Kiwifruit nutrition: what it takes up when, and why calcium is decided in fifty days
Savvas Pastopoulos, Agronomist MSc — Pastopoulos Agricultural Ltd, Neos Mylotopos, Pella
Kiwifruit takes up more calcium than any other element, but almost all of it ends up in the leaves. Very little reaches the fruit, and it does so within a narrow window of about fifty days after fruit set. Whatever is done afterwards does not change it. This article shows how many kilograms of each element actually leave with the harvest, when the vine asks for them, why excess nitrogen costs storage life, and at which point a recent 2025 study contradicts some of the traditional recommendations.
A kiwifruit vine accumulates around 190 kg of calcium per hectare in the canopy, but only 8 kg leave with the fruit. Adding calcium to the soil late in the season does not reach the fruit, because the route has already closed. Fruit calcium is not a question of quantity in the soil but a question of timing and transpiration.
1. How much is taken up and where it ends up
Kiwifruit is among the most demanding species in terms of nutrients. In a mature ‘Hayward’ block on pergola, the canopy — leaves, fruit and shoots together — accumulates the following quantities in one year [1].
| Element | Canopy (kg/ha) | Where it ends up |
|---|---|---|
| Calcium (Ca) | 190 | Between 85 and 93% stays in the leaves. The fruit takes very little. |
| Potassium (K) | 174 | Mobile. About half ends up in the fruit. |
| Nitrogen (N) | 110 | Mobile, also redistributed to the reserves. |
| Magnesium (Mg) | about 45 | Value read off a graph, indicative. |
| Phosphorus (P) | about 8 | The smallest quantity, but the fruit takes a large share of it. |
Values for Actinidia deliciosa ‘Hayward’, pergola, yield around 35 tonnes per hectare. The last two rows are read off a graph and are given as an order of magnitude.
An earlier study on five-year-old ‘Hayward’ vines measured annual uptake by the whole plant: nitrogen 141, potassium 169, calcium 161, sulphur 32, magnesium 28 and phosphorus 19 kg per hectare [2]. The two sets do not coincide because they do not measure the same thing — one the canopy, the other the whole plant — but they agree on the essential point: calcium, potassium and nitrogen are the three large quantities, with phosphorus and magnesium far below.
2. What actually leaves with the harvest
Accumulation in the canopy is not the same as loss. The leaves fall and the prunings return to the soil; whatever is inside them is recycled. The only thing lost for good is what is loaded onto the truck. At a yield of around 25 tonnes per hectare, the fruit removes [1]:
| Element | kg/ha | Comment |
|---|---|---|
| Potassium (K) | 81 | The element removed in the largest quantity. |
| Nitrogen (N) | 46 | Far less than is usually applied. |
| Calcium (Ca) | 8 | Of the 190 kg in the canopy, only eight leave. |
| Magnesium, phosphorus and sulphur | 4 to 6 | Small quantities, each within this range. |
These values are indicative and do not constitute a fertiliser recommendation.
The conversion with the most practical value is per tonne of fruit, because it transfers to any yield level:
81 kg K ÷ 25 tonnes = 3.2 kg potassium per tonne of fruit46 kg N ÷ 25 tonnes = 1.8 kg nitrogen per tonne of fruit8 kg Ca ÷ 25 tonnes = 0.3 kg calcium per tonne of fruit
With these coefficients, an orchard yielding 35 tonnes per hectare loses about 112 kg of potassium and 63 kg of nitrogen per hectare a year. The rest of the fertiliser does not go to the fruit: it goes to the canopy, to the vine's reserves, or it is lost.
Sources: 1
3. The course of the season
Accumulation is not uniform. Nitrogen and potassium rise almost linearly until about 120 days after full bloom and slow down around day 140. Calcium rises steadily throughout the season, but — and this is the critical point — almost all of that increase is in the leaves, not in the fruit [1].
For the mobile elements, most of the demand has already been expressed by fruit set. The crop's nutrition manual places more than 80% of the accumulation of potassium, nitrogen, zinc and copper before that point [3]. In practice, a fertiliser programme that starts late has already missed most of the window.
The weight of the fertiliser programme falls between bud break and about two months after bloom. Late fertilisation has a place only for building reserves in the wood and roots, not for feeding the fruit — and, as section 6 shows, late nitrogen also has a cost.
4. Calcium and the 50-day window
Calcium behaves entirely differently from the other elements, and the reason is anatomical. The calcium ion has almost no mobility in the phloem. It moves towards the fruit only through the xylem, carried by the water flow that transpiration pulls and root pressure pushes [1].
The young fruit transpires, because its skin has not yet hardened. As hardening proceeds, surface conductance falls and hydraulic resistance rises after about day 60. From that point on, calcium flow into the fruit essentially stops.
| Period | What happens to fruit calcium |
|---|---|
| Up to ~50 days from fruit set | About 80% of the fruit's total calcium enters. The mass per fruit peaks around day 55. |
| 50 to 80 days | The inflow slows as the skin hardens and transpiration falls. |
| 80 days to harvest | Minimal addition. The concentration in fact falls, because the fruit keeps growing while the calcium does not increase. |
For comparison, in the mobile potassium and nitrogen the corresponding proportion entering during Stage I is only 37 to 55% [1]. Those keep being supplied all season; calcium does not.
In the leaves the calcium concentration rises throughout the season, because the leaf transpires continuously and strongly. That is why they hold 85 to 93% of the canopy's calcium. The fruit is not a competitive sink for calcium — it is the weak recipient of a flow controlled elsewhere.
Sources: 1
5. What controls fruit calcium — and what does not
Because calcium reaches the fruit only with the water flow, anything that affects transpiration affects it too. The 2014 review records the following factors [1].
| Factor | Mechanism and practical significance |
|---|---|
| Light in the fruiting zone | Promotes xylem development in the pedicel and in the fruit. Fruit in well-lit positions measured 40 to 80% higher in calcium. Summer pruning that opens the canopy helps. |
| Dryness of the air | The principal driver of fruit transpiration. In dry air the fruit drew in more calcium, about 0.21% of dry matter against 0.17% in humid air. |
| Air movement in the canopy | Thins the boundary layer around the fruit and raises transpiration, but only in the young fruit. After the skin hardens there is no response. |
| Potassium | Competes with calcium in root uptake and at the membrane, and accelerates fruit expansion, diluting it. |
| Irrigation | Drought closes the stomata and cuts the flow; over-irrigation dilutes the soil solution. Steady, moderate moisture is wanted in the early stage. |
| pH and an active root system | Calcium is taken up as a soluble ion and its availability depends on pH. The surface layer of the soil is the richest, so the active roots near the surface are what count. |
An experimental study on a green-fleshed cultivar on calcareous soil tested precisely these levers. The result was partly negative: the distribution of calcium in the fruit was affected neither by applications of calcium, potassium and nitrogen, nor by increased light [4].
The central point was confirmed, however: reducing fruit transpiration reduced its calcium. And something emerged that softens the strict picture of the 50-day window — when transpiration was restored, the fruit was able to regain calcium even close to harvest.
The practical conclusion changes in tone: the mechanism is transpiration, not the amount of fertiliser. A programme that targets fruit calcium with more kilograms of calcium in the soil will most probably see no difference; a programme that looks after transpiration might.
6. Nitrogen and storage life
Here the circle closes, and here lies the most directly applicable finding. Excess nitrogen does not merely deprive the fruit of calcium: it measurably shortens storage life.
The mechanism is indirect. High nitrogen gives vigorous shoots, vigorous shoots give more leaf area, and leaf transpiration pulls the xylem stream — and the calcium with it — towards the leaves instead of the fruit. In apple, nitrogen up to 150 kg per hectare gave 30% lower fruit calcium [1].
| Study | Finding |
|---|---|
| Pacheco and co-workers (2008) | Raising nitrogen from 30 to 90 kg per hectare raised fruit nitrogen from 165 to 190 mg per 100 g and gave about 10% lower firmness. Both nitrogen and potassium reduced flesh firmness [5]. |
| Johnson and co-workers (1997) | Heavy nitrogen fertilisation raised leaf nitrogen to 3% against 1.6% in the control. After six months in storage, the unfertilised fruit were twice as firm [6]. |
Two further points from the same literature. First, the nitrogen-to-calcium ratio in sound fruit runs around 5.1 — what counts is the balance, not each element separately. Second, of all the nutritional disorders examined in the crop's manual, the only one that seriously harmed storage was boron toxicity, which dropped firmness from 5 to 2 and caused premature ripening [3].
Late nitrogen fertilisation to build reserves is legitimate practice. The point is that the nitrogen should not end up in the fruit close to harvest. Because demand peaks early, the end of the season is not the time for nitrogen that will be loaded into the fruit — that is how both the calcium and the storage life are protected.
7. The yellow-fleshed cultivars
All of the above concerns green-fleshed ‘Hayward’. As the yellow-fleshed cultivars expand, a more recent three-year study on a six-year-old Zesy002 block in northern Italy is worth attention; it measured annual uptake per vine [7].
| Element | grams per vine | Recycling |
|---|---|---|
| Calcium (Ca) | 235 | More returns to the soil than is removed. |
| Nitrogen (N) | 175 | About half removed, half recycled. |
| Potassium (K) | 138 | About half removed, half recycled. |
| Magnesium (Mg) | 48 | More returns than leaves. |
| Sulphur (S) | 17 | About half and half. |
| Phosphorus (P) | 16 | About half and half. |
The study gives its values per vine, so conversion to a per-hectare figure depends on the planting density of the particular block. For 4 × 4 metre spacing:
4 m × 4 m = 16 m² per vine10,000 m² ÷ 16 m² = 625 vines per hectare175 g N × 625 = 109,375 g ≈ 109 kg of nitrogen per hectare
Indicative calculation for a density of 625 vines per hectare. At any other density the result changes proportionally.
The 109 kg of nitrogen per hectare is total uptake, of which about half returns to the soil in the leaves and prunings. That leaves around 55 kg per hectare genuinely removed. The measurement on green-fleshed ‘Hayward’ gave 46 kg per hectare in the fruit. Two different cultivars, two different countries, two different methods — and the same order of magnitude. That is a solid basis on which to judge whether a fertiliser programme is excessive.
8. Practical conclusions
What to take away
- The weight of the fertiliser programme falls early. More than 80% of the accumulation of potassium, nitrogen, zinc and copper has taken place by fruit set [3].
- Fruit calcium is decided in the first 50 days or so after fruit set, while the skin still transpires. Anything aimed at fruit calcium is meaningful only within that interval [1].
- Fruit calcium is not raised by more calcium in the soil. The 2025 trial found no effect from applications of calcium, potassium or nitrogen, or from increased light. The lever is transpiration [4].
- Nitrogen in moderation. The fruit removes around 46 kg per hectare. Excess diverts calcium to the leaves and reduces firmness and storage life [5] [6].
- Care with potassium after bloom. It competes with calcium uptake at exactly the time when calcium is accumulating in the fruit.
- Boron is the element with the narrowest margin. It is the only nutritional disorder that seriously harmed storage in the crop's manual [3].
- Good pollination. A high seed number per fruit is associated with greater calcium accumulation [1].
- Summer pruning and irrigation. Fewer vigorous shoots, more light in the fruiting zone, and steady moderate moisture in the early stage.
It is meaningful only before about day 50 from fruit set, while the skin is still permeable. Later, the fruit is effectively closed.
Spraying vines carrying fruit can cause marking of the fruit and scorching of the foliage. A prior test on a limited number of vines is required, with the same product and the same dose, and a wait of 5 to 7 days before general application. Spraying is done in the late afternoon or early morning, never in extreme heat and never on foliage under water stress.
A nutrition programme does not follow from these figures but from soil and leaf analysis of the particular holding. The values here give the order of magnitude and, above all, the timing. For the physiology of the fruit itself there is a separate article on the blog, The physiology of kiwifruit.
9. Sources
The numbers in the text refer to the list below.
- [1] Montanaro, G., Dichio, B., Lang, A., Mininni, A. N., Nuzzo, V., Clearwater, M. J. and Xiloyannis, C. (2014). Internal versus external control of calcium nutrition in kiwifruit. Journal of Plant Nutrition and Soil Science 177(6):819–830. Source for canopy accumulation (Ca 190, K 174, N 110 kg/ha), for removal in the fruit, for the 80% of calcium entering by day 50, for the 37–55% of K and N in Stage I, for the 85% of calcium in the leaves, for the factors of light, air dryness, air movement, potassium and irrigation, and for the 30% lower calcium in apple.
- [2] Buwalda, J. G. and Smith, G. S. (1987). Accumulation and partitioning of dry matter and mineral nutrients in developing kiwifruit vines. Tree Physiology 3(4):295–307. Source for the annual whole-plant uptake in five-year-old ‘Hayward’: N 141, K 169, Ca 161, S 32, Mg 28 and P 19 kg per hectare, and for the estimate that the leaves hold 93% of the calcium.
- [3] Smith, G. S., Asher, C. J. and Clark, C. J. (1987). Kiwifruit Nutrition: Diagnosis of Nutritional Disorders, 2nd revised edition, Agpress Communications, Wellington. Source for the timing of accumulation (more than 80% of K, N, Zn and Cu by fruit set) and for the finding that boron toxicity was the only nutritional disorder that seriously harmed storage, dropping firmness from 5 to 2.
- [4] Baldi, E., Toselli, M., Bonora, A., Boini, A., Quartieri, M., Germani, M., Polidori, G. and Corelli Grappadelli, L. (2025). Agronomic strategies to manipulate kiwifruit calcium content to understand its role in fruit physiology. Horticulturae 11(3):237. Source for the finding that calcium distribution in the fruit was not affected by applications of calcium, potassium and nitrogen or by increased light, that reducing fruit transpiration reduces its calcium, and that accumulation can recover when transpiration is restored, even close to harvest.
- [5] Pacheco, C. and co-workers (2008). Influence of nitrogen and potassium on yield, fruit quality and mineral composition of kiwifruit. Source for the rise in fruit nitrogen from 165 to 190 mg per 100 g with nitrogen from 30 to 90 kg per hectare, for the roughly 10% lower firmness, and for the finding that both nitrogen and potassium reduced flesh firmness.
- [6] Johnson, R. S. and co-workers (1997). Nitrogen effects on kiwifruit storage quality. Acta Horticulturae 444:285–289. Source for leaf nitrogen of 3% against 1.6% and for the finding that after six months in storage the unfertilised fruit were twice as firm.
- [7] Seasonal uptake and partitioning of macro- and micronutrients in yellow-fleshed kiwifruit (Actinidia chinensis var. chinensis) (2025). Horticulturae 11(9):1003. Source for the annual uptake per vine in cultivar Zesy002 (N 175, Ca 235, K 138, Mg 48, S 17 and P 16 grams) and for the split between the fraction removed and the fraction returned to the soil.
This text is general technical information and not individual agronomic advice. The values, timings and ratios come from published studies under specific conditions of cultivar, rootstock, soil, climate and yield target; they are indicative and do not constitute a fertiliser recommendation. A nutrition programme is set after soil and leaf analysis and an on-site assessment by a licensed agronomist.
Every fertiliser or soil amendment is used in accordance with its marketing authorisation and its label; in Greece the authorisations are checked in the fertiliser register of the Ministry of Rural Development and Food. Plant protection products are used solely on the basis of the national authorisations and the label; in Greece these are checked at fyto.plantprotect.gr.
Foliar sprays: spraying vines carrying fruit can cause marking of the fruit and scorching of the foliage. A prior test on a limited number of vines is required, with the same product and the same dose, and a wait of 5 to 7 days before general application. Spraying is done in the late afternoon or early morning, never in extreme heat and never on foliage under water stress.
Every intervention is carried out at the user's own responsibility. Pastopoulos Agricultural Ltd accepts no liability for the use of the information in this text.
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