Nutrition
Potato is a crop with a high demand for nutrients. Unlike cereals, where we harvest seeds, potatoes produce their yield in the form of underground tubers—highly modified stems. This process requires enormous amounts of energy, which the plant obtains through photosynthesis in its leaves (foliage). This is precisely why the connection "foliage → photosynthesis → tubers" is the key logic underpinning the entire potato nutrition system.
"Tuber yield is directly dependent on the abundant supply of carbohydrates that the foliage supplies to the tubers" (Swiader, 1992).
When we talk about potato nutrition, we are essentially solving the problem of how to provide the plant with sufficient "building materials" and "fuel" to produce maximum yields of high-quality tubers. Each element plays its role, and an imbalance of any one can become a limiting factor.
In this article, we will break down which nutrients potatoes need, when and in what quantities to apply them, how to recognize deficiencies, and how to avoid excess.
1. How Potatoes Form Yield Through Nutrition
The Central Logic: Foliage → Photosynthesis → Tubers
To understand why potatoes need certain fertilizers, you need to grasp a simple scheme:
1. Nutrients (especially nitrogen and magnesium) are necessary for the growth of healthy, vigorous foliage.
2. Healthy foliage effectively captures sunlight and carries out photosynthesis—converting carbon dioxide and water into carbohydrates (sugars, starch).
3. Carbohydrates are transported from the leaves downward to the underground organs, where they accumulate in the tubers.
This process requires balance. If the foliage is too vigorous, it consumes too many resources—tubers become smaller. If the foliage is weak, there is little photosynthesis, and tubers simply have nothing to form from.
"Once tubers begin to form, they compete directly for carbohydrates with other parts of the plant. Since leaves and stem growing points are closer to the carbohydrate source, tubers receive only the surplus that was not used by the above-ground parts" (Swiader, 1992).
What the Plant Removes from the Soil
Potato is a "hungry" crop. From each hectare yielding 30–40 tons, the plant removes:
| Element | Amount per 1 ton of tubers (kg) | Approximately per 30 t/ha (kg) |
|---|---|---|
| Nitrogen (N) | 4.5–5.9 | 135–177 |
| Phosphorus (P₂O₅) | 0.6–1.1 | 18–33 |
| Potassium (K₂O) | 7.1–10.7 | 213–321 |
| Calcium (Ca) | 0.1–0.15 | 3–4.5 |
| Magnesium (Mg) | 0.25–0.45 | 7.5–13.5 |
(Dean, 1994; Naumann & Pawelzik, 2023)
It's important to understand: these figures are only guidelines. Actual consumption depends on variety, weather conditions, soil type, and target yield.
2. The Role of Nitrogen
Why Nitrogen Is the Main Driver of Growth
Nitrogen is the element without which potatoes cannot build a single green leaf. It is a component of chlorophyll (the substance that captures sunlight), all proteins, amino acids, enzymes, and nucleic acids. Essentially, nitrogen is the "building material" for the entire foliage. Without it, there will be no powerful photosynthesis, and consequently, no high tuber yield.
"One of the most important macronutrients for plant growth, biomass formation, as well as yield and quality is nitrogen, since it is a constituent of chlorophyll, proteins, amino acids, nucleic acids, and coenzymes" (Naumann & Pawelzik, 2023).
Potatoes consume nitrogen in large quantities: for each ton of tubers, 4.5 to 5.9 kg of nitrogen is removed (Dean, 1994). For a yield of 30 t/ha, this amounts to 135–180 kg/ha—and that's just the tubers, not counting the foliage. This is why nitrogen fertilizers are the foundation of potato nutrition, but they are also the most common cause of errors.
Forms of Nitrogen: What Is Available to the Plant
In soil, nitrogen exists in different forms. Two main forms are important for potatoes:
- Nitrate nitrogen (NO₃⁻) — the most accessible form. It is highly soluble in water and easily absorbed by roots. Potatoes prefer nitrates.
- Ammonium nitrogen (NH₄⁺) — also absorbed, but in smaller amounts and often less tolerated, especially at high concentrations.
"The most important forms of nitrogen for plant nutrition in agricultural conditions are nitrate and ammonium nitrogen" (Naumann & Pawelzik, 2023).
A characteristic of potatoes: they use the nitrate form better, especially at low soil temperatures. Below +13 °C, nitrate absorption slows significantly, while ammonium may be absorbed more actively (Dean, 1994). Therefore, in spring, in cold soil, it is useful to use fertilizers with a predominance of nitrates (for example, calcium nitrate).
The Nitrogen Cycle: What Happens to Nitrogen in the Soil
To understand when and how much nitrogen to apply, it is useful to know where it can "escape" from control.
1. Mineralization — organic matter (humus, plant residues) decomposes by microorganisms, and nitrogen is converted into mineral forms (ammonium, then nitrates). This is a natural source of nitrogen that works throughout the summer.
2. Nitrification — ammonium is oxidized by bacteria to nitrites, then to nitrates. Nitrates are mobile and easily accessible to plants.
3. Immobilization — microorganisms themselves consume nitrogen from the soil, especially if fresh organic matter (straw, sawdust) is added. At this point, nitrogen temporarily becomes unavailable to the potato.
4. Denitrification — in waterlogged, oxygen-depleted soil, nitrates are reduced to gaseous nitrogen (N₂, N₂O) and volatilize into the atmosphere. This is the main nitrogen loss on heavy soils.
5. Leaching — nitrates are very mobile and are easily washed by rainfall into deep layers, beyond the root zone. Especially severe on light sandy soils.
Knowledge of these processes suggests:
- Nitrogen should be applied in split doses to match the plant's peak consumption periods.
- Do not apply large doses before rain.
- On light soils, fertilizing should be more frequent but in smaller portions.
Nitrogen Deficiency: How to Recognize It
Nitrogen deficiency manifests quite clearly:
- Old (lower) leaves begin to yellow — chlorophyll breaks down, and nitrogen is moved to young leaves.
- Yellowing gradually rises up the plant.
- Foliage becomes pale green, growth slows.
- Flowering is weak or absent.
- Tubers become smaller, yield decreases.
"Signs of nitrogen deficiency usually appear first on old leaves, which turn yellow. General yellowing moves upward through the plant, and with severe nitrogen starvation, young leaves are also affected" (Dean, 1994).
If you see such signs in early to mid-summer—this is a signal for an urgent nitrogen application. It is better to use the nitrate form (calcium nitrate), as it acts quickly.
Nitrogen Excess: Danger to the Harvest
Overfeeding with nitrogen is as serious a mistake as starvation. Excess nitrogen causes:
1. Excessive foliage growth — the plant goes into "luxuriant growth." Stems become thick, leaves large and dark green, but tubers are set late and fill poorly.
"At high nitrogen rates, tuber formation and maturation are delayed, and the accumulation of dry matter in tubers is reduced" (Welbaum, 2015).
2. Reduced starch content — tubers become watery, store poorly, and darken when fried.
3. Increased disease susceptibility — loose, succulent foliage is an ideal environment for late blight and other fungal diseases.
4. Nitrate accumulation in tubers — this can be a health concern, especially in early potatoes.
"Excessive fertilizer application, especially nitrogen, can delay tuber formation and plant maturation, as well as reduce the accumulation of dry matter in tubers" (Welbaum, 2015).
How to tell there's too much nitrogen?
- Foliage is bright green, vigorous, but flowering is delayed or weak.
- Leaves are soft, succulent, and do not yellow toward the end of the season.
- Tubers are often small with low starch content.
In this case, all nitrogen fertilizing should be stopped to allow the plant to redirect resources to the tubers.
Nitrogen and Tuber Quality
Nitrogen affects not only yield size but also what the tubers will be like:
| Parameter | With adequate nitrogen | With excess nitrogen |
|---|---|---|
| Starch content | Optimal | Reduced |
| Storage quality | Good | Deteriorates |
| Resistance to mechanical damage | Moderate | Decreases |
| Reducing sugar content | Normal | Increases → dark chips and fries |
| Asparagine content (acrylamide precursor) | May increase with excess | Increases |
"With increasing nitrogen nutrition, the content of free amino acids increases... providing important precursors for glycoalkaloid synthesis" (Naumann & Pawelzik, 2023). "Reducing nitrogen fertilizer increased reducing sugar concentration by 60–100% depending on the variety" (Naumann & Pawelzik, 2023).
This means that for processing (chips, fries), strict nitrogen control is needed—its excess worsens the color of the finished product.
Practical Recommendations for Nitrogen Application
General Rates
For gardeners and small farmers, the guideline is the planned yield. On average, 100 to 200 kg/ha of active nitrogen is recommended (approximately 10–20 g per 1 m²). On light sandy soils, rates are higher; on heavy soils, lower.
"Recommended nitrogen rates range from 100–200 kg/ha (sometimes up to 300 kg/ha)" (Naumann & Pawelzik, 2023).
Important: adjust the rate considering the previous crop. After legumes (peas, clover), the nitrogen share can be reduced by 20–30%, as some nitrogen remains in the soil.
Split Application — Key to Success
Nitrogen should not be applied all at once. The best strategy is to divide it into 2–3 applications:
1. At planting — 30–40% of the total rate. This gives a start for root development and early leaves.
2. Emergence to row closure (2–3 weeks after emergence) — another 30–40%. This is the period of rapid foliage growth.
3. Budding to early flowering — the remaining 20–30%. This is when tubers have already set and are beginning to grow, but nitrogen is still needed to maintain photosynthesis.
"Studies with labeled nitrogen have shown that the most effective nitrogen uptake occurs from emergence to the early tuber bulking stage... The highest percentage of nitrogen was absorbed when applied before July 13" (Dean, 1994).
Important rule: the last nitrogen application should be no later than 30–45 days before the planned harvest. If nitrogen is applied later, the tubers will not have time to mature, will store poorly, and nitrate levels may remain high.
"For proper maturation of plants and tubers, the soil nitrogen supply must be depleted, and seasonal applications should be stopped 30–45 days before harvest" (Pavek, 2020).
Which Nitrogen Form to Choose
For potatoes, fertilizers with nitrate form are preferred:
- Calcium nitrate (15.5% N) — an excellent option, especially in spring. Contains calcium, which benefits tuber quality.
- Ammonium nitrate (34% N) — contains half the nitrogen in nitrate form and half in ammonium form. Works well but acidifies the soil.
- Urea (46% N) — requires incorporation into the soil to avoid ammonia losses. On light soils, less effective due to leaching.
- Compound fertilizers (nitroammophoska, diammophoska) — convenient, but it's important to check the NPK ratio. For potatoes, potassium should be at least equal to nitrogen (e.g., 1:1:1.5 or 1:1:2).
How to Monitor Nitrogen Nutrition
The simplest way is to observe the color and condition of the foliage:
- Pale green, small leaves → nitrogen deficiency.
- Bright green, vigorous, but weak flowering → overfeeding.
- Lower leaves yellowing, but upper leaves green → developing deficiency.
Nitrogen and Neighboring Elements
Nitrogen works closely with potassium and magnesium. With excess nitrogen, potassium deficiency often occurs because the plant begins to "luxuriate" and consumes more potassium to maintain turgor. Therefore, always monitor the balance: for each part of nitrogen, there should be at least 1–1.5 parts of potassium (K₂O) and magnesium in reasonable proportions.
Key Takeaways on Nitrogen
1. Nitrogen is the foundation of growth, but overfeeding is worse than starvation. Base your decisions on the condition of the foliage, not just standard rates.
2. Apply in split doses: start → growth → budding. Do not give all nitrogen at once.
3. Stop fertilizing one month before harvest. This preserves quality and storage ability.
4. Prefer nitrate forms (calcium nitrate) in spring and on light soils.
5. Monitor balance with potassium and magnesium. Nitrogen is not self-sufficient.
In the next chapter, we will discuss the role of phosphorus and why it is critical for the early development of potatoes.
3. Phosphorus: Starter and Root System
Why Phosphorus Is the Plant's "Energy Currency"
If nitrogen is responsible for green mass growth, phosphorus is responsible for the energy of all processes in the plant. It is a component of ATP—the universal molecule that transfers energy from photosynthesis and respiration to any "consumers": protein synthesis, cell division, uptake of other elements, and transport of substances through vessels. Without phosphorus, no energy-consuming process works.
"Phosphorus participates in cellular energy transfer from ATP to ADP, is a component of nucleic acids, coenzymes, and phospholipids of biomembranes" (Naumann & Pawelzik, 2023).
A distinctive feature of potatoes is that their root system is weak, fibrous, and shallow. Most roots are in the top 30-centimeter layer and cannot "mine" phosphorus from deeper horizons. Therefore, phosphorus must be available immediately at the root from the very first days of the plant's life.
The Role of Phosphorus: From Roots to Tubers
1. Stimulation of the Root System
Phosphorus is especially important in the earliest stages. It promotes cell division in root meristems, root elongation, and the formation of lateral roots. A plant with good phosphorus nutrition explores the soil volume faster, more efficiently using moisture and other elements.
"Phosphorus is particularly involved in early growth stages; it is necessary for tuber formation and promotes tuber maturation during the growth process" (Naumann & Pawelzik, 2023).
2. Energy for Emergence
After planting, potatoes initially use reserves in the mother tuber, but within 2–3 weeks, their own roots appear. For these roots to grow quickly and provide nutrition for shoot development, ATP energy is needed—and that requires phosphorus. This is why phosphorus fertilizers are applied in rows at planting: roots immediately encounter them in the zone of availability.
3. Stolon Formation and Tuber Initiation
Stolons are underground shoots on the ends of which tubers form. Their growth and branching, as well as tuber initiation itself, require intensive cell division and energy expenditure, which is impossible without phosphorus.
"Phosphorus demand is relatively low in the first weeks of growth but increases significantly after tuber formation during the maturation period" (Naumann & Pawelzik, 2023).
4. Starch Quality
Phosphorus even affects starch structure. In potato starch, phosphorus is present as covalently bound residues (glucose-6-phosphate), which gives starch special properties: higher viscosity, swelling capacity. This is important for processing and culinary quality.
"Increased phosphorus availability in the soil can lead to increased phosphorus concentration in the tuber... leading to higher amylose content and altered thermal properties of starch" (Naumann & Pawelzik, 2023).
Phosphorus Deficiency: How to Recognize It
Phosphorus starvation does not manifest as obviously as nitrogen deficiency, but it has characteristic signs:
- Leaves become dark green, sometimes with a purple or bluish tinge — especially on the underside and along veins. This is due to anthocyanin accumulation.
- Plant growth is slowed, foliage looks stunted, stems may be stiff and upright.
- Flowering is weak, tuber set is delayed.
- The root system is poorly developed, roots are thin and weak.
"Under phosphorus deficiency, leaves become dark green and may develop a purple tint. Leaf growth may be reduced, leaves and stems are stiff and upright" (Dean, 1994).
Important: a purple tint on leaves can also appear at low temperatures (even with sufficient phosphorus), as cold disrupts phosphorus uptake into tissues. Therefore, the diagnosis should be made by evaluating a combination of signs in warm weather.
Phosphorus Excess: Is It Dangerous?
Phosphorus excess is less common than deficiency, but it is also undesirable:
- High phosphorus concentrations can cause zinc and iron deficiencies due to antagonism.
- On some soils, excess phosphorus enhances the fixation of other micronutrients (copper, manganese).
- Excess phosphorus is economically unjustified and environmentally risky (runoff into water bodies).
"Phosphorus is a major limiting factor in potato production. Due to the high phosphorus demand and relatively low use efficiency, it is often applied in excess" (Naumann & Pawelzik, 2023).
However, for most gardeners, the problem is precisely a lack of phosphorus, so it is important not to forget about it.
Phosphorus Behavior in Soil
Phosphorus is a low-mobility element. Unlike nitrates, it is practically not leached, but it also does not migrate to roots. Roots must "find" phosphorus themselves. That is why:
- Broadcast application (surface) of phosphorus is ineffective — most of it remains unavailable.
- Row application (in holes or furrows) is the most reliable method — the fertilizer is placed directly at the roots.
- Phosphorus is easily bound with calcium (on alkaline soils), aluminum, and iron (on acidic soils), becoming insoluble forms.
"Phosphorus is poorly soluble and very immobile, so roots must grow toward phosphorus to absorb it" (Naumann & Pawelzik, 2023).
Optimal pH for Phosphorus
Phosphorus is best available at soil pH 6.0–7.5. Below pH 5.5, it binds with aluminum and iron; above pH 7.5, with calcium. Therefore, monitoring acidity is an important part of phosphorus nutrition.
"Neutral soil pH in the range of 6.0–7.5 is optimal for phosphorus solubility, while pH outside this range can impair phosphorus availability" (Naumann & Pawelzik, 2023).
Practical Recommendations for Phosphorus Application
When and How Much
The main phosphorus dose is applied at planting. The approximate rate for potatoes is 60 to 120 kg/ha P₂O₅ (approximately 6–12 g/m²). The exact figure depends on soil type:
| Soil Type | Recommended P₂O₅ Rate |
|---|---|
| Sandy, poor | 100–120 kg/ha |
| Loamy, medium | 80–100 kg/ha |
| Rich, chernozem | 40–60 kg/ha |
Adapted from Rosen et al. (2014), cited in Naumann & Pawelzik (2023).
How to Apply
1. Row application (in the hole or furrow) — the best option. Phosphorus is placed at a depth of 5–10 cm, next to the tuber. Roots immediately enter a zone with high phosphorus concentration.
2. Pre-plant incorporation under tillage — acceptable, but less effective. In this case, phosphorus is evenly distributed in the plow layer, and roots need more time to "find" it.
3. Foliar phosphorus applications — practically useless, as phosphorus penetrates leaves poorly. Used rarely, only in emergency cases and in chelated form.
"Band application may be preferred for phosphorus use efficiency since it is usually placed close to the roots" (Naumann & Pawelzik, 2023).
Compatibility with Other Fertilizers
Phosphorus combines well with nitrogen and potassium in compound fertilizers (e.g., nitroammophoska — 10:10:10 or 15:15:15). But remember that phosphorus in such mixtures is still less mobile, so row application is preferred.
Phosphorus and Micronutrients: Balance
High phosphorus doses can reduce the availability of zinc and iron. This is especially noticeable on alkaline soils. Therefore, if the soil is rich in phosphorus and plants show signs of chlorosis (yellowing between veins on young leaves), it is worth checking not only iron and zinc but also the phosphorus level.
"Elevated phosphorus levels in the soil can reduce the availability of zinc and iron" (Naumann & Pawelzik, 2023).
How to Monitor Phosphorus Nutrition
Visually assessing phosphorus levels is difficult—deficiency symptoms do not appear immediately and may be non-specific. The most reliable method is soil analysis. It is done in autumn or early spring to adjust fertilizer rates.
During the growing season, you can look at:
- leaf color (absence of purple tint);
- growth rates (normal development);
- size and number of tubers.
If deficiency is suspected in the middle of the season, applying phosphorus in rows is no longer possible, but liquid phosphorus fertilizers can be used with irrigation (drip irrigation)—they provide localized application to the root zone.
Key Takeaways on Phosphorus
1. Phosphorus is energy for root growth, emergence, and tuber set. Without it, potato establishment will be weak.
2. Apply phosphorus at planting, in rows (locally). This is the only way to ensure its availability to the weak root system.
3. Maintain pH 6.0–7.5 — only in this range is phosphorus soluble and available.
4. Deficiency symptoms: dark green or purple leaves, stunted growth. Do not confuse with cold stress.
5. Phosphorus excess is rare, but it can cause zinc and iron deficiencies.
6. Foliar phosphorus applications are inefficient — only root feeding works.
In the next chapter, we will cover potassium—the main element for tuber formation and bulking, the foundation of potato quality.
4. Potassium: The Main Element for Tuber Formation
Why Potassium Is the King of Potatoes
If nitrogen is the "fuel" for foliage growth, and phosphorus is the "battery" for energy, then potassium is the "builder" of tubers. It is potassium that most determines how much starch accumulates in tubers, how dense, flavorful, and long-storing they will be.
"Potassium is the most important of all macronutrients in potato production, both in terms of yield and quality. It has the highest concentration in tubers and leaves" (Naumann & Pawelzik, 2023).
Potato is an absolute champion in potassium consumption among vegetable crops. For each ton of tubers, 7.1 to 10.7 kg K₂O is removed—more than nitrogen and phosphorus combined (Dean, 1994). For a yield of 30 t/ha, this is 213–321 kg K₂O. Potassium enters tubers in enormous quantities, and its content in the flesh can reach 400–600 mg per 100 g of fresh weight (Naumann & Pawelzik, 2023).
Potassium Functions: From Photosynthesis to Starch
Potassium participates in almost all key processes leading to yield.
1. Osmoregulation and Water Regime
Potassium is the main osmotically active ion in cells. It regulates water uptake, maintains turgor (elasticity) of tissues. With sufficient potassium, leaves remain elastic, stems strong, and tubers juicy but not watery.
"Potassium is a key nutrient in osmoregulation; this function is essential for cell elongation, turgor regulation, and stomatal movement" (Naumann & Pawelzik, 2023).
2. Photosynthesis and Gas Exchange
Potassium regulates the opening and closing of stomata—microscopic "windows" on leaves through which carbon dioxide enters for photosynthesis. Under potassium deficiency, stomata close, and photosynthesis slows.
"Potassium is also actively involved in photosynthesis through its function in stomatal regulation. Precise stomatal movement is necessary for the uptake of sufficient carbon dioxide" (Naumann & Pawelzik, 2023).
3. Sugar Transport from Leaves to Tubers
Potassium is required for phloem loading with sucrose. It creates the osmotic gradient that "pulls" carbohydrates from the leaves downward to the growing tubers.
"Potassium establishes the necessary osmotic pressure for sucrose distribution in the phloem" (Naumann & Pawelzik, 2023). "Under potassium deficiency, phloem loading and transport are impaired, leading to sucrose accumulation in leaves" (Koch et al., 2019, cited in Naumann & Pawelzik, 2023).
4. Activation of Starch Synthesis Enzymes
Potassium activates starch synthase—the key enzyme that converts sugars into starch. Without potassium, this process slows, and tubers accumulate more sugars rather than starch.
"Potassium is required for starch synthase activity, and its deficiency can limit starch synthesis" (Naumann & Pawelzik, 2023).
5. Increased Resistance to Stress and Diseases
Plants well-supplied with potassium tolerate drought, frost, pest attacks, and diseases better. This is because potassium strengthens cell walls and participates in the synthesis of protective compounds.
"Since potassium fulfills many essential functions in plant metabolism, it is evident that its status affects crop resistance to both abiotic and biotic stresses" (Naumann & Pawelzik, 2023).
How Potassium Affects Tuber Quality
1. Starch and Dry Matter
Potassium directly affects starch percentage. Optimal potassium nutrition gives tubers with high density and good mealiness when cooked. However, potassium excess can reduce starch content, as water uptake into cells increases.
"Although photosynthesis and plant growth are favorably stimulated by potassium, tuber dry matter begins to decline at high levels of potassium nutrition" (Naumann & Pawelzik, 2023).
The sweet spot: moderate potassium doses improve quality, high doses can worsen it.
2. Reduced Flesh Discoloration
Potassium reduces the tendency of tubers to:
- Discoloration upon cutting (enzymatic oxidation) — through reduced polyphenol oxidase activity.
- After-cooking darkening — by increasing citric acid content, which binds iron ions, preventing their reaction with chlorogenic acid.
"Potassium can indirectly reduce melanin formation and thus the development of black spot, as well as after-cooking darkening" (Naumann & Pawelzik, 2023).
3. Resistance to Bruising
With sufficient potassium, tuber cells have higher turgor and elasticity, reducing the risk of black spot (bruising) during harvest and transport.
"Increased cell turgor as a result of adequate potassium nutrition potentially reduces the risk of internal enzymatic browning caused by mechanical stress" (Naumann & Pawelzik, 2023).
4. Reduced Reducing Sugars
Potassium promotes more complete conversion of sugars into starch. This is especially important for processing: the less reducing sugars (glucose, fructose), the lighter the color of chips and fries, and the lower the acrylamide formation during frying.
"In controlled environment experiments, a clear correlation was demonstrated between increased potassium nutrition and reduced reducing sugar levels" (Naumann & Pawelzik, 2023).
5. Reduced Glycoalkaloids
There is evidence that potassium reduces solanine and chaconine content—toxic substances that impart a bitter taste.
"Effect of potassium fertilization on glycoalkaloid content in tubers: with increasing potassium nutrition, their content decreased" (Naumann & Pawelzik, 2023, citing Ahmed & Müller, 1979).
6. Storage Quality
Potassium increases tuber resistance to desiccation and rot during long-term storage.
"Moderate potassium rates (0–240 kg K₂O/ha, optimum 160 kg/ha) were suitable for reducing fresh weight loss in two medium-early varieties over 6 months of storage" (Naumann & Pawelzik, 2023).
Potassium Deficiency: How to Recognize It
Potassium starvation manifests characteristically:
- Old leaves — first along the edges, then between the veins — turn yellow, then brown and die ("marginal scorch," edge necrosis).
- Leaves may become wrinkled, curl, internodes shorten.
- Foliage acquires a bronze tint.
- Tubers form small, with low starch content, often watery.
- Plants become more susceptible to drought and diseases.
"Under potassium deficiency, foliage becomes dark green and bronzing may develop, starting from the tips and edges of leaves. Foliage may be wrinkled and veins sunken" (Dean, 1994).
Important: symptoms of potassium starvation are often confused with moisture deficiency or diseases. If you see "scorch" on leaf edges with normal watering—likely potassium deficiency.
Potassium Excess: Possible Problems
Although potassium overdose is less common than deficiency, it also has consequences:
- Reduced starch content due to excessive water accumulation ("dilution").
- Competitive suppression of magnesium — potassium and magnesium compete for root uptake. At very high potassium levels, magnesium starvation may develop, especially on light soils.
- Flavor changes — tubers may become less mealy.
"Very high potassium accumulation in tubers leads to increased water uptake... which can lead to starch dilution" (Naumann & Pawelzik, 2023).
Therefore, it is important to maintain a balance: potassium is needed, but in reasonable limits.
Forms of Potassium Fertilizers: Sulfate or Chloride?
This is one of the most important practical questions for potato growers.
Potassium Chloride (KCl) — Cheap but with a Caveat
- Pros: affordable price, high potassium concentration (60% K₂O).
- Cons: contains chlorine (Cl⁻), which potatoes do not like. Chlorine can:
- Reduce tuber starch content by 1–2% compared to the sulfate form.
- Impair taste and storage quality.
- Delay growth on light sandy soils.
"Application of KCl reduced starch content in tubers by about 2%, resulting in starch yield approximately 1 t/ha lower than with the sulfate form" (Naumann & Pawelzik, 2023).
Potassium Sulfate (K₂SO₄) — Preferred Choice
- Pros: chlorine-free, gives higher tuber quality (starch content, taste, storage quality), contains sulfur (18% S), which is important for protein synthesis.
- Cons: more expensive, lower potassium concentration (about 50% K₂O).
"In principle, chloride-free fertilizers are recommended for potatoes" (Haberland, 2020, cited in Naumann & Pawelzik, 2023).
Practical takeaway: if you have a choice—use potassium sulfate. Especially on light soils and for processing varieties. Potassium chloride can be used in small doses on heavy soils where chlorine is leached, but the risk remains.
When and How to Apply Potassium
Optimal Rates
Recommendations vary greatly by region, soil type, and target yield. Approximate rates:
| Conditions | K₂O Rate (kg/ha) |
|---|---|
| Light sandy soils, high yield | 250–350 |
| Medium loams | 200–280 |
| Rich soils, medium yield | 150–200 |
| Table varieties (quality more important than yield) | 200–250 |
| Processing varieties (starch) | 180–220 |
Source: compiled data from Naumann & Pawelzik (2023), Dean (1994).
Timing of Application
Potassium is low-mobility in soil (although less so than phosphorus), so the main dose is applied before planting or in rows at planting. This ensures availability in the root zone throughout the growth period.
- In autumn under tillage — on heavy soils, to allow potassium to stabilize.
- In spring before planting — on light soils, to avoid leaching.
- In rows at planting — especially effective for potassium sulfate, as it is immediately placed at the roots.
"Usually all potassium is broadcast before planting" (Naumann & Pawelzik, 2023).
Split application of potassium is generally not required, as potassium moves slowly in the soil. However, on very light sandy soils, part of the potassium can be applied as a topdressing at the budding stage (via drip irrigation).
Combination with Nitrogen
Potassium and nitrogen work in pairs. For each unit of nitrogen (N), it is desirable to have 1–1.5 units of K₂O. For example, with a nitrogen rate of 150 kg/ha, potassium should be 180–220 kg/ha. This ratio maintains the balance between foliage growth and tuber bulking.
Potassium and Magnesium: A Balance to Remember
Potassium and magnesium are antagonists. High potassium doses can suppress magnesium uptake, and vice versa. This is especially relevant on light soils poor in magnesium.
"Potassium-induced magnesium deficiency is one of the most commonly observed phenomena... in potato experiments, a significant reduction in leaf magnesium concentration was found with higher potassium nutrition" (Naumann & Pawelzik, 2023).
Recommendation: maintain a K:Mg ratio of approximately 3:1. If you apply high potassium rates, watch for symptoms of magnesium deficiency (yellowing between veins on old leaves). In this case, a foliar application of magnesium sulfate will help.
How to Monitor Potassium Nutrition
Visual Signs
- Absence of "marginal scorch" and bronzing on leaves.
- Normal leaf turgor even in heat (potassium helps retain water).
Soil Analysis
The most accurate method is soil analysis for exchangeable potassium. Optimal values for potatoes are 150–250 mg/kg (by Kirsanov method or AAS). Below this—needs fertilizing; above—the rate can be reduced.
Petiole Analysis
During the season, you can monitor potassium content in dry matter of petioles (normally at least 6–8% K at the bulking stage). For amateurs, this is more complex, but with laboratory access, it's useful.
Key Takeaways on Potassium
1. Potassium is the most important element for tubers. It determines starch content, taste, storage quality, and bruise resistance.
2. Apply potassium sulfate, not potassium chloride. Chlorine reduces quality, especially on light soils and for processing.
3. Potassium rate should be at least equal to nitrogen, preferably 20–30% higher. Balance N:K₂O ≈ 1:1.2–1.5.
4. Apply potassium mainly before planting or in rows. Split application is rarely needed.
5. Do not overfeed potassium — excess reduces starch content and can cause magnesium deficiency.
6. Watch for signs of potassium deficiency: marginal scorch of old leaves, bronze coloring, weak turgor.
7. Potassium increases resistance to drought and diseases — it is not just a fertilizer but also an "insurance" for the crop.
In the next chapter, we will cover the role of calcium, magnesium, and micronutrients—elements often overlooked but critically important for tuber quality.
5. Calcium, Magnesium, and Micronutrients: Quality and Resilience
Why These Elements Are Not "Secondary" But "Quality" Elements
Nitrogen, phosphorus, and potassium are the foundation of yield. But it is calcium, magnesium, and micronutrients that often determine whether the yield will be high-quality: dense, flavorful, long-storing, and resistant to diseases and mechanical damage. Without them, even with ideal NPK, tubers can be watery, susceptible to rots, darken when cooked, or have internal defects.
"Potassium, calcium, and magnesium especially affect tuber quality, but other macronutrients such as nitrogen, phosphorus, and sulfur, as well as micronutrients such as iron, zinc, boron, and manganese, perform important functions in plant metabolism and therefore also affect quality" (Naumann & Pawelzik, 2023).
In this chapter, we will cover why potatoes need each of these elements, how to recognize deficiencies, and how to properly address them.
Calcium (Ca): Cell Strength and Disease Protection
Calcium is a building material for cell walls and membranes. It makes tissues strong, resistant to pathogen penetration and mechanical damage.
Calcium Functions in Potatoes
1. Strengthening cell walls. Calcium binds pectins, forming a strong "framework" for cells. This is especially important for tuber skin—its strength and resistance to cuts during harvest.
"Calcium is important for cell wall and membrane stability" (Naumann & Pawelzik, 2023).
2. Disease resistance. Calcium reduces susceptibility to bacterial soft rot (Erwinia carotovora) and other infections. The higher the calcium content in tissues, the harder it is for bacteria to break down cell walls.
"The severity of bacterial soft rot caused by Erwinia carotovora decreased with increasing calcium concentration in tubers" (Naumann & Pawelzik, 2023).
3. Prevention of internal defects. Calcium deficiency is one of the causes of internal brown spot (rusty spot) and hollow heart in tubers. These defects significantly reduce marketable quality.
"Calcium deficiency can lead to various physiological disorders, including internal brown spot and hollow heart" (Naumann & Pawelzik, 2023).
4. Regulation of cellular signals. Calcium acts as a "second messenger," transmitting signals about stresses (drought, cold, pathogen attack) within the plant.
Why Potatoes Often Lack Calcium
Calcium enters the plant only through roots, and its transport is via the xylem—mainly with water flow that evaporates through leaves. Tubers transpire little water, so calcium enters them poorly. Most calcium is taken up by the foliage.
"Calcium is not mobile in the phloem and therefore cannot be transported from old leaves to young ones... Due to low transpiration, the tuber is one of the calcium-deficient organs" (Naumann & Pawelzik, 2023).
This means that relying on "redistribution" of calcium within the plant is futile. Calcium must reach the tubers directly through stolon roots.
How to Address Calcium Deficiency
- Liming acidic soils. Applying dolomitic or limestone flour raises pH to 6.0–6.5 and provides the plant with calcium. Important: lime is applied in autumn or to the preceding crop, allowing time to act.
- Calcium nitrate (Ca(NO₃)₂) — a nitrogen-calcium fertilizer. Provides calcium in available form and simultaneously nitrogen. Especially effective in early growth topdressings.
- Foliar applications with chelated calcium — can help in emergency cases, especially during tuber bulking.
"Application of calcium in close proximity to tubers and stolons can cause a threefold increase in calcium concentration in tubers" (Palta, 2010, cited in Naumann & Pawelzik, 2023).
Important: excessive liming (pH above 7.0) can increase common scab development, so monitor pH.
Calcium Deficiency Symptoms
- Young leaves (growing points) lighten at the edges, then edges die, leaves curl and become brittle.
- Stem tips may die.
- Tubers show brown spots in the flesh, sometimes cavities (hollow heart).
- Tuber skin becomes thin and easily damaged.
"Calcium deficiency appears first on young apical leaves. They may have a light green border along the edges, which then becomes necrotic, causing them to shrivel or curl" (Dean, 1994).
Magnesium (Mg): Heart of Photosynthesis and Carbohydrate Transport
Magnesium is the central atom in the chlorophyll molecule. Without magnesium, there is no green color, no photosynthesis, no sugars for tubers. Additionally, magnesium activates many enzymes and participates in phloem loading with sucrose.
Magnesium Functions
1. Photosynthesis. Magnesium is a component of chlorophyll and activates the key enzyme Rubisco (RuBisCO), which fixes carbon dioxide.
"Magnesium has an important role in photosynthesis as the central atom of chlorophyll" (Naumann & Pawelzik, 2023).
2. Carbohydrate transport. Magnesium is required for sucrose loading into the phloem. Under its deficiency, sugars accumulate in leaves and do not reach tubers.
"Magnesium is required for phloem loading with sucrose, as it allosterically activates ATPases that create the proton gradient" (Naumann & Pawelzik, 2023).
3. Reduced nitrate content. Magnesium improves protein synthesis, reducing nitrate accumulation in tubers.
"Increasing magnesium nutrition reduced nitrate content in tubers by an average of 15% compared to the no-magnesium treatment" (Naumann & Pawelzik, 2023).
4. Increased cell strength. Magnesium participates in pectin formation, making tissues more elastic.
Magnesium Deficiency: How to Recognize It
- On old (lower) leaves, interveinal chlorosis appears—yellow spots between veins, while veins themselves remain green.
- Later, yellowing turns brown, leaves die.
- Growth slows, tuber starch content decreases.
"Magnesium deficiency first appears on lower leaves. They become chlorotic at the tips and edges, then full interveinal chlorosis develops" (Dean, 1994).
How to Address Magnesium Deficiency
- Dolomitic flour (MgCO₃·CaCO₃) — when liming acidic soils, simultaneously provides magnesium and calcium.
- Magnesium sulfate (MgSO₄·7H₂O) — Epsom salt — fast-acting fertilizer, applied both to soil (20–40 kg/ha MgO) and foliarly (1–2% solution).
- Potassium magnesium sulfate (K₂SO₄·MgSO₄) — combines potassium and magnesium, especially useful on light soils.
- Foliar applications — most effective when symptoms appear, as magnesium penetrates leaves well.
The Potassium-Magnesium Ratio
Potassium and magnesium are antagonists. At high potassium doses, magnesium uptake decreases. Therefore, it is important to maintain a K:Mg ratio of approximately 3:1 (as K₂O:MgO). For example, with 240 kg K₂O, apply about 80 kg MgO.
"The amount of potassium applied usually determines the corresponding magnesium fertilization... a ratio of 3:1 is often used" (Haberland, 2020, cited in Naumann & Pawelzik, 2023).
Micronutrients: Small Doses, Big Effects
Micronutrients are needed by plants in quantities hundreds of times smaller than macronutrients, but their deficiency leads to serious metabolic disorders and deterioration of tuber quality.
Iron (Fe) — Chlorophyll and Respiration
Iron participates in chlorophyll synthesis, is a component of respiratory and photosynthetic enzymes (cytochromes, ferredoxin). It also plays a role in oxidation-reduction reactions.
Deficiency symptoms: chlorosis of young leaves — veins remain green, while tissue between them turns yellow or white (unlike magnesium, where chlorosis starts on old leaves).
"The primary symptom of iron deficiency is loss of color (chlorosis) between veins. Veins and leaf edges may remain green, while interveinal tissue becomes almost white" (Dean, 1994).
How to address: on alkaline soils, iron is unavailable. Apply iron chelate (foliarly) or acidify the soil (ammonium sulfate, sulfur). Iron sulfate (FeSO₄) in soil binds quickly, so chelated forms are more effective.
Zinc (Zn) — Protein and Hormone Synthesis
Zinc activates many enzymes, participates in protein synthesis, auxins (growth hormones), and protection against oxidative stress. It also increases disease resistance.
"Zinc is an important micronutrient for plant growth and development; it participates as a cofactor in approximately 300 enzymatic reactions" (Naumann & Pawelzik, 2023).
Deficiency symptoms:
- Young leaves are small, deformed, often with chlorosis.
- Internodes shorten, rosetting.
- Necrotic spots may appear on leaves.
"Under zinc deficiency, lower leaves may be chlorotic, and young leaves are reduced in size. Internodes shorten, and irregular necrotic spots appear on young leaves" (Dean, 1994).
How to address: soil application of zinc sulfate (5–10 kg/ha Zn) or foliar application with zinc chelate. On alkaline soils, soil application is less effective—use chelates.
Boron (B) — Growing Point Growth and Cell Wall Formation
Boron participates in cell wall formation, especially in growing tissues (growing points, flowers, ovaries). It is also important for sugar transport and nucleic acid synthesis.
Deficiency symptoms:
- Growing point death — the stem tip browns and dies.
- Young leaves become light, curled.
- Flowering weak, ovaries drop.
- Tubers may have cracks, brown spots.
"Young leaves and apical growth are the first to show boron deficiency effects. Apical leaves may be lighter, and the tip may brown and die" (Dean, 1994).
How to address: boric acid (0.1–0.2% solution) is applied foliarly—this is the most effective and safe method. Boron excess is toxic, so doses must be strictly observed (1–2 kg/ha boron). Soil boron application is rare, only based on analysis results.
Important: boron is very toxic in overdose (the difference between deficiency and toxicity is very small). Always follow dosage instructions.
Manganese (Mn) — Photosynthesis and Disease Resistance
Manganese is a component of the oxygen-evolving complex of photosystem II; without it, photosynthesis is impossible. It also activates enzymes involved in lignin synthesis and pathogen resistance.
Deficiency symptoms:
- Similar to iron deficiency: chlorosis between veins on young leaves, but with brown speckling and spots.
- On acidic soils, manganese excess (toxicity) appears as small brown dots on old leaves.
"Manganese deficiency can look similar to iron deficiency. Similar chlorosis develops, but brown spots also appear, which may be extensive" (Dean, 1994).
How to address: manganese sulfate applied to soil (10–20 kg/ha) or foliarly (0.1–0.2% solution). On acidic soils, manganese is often in excess, so check pH before application.
When Micronutrients Become Critical
Micronutrients are generally not needed on fertile, well-cultivated soils with neutral pH. However, their deficiency often occurs in the following cases:
- Sandy, organic-poor soils — low initial micronutrient reserves.
- Strongly acidic soils (pH < 5.5) — mobility of some elements (manganese, iron) is high, but availability of boron and zinc may be reduced.
- Strongly alkaline soils (pH > 7.5) — iron, zinc, manganese become unavailable (chlorosis is common).
- High phosphorus rates — phosphorus and zinc compete for uptake.
- High potassium rates — reduces magnesium availability.
- Intensive high-yielding varieties — remove more micronutrients with the crop.
"Micronutrients are generally not directly recommended, as often there are sufficient amounts in the soil. Nevertheless, some regions of the world—for example, more than 50% of Indian soils—suffer from iron deficiency" (Naumann & Pawelzik, 2023).
Practical Recommendations for Application
Calcium and Magnesium
- Liming: at pH < 5.5, apply dolomitic flour (contains Ca and Mg) in autumn, 3–5 kg per 10 m² (depends on acidity).
- Epsom salt (magnesium sulfate): at first signs of magnesium deficiency—foliar application with 1–2% solution (100–200 g per 10 L water).
- Calcium nitrate: spring application (10–15 g/m²) provides both nitrogen and calcium.
Micronutrients (as needed)
- Compound fertilizers with micronutrients — the simplest way for prevention.
- Foliar applications with chelates (iron, zinc, manganese, boron) — during active foliage growth and early tuber bulking. Dosages strictly according to instructions.
- Boric acid: 0.1–0.2% solution (1–2 g per 1 L water), spraying at the budding stage.
Important: most micronutrients (except boron) are well absorbed through leaves in chelated form. Soil application is often less effective due to soil binding.
Key Takeaways on Calcium, Magnesium, and Micronutrients
1. Calcium — tuber strength and health. Apply calcium nitrate in spring or lime soil in autumn. Do not allow deficiency—it leads to internal defects and rots.
2. Magnesium — heart of photosynthesis. Maintain K:Mg ≈ 3:1 balance. At symptoms—foliar application with magnesium sulfate.
3. Iron and zinc — critical on alkaline soils. Symptoms — chlorosis of young leaves. Use chelates foliarly.
4. Boron — important for growing points and flowering. Doses are small, overdose dangerous. Best form is foliar (boric acid).
5. Manganese — on acidic soils often in excess, on alkaline — deficient. Recognize by brown speckling between veins.
6. Always check soil pH — it affects the availability of almost all micronutrients. Optimal pH for potatoes is 5.8–6.5.
7. Prevention is better than cure. Use compound fertilizers with micronutrients — it's more reliable than reacting to symptoms.
Deficiency Symptoms Table (Quick Reference)
| Element | Primary Symptom | On Which Leaves | What to Do |
|---|---|---|---|
| Nitrogen (N) | Lower leaves yellow | Old | Nitrate topdressing |
| Phosphorus (P) | Dark green/purple leaves | All | Row application |
| Potassium (K) | Marginal scorch, bronzing | Old | Potassium sulfate |
| Calcium (Ca) | Growing point death, brittleness | Young | Calcium nitrate, lime |
| Magnesium (Mg) | Interveinal chlorosis | Old | Magnesium sulfate (foliar) |
| Iron (Fe) | Interveinal chlorosis (veins green) | Young | Iron chelate (foliar) |
| Zinc (Zn) | Small deformed leaves | Young | Zinc chelate (foliar) |
| Boron (B) | Growing point death | Young | Boric acid (foliar) |
| Manganese (Mn) | Brown speckling between veins | Young | Manganese sulfate (foliar) |
6. Fertilizing by Growth Stages: A Practical Strategy
Why It's Important to Synchronize Fertilizing with Growth Stages
Potato is a plant with a pronounced dynamic of nutrient consumption. What is critical at the beginning of growth becomes useless or even harmful during tuber bulking. Knowing when to give the right element is the key skill of a successful potato grower.
"Adequate water application during the growing season is crucial for optimizing crop production. Both timing and amount of water must be considered in a proper irrigation program" (Dean, 1994). The same applies to nutrients.
The entire potato cycle can be divided into five key stages, each requiring its own fertilizer "menu." In this chapter, we will provide a step-by-step fertilizing strategy for gardeners and small farmers.
Stage 1: Planting and Emergence (Start)
Duration: from planting to full emergence (usually 2–4 weeks after planting, depending on soil temperature).
What Happens to the Plant
- The mother tuber provides energy and nutrition for the first shoots.
- Own roots begin to form—they will feed the plant throughout the season.
- Primordia of future stolons and tubers are just beginning to form.
Which Elements Are Critical
- Phosphorus — energy for cell division of roots and stolons.
- Nitrogen — in a small starter dose for the first foliage push.
- Potassium — in moderate amounts for osmoregulation and overall resilience.
Practical Strategy
1. Phosphorus — must be in rows (locally). Apply superphosphate or a compound fertilizer with high phosphorus content directly in the hole or furrow at planting. This is the only way to guarantee phosphorus availability to weak young roots.
"Phosphorus fertilizers are often applied as a band at planting, which gives an advantage in use efficiency, as they are usually placed close to the roots" (Naumann & Pawelzik, 2023).
2. Starter nitrogen. Apply 20–30% of the total seasonal nitrogen rate. Use calcium nitrate — it provides nitrogen in nitrate form, available even in cool soil, and supplies calcium.
3. Potassium. If you didn't apply potassium in autumn, add it to the rows with phosphorus. Use potassium sulfate (not chloride). Rate — approximately 30–40% of the total potassium rate.
4. Magnesium and micronutrients at the start are generally not needed unless the soil has a confirmed deficiency.
Approximate Rates per 1 m² (for Gardeners)
| Element | Fertilizer | Rate per 1 m² |
|---|---|---|
| Phosphorus (P₂O₅) | Superphosphate (double) | 5–7 g |
| Nitrogen (N) | Calcium nitrate | 5–8 g |
| Potassium (K₂O) | Potassium sulfate | 8–12 g |
Important: do not allow fertilizer to contact the seed tuber—this can cause burn. Place it 3–5 cm to the side and slightly deeper.
"Fertilizer should be placed 5–10 cm to the side and 5 cm deeper than the seed tuber to avoid burning developing sprouts and roots" (De Jong et al., 2011).
Stage 2: Active Foliage Growth (Emergence — Row Closure)
Duration: from emergence to when foliage closes between rows (usually 4–6 weeks after emergence, plant height 20–30 cm).
What Happens to the Plant
- Rapid growth of stems and leaves.
- Formation of the photosynthetic apparatus—the foundation of the future crop.
- Root system actively expands.
- By the end of this stage, stolons begin to form and tubers are initiated.
Which Elements Are Critical
- Nitrogen — the main "builder" of foliage. Nitrogen consumption peaks in this stage.
- Potassium — for turgor, water transport, and preparation for tuberization.
- Magnesium — for chlorophyll and photosynthesis.
- Phosphorus — continues to support root growth and stolon initiation.
Practical Strategy
1. Nitrogen application — apply the second portion of nitrogen (30–40% of the total rate). Use calcium nitrate or ammonium nitrate. If the soil is moist and growth is active, it can be applied dry under cultivation or with irrigation.
2. Potassium. If potassium was not applied at planting, now is the time to provide it as a topdressing. Use potassium sulfate or potassium magnesium sulfate (if magnesium is also needed).
3. Magnesium. If there are signs of deficiency (yellowing between veins on lower leaves), apply a foliar spray with 1–2% magnesium sulfate solution.
4. Phosphorus. At this stage, phosphorus is not as critical as at the start. If you applied it at planting, no additional is needed. But if planting was without phosphorus, you can apply liquid phosphorus fertilizer with irrigation (but efficiency will be lower).
Approximate Rates per 1 m² (Foliar or Irrigation Topdressing)
| Element | Fertilizer | Rate per 1 m² |
|---|---|---|
| Nitrogen (N) | Calcium nitrate | 8–12 g |
| Potassium (K₂O) | Potassium sulfate | 5–8 g |
| Magnesium (MgO) | Magnesium sulfate (foliar) | 2–3 g (1–2% solution) |
Stage 3: Budding — Early Flowering (Peak Consumption)
Duration: from bud appearance to mass flowering (usually 1.5–2 weeks).
What Happens to the Plant
- Tuber initiation and active growth begin precisely in this stage.
- Consumption of all elements reaches its maximum.
- The plant shifts from foliage growth to tuber bulking—this is a critical transition.
"The most effective nitrogen uptake occurs from emergence to the early tuber bulking stage... The highest percentage of nitrogen was absorbed when applied before July 13" (Dean, 1994).
Which Elements Are Critical
- Nitrogen — still needed to maintain photosynthesis, but not for foliage growth.
- Potassium — the main element for tuber bulking, consumption peaks.
- Phosphorus — for energy of tuber growth and continued cell division.
- Boron — important for flowering, tuber set, and sugar transport.
- Calcium — begins to enter tubers through stolon roots.
Practical Strategy
1. Nitrogen — last application. Apply the remaining 20–30% of nitrogen. After this stage, nitrogen fertilizing stops.
2. Potassium — main dose. If you haven't applied all potassium at planting, now is the time to give it as a topdressing. It's better to use potassium fertilizers with irrigation (fertigation) or under cultivation.
3. Phosphorus. At this stage, phosphorus can be given in small amounts, especially if the soil is poor or the variety is high-yielding. Best applied as part of compound fertilizers.
4. Boron — foliar application. Spraying with 0.1–0.2% boric acid solution at the budding stage significantly improves flowering and tuber set. This is one of the most effective techniques for improving quality.
"Boron spraying significantly reduced tuber discoloration and increased ascorbic acid concentration" (Naumann & Pawelzik, 2023, citing Mondy & Munshi, 1993).
5. Calcium. If you didn't apply calcium nitrate in spring, now you can do a foliar application with chelated calcium (to prevent hollow heart and increase skin strength).
Approximate Rates per 1 m²
| Element | Fertilizer | Rate per 1 m² | Method |
|---|---|---|---|
| Nitrogen (N) | Ammonium nitrate | 5–8 g | Into soil under cultivation/irrigation |
| Potassium (K₂O) | Potassium sulfate | 8–12 g | Into soil or with irrigation |
| Boron (B) | Boric acid | 0.1–0.2 g (1–2 g per 10 L water) | Foliar (spraying) |
Stage 4: Tuber Bulking (Active Tuber Growth)
Duration: from mass flowering to the beginning of foliage yellowing (3–6 weeks, depending on variety and weather).
What Happens to the Plant
- Tubers rapidly gain weight and starch.
- Foliage no longer grows; its only function is photosynthesis for tubers.
- Potassium and phosphorus consumption remain high, while nitrogen consumption drops sharply.
Which Elements Are Critical
- Potassium — remains the main element. It ensures phloem loading with sucrose and starch synthesis.
- Phosphorus — for energy support of tuber growth.
- Calcium — for skin strengthening and preventing internal defects.
- Nitrogen — NOT needed. Applying nitrogen at this stage worsens quality and reduces storage ability.
"To ensure proper plant and tuber maturation, the soil nitrogen supply must be depleted, and seasonal applications should be stopped 30–45 days before harvest" (Pavek, 2020).
Practical Strategy
1. Nitrogen — completely exclude. No nitrogen fertilizing! If you see foliage beginning to yellow early, it's a natural maturation process, not nitrogen deficiency.
2. Potassium — continue applying (if possible). At this stage, potassium can be given through irrigation (fertigation) or dry under cultivation. This will support bulking.
3. Phosphorus. If the soil is poor, a small amount of phosphorus can be applied with irrigation, but generally, phosphorus applied at planting fully covers needs.
4. Calcium and magnesium. If there are signs of deficiency, apply foliar applications with chelated forms. Calcium is especially important for skin density.
Approximate Rates per 1 m² (if needed)
| Element | Fertilizer | Rate per 1 m² | Method |
|---|---|---|---|
| Potassium (K₂O) | Potassium sulfate | 5–8 g | With irrigation (fertigation) |
| Calcium (Ca) | Calcium chelate | per instructions | Foliar (spraying) |
Stage 5: Maturation and Preparation for Harvest
Duration: the last 2–3 weeks before harvest, when foliage yellows and dies.
What Happens to the Plant
- Photosynthesis gradually declines.
- Tubers reach final size and accumulate maximum starch.
- Skin thickens and becomes resistant to damage.
Which Elements Are Critical
- No fertilizing is needed. All processes are winding down; the plant uses internal reserves.
Practical Strategy
- Stop all fertilizing 3–4 weeks before harvest.
- If you use drip irrigation, water only as needed (do not overwater).
- During this period, you can apply a foliar calcium application (if skin is weak) 2–3 weeks before harvest—this may improve storage quality.
Summary Table of Fertilizing by Stages (for Gardeners and Small Farmers)
| Growth Stage | What to Apply | Rate per 1 m² | Application Method | Note |
|---|---|---|---|---|
| Planting | Phosphorus (superphosphate) | 5–7 g (as P₂O₅) | In rows/holes | Avoid tuber contact |
| Nitrogen (calcium nitrate) | 5–8 g (as N) | In rows/holes | 20–30% of seasonal rate | |
| Potassium (potassium sulfate) | 8–12 g (as K₂O) | In rows/holes | 30–40% of seasonal rate | |
| Active foliage growth | Nitrogen (calcium/ammonium nitrate) | 8–12 g (as N) | Into soil under cultivation or with irrigation | 30–40% of rate |
| Potassium (potassium sulfate) | 5–8 g (as K₂O) | Into soil or with irrigation | If not applied at planting | |
| Magnesium (magnesium sulfate) | 2–3 g | Foliar (1–2% solution) | If deficiency signs | |
| Budding – flowering | Nitrogen (ammonium nitrate) | 5–8 g (as N) | Into soil with irrigation | Last nitrogen application |
| Potassium (potassium sulfate) | 8–12 g (as K₂O) | Into soil or with irrigation | Main potassium application | |
| Boron (boric acid) | 0.1–0.2 g | Foliar (1–2 g per 10 L) | Important for set | |
| Calcium (chelate) | per instructions | Foliar | As needed | |
| Tuber bulking | Potassium (potassium sulfate) | 5–8 g (as K₂O) | With irrigation (fertigation) | If possible |
| Nitrogen — do not apply! | — | — | — | |
| Maturation | Stop fertilizing | — | — | 3–4 weeks before harvest |
Additional Tips for Managing Fertilizing
1. Adjusting for Weather
- In rainy seasons, nitrogen and potassium can be leached—increase the frequency of applications (but not the dose).
- In dry seasons, foliar applications (especially micronutrients) are more effective than soil applications.
- In cool springs (soil < +10 °C), use nitrate forms of nitrogen (calcium nitrate)—they are better absorbed.
2. Signs of Overfeeding (Nitrogen Excess)
- Foliage is dark green, vigorous, but flowering is weak or delayed.
- Tubers are small, watery.
- Late blight infection is more severe than usual.
What to do: stop all nitrogen fertilizing, increase potassium (to shift to bulking).
3. Signs of Underfeeding
- Pale, small foliage — nitrogen deficiency.
- Purple tint — phosphorus deficiency.
- Marginal scorch — potassium deficiency.
- Yellowing between veins — magnesium or iron deficiency.
What to do: apply the corresponding elements in available form (foliarly for quick effect).
4. Foliar Applications — When and How
- Foliar applications are effective for micronutrients (boron, zinc, iron, manganese) and magnesium.
- For macronutrients (NPK), foliar application is less effective but can help in emergency deficiency.
- Best time — morning or evening, in calm weather.
- The solution should be fresh, concentration strictly per instructions (excess burns leaves).
5. Timing Guidelines
- Starter fertilizers — at planting.
- First topdressing — 3–4 weeks after emergence (height 15–20 cm).
- Second topdressing — at budding stage (1–2 weeks before flowering).
- Third (only potassium) — at the start of tuber bulking (2–3 weeks after flowering).
- Last topdressing (any) — no later than 30 days before harvest.
"For proper plant and tuber maturation, the soil nitrogen supply must be depleted, and seasonal applications should be stopped 30–45 days before harvest" (Pavek, 2020).
Key Takeaways on Fertilizing Strategy
1. Five stages — five different "menus." You can't use one scheme for the entire season.
2. Phosphorus — only at the start, in rows. This is the key to rapid root development.
3. Nitrogen — split, until flowering. Last application at budding stage. After flowering — only potassium.
4. Potassium — the main element at all stages. Apply it at planting, during foliage growth, and during bulking. Use sulfate form.
5. Micronutrients — as needed. Boron — definitely at budding. Iron, zinc, manganese — when chlorosis symptoms appear.
6. Monitor weather and plant condition. Adjust doses and timing to the specific conditions of your site.
7. Stop all fertilizing one month before harvest. This preserves tuber quality and storage ability.
7. How to Tell When Nutrition Is Lacking: Field Diagnostic Guide
Why Learning to "Read" the Plant Is the Most Important Skill
Laboratory analysis of soil and petioles is reliable but expensive and not always available to amateur gardeners. Meanwhile, the plant itself sends us signals about its condition every day. Leaf color, shape, the pattern of dieback—all of this is a language in which potatoes communicate their needs.
"Observing the plant is the best way to adjust fertilizing in time" (Dean, 1994).
In this chapter, we will learn to correctly "read" these signals, distinguish similar symptoms, and make the right decisions. This is the most practical section of the article—a kind of field guide you can take with you to your plot.
General Principles of Diagnosis
Before examining specific symptoms, remember a few important rules:
1. Deficiency Shows Differently on Old and Young Leaves
- Mobile elements (nitrogen, potassium, magnesium, phosphorus) can move from old leaves to young ones. Therefore, their deficiency appears first on old (lower) leaves.
- Immobile elements (calcium, iron, boron, zinc, manganese) cannot be redistributed. Their deficiency appears on young (upper) leaves and at growing points.
This is the first thing to determine when inspecting a plant: "Which leaves are affected first?"
2. Symptoms Can Be Similar
Chlorosis (yellowing) is a common sign of many deficiencies. It is important to look at the pattern of yellowing:
- Uniform yellowing of the entire leaf → often nitrogen.
- Yellowing between veins (veins green) → often magnesium, iron, or manganese.
- Yellowing from the edges → often potassium.
3. Consider Weather and Time of Season
- Cold weather itself can cause purple leaf tint and growth slowdown—this is not always phosphorus deficiency.
- Natural yellowing of lower leaves at the end of the season is normal, not nitrogen deficiency.
- Drought intensifies potassium deficiency symptoms because potassium regulates water balance.
"Potassium deficiency symptoms can be confused with moisture deficiency or diseases. If you see 'scorch' on leaf edges with normal watering—likely potassium deficiency" (Naumann & Pawelzik, 2023).
4. Compare Plants in the Field
Problems often don't start uniformly across the entire area but on specific spots:
- On light sandy soils—symptoms appear faster due to leaching.
- On compacted areas—roots work less efficiently.
- After rain—nitrogen leaching may appear.
Visual Diagnosis: From Symptom to Solution
Below is a detailed reference for each element with symptom descriptions, distinguishing features, and recommended actions.
Nitrogen (N) — "Pallor and Yellowing from Below"
| Feature | Description |
|---|---|
| Where it appears | On old (lower) leaves |
| Primary symptom | Leaves become pale green, then uniformly yellow |
| Pattern | Yellowing moves from lower leaves upward |
| Additional signs | Growth slowed, stems thin, tubers small |
"Signs of nitrogen deficiency usually appear first on old leaves, which turn yellow. General yellowing moves upward through the plant" (Dean, 1994).
What to do: urgent nitrogen application in nitrate form (calcium nitrate, 10–15 g per 1 m²). Repeat after 7–10 days if needed.
What not to do: do not apply large nitrogen doses at the end of the season—this will worsen storage quality.
Phosphorus (P) — "Purple" and "Frozen" Growth
| Feature | Description |
|---|---|
| Where it appears | On all leaves, but more strongly on old ones |
| Primary symptom | Leaves become dark green with a purple or bluish tint |
| Pattern | Tint appears on the underside and along veins |
| Additional signs | Plants stunted, stems stiff, roots weak |
"Under phosphorus deficiency, leaves become dark green and may develop a purple tint. Leaf growth may be reduced, leaves and stems are stiff and upright" (Dean, 1994).
Important: purple tint can also appear with cold (even with sufficient phosphorus). The diagnosis of "phosphorus deficiency" is made if the symptom persists in warm weather (above +15 °C).
What to do: phosphorus is low-mobility; it must be applied to the root zone. In mid-season, this is difficult—better to apply liquid phosphorus fertilizer with irrigation (but efficiency will be lower than at planting). For next season—be sure to apply phosphorus in rows at planting.
Potassium (K) — "Marginal Scorch" and Bronzing
| Feature | Description |
|---|---|
| Where it appears | On old (lower) leaves |
| Primary symptom | Yellowing and dieback of leaf edges ("marginal scorch"), then between veins |
| Pattern | Leaf edges turn brown, as if burned |
| Additional signs | Leaves wrinkled, bronze tint, weak turgor |
"Under potassium deficiency, foliage becomes dark green and bronzing may develop, starting from the tips and edges of leaves. Foliage may be wrinkled and veins sunken" (Dean, 1994).
What to do: apply potassium sulfate (10–15 g per 1 m²). For severe deficiency—foliar application with 0.5–1% potassium sulfate solution (50–100 g per 10 L water).
Calcium (Ca) — Brittleness and Internal Defects
| Feature | Description |
|---|---|
| Where it appears | On young (upper) leaves and growing points |
| Primary symptom | Light border along the edges of young leaves, then edge dieback |
| Pattern | Leaves curl, become brittle, growing point dies |
| Additional signs | Tuber skin thin, internal brown spots, hollow heart |
"Calcium deficiency appears first on young apical leaves. They may have a light green border along the edges, which then becomes necrotic, causing them to shrivel or curl" (Dean, 1994).
What to do: calcium nitrate (soil application 10–15 g per 1 m²) or foliar application with calcium chelate. Liming acidic soils in autumn is the best prevention.
Magnesium (Mg) — Interveinal Chlorosis on Old Leaves
| Feature | Description |
|---|---|
| Where it appears | On old (lower) leaves |
| Primary symptom | Interveinal chlorosis — tissue between veins yellows, veins remain green |
| Pattern | Yellowing starts from edges and tips, spreads toward the center |
| Additional signs | With severe deficiency, leaves brown and die |
"Magnesium deficiency first appears on lower leaves. They become chlorotic at the tips and edges, then full interveinal chlorosis develops" (Dean, 1994).
What to do: foliar application with 1–2% magnesium sulfate solution (100–200 g per 10 L water). For severe deficiency—repeat after 7–10 days. To soil — magnesium sulfate (20–40 g per 1 m²) or dolomitic flour (for liming).
Iron (Fe) — Chlorosis on Young Leaves (Veins Green)
| Feature | Description |
|---|---|
| Where it appears | On young (upper) leaves |
| Primary symptom | Interveinal chlorosis — tissue between veins yellows or whitens, veins remain bright green |
| Pattern | Contrast between green veins and pale tissue is very sharp |
| Additional signs | With severe deficiency, leaves become almost white |
"The primary symptom of iron deficiency is loss of color (chlorosis) between veins. Veins and leaf edges may remain green, while interveinal tissue becomes almost white" (Dean, 1994).
What to do: foliar application with iron chelate (per instructions). Soil application of iron sulfate is largely ineffective on alkaline soils. For prevention—maintain pH no higher than 6.5–7.0.
Zinc (Zn) — Small Deformed Leaves
| Feature | Description |
|---|---|
| Where it appears | On young (upper) leaves |
| Primary symptom | Young leaves small, deformed, often with chlorosis |
| Pattern | Internodes shorten, leaves grow in rosettes |
| Additional signs | Necrotic spots may appear on leaves |
"Under zinc deficiency, lower leaves may be chlorotic, and young leaves are reduced in size. Internodes shorten, and irregular necrotic spots appear on young leaves" (Dean, 1994).
What to do: foliar application with zinc chelate (per instructions). On alkaline soils, soil application is less effective. Check pH—if above 7.0, it needs to be reduced.
Boron (B) — Growing Point Death
| Feature | Description |
|---|---|
| Where it appears | On young (upper) leaves and growing points |
| Primary symptom | Growing point death — stem tip browns and dries |
| Pattern | Young leaves pale, curled, brittle |
| Additional signs | Flowering weak, ovaries drop, tubers with cracks |
"Young leaves and apical growth are the first to show boron deficiency effects. Apical leaves may be lighter, and the tip may brown and die" (Dean, 1994).
What to do: foliar application with 0.1–0.2% boric acid solution (1–2 g per 1 L water). Spraying at budding stage is the best prevention.
Warning: boron excess is toxic! Never exceed recommended dosages.
Manganese (Mn) — Chlorosis with Brown Speckling
| Feature | Description |
|---|---|
| Where it appears | On young (upper) leaves |
| Primary symptom | Interveinal chlorosis with brown spots and speckling between veins |
| Pattern | Similar to iron deficiency but with noticeable brown necrosis |
| Additional signs | On acidic soils—manganese excess (toxicity) |
"Manganese deficiency can look similar to iron deficiency. Similar chlorosis develops, but brown spots also appear, which may be extensive" (Dean, 1994).
What to do: foliar application with 0.05–0.1% manganese sulfate solution (5–10 g per 10 L water). On alkaline soils, manganese is unavailable—chelated forms are needed.
Quick Reference: Primary Diagnosis Table
| Symptom | Primary Deficiency | Secondary Candidate |
|---|---|---|
| Lower leaves uniformly yellow | Nitrogen | Sulfur (rare) |
| Lower leaves purple or dark green | Phosphorus | Cold, stress |
| Edges of lower leaves "scorched," bronze | Potassium | Drought |
| Yellowing between veins on old leaves | Magnesium | — |
| Yellowing between veins on young leaves | Iron | Manganese, zinc |
| Young leaves small, deformed | Zinc | Boron (if dying) |
| Growing point death, brittle leaves | Boron | Calcium |
| Brown spots between veins on young leaves | Manganese | — |
| Thin skin, internal spots in tubers | Calcium | — |
How to Distinguish Nutrient Deficiency from Other Problems
Not all changes in the plant are related to nutrition. Here are some common diagnostic errors:
Magnesium Deficiency vs Iron Chlorosis
- Magnesium: chlorosis on old leaves.
- Iron: chlorosis on young leaves (top).
Phosphorus Deficiency vs Cold Stress
- Cold: purple tint disappears when temperatures warm.
- Phosphorus deficiency: purple tint persists in warm weather.
Potassium Deficiency vs Verticillium Wilt
- Potassium: marginal scorch, yellowing starts at edges.
- Verticillium: yellowing between veins, often one-sided, accompanied by wilting.
Nitrogen Deficiency vs Natural Maturation
- Nitrogen deficiency: yellowing begins in mid-summer, foliage pale, growth slowed.
- Natural maturation: yellowing begins at the end of the season (3–4 weeks before harvest), foliage yellows evenly from top to bottom.
"Sulfur deficiency symptoms are similar to nitrogen starvation, except that they appear first on young leaves. General yellowing goes from top to bottom, without necrotic changes" (Dean, 1994).
Practical Steps When a Problem Is Found
Step 1. Inspect the Plant
- Which leaves are affected — old or young?
- What is the pattern of yellowing — uniform or patchy?
- Is there edge dieback or spots?
Step 2. Assess the Conditions
- What was the weather in the last 2 weeks? (cold, drought, rain)
- Was there overwatering or drying out?
- What is the soil type on this spot? (sand, loam)
Step 3. Recall What and When Was Applied
- Did you apply phosphorus at planting?
- When was the last nitrogen application?
- Did you apply potassium and magnesium?
Step 4. Make a Decision
- If deficiency is clear and the season allows — urgent application (preferably foliar for quick effect).
- If symptoms are mild — wait 5–7 days and observe.
- If the season is nearing its end — fertilizing is no longer needed, especially nitrogen.
Prevention: Better to Prevent Than to Cure
The most effective way to avoid deficiencies is planned application of all elements at the right times.
Before Planting
- Get a soil analysis (at least for pH and major elements).
- Apply phosphorus and potassium in rows at planting (this is the foundation).
- On acidic soils—apply liming in autumn.
During the Season
- Keep a fertilizing calendar—record what and when was applied.
- Inspect plants once a week (especially during active growth and bulking stages).
- At the first signs—act immediately, don't wait for the problem to worsen.
At the End of the Season
- Assess which elements were deficient this year.
- Adjust your fertilizer plan for next season (e.g., increase potassium rate or add magnesium).
"The effectiveness of nutrient management is determined by careful planning based on soil analysis, plant observation, and adjusting the program according to the real situation in the field" (Dean, 1994).
Conclusion: Seven Key Takeaways on Potato Nutrition
We've traveled from how potatoes form their yield to practical deficiency diagnosis. Here are seven key lessons to remember:
1. Potato yield is the work of the foliage
Without healthy, powerful leaf surface, there will be no tubers. Healthy foliage requires a balanced set of nutrients, not just nitrogen.
2. Balance is more important than quantity
Overfeeding nitrogen, potassium deficiency, ignoring micronutrients—these are typical mistakes. It is better to apply moderately but precisely according to growth stages.
3. Potassium is the main element for tubers
Potassium should be at least equal to nitrogen, often more. And definitely in sulfate form, chlorine-free.
4. Phosphorus — only at the start, only in rows
Phosphorus is low-mobility. Broadcasting it is a waste of fertilizer. In rows at planting is the only reliable method.
5. Calcium and magnesium — quality and storage
Without calcium—thin skin and internal defects. Without magnesium—weak photosynthesis. Their deficiency is often underestimated, but it shouldn't be.
6. Micronutrients solve quality problems
Boron at budding, iron and zinc for chlorosis—these techniques can dramatically improve tuber quality at minimal cost.
7. Look at the plant, not at the table
The most accurate diagnosis is observation of the foliage. Every leaf is an indicator of the plant's condition. Learn to "read" it—and your harvest will be consistently high and high-quality.
Quick Reference Card for the Garden
STEP 1: Which leaves have the problem?
- Old (lower) → N, P, K, Mg
- Young (upper) → Ca, Fe, Zn, B, Mn
STEP 2: What does the symptom look like?
- Uniform yellowing → N
- Dark green/purple → P
- Yellow/brown edges ("scorch") → K
- Yellowing between veins (old leaves) → Mg
- Yellowing between veins (young leaves) → Fe
- Small deformed leaves → Zn
- Growing point death → B
- Brown spots between veins → Mn
STEP 3: What to do?
- Nitrogen, potassium, magnesium → foliar application + to soil
- Phosphorus → only to soil (in rows at planting)
- Micronutrients → foliar (chelates or sulfates at low concentration)
References
- Dean, B.B. (1993). ‘Cultivation, Fertilization, and Irrigation’, in Managing the Potato Production System. New York: Routledge, pp. 69-84.
- Jong, H.De., Sieczka, J.B., Jong, W.De. (2011). ‘Soils and Fertility’, in The Complete Book of Potatoes. What Every Grower and Gardener Needs to Know. Portland, London: Timber Press, pp. 72-76.
- Kemble, J.M., Bertucci, M.B., Jennings, K.M., Meadows, I.M., Rodrigues, C., Walgenbach, J.F., Wszelaki, A.L. (2022). Southeast U.S. Vegetable Crop Handbook. 23rd edition : Great American Media Services.
- Naumann, M., Pawelzik, E. (2023). ‘Nutrient management in potato’, in Potato Production Worldwide. : Elsevier, 101-120.
- Pavek, M.J. (2014). ‘Commercial potato production and cultural management.’, in The potato: botany, production and uses. Wallingford: CABI, 83-102.
- Swiader, J.M., Ware, G.W., McCollum, J.P. (1992). ‘Potatoes’, in Producing Vegetable Crops. Danville, Illinois: Interstate Publishers, pp. 435-458.
- Welbaum, G.E. (2015). ‘Family Solanaceae’, in Vegetable production and practices. Boston, MA: CABI, ch. 11.
