Nutrition (fertilizers and dressings)

1.1. Macronutrients

Nitrogen (N) — the main element for growth. It is a component of proteins, amino acids, chlorophyll, and nucleic acids. Nitrogen ensures vigorous shoot growth, the formation of large leaves, and the setting of fruit buds (Agustí, 2010; Potapov et al., 2000). However, it is important not to overdo nitrogen: excess causes “vegetative overgrowth” — trees grow vigorously at the expense of fruiting, wood matures poorly and suffers from frost (Agustí, 2010; Krivko, 2014).

Phosphorus (P) — is responsible for root system development, flowering, and fruit ripening. It participates in energy metabolism (ATP) and the transfer of genetic information. Phosphorus deficiency slows growth, reduces fruit set, and lowers sugar content. Phosphorus fertilizers are best applied in autumn or spring as part of complex fertilizing (Westwood, 1993; Srivastava et al., 2020).

Potassium (K) — the “quality element”. It increases resistance to drought, disease, and frost, improves fruit colour, taste, and storage life. Potassium activates enzymes, regulates water balance and sugar transport. In plums, especially during fruit filling, the need for potassium is very high. Potassium deficiency shows as marginal leaf scorch and smaller fruit (Agustí, 2010; Gull et al., 2023; Westwood, 1993).

Calcium (Ca) — a building material for cell walls. It ensures tissue strength, disease resistance, and good fruit storage. Calcium moves poorly within the plant, so its deficiency often appears on young leaves and fruit (bitter pit, cracking). Calcium application is especially important on acidic soils (Srivastava et al., 2020; Westwood, 1993).

Magnesium (Mg) — the central element of the chlorophyll molecule. It participates in photosynthesis and activates many enzymes. Magnesium deficiency causes interveinal chlorosis on old leaves, premature leaf drop, and smaller fruit. Magnesium deficiency is especially common on light sandy soils (Agustí, 2010; Westwood, 1993).

1.2. Micronutrients

Iron (Fe) — necessary for chlorophyll synthesis and respiration. Its deficiency (chlorosis) appears as yellowing of young leaves with green veins. This is often related to alkaline soil reaction or excess lime. Applying iron chelates or organic matter helps correct the situation (Agustí, 2010; Srivastava et al., 2020).

Zinc (Zn) — affects the synthesis of auxins (growth hormones), proteins, and stress tolerance. Its deficiency causes “rosetting” — small deformed leaves gathered in rosettes at shoot tips, and fruit become misshapen. Plums on carbonate soils are especially sensitive to zinc deficiency (Westwood, 1993; Srivastava et al., 2020).

Boron (B) — critically important for flowering and fertilization. It stimulates pollen germination, pollen tube growth, and fruit set. Boron deficiency leads to fruitlet drop, fruit deformation and cracking, and dieback of terminal buds. Boron is immobile, so it is applied as foliar sprays before and after flowering (Agustí, 2010; Srivastava et al., 2020).

Manganese (Mn) — participates in photosynthesis and respiration. Its deficiency resembles iron chlorosis but appears on old leaves and is more common on neutral and alkaline soils. Applying manganese sulfate or chelates solves the problem (Westwood, 1993).

1.3. Nutritional Features in Different Developmental Periods

The plum’s nutrient requirements change with the seasons.

Important: in the first half of summer, plum actively consumes nitrogen and potassium; in the second half — potassium and phosphorus. Excess nitrogen late in summer delays wood maturation and reduces winter hardiness (Agustí, 2010; Krivko, 2014).

Summary for Chapter 1:

Successful plum growing depends on balanced nutrition. The main macronutrients (N, P, K, Ca, Mg) and micronutrients (Fe, Zn, B, Mn) each perform unique functions. Understanding the tree’s seasonal needs will help you avoid mistakes and obtain stable, high-quality harvests.

In the next chapter, we will explain how to determine which elements your plum lacks and how to properly diagnose its nutrition.

2.1. Soil Analysis

Soil analysis gives an idea of which elements are available to the roots, as well as the acidity (pH) and organic matter content. It is the most reliable way to determine whether the soil needs liming and to assess phosphorus and potassium reserves.

What to check:

• Soil pH — the optimal range for plum is 6.0–7.5. In acidic conditions (pH < 5.5), many elements (phosphorus, calcium, magnesium) become less available, while aluminium and manganese become toxic. On alkaline soils (pH > 7.5), iron, zinc, and manganese deficiencies are common (Westwood, 1993; Srivastava et al., 2020).
• Available phosphorus and potassium — for plum, the optimal level of P₂O₅ is 10–20 mg per 100 g of soil, and K₂O — 15–30 mg per 100 g of soil (Potapov et al., 2000).
• Humus content — preferably at least 2–3%.

How often to test: soil analysis is recommended every 3–4 years, preferably in autumn or early spring before fertilizing. Samples are taken from a depth of 20–40 cm from several places in the garden (at least 5–10 points per plot) and mixed for an average sample.

2.2. Leaf Analysis

Leaf analysis is a more accurate indicator of the tree’s actual nutrition status, because it reflects which elements have actually been absorbed by the plant, not just present in the soil.

When to sample: for plum, as for other stone fruits, the optimal time is mid-July to August (during active growth and flower bud initiation) (Agustí, 2010; Westwood, 1993).

How to sample: collect middle leaves from current-season shoots (not root suckers or terminal ones) from all sides of the canopy, from several trees of the same variety and age. A total of about 50–100 leaves is needed.

Optimal values for plum (in % on dry matter) (Agustí, 2010; Potapov et al., 2000; Srivastava et al., 2020):

Deviations from these values indicate a deficiency or excess of the element.

2.3. Visual Signs of Deficiency and Excess

For the home gardener, the most accessible method is regular tree inspection. Plums respond well to nutrition, and many problems can be seen by appearance. Below are typical symptoms for the main elements.

2.4. Summary Table of Visual Diagnosis for Plum

Important note: many symptoms can be caused not only by nutrient deficiency but also by diseases, pests, drought, waterlogging, or frost damage. Therefore, when suspicious signs appear, it is worth excluding other factors, and if necessary, confirm the diagnosis with soil or leaf analysis.

Summary for Chapter 2:

Regular observation of the plum is the most accessible and quick way to spot nutritional problems. Pay attention to leaf colour, shape and condition, shoot growth, and fruit quality. When in doubt, use soil and leaf analysis — it will help determine exactly which elements need adjustment.

3.1. Why Is Organics So Important for Plum?

Organic matter, once in the soil, undergoes a complex decomposition process involving bacteria, fungi, and worms. The result is humus — a stable fertile substance that:

• Improves soil structure: loosens clay soils and binds sandy soils, creating a crumbly, water-permeable structure (Martin, 2019; Westwood, 1993).
• Increases water-holding capacity: the organic “sponge” retains moisture in the root zone, reducing the need for irrigation.
• Serves as a slow-release reservoir of nutrients: as humus mineralises, nitrogen, phosphorus, potassium, and micronutrients are gradually released in forms available to roots (Agustí, 2010; Srivastava et al., 2020).
• Activates soil life: organic fertilizers are food for earthworms and beneficial bacteria, which in turn suppress soil pathogens and improve root nutrition (Martin, 2019).

Plum is particularly grateful for organics: it responds well to improved soil structure, and its root system, especially on clonal rootstocks, is concentrated in the upper layers where most organic matter is found (Potapov et al., 2000; Trunov & Samoshchenkov, 2012).

3.2. Compost — “Black Gold” of the Garden

Compost is a homogeneous, dark, crumbly mass with a pleasant earthy smell, processed by microorganisms. It is the ideal organic fertilizer for plum, as it is balanced in elements, free of weed seeds (if properly made), and safe for roots.

What can be composted: plant residues (mown grass, leaves, vegetable tops, weed-free weeds), food waste (vegetable peelings, eggshells, tea leaves), straw, sawdust (in small amounts), paper and cardboard. It is important to maintain a carbon-to-nitrogen ratio (C:N) of about 30–40 parts “brown” (dry) materials to 1 part “green” (wet) materials (Martin, 2019).

How to apply compost under plum:

• At planting: mix 1–2 buckets of well-rotted compost with soil in the planting hole. This gives the young tree starter nutrition for 2–3 years.
• Under mature trees: apply compost in spring or autumn at the rate of 30–50 kg (3–5 buckets) per mature tree (Agustí, 2010; Krivko, 2014). Spread evenly over the canopy projection (root zone) and incorporate into the surface soil layer to a depth of 10–15 cm, or use as mulch.
• Frequency: once every 2–3 years. In intensive orchards with high planting density on dwarf rootstocks, compost can be applied annually (Trunov & Samoshchenkov, 2012).

Important: compost must be fully mature (cured), otherwise it will continue to decompose in the soil, consuming nitrogen and inhibiting roots.

3.3. Manure and Farmyard Manure

Manure (cattle, horse, goat) is a classic organic fertilizer. However, fresh manure is not used for plum: it contains many weed seeds, can burn roots, and releases ammonia during decomposition. Only well-rotted manure (age 1–2 years) or farmyard manure compost — fully decomposed mass — is used.

Characteristics of different types of manure:

• Horse manure — the “hottest”, contains more nitrogen and phosphorus, decomposes quickly, loosens heavy soils well. Recommended for mulching and autumn application.
• Cattle manure — “cooler”, rich in potassium and organic matter, structures the soil well.
• Poultry manure — concentrated nitrogen-phosphorus fertilizer. Fresh, it is very aggressive (can burn roots). It is used only in highly diluted form (1:20) or after composting with straw and sawdust. For plum, granulated poultry manure or manure infusion is better (Krivko, 2014).

Rates of manure and farmyard manure application:

• Under mature plum — 30–40 kg of rotted manure or 20–30 kg of farmyard manure per tree, once every 3–4 years (Agustí, 2010).
• Better applied in autumn so that the fertilizer “matures” over winter and starts supplying nutrients in spring.
• Application method: spread evenly over the root zone and incorporate to a depth of 15–20 cm (the depth of the main root mass) (Westwood, 1993).

Important: manure and farmyard manure are valuable sources of potassium, magnesium, and micronutrients. They are especially useful on light sandy soils where nutrients are easily leached.

3.4. Green Manures (Cover Crops)

Green manures are fast-growing plants (usually legumes or cereals) sown in inter-rows or root zones, then ploughed in or mowed as green fertilizer. For plum, this is an excellent way to improve soil without hauling heavy organics.

Best green manures for plum:

• Legumes: vetch, peas, lupin, sweet clover, clover, sainfoin. They enrich the soil with nitrogen through symbiosis with nodule bacteria (atmospheric nitrogen fixation). Ploughing in 1 tonne of legume green manure supplies about 30–40 kg of nitrogen to the soil (Potapov et al., 2000; Trunov & Samoshchenkov, 2012).
• Cereals: oats, rye, barley — provide a lot of organic mass, improve soil structure, but do not fix nitrogen.
• Brassicas: white mustard, phacelia, rapeseed — grow quickly, suppress weeds, sanitise the soil from pathogens, and work well as predecessors.

Agronomy: sow green manures in spring or summer in inter-rows (or every other row). At the flowering stage (for legumes) or early budding, mow them (with a rotary mower) and incorporate into the soil to a depth of 12–15 cm (Agustí, 2010; Krivko, 2014). Alternatively, leave the mowed mass on the surface as mulch.

For plum, particularly effective are: vetch-oat mixture, mustard, phacelia, which grow rapidly and combine well with fruit orchards.

3.5. Herbal Infusions (“Liquid Green Fertilizer”)

This simple and accessible method allows you to quickly obtain a liquid organic fertilizer rich in nitrogen and micronutrients. It is often called “herbal fermentation” or “compost tea” (Krivko, 2014; Martin, 2019).

Preparation: place any green mass (weeds, nettles, mown grass, vegetable tops) in a plastic barrel (not metal!) and fill with water. To speed up fermentation, add a nitrogen source (urea — 1 tablespoon per barrel, or manure slurry, or old compost). Cover the barrel with a lid and leave in the sun for 10–15 days. Stir occasionally. When fermentation is complete (liquid becomes clear, no foam), the infusion is ready.

Use: dilute the finished infusion with water at 1:10 and water the soil under the plum at the rate of 1–2 buckets per mature tree. The sediment (fermented grass) is used as mulch. Such feeding is done 1–2 times per season — in spring, during active shoot growth.

Important: the infusion is rich in nitrogen and works well in the first half of summer. Do not apply it after July to avoid prolonged shoot growth.

3.6. Mulching as an Element of Organic Nutrition

Mulch from organic materials (mown grass, leaves, straw, bark, wood chips, sawdust, compost) performs several functions at once: protects the soil from overheating and drying out, suppresses weeds, and gradually decomposes, enriching the soil with organic matter and nutrients.

For plum, the following are particularly useful:

• Compost mulch — provides both protection and nutrition.
• Freshly mown grass — decomposes well over the season.
• Shredded branches (ramial wood chips) — decompose slowly, enrich the soil with lignin and create favourable conditions for saprophytic fungi (Martin, 2019).

Mulching rules: apply mulch in a layer 5–10 cm thick, but not directly against the trunk (leave 5–10 cm free space to avoid collar rot). Renew the mulch layer once a year (preferably in spring).

3.7. Comparative Characteristics of Organic Fertilizers

Summary for Chapter 3:

Organic fertilizers are the foundation of long-term fertility in the orchard. Compost and farmyard manure create a favourable environment for roots, improve soil, and feed the plum slowly but steadily. Green manures and herbal infusions complement the system, especially during active growth. The main rule — organics should not completely replace mineral fertilizers but work with them in synergy. In the next chapter, we will tackle mineral fertilizers: which elements to apply when, and in what doses.

4.1. General Principles of Applying Mineral Fertilizers for Plum

Before moving to specific elements, remember a few rules:

1. Mineral fertilizers are not a replacement for organics, but a supplement. Organics create a “background” of fertility, while mineral fertilising provides “quick” corrections at critical moments (Agustí, 2010; Srivastava et al., 2020).

2. Doses are calculated based on tree age, yield, and soil fertility. The older and more productive the tree, the more nutrients it removes (Potapov et al., 2000; Trunov & Samoshchenkov, 2012).

3. Nitrogen is applied only in spring and the first half of summer. Autumn nitrogen application delays wood maturation and reduces winter hardiness.

4. Phosphorus and potassium can be applied in both spring and autumn. Autumn application of phosphorus-potassium fertilizers promotes the accumulation of reserve substances in roots and flower bud initiation (Westwood, 1993; Krivko, 2014).

5. Prefer chloride-free fertilizers. Plum, like most stone fruits, is sensitive to chloride, so potassium fertilizers should be used as potassium sulfate (K₂SO₄) rather than potassium chloride (KCl) (Agustí, 2010).

4.2. Nitrogen Fertilizers

Nitrogen (N) is the most important element for growth. For plum, as for other stone fruits, nitrogen nutrition critically affects tree size, fruit quantity, and quality (Agustí, 2010; Trunov & Samoshchenkov, 2012).

Forms of nitrogen fertilizers:

Recommended nitrogen rates for plum:

• Young trees (up to 5 years) — 30–60 kg a.i./ha (equivalent per tree: 50–100 g urea or 70–150 g ammonium nitrate) (Agustí, 2010; Potapov et al., 2000).
• Bearing trees (over 5 years) — 60–90 kg a.i./ha (or 150–200 g urea per tree). At high yields (>30 t/ha), increase the rate to 100–120 kg a.i./ha (Trunov & Samoshchenkov, 2012).
• On poor soils and where inter-rows are grassed — increase the rate by 20–30 %.

Timing of nitrogen application:

• Spring application (before flowering and immediately after flowering) — 60–70 % of the annual rate.
• Second application (at the stage of intensive fruitlet growth, June) — 20–30 % of the rate.
• Third application (after harvest, to restore the tree) — 10–20 %, but only in southern regions with warm autumns (Krivko, 2014; Trunov & Samoshchenkov, 2012).

Important: nitrogen fertilizers are applied only in dissolved form or incorporated into the soil (depth 10–15 cm) with obligatory watering. Surface broadcasting without incorporation leads to nitrogen losses (up to 50 % as gaseous ammonia) (Bryla, 2020).

4.3. Phosphorus Fertilizers

Phosphorus (P) is responsible for root system development, flowering, fruit set, and fruit taste. Plum removes comparatively little phosphorus (about 1–2 kg P₂O₅ per 1 t of fruit), but its deficiency sharply reduces yield (Westwood, 1993).

Forms of phosphorus fertilizers:

Recommended phosphorus rates for plum:

• Main application (autumn or spring) — 60–90 kg P₂O₅ a.i./ha (or 150–200 g double superphosphate per mature tree) (Agustí, 2010; Potapov et al., 2000).
• On poor soils and at high yields — up to 120 kg P₂O₅ a.i./ha (Trunov & Samoshchenkov, 2012).

Timing and methods of application:

• Phosphorus fertilizers are immobile in soil. Apply them at the depth of the main root mass (20–30 cm) in autumn or early spring.
• On acidic soils, phosphorus is fixed by aluminium and iron, so superphosphate efficiency increases with liming. On alkaline soils, phosphorus is fixed by calcium — here, ammonium phosphate or diammonium phosphate is preferred (Srivastava et al., 2020).

4.4. Potassium Fertilizers

Potassium (K) is the “king of quality” for plum. It improves colour, taste, sugar content, and storage life of fruit, increases resistance to drought, diseases, and winter frost. Plum removes a lot of potassium with the harvest — about 10–15 kg K₂O per 1 t of fruit (Agustí, 2010; Gull et al., 2023).

Forms of potassium fertilizers:

Recommended potassium rates for plum:

• Main application — 80–120 kg K₂O a.i./ha (or 150–200 g potassium sulfate per mature tree) (Agustí, 2010; Trunov & Samoshchenkov, 2012).
• At high yields (>30 t/ha) — up to 150 kg K₂O a.i./ha.
• On light sandy soils, increase the potassium rate by 20–30 %, but apply in split doses, as potassium is easily leached (Westwood, 1993).

Timing of potassium application:

• Spring (before flowering) — 30 % of the rate.
• Summer (during fruit growth) — 50 % of the rate. This is the critical period when potassium actively moves into the fruit.
• Autumn (after harvest) — 20 % of the rate for flower bud initiation and winter preparation (Krivko, 2014).

Important: potassium and magnesium are antagonists. At high potassium doses, magnesium deficiency may occur. Therefore, on magnesium-poor soils, potassium magnesium sulfate is preferred (Srivastava et al., 2020).

4.5. Magnesium and Calcium Fertilizers

Although magnesium and calcium are often classified as micronutrients, their role for plum is enormous. Magnesium is in chlorophyll, and calcium is in cell walls and determines fruit storage life (Westwood, 1993; Srivastava et al., 2020).

Magnesium fertilizers:

• Magnesium sulfate (MgSO₄·7H₂O) — 10–16 % Mg, 20–22 % S. Applied when magnesium is deficient (especially on light soils) at a rate of 30–50 g per tree (foliar) or as part of potassium magnesium sulfate.
• Dolomite lime (CaCO₃·MgCO₃) — simultaneously a liming agent and magnesium fertilizer. Applied at pH < 5.5 at 2–4 kg/m² (Agustí, 2010).

Calcium fertilizers:

• Lime, dolomite — for acid soil neutralisation.
• Gypsum (CaSO₄) — does not affect pH, useful on alkaline soils for improving structure.
• Calcium nitrate (Ca(NO₃)₂) — contains nitrogen and calcium, good for spring fertilising (Bryla, 2020).

4.6. Compound Fertilizers

For the home gardener, compound fertilizers are often more convenient than mixtures of individual elements. They contain a balanced NPK set and micronutrients in chelated form (readily available to plants).

Types of compound fertilizers:

• Nitrophoska (NPK 16–16–16 or 15–15–15) — universal, contains N, P, K in equal proportions. Suitable for spring application (before flowering).
• NPK with higher phosphorus and potassium (e.g., 10–15–20) — used for autumn fertilising.
• Water-soluble fertilizers (crystallins, polyfids, Master, etc.) — special compound fertilizers for fertigation and foliar feeding. Contain micronutrients in chelated form (iron, zinc, copper, manganese) and are ideal for precise nutrition (Krivko, 2014; Srivastava et al., 2020).

Examples of brands: “Crystalon”, “Master”, “Polyfeed”, “Nutrivant”, “Agricola”. In Russia, domestic analogues are available: “Aquarin”, “Rastvorin”, “Urozhay-super”.

How to choose a compound fertilizer for plum:

4.7. Annual Mineral Nutrition Schedule for Plum (Approximate rates per mature tree, 8–12 years old)

Important: all doses are approximate. Exact doses depend on tree condition, soil fertility, and yield. The best approach is to adjust nutrition based on soil and leaf analysis results.

5.1. Spring Feeding (Before Flowering and at Early Bud Break)

Goal: to ensure a strong start to vegetation, shoot and leaf growth, good flowering, and fruit set.

In spring, the plum awakens from winter dormancy. At this time, its need for nitrogen is maximal, because nitrogen stimulates cell division, new shoot and leaf growth. Additionally, for successful flowering and pollination, boron (B) is important, as it improves pollen germination and pollen tube growth (Agustí, 2010; Ziogas et al., 2020).

Timing: the first feeding is done before bud break (when buds are just beginning to swell) or at the very beginning of bud break. The second — 10–14 days after the first, but before flowering.

What to apply:

1. Nitrogen fertilizers (main element):

• Urea — 150–200 g per mature tree (8–12 years) (Agustí, 2010).
• Or ammonium nitrate — 200–250 g per tree.
• Or nitrophoska (NPK 16–16–16) — 250–300 g per tree (also contains phosphorus and potassium, useful in spring).
• Boron (for flowering) — if the previous year had poor fruit set or fruitlet drop. Use:
• Foliar spray with 0.1–0.2 % boric acid solution (10–20 g per 10 L water) on buds (before flowering) (Ziogas et al., 2020; Srivastava et al., 2020).
• Or add a boron-containing fertilizer (“Boroplus”, “Master Bor”) to the tank mix.

Application method: nitrogen fertilizers are best incorporated into the soil of the root zone to a depth of 10–15 cm (under the canopy projection) with obligatory watering. Boron feeding — by spraying (foliar) in the evening.

Important: do not overdo nitrogen! Excess nitrogen in spring can cause excessive shoot growth at the expense of flowering and fruit set, and can provoke fungal diseases. The nitrogen rate for a mature tree should not exceed 200 g a.i. per spring (Agustí, 2010; Trunov & Samoshchenkov, 2012).

5.2. Feeding After Flowering (Fruitlet Formation Phase)

Goal: to support fruitlet growth, reduce drop, ensure stone formation, and initiate flower buds for next year’s crop.

Soon after flowering, the plum enters a phase of intensive cell division in fruit and stone formation (this period lasts about 4–6 weeks after flowering). During this time, the tree needs nitrogen (for fruitlet growth), phosphorus (for roots and energy metabolism), potassium (for carbohydrate transport to fruit), and calcium (for cell wall strength, reducing cracking and improving storage) (Westwood, 1993; Srivastava et al., 2020).

Timing: 10–15 days after mass flowering (when petals have fallen and fruitlets are clearly visible).

What to apply:

1. Compound fertilizer with a predominance of phosphorus and potassium (e.g., NPK 12–18–15 or 10–20–15) — 200–250 g per mature tree. You can use nitrophoska, but a special “fruit” fertilizer is better.

2. Calcium nitrate (if soil is acidic or there are signs of calcium deficiency) — 50–100 g per tree (foliar spray with 1 % solution) or dry with irrigation (Bryla, 2020).

3. Micronutrients — especially boron (if not applied in spring), zinc, and manganese. Better applied foliarly in chelated form (e.g., “Tsitovit”, “Microvit”, “Aquamix”) (Ziogas et al., 2020).

Application method: compound fertilizer can be spread over the root zone and incorporated to a depth of 10 cm followed by watering. Foliar micronutrient sprays are applied to leaves (lower side) in cloudy weather or in the evening.

Important: during this period, liquid organic fertilisers are very useful — cow manure infusion (1:10) or herbal infusion (1:10) — 1–2 buckets per tree. They provide nitrogen, micronutrients, and improve soil microflora (Krivko, 2014).

5.3. Summer Feeding (Fruit Filling and Ripening Phase)

Goal: to ensure maximum fruit size, sugar accumulation, improved colour, and better storage life.

From mid-June to the end of July, the plum enlarges its fruit. During this period, the need for nitrogen drops sharply (excess nitrogen leads to “watery” fruit and poor wood maturation), while the need for potassium peaks. Potassium is responsible for transporting assimilates from leaves to fruit, increasing sugar content and colour (Agustí, 2010; Gull et al., 2023). Calcium is also important — it reduces cracking and improves storage (Srivastava et al., 2020).

Timing: the first summer feeding — 3–4 weeks after flowering (June); the second — 3–4 weeks before harvest (July) (for late varieties). For early varieties, the main summer feeding falls in June – early July.

What to apply:

1. Potassium fertilizers (chloride-free!):

• Potassium sulfate — 150–200 g per mature tree (Agustí, 2010).
• Or potassium magnesium sulfate — 200–250 g per tree (also supplies magnesium).
• Or monopotassium phosphate — 50–100 g per tree (in dissolved form for fast action).
• Calcium (foliar spray) — 1 % solution of calcium chloride or calcium nitrate (100–150 g per 10 L water) on leaves, especially before ripening (Bryla, 2020; Ziogas et al., 2020). This reduces cracking and improves storage.
• Nitrogen — only at the very beginning of summer (until June 20), if shoots are growing weakly. A small dose — 30–50 g urea per tree. After mid-June, nitrogen is excluded!

Application method: potassium fertilizers — dry incorporated into soil with watering, or as a solution (root watering). Calcium — only foliar (through leaves, because calcium hardly moves downward in the phloem). Spray in the evening.

Important: in hot, dry weather, the potassium rate can be slightly increased (up to 250 g), but do not exceed recommended doses. Excess potassium can cause magnesium and calcium deficiency (Westwood, 1993).

5.4. Autumn Feeding (After Harvest)

Goal: to restore the tree’s strength after fruiting, initiate flower buds for the next crop, and prepare the tree for winter.

In autumn, the plum ends its vegetation. At this time, flower buds for the next year are initiated, and reserve carbohydrates accumulate in roots and wood. These processes require phosphorus (energy) and potassium (winter hardiness). Nitrogen in autumn is strictly forbidden, as it stimulates shoot growth, hindering wood maturation and reducing frost resistance (Agustí, 2010; Krivko, 2014).

Timing: after harvest, about 3–4 weeks before stable frosts (in temperate zones — September to early October). In southern regions — October to November.

What to apply:

1. Phosphorus fertilizers:

• Double superphosphate — 150–200 g per mature tree (Agustí, 2010).
• Or single superphosphate — 300–400 g per tree.
• Potassium fertilizers (chloride-free):
• Potassium sulfate — 150–200 g per tree.
• Or potassium magnesium sulfate — 200–300 g per tree.
• Organic fertilizers (if not applied earlier in autumn) — 2–3 buckets of compost or farmyard manure per tree (alternate with minerals — one year organics, the next minerals) (Westwood, 1993).

Application method: phosphorus and potassium are immobile, so incorporate them into the soil to a depth of 20–25 cm (under the canopy projection). This can be combined with digging the root zone. If applying organics, mix with soil.

Important: autumn fertilization is the main application for the entire next season. Phosphorus and potassium applied in autumn will gradually be released and become available to roots as early as early spring. Autumn feeding is especially important for old trees and at high yields.

5.5. Summary Feeding Calendar for Plum (Example for a mature tree in temperate zones)

Summary for Chapter 5:

Seasonal feeding of plum is a flexible system that adapts to the tree’s developmental phases.

• Spring — nitrogen and boron for growth and flowering.
• After flowering — complex nutrition (N, P, K, Ca, micro) for fruitlets.
• Summer — potassium and calcium for fruit quality; nitrogen minimised.
• Autumn — phosphorus and potassium for winter hardiness and future yield.

By following this schedule and adjusting doses according to tree condition and soil, you will provide your plum with everything it needs at every stage of its life.

6.1. Soil Application — The Main Method

This is the classic, most common method. Fertilizers are incorporated directly into the soil, into the active root absorption zone. For plum, whose root system (especially on clonal rootstocks) is mainly in the upper 30–60 cm layer, soil application remains the baseline.

Advantages:

• Provides long-term, gradual nutrition.
• Improves soil physical and chemical properties (especially with organics).
• Suitable for all types of fertilizers — organic, mineral, slow-release.

Disadvantages:

• Some nutrients can be fixed by soil and become unavailable (especially phosphorus and potassium on certain soil types) (Westwood, 1993).
• With uneven application, roots may “avoid” areas with high fertilizer concentration.

Technique of soil application:

• Root zone — the main application area. For a mature plum, the canopy projection roughly corresponds to the main root mass. Fertilizers are spread evenly over the entire root zone (or in a circular trench along the canopy periphery).
• Depth of incorporation depends on fertilizer type and season:
• Nitrogen (urea, ammonium nitrate) — incorporate to a depth of 10–15 cm, as nitrogen is mobile and needs to be placed in the active root zone (Agustí, 2010; Krivko, 2014).
• Phosphorus and potassium — to a depth of 20–25 cm, as they are immobile and should be placed where roots will “mine” them throughout the season (Westwood, 1993; Potapov et al., 2000).
• Organics — incorporate to a depth of 15–20 cm (preferably in autumn) so that they partially decompose by spring.
• Methods of incorporation:
• Broadcasting over the surface followed by digging or loosening. This is the simplest method for home gardens.
• Furrow application — cut 2–4 furrows 20–25 cm deep along the canopy periphery, apply fertilizers (especially phosphorus-potassium) and cover with soil. This method is more precise and economical, especially for large trees (Krivko, 2014).
• Local application (into holes or pits) — for targeted delivery to absorbing roots. Used for correcting deficiencies.

Important: soil application should always be accompanied by watering, especially for mineral fertilizers. Watering “pulls” dissolved substances into the root zone and prevents root burn at high concentrations (Bryla, 2020).

6.2. Fertigation — Application with Irrigation Water

Fertigation is a method where fertilizers are applied together with irrigation water, usually through drip irrigation or sprinkler systems. In intensive orchards with drip irrigation, fertigation becomes the main method of supplying mineral fertilizers, especially nitrogen and potassium, which are highly soluble.

Advantages:

• High efficiency — fertilizers reach roots directly with moisture, uptake reaches 80–90 % (compared to 50–60 % with dry application) (Bryla, 2020; Trunov & Samoshchenkov, 2012).
• Uniformity — nutrition is distributed throughout the root zone.
• Possibility of frequent small-dose feedings — ideal for stone fruits, which are sensitive to sharp changes in salt concentration.
• Water and fertilizer savings — losses to evaporation and soil fixation are minimal (Krivko, 2014).

Disadvantages:

• Requires a drip irrigation system and clean water (free of suspended solids).
• Not all fertilizers are suitable for fertigation — need fully soluble, residue-free (crystalline or liquid compound fertilizers).
• Requires regular checking of water pH and fertilizer compatibility.

Which fertilizers are used for fertigation:

• Nitrogen: urea, ammonium nitrate, calcium nitrate, potassium nitrate.
• Phosphorus: monopotassium phosphate (MKP), ammonium phosphate (MAP), polyphosphates (less prone to precipitation).
• Potassium: potassium sulfate (soluble but slow), potassium nitrate.
• Compound: water-soluble fertilizers with micronutrients in chelated form — “Crystalon”, “Master”, “Polyfeed”, “Aquarin”, etc. (Krivko, 2014; Srivastava et al., 2020).

Rules for fertigation:

• Fertilizers are applied only in dissolved form via injectors or tank mixtures.
• Solution concentration should not exceed 0.5–1 % (i.e., 5–10 g fertilizer per 1 L water) to avoid root burn.
• Fertigation is carried out in the middle or at the end of the irrigation cycle — first give clean water to wet the soil, then the fertilizer solution, then clean water again to flush the system (Bryla, 2020).
• For plum, fertigation is especially effective during the active fruit growth period (June–July) and when applying potassium in readily available form.

6.3. Foliar Sprays (Through Leaves)

Foliar feeding is spraying the canopy with fertilizer solutions. Through stomata and the cuticle, nutrients penetrate directly into plant tissues, bypassing the soil. This method does not replace root nutrition, but serves as a quick and precise way to correct deficiencies, especially of micronutrients (Ziogas et al., 2020; Srivastava et al., 2020).

Advantages:

• Quick action — elements enter the plant within hours, critical in acute deficiencies.
• High uptake — up to 80–95 % of applied elements (especially micronutrients) (Ziogas et al., 2020).
• Targeted action — e.g., boron spray directly at flowering, or calcium during fruit filling.
• Independent of soil conditions — works even on alkaline soils where iron and zinc are unavailable to roots (Agustí, 2010).

Disadvantages:

• Short-lived effect — elements do not accumulate in soil, requiring repeated treatments.
• Risk of leaf burn at overdose or wrong concentration.
• Weather-dependent — sprays are effective only in cloudy weather or in the evening, at temperatures below 25 °C and no wind.

Which elements are applied foliarly:

• Macronutrients: urea (0.3–1 % solution), magnesium sulfate (1–2 %), calcium nitrate or calcium chloride (0.5–1 %) — for quick correction of calcium deficiency (especially fruit cracking) (Ziogas et al., 2020; Bryla, 2020).
• Micronutrients: boron (0.1–0.2 % boric acid), zinc (0.05–0.1 % zinc sulfate), iron (0.1–0.2 % iron chelate), manganese (0.05–0.1 % manganese sulfate). Better to use chelated forms — they do not cause burns and penetrate leaves better (Srivastava et al., 2020; Ziogas et al., 2020).

Timing of foliar sprays for plum:

• Boron — before flowering (bud swelling) and immediately after flowering, to improve fruit set (Agustí, 2010).
• Calcium — during fruit growth (June–July) — 2–3 treatments at 10–14 day intervals, to strengthen fruit tissues and reduce cracking (Srivastava et al., 2020; Ziogas et al., 2020).
• Zinc and iron — in spring, on young leaves, to correct chlorosis and rosetting (Westwood, 1993).
• Urea — early spring (on dormant buds) or autumn (after leaf fall) at high concentration (3–5 %) — for fungicidal treatment and additional nutrition.
• Complex micronutrients — 1–2 times per season (during shoot growth and fruit filling) as part of a tank mix with fungicides (Krivko, 2014).

Technique for foliar sprays:

• Spray in the evening (after 6–7 pm) or early morning, when stomata are open and evaporation is minimal.
• Add a sticker/spreader (soap, “Liposam”, “Trend”, etc.) — this increases leaf wetting and efficiency by 1.5–2 times (Krivko, 2014).
• Treat both sides of leaves, especially the lower side, where more stomata are present (Martin, 2019).
• Concentrations for foliar sprays are always lower than for root application, and depend on the element (see table below).

Recommended concentrations for foliar sprays of plum (Ziogas et al., 2020; Krivko, 2014; Srivastava et al., 2020):

6.4. Comparison of Application Methods and Their Combination

Recommendation for combination:

• Autumn and spring — main soil application of phosphorus and potassium (incorporated to depth).
• Early spring (before flowering) — nitrogen and boron: can be through soil (urea) + foliar boron spray.
• After flowering and during fruit filling — fertigation with potassium and compound fertilizers (if drip irrigation available) or soil application with watering.
• During summer — foliar sprays with calcium and micronutrients (on leaves), especially when deficiency signs appear.
• Autumn — phosphorus and potassium via soil (main application) + (optional) urea spray after leaf fall to increase winter hardiness (Ziogas et al., 2020; Krivko, 2014).

Important: do not rely entirely on any single method. A combination of soil nutrition (provides long-term background) and foliar sprays (quickly corrects bottlenecks) gives the best results. In intensive orchards, fertigation becomes the link between them (Bryla, 2020; Srivastava et al., 2020).

7.1. Excessive Nitrogen Nutrition

What is the mistake: over-application of nitrogen fertilizers (especially in spring and early summer) or using nitrogen in too high doses, as well as late-summer and autumn nitrogen application.

Why it is harmful:

• Nitrogen stimulates vigorous vegetative growth. The tree “overgrows” — shoots become long, thick, with long internodes, leaves large, dark green, but loose (Agustí, 2010; Krivko, 2014).
• All energy goes to growth, not to flower bud initiation. The result — reduced yield, poor fruit set, fruitlet drop (Agustí, 2010).
• Fruit become watery, less sweet, poorly coloured, and storage life decreases (Westwood, 1993; Srivastava et al., 2020).
• Wood does not mature by autumn, sharply reducing winter hardiness. Plum may freeze even at moderate frosts (Agustí, 2010; Potapov et al., 2000).
• Susceptibility to diseases (especially monilinia and bacterial canker) and pests (aphids, mites) increases (Gull et al., 2023).

How to avoid:

• Apply nitrogen strictly according to needs: for a mature plum (8–12 years), the optimal dose is 150–200 g urea (or 200–250 g ammonium nitrate) over the spring-summer period, divided into 2–3 applications (Agustí, 2010; Trunov & Samoshchenkov, 2012).
• Never apply nitrogen after mid-July (for southern regions — after late July). In temperate zones — after June 20 (Krivko, 2014).
• Maintain NPK balance: spring — nitrogen + phosphorus + potassium, summer — emphasis on potassium, autumn — only phosphorus and potassium, no nitrogen (Bryla, 2020; Westwood, 1993).
• Use slow-release nitrogen forms (e.g., urea-formaldehyde fertilizers) or organics that release nitrogen gradually.

Signs of nitrogen overfeeding: dark green, “fat” leaves, vigorous shoots with long internodes, poor flowering, small and watery fruit, prolonged growth into autumn.

7.2. Unbalanced Fertilizer Application (Element Imbalance)

What is the mistake: focusing on only one element (often nitrogen or potassium) and ignoring others, especially micronutrients, or applying elements in the wrong ratio, causing antagonism (e.g., excess potassium causes magnesium deficiency, excess nitrogen causes calcium deficiency) (Westwood, 1993; Srivastava et al., 2020).

Why it is harmful:

• Excess potassium with insufficient magnesium can cause chlorosis (interveinal yellowing of old leaves), and also impair calcium uptake, leading to fruit cracking and reduced storage (Agustí, 2010; Gull et al., 2023).
• Excess phosphorus can cause zinc and iron deficiency (especially on alkaline soils), showing as chlorosis of young leaves (Westwood, 1993; Srivastava et al., 2020).
• Ignoring micronutrients (boron, zinc, manganese, iron) leads to characteristic deficiencies: poor fruit set, rosetting, chlorosis, small misshapen fruit (Agustí, 2010; Ziogas et al., 2020).
• Unbalanced NPK ratio weakens the tree, reduces stress and disease resistance, impairs fruit taste and market quality (Potapov et al., 2000; Srivastava et al., 2020).

How to avoid:

• Apply compound fertilizers (nitrophoska, crystallins, polyfids) with micronutrients, especially at critical stages (flowering, fruit set, filling) (Krivko, 2014).
• Regularly (every 2–3 years) conduct soil and leaf analysis to adjust nutrition based on actual data (Westwood, 1993; Bryla, 2020).
• Consider element antagonism:
• Potassium and magnesium — antagonists; at high potassium doses, apply additional magnesium (potassium magnesium sulfate).
• Nitrogen and calcium — excess nitrogen reduces calcium supply to fruit; therefore, during fruit growth, foliar calcium sprays are useful.
• Phosphorus and zinc — high phosphorus doses may block zinc (Westwood, 1993; Srivastava et al., 2020).
• Apply micronutrients (especially boron, zinc, iron, manganese) in chelated form, via foliar sprays (Ziogas et al., 2020).

Signs of imbalance: symptoms of multiple element deficiencies simultaneously (e.g., interveinal chlorosis on old leaves and yellowing of young leaves), deterioration of fruit quality, reduced overall tree vitality.

7.3. Violation of Timing and Application Methods

What is the mistake: applying fertilizers at the wrong time (e.g., nitrogen in autumn, phosphorus in spring without incorporation, foliar sprays in sunny weather), incorrect depth, mixing incompatible fertilizers, applying high concentrations to the root zone (root burn).

Why it is harmful:

• Autumn nitrogen application — the most dangerous mistake: nitrogen stimulates growth, the tree does not enter dormancy, shoots freeze, winter hardiness drops sharply (Agustí, 2010; Krivko, 2014).
• Surface application of phosphorus (without incorporation) — phosphorus hardly moves in soil, stays in the top layer and becomes unavailable to most roots (Westwood, 1993; Bryla, 2020).
• Foliar sprays in hot sunny weather or at high concentration — cause leaf burn (necrosis, marginal burns), reduce photosynthesis (Ziogas et al., 2020; Srivastava et al., 2020).
• Mixing incompatible fertilizers (e.g., superphosphate with ammonium nitrate or calcium nitrate) — leads to chemical reactions, precipitation, and nutrient loss (Krivko, 2014).
• Applying concentrated solutions directly to the trunk — burn of root collar and roots, which can kill the tree (Martin, 2019).

How to avoid:

• Strictly follow the seasonal calendar (see Chapter 5):
• Nitrogen — only spring and first half of summer.
• Phosphorus and potassium — autumn + spring (incorporated to depth).
• Organics — autumn or spring, but always incorporated.
• Depth of incorporation:
• Nitrogen fertilizers — 10–15 cm.
• Phosphorus-potassium — 20–25 cm (along canopy periphery, in furrows or holes).
• Foliar sprays — perform only in the evening (after 6–7 pm) or early morning, in cloudy weather, at temperatures below 25 °C (Ziogas et al., 2020).
• Do not mix everything. Check fertilizer compatibility (compatibility tables are usually on the packaging or available in handbooks). Basic rules (Krivko, 2014):
• Superphosphate should not be mixed with ammonium nitrate and urea (dry) — nitrogen is lost.
• Calcium nitrate should not be mixed with superphosphate and potassium sulfate (precipitate forms).
• Urea can be mixed with most fertilizers except superphosphate and alkaline forms.
• Apply fertilizers not at the centre of the root zone, but at the periphery — where active absorbing roots are located (Westwood, 1993).
• After applying mineral fertilizers, be sure to water (at least 2–3 buckets of water per 1 m²) to dissolve and “pull” salts into the root zone (Bryla, 2020).

Signs of timing violations: prolonged autumn growth, shoot freezing, weak flowering next spring, leaf burns after spraying.

7.4. Additional Mistakes

• Ignoring organic fertilizers. On minerals alone, soil gradually depletes, structure deteriorates, microbiological activity declines. Organics are the basis of long-term fertility (Martin, 2019; Srivastava et al., 2020).
• Applying fresh manure. Fresh manure contains a lot of ammonia, burns roots, attracts pests, and contains weed seeds. Use only rotted manure or compost (Agustí, 2010; Krivko, 2014).
• Neglecting soil and leaf analysis. Without diagnosis, you work blindly. Regular analysis (at least once every 3 years) will help precisely adjust nutrition (Westwood, 1993; Bryla, 2020).
• Excess micronutrients (especially boron). Boron toxicity shows as marginal leaf scorch, tissue necrosis, gum exudation. Boron is applied only when needed and at strictly recommended concentrations (0.1–0.2 %) (Krivko, 2014; Srivastava et al., 2020).
• One-sided nutrition (only nitrogen or only potassium). All elements are important and interconnected. Balanced nutrition is the key to tree health.

7.5. Quick Checklist: How to Avoid Mistakes