent, macronutrients through the leaf surface. Zucchini leaves are large and absorb nutrients well.

How and when. Dissolve the fertiliser in water (usually at a 10‑fold lower concentration than for root feeding) and spray the leaves on the upper and, importantly, lower side. Do this in the morning or evening, in cloudy weather, so that the solution does not dry out instantly. It is especially effective during budding and after stress (drought, cold, pest damage).

Why it works. Through leaves, nutrients are absorbed tens of times faster than through roots. This is the only way to deliver micronutrients (boron, iron, zinc, manganese, molybdenum) to growing points and ovaries when the root system for some reason works poorly (cold soil, root damage). Foliar feeding does not work for severe nitrogen or potassium deficiency, but is ideal for treating chlorosis and stimulating flowering (Tarakanov and Mukhin, 2003; Wien and Stützel, 2020).

Limitations. Cannot replace basic nutrition – it gives only a quick but short‑term effect. Ineffective in dry weather (dries quickly) or in rain (washed off). Requires care: high concentration causes leaf burns.

Practical summary: The best strategy for zucchini is a combination of all four methods:

1. Basic application – creates base fertility.

2. Root feeding – provides regular balanced nutrition (alternate organic and mineral) during active growth.

3. Fertigation (if you have drip irrigation) – the “gold standard” for continuous fruiting.

4. Foliar feeding – as “first aid” with micronutrients, especially for ovaries and for plant recovery after stress.

Remember the main rule: it is better to under‑feed than to over‑feed. When working with solutions, always strictly observe the dosages indicated on the package, and for foliar feeding, make the concentration even 2 times weaker.

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Organic Fertilisers: The Foundation of Fertility

Organic fertilisers include all natural‑origin fertilisers: manure, compost, humus, peat, green manures, ash, bone meal, etc. Their main value is not so much in the specific NPK content, but in their complex effect on the soil.

Why organic matter is important for zucchini.

• Improves soil structure. Manure and compost make heavy clay soils more loose and air‑permeable, while binding light sandy soils and increasing their moisture‑holding capacity. Zucchini’s root system, which requires good aeration, develops much better in such soil (Nonnecke, 1989).
• Serves as a source of carbon dioxide. As organic matter decomposes, it releases CO₂, which is intensively absorbed by leaves during photosynthesis. This is especially important for heat‑loving crops such as zucchini (Karataev and Sovetkina, 1984).
• Gradual release of nutrients. Unlike mineral salts, organic matter works as a “delayed‑release capsule.” Nutrients are released gradually, as mineralisation proceeds, and the plant receives them throughout the season without the risk of “overfeeding.”
• Enriches microflora. Organic matter is “food” for beneficial soil bacteria and worms, which actively convert it into plant‑available humus.

How to apply.

• In autumn, under digging, apply fresh manure (30–40 kg per 10 m²) or semi‑rotted compost. Over winter they will decompose and become available to roots.
• In spring, in planting holes, use only well‑rotted humus or well‑matured compost (a handful per hole). Fresh manure should not be used in spring – it can burn roots and provoke diseases (Tarakanov and Mukhin, 2003).
• During the season, use liquid organic feeds: cow manure infusion (1:10) or poultry manure (1:20), herbal infusion from nettle and dandelion. This is an ideal way to support zucchini during mass fruiting.

Mineral Fertilisers: A Precise and Fast Tool

Mineral fertilisers are purified and concentrated salts containing specific elements in a plant‑available form. They are indispensable when you need to quickly replenish a specific deficiency or give a powerful boost at a key development moment.

Why mineral fertilisers are important for zucchini.

• Fast action. Nitrogen in nitrate or ammonium form is absorbed in hours, potassium in 2–3 days. This allows an immediate response to changes in the plant’s condition (Wien and Stützel, 2020).
• Precise dosage. You know exactly how many grams of nitrogen, phosphorus, or potassium you are giving. This is especially important when you need to adjust nutrition during flowering (increase phosphorus and potassium) or avoid nitrogen imbalance.
• Ease of use. Mineral fertilisers are easy to apply both in dry form (under digging, in holes) and as solutions for root and foliar feeding. They do not require decomposition time.

How to apply.

• Basic application (autumn or spring): complex fertilisers like nitroammophoska (NPK 16:16:16) or more specialised ones for cucurbits. Dose – 30–40 g per 1 m².
• Feedings during growth: urea or ammonium nitrate – for rapid green growth.
• Feedings during flowering and fruiting: monopotassium phosphate or potassium sulfate – for fruit quality; calcium nitrate – for blossom‑end rot prevention.
• Foliar feeding: iron chelate, boric acid, manganese sulfate – for treatment and prevention of chlorosis and poor fruit set (Meshkov et al., 2017).

Organic vs. Mineral: Strengths and Compromises

The Golden Mean: An Integrated Approach

For zucchini, the optimal feeding strategy is a combination of organic and mineral fertilisers. Organic matter creates a “buffer” and a constant background of nutrition, while mineral feeds act as “triggers” that initiate flowering or accelerate fruit filling.

Practical seasonal schedule:

1. Autumn: apply manure or compost (3–5 kg/m²) and superphosphate (40–50 g/m²) under digging.

2. Spring: in the hole – humus + 30 g nitroammophoska.

3. 4–6 leaf stage: root feeding with cow manure infusion (1:10) – a source of nitrogen.

4. Budding stage: root feeding with ash infusion (1 cup per 10 L of water) – a source of potassium, calcium, and micronutrients, plus foliar feeding with boric acid (1 g/10 L).

5. Mass fruiting: alternate root feeding: one – organic infusion (herbal or cow manure), after 7–10 days – mineral solution (monopotassium phosphate – 15–20 g per 10 L of water).

6. At the first signs of chlorosis: foliar feeding with iron chelate.

Important to remember:

• Fresh manure is not only a source of nutrition but also weed seeds and pathogens. Use only well‑rotted (at least one year old).
• Wood ash is an excellent potassium‑phosphorus fertiliser, but it alkalises the soil. On alkaline soils, apply with caution and do not mix with ammonium fertilisers (nitrogen is lost).
• Mineral fertilisers containing chlorine (potassium chloride) are not liked by zucchini. Preference – sulfates.

Bottom line: Organic and mineral fertilisers are not enemies but allies in obtaining a rich harvest. The gardener’s task is not to choose between them, but to combine them wisely, using the strengths of each. In the next chapter, we will analyse typical mistakes to avoid so that your feeding brings benefit, not harm.

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❌ Mistake 1. Excess nitrogen (“luxuriant growth” of the bush)

How it shows. The plant looks lush and healthy: leaves are large, dark green, stems are powerful. But flowering is weak, ovaries form late and in small numbers, and many drop off. The zucchini is “overgrowing” – spending all its energy on green mass at the expense of fruiting.

Why it happens. Most often due to excessive feeding with fresh manure or poultry manure, as well as frequent watering with urea or ammonium nitrate. Excess nitrogen stimulates vegetative growth and suppresses generative development (flowering and fruiting) (Karataev and Sovetkina, 1984).

What to do. Stop nitrogen feeding. Emphasise potassium‑phosphorus (ash, potassium sulfate, monopotassium phosphate). You can do a foliar spray with monopotassium phosphate (15 g per 10 L of water) – this will switch the plant’s “attention” to fruiting. If the situation is critical (huge bush and not a single ovary), try pinching the growing point of the main shoot.

❌ Mistake 2. Potassium deficiency (“marginal burn” and “pear‑shaped” fruits)

How it shows. Old leaves turn yellow at the edges, dry out, and curl upward (“marginal burn”). Fruits often have a pear‑shaped form: wide near the stalk and sharply narrowing toward the tip. Zucchinis lose firmness, become watery, and store poorly.

Why it happens. Potassium is the most consumed element in zucchini during fruiting. Its removal with the harvest is very high (Nonnecke, 1989). If we do not replenish it, the plant starts “pulling” potassium from old leaves and redirecting it to fruits. This is an SOS signal: the plant is starving.

What to do. Regularly feed with potassium fertilisers during mass fruiting: once every 7–10 days. Use potassium sulfate (1 tablespoon per 10 L of water), wood ash (1 cup per 10 L of water), monopotassium phosphate (15–20 g per 10 L of water). Ideally, alternate root potassium feeds with foliar ones.

❌ Mistake 3. Overfeeding (“fat soil” and root burn)

How it shows. The plant may “burn” – leaf edges turn brown and die. In severe cases, roots get chemical burns, and the plant begins to wilt even in moist soil. Yield decreases, and fruits accumulate excess nitrates (Tarakanov and Mukhin, 2003).

Why it happens. Too high a concentration of fertilisers in the soil (especially mineral salts) increases the osmotic pressure of the soil solution. Roots cannot absorb water, and physiological drought occurs (Wien and Stützel, 2020). Overfeeding often happens when organic and mineral fertilisers are applied simultaneously in high doses, or when dilution instructions are violated.

What to do. In case of excess salts – water the soil abundantly with clean water (to “flush” the top layer), if there is drainage. Temporarily stop feeding until the plant recovers. In future, strictly observe dosages: for root feeding – 1–2% solution (10–20 g per 10 L of water), for foliar – 0.1–0.5% (1–5 g per 10 L of water). Never apply dry fertiliser to dry soil – water first.

❌ Mistake 4. Late nitrogen feeding (when fruits are already filling)

How it shows. In late summer and early autumn, when you continue to water zucchini with urea, you stimulate new green growth instead of allowing the fruits to ripen. Zucchinis become watery, less sweet, store poorly, and accumulate nitrates.

Why it happens. After mass fruiting begins, nitrogen becomes the “enemy” of fruit quality. It delays the transport of carbohydrates to fruits, stimulating their consumption for leaf growth (Kotov and Adritskaya, 2016).

What to do. Stop all nitrogen feeding 3–4 weeks before the end of harvest. During ripening, use only phosphorus‑potassium feeds (ash, potassium sulfate). They promote sugar accumulation and improve fruit storage.

❌ Mistake 5. Ignoring deficiency signs on leaves

How it shows. You see that leaves are turning yellow, drying out, spots appear, but you do not know what it means and continue watering with “universal” fertiliser. As a result, the situation may worsen (Meshkov et al., 2017).

Why it happens. We often “treat” the wrong thing. Yellow leaves are not always nitrogen deficiency. It could be magnesium, iron, manganese, calcium, or potassium deficiency. Ignoring symptoms leads to applying some elements while the plant suffers from a lack of others.

What to do. Learn to “read” the plant (see chapters 2 and 3). If you suspect a specific deficiency – apply targeted feeding (foliar). If unsure – use complex micronutrient fertilisers with chelates (e.g., “Tsitovit,” “Ferovit”). They contain a balanced set of micronutrients in available form.

❌ Mistake 6. Feeding on dry soil

How it shows. You apply fertiliser (especially dry) to dry soil, and the plant does not absorb it. Moreover, dry granules can draw moisture from the roots, causing burns. Watering after dry application does not always save the situation.

Why it happens. Roots absorb nutrients only from solution. If the soil is dry and the fertiliser does not dissolve, it remains unavailable.

What to do. Always apply fertiliser to moist soil: after watering or rain. If using dry granules – water the area first, spread the fertiliser, work it into the soil, and water again. For liquid feeds – prepare the solution and water onto moist soil.

❌ Mistake 7. Feeding a diseased or weakened plant

How it shows. You see the plant wilting, turning yellow, or infested with pests, and you try to “heal” it with fertiliser. This often worsens the problem.

Why it happens. In a diseased plant, root or leaf functions are impaired. It cannot absorb the feed, and excess salts only increase stress. Root feeding of a diseased plant is useless and sometimes harmful (e.g., with root rots).

What to do. First solve the problem: cure the disease or remove the pest. Restore watering and root aeration (loosening). Only when the plant revives and starts growing – begin feeding, but at half the dose.

❌ Mistake 8. Using chlorine‑containing fertilisers

How it shows. Leaves turn yellow at the edges, curl, and the plant looks depressed.

Why it happens. Zucchini, like all cucurbits, does not tolerate chlorine well. Potassium chloride, sylvinite, and other chlorine‑containing potassium fertilisers can cause toxicity (Karataev and Sovetkina, 1984).

What to do. Choose chlorine‑free forms: potassium sulfate, potassium magnesium sulfate (Kalimag), wood ash, monopotassium phosphate. The package should state “chlorine‑free” or “chlorine content not more than 3%.”

❌ Mistake 9. Violating the timing and frequency of feeding

How it shows. Uneven development, “hunger” during fruiting, or conversely, “overfeeding” at the beginning of the season.

Why it happens. We feed zucchini “when we remember,” not on a schedule. We ignore growth phases (see chapter 4).

What to do. Make a simple feeding plan for the season, taking into account growth phases. Basic application – before planting. Starter feeding – 2 weeks after emergence. Enhanced potassium‑phosphorus – during flowering and mass fruiting. Stop nitrogen feeding a month before harvest. And most importantly: regularity, not single “shock” doses.

Bottom line: The most dangerous mistakes are excess nitrogen, potassium deficiency, overfeeding, and late feeding. To avoid them, observe your plants carefully, follow dosages, do not mix incompatible products, and always apply fertilisers to moist soil. And remember: it is better to under‑feed than to over‑feed. In the next, concluding chapter, we will summarise and draw up a brief action plan for obtaining an abundant and high‑quality zucchini harvest.