Olive nutrition: what actually leaves the grove

Savvas Pastopoulos, Agronomist MSc, Pastopoulos Geoponiki L.P., Neos Mylotopos, Pella

The olive removes far fewer nutrients from the field than established practice assumes. Long-term studies in Spain measured exactly how much leaves with the fruit and with the prunings, and showed that in rainfed groves the element most often short is potassium. Nitrogen, applied in the largest quantities, is usually unnecessary, and in excess it degrades the oil. What follows are the real annual removals in kilos per hectare, the limits of leaf analysis, and what actually causes alternate bearing.

1. How many nutrients actually leave the grove

The only quantity that must in every case be returned to the soil is the quantity taken out of it. For the olive, that quantity was measured in two long-term studies in Andalusia, on rainfed trees of the cultivar Picual, with nutrients determined separately in the fruit and in the whole of the pruning material [1].

The result overturns the common view. At a mean yield of 8.2 tonnes per hectare, the annual removals were:

Values from the experimental site at Cabra, Córdoba, a seven-year mean.

The composition of the fruit

Separately from the prunings, the fruit itself contains, per tonne of fresh weight [1]:

More than half of the mineral content of the olive is potassium. That alone explains why potassium is the element exhausted first in a productive grove, and why it was the only one that had to be applied over the seven years of the Spanish study.

Sources: 1 13

2. Annual requirements in kilos per hectare

The values above are on an element basis. Fertilisers, however, are sold as oxides. The conversion:

So for a yield of 8.2 tonnes of fruit per hectare and with the prunings removed from the field, the annual removal in fertiliser units is:

The values are indicative and refer to removal, not to a fertiliser recommendation. The final rate is set after leaf and soil analysis, and after on-site assessment by an agronomist.

Checking the arithmetic

The values in the table check internally. Fruit potassium is 4.42 kg per tonne; times 8.2 tonnes per hectare gives 36.2 kg per hectare. Total potassium removal is 45.5, so the prunings contribute 9.3 kg, consistent with the finding that potassium leaves mainly with the fruit. Conversely, fruit calcium gives only 2.4 kg per hectare out of 57.9 in total, that is, 96% of the calcium leaves with the branches [1]. The two independent tables in the paper agree on every element.

Sources: 1

3. What changes if the prunings stay in the field

Here lies the largest practical difference, and it is often ignored. If the prunings are chipped and left in the grove, their whole share drops out of the calculation. Real removal is then limited to what left in the crates:

Calculated for 8.2 tonnes of fruit per hectare, from the fruit composition in section 1.

Sources: 1 2

4. Leaf analysis and its limits

Leaf analysis is the only reliable method of diagnosing the nutritional status of the tree. Soil analysis is not accurate enough for the olive and serves mainly for pH, salinity and calcium carbonate [3].

Sampling

• Timing: July, or at least five to eight weeks after full bloom. Concentrations of most elements stabilise in the leaf at that point.
• Position: fully expanded, healthy leaves from the middle towards the base of current-season shoots that carry no fruit. This point matters: a fruiting shoot gives entirely different values.
• Number: about 100 leaves per sample, from 20 to 25 trees representing a uniform block of up to 10 hectares.
• Handling: washing, drying and dispatch to a laboratory. Leaves carrying spray residue give falsely high copper, zinc and manganese.

Reference values for a July sample

Reference values for July samples, after the University of California [3], in agreement with the values used in the Spanish studies [1]. The toxicity column comes from a Californian compilation and has not been experimentally validated, as explained in section 7.

The critical finding on nitrogen

The 1.40% nitrogen deficiency threshold comes from work published in 1983. Later long-term measurements showed that no reduction in yield or growth was recorded in trees with lower concentrations, which moves the real deficiency limit for mature trees below 1.20% [4]. That is, a proportion of the analyses read today as "nitrogen deficiency" are in fact nothing of the kind.

Sources: 1 3 4 7

5. Nitrogen, the element applied most and costing most

Over the seven years of the Spanish studies, the control trees received no nitrogen at all, because leaf analysis never indicated a need. Leaf nitrogen concentration stayed above the deficiency limit throughout, and the nitrogen balance of the grove was found to be positive: inputs from mineralisation of organic matter, from rainfall and from irrigation water more than covered the outputs [4] [6].

What over-fertilising with nitrogen costs

• Oil quality. Over-fertilising with nitrogen reduces polyphenols and hence the oxidative stability of the oil, raises free acidity and lowers oleic acid [5]. Polyphenols are the principal antioxidant of olive oil and the basis of every quality claim made for it.
• Oil biosynthesis. Recent work showed that elevated nitrogen within the fruit directly impairs oil biosynthesis [11]. The effect, in other words, is not only indirect through vegetative growth.
• Flower quality. Nitrogen status affects flower quality and ovule longevity [8], and therefore fruit set.
• Growth at the expense of cropping. Very fertile conditions cause excessive vegetative growth and weaker flowering and fruit set [3].
• Environment. Nitrate leaching to groundwater, an issue with a regulatory dimension in vulnerable zones.

How much nitrogen, when it is needed

When analysis shows a real deficiency, American recommendations for mature trees run from 40 to 100 pounds of elemental nitrogen per acre per year [3]. The conversion gives:

That is, 45 to 112 kg of nitrogen per hectare. The lower end coincides almost exactly with the removal figure of 54.4 kg. The upper end is twice the removal and is justified only in young plantings building a framework, or on demonstrated deficiency.

Timing in Macedonia

Bud differentiation takes place in March and April, provided sufficient winter chilling has occurred. Base fertilisation should not be applied too early, so that frost susceptibility is not increased. At soil temperatures below 15 °C nitrogen uptake is limited and the risk of nitrate leaching is high, so in base-dressing fertilisers the proportion of nitrate nitrogen is kept as low as possible [13].

Sources: 3 4 5 6 8 11 13

6. Potassium, deficiencies are frequent in rainfed fields

Over the seven years of the Spanish studies, potassium was the only element that had to be applied. Leaf values sat almost every year below the sufficiency limit of 0.80% but above the deficiency limit of 0.40% [1].

The reason is twofold. First, more than half the mineral content of the fruit is potassium, so every bearing year empties the store. Second, on calcareous soils potassium is fixed and becomes hard to access, which is why deficiency appears even on soils with high total potassium [9].

How it is corrected

The rates refer to potassium sulphate containing 50% K₂O and are indicative. The conversion from the American recommendation: 500 pounds per acre equals 560 kg per hectare [3].

Potassium and drought

Potassium regulates stomatal aperture and cell turgor. In groves with low rainfall or recurring drought, potassium sufficiency is associated with better maintenance of turgor and greater resistance to water stress [12]. In rainfed conditions, then, potassium is not only a question of yield, it is a question of surviving the season.

Sources: 1 3 9 12 17

7. Boron and fruit set

Boron is removed in a minimal quantity, 0.11 kg per hectare per year [1], and yet it is the micronutrient of greatest practical importance in the olive. The reason is its part in pollen tube germination and in flower fertility.

Deficiency symptoms

• Deformed fruit, with the characteristic appearance described as "monkey face"
• Short shoots and dieback of tips
• Rough bark
• Leaves with a yellow band between a brown tip and a green base

The importance of foliar boron

Applying boron before bloom has significantly improved fruit set even on trees with no visible symptoms, at low but not deficient leaf boron values [3] [10]. That is, the band between 14 and 19 ppm, typically read as "marginal", is in practice a band of lost yield.

Rates

Converted from 25 to 50 pounds per acre [3]. The values are indicative; the final rate is set after leaf analysis and on-site assessment by an agronomist.

Sources: 1 3 10 18 19 20 21

8. Calcium, magnesium and micronutrients

Calcium. It is the element removed in the largest quantity, but almost entirely with the prunings. On the calcareous soils of the Mediterranean, calcium deficiency is rare and appears only on acid or non-calcareous soils [1]. On the Giannitsa plain, with pH usually above 7.5, calcium is not a limiting factor.

Magnesium. It is removed at fourteen times less than the major elements. A problem appears mainly as antagonism, when potassium fertilisation is excessive and displaces magnesium at the root.

Micronutrients. Copper, manganese and zinc are removed in quantities 500 to 1,000 times smaller than the macronutrients [1]. They are a problem only when they are fixed in the soil, not when they are genuinely absent. On calcareous soils with high pH, iron and zinc are fixed and correction is made foliarly or with chelated forms, never by simply raising the soil rate.

Sources: 1

9. Alternate bearing, what actually causes it

Alternate bearing is the alternation of a year of heavy production with a year of little or none. The olive is among the species with the strongest tendency, and the phenomenon governs the economics of a grove more than any other factor.

The old explanation and the current one

The widespread explanation attributes the phenomenon to exhaustion: the tree spends its reserves in the bearing year and has nothing to give the next. That explanation is not wrong, but it is secondary.

The current picture places the main weight on hormonal inhibition of floral differentiation. The seeds inside the developing fruit produce molecular signals, of which the gibberellins are the most studied, which inhibit floral induction for the following year [14]. The mechanism works along two routes: the fruit inhibits vegetative growth, so fewer nodes capable of flowering are created, and at the same time it inhibits floral differentiation itself.

The time window

Two FLOWERING LOCUS T-type genes have been identified in olive, OeFT1 and OeFT2. Their expression in mature leaves rises from December to February, that is, immediately before inflorescence initiation [15]. This means that the decision about the following year's crop is taken during winter, and depends both on chilling and on whether fruit is still on the tree.

Where nutrition comes in

Gene expression analysis in leaves from "on" and "off" years showed that nutrition metabolism has a notable part in alternate bearing, alongside hormonal control and flowering processes [14]. In the "off" year, with little fruit, the tree consumes few carbohydrates and accumulates starch, which feeds the next bearing year.

Nutrition, then, is not the cause, but it is the factor that determines how deep the dip will be. A tree entering an bearing year with an empty potassium store produces smaller fruit and exhausts itself further.

Sources: 14 15

10. What fertilisation can and cannot do about alternate bearing

Sources: 3 5 14

11. A programme for the year that carries a crop

The quantity is calculated from the expected yield of the current year, not the last one. The estimate is made after fruit set, when the crop load is visible.

A year with a crop

• Potassium: the critical requirement. Cover at least the removal, that is, 4.42 kg K per tonne of expected fruit, erring towards the upper end on calcareous soil.
• Nitrogen: only if the previous July's leaf analysis showed a value below 1.40%. Otherwise no addition.
• Boron: foliar before bloom, provided analysis showed a value below 19 ppm, that is, below the sufficiency threshold of the table in section 4.
• Harvest: as early as the market allows.

A year without a crop

• Reduced fertilisation. Without fruit, removal is limited almost entirely to the prunings. The same rate as in the bearing year is waste and feeds vegetative growth.
• Leaf analysis in July. The "off" year is the right one for measuring true nutritional status, without the distortion of crop load.
• Organic matter. The right year for manure or compost, so that it mineralises in good time.
• Renewal pruning and chipping of the prunings on site.

12. The key points

• Annual removals by the olive are limited. 54.4 kg nitrogen, 45.5 potassium and 57.9 calcium per hectare per year, at a yield of 8.2 tonnes [1].
• Over half the mineral content of the fruit is potassium, 4.42 kg per tonne. It is the element exhausted first.
• Chipping the prunings halves the nitrogen requirement and practically eliminates the calcium requirement.
• Nitrogen is usually a problem of excess, not of shortage. Seven years without nitrogen fertilisation, without loss of yield [4].
• Over-fertilising with nitrogen degrades the oil, fewer polyphenols, lower oxidative stability [5].
• Potassium deficiencies are frequent in rainfed groves, particularly on calcareous soils that fix it [9].
• Boron before bloom improves fruit set even without symptoms [3] [10]. The olive is moderately tolerant to boron, and documented toxicity comes from the irrigation water and from excessive soil application, not from a foliar programme [18] [19].
• Alternate bearing is hormonal, not primarily nutritional. The seed in the fruit inhibits the following year's flowering [14]. Fertilisation narrows the swing; it does not remove it.
• Early harvest is the most effective intervention against alternate bearing, and it raises oil quality at the same time.

13. Sources

The numbers in the text refer to the list below.

• [1] Fernández-Escobar, R. et al. (2015). Nutrient Removal from Olive Trees by Fruit Yield and Pruning. HortScience 50(3):474-478. The primary source for all the removal figures: 57.9 kg calcium, 54.4 nitrogen, 45.5 potassium, 6.87 phosphorus, 3.79 magnesium per hectare per year; the fruit composition of 4.42 grams of potassium and 2.87 of nitrogen per kilo of fresh weight; and the finding that potassium was the only element requiring application over seven years.
• [2] Rodrigues, M.A. et al. (2012). Fertilizer recommendations for olive based upon nutrients removed in crop and pruning. Scientia Horticulturae 142:205-211. Source for the methodology of calculating fertilisation from removal and for confirmation of the values in younger plantings.
• [3] University of California Cooperative Extension (2006). Fertility Management for Oil Olives. First Press 1(3):1-2. Source for the table of critical July leaf values, for the sampling procedure of 100 leaves from non-fruiting shoots, for the nitrogen rates of 40-100 pounds per acre, for the corrective potassium and boron rates, and for the observation that very fertile conditions reduce flowering and fruit set.
• [4] Fernández-Escobar, R. et al. (2009). Long-term effects of N fertilization on cropping and growth of olive trees and on N accumulation in soil profile. European Journal of Agronomy 31:223-232. Source for the seven years without nitrogen fertilisation and without loss of yield, and for the challenge to the 1.40% deficiency threshold in favour of a value below 1.20% in mature trees.
• [5] Fernández-Escobar, R. et al. (2006). Olive oil quality decreases with nitrogen over-fertilization. HortScience 41:215-219. Source for the reduction in polyphenols and oxidative stability, the rise in free acidity and the fall in oleic acid under nitrogen over-fertilisation; and for the tenfold increase in fertiliser cost without benefit.
• [6] Fernández-Escobar, R. et al. (2012). An approach to nitrogen balance in olive orchards. Scientia Horticulturae 135:219-226. Source for the positive nitrogen balance of the olive grove, that is, that inputs more than cover outputs even without fertilisation.
• [7] Fernández-Escobar, R. et al. (2009). Foliar diagnosis as a guide to olive fertilization. Spanish Journal of Agricultural Research 7:212-223. Source for the use of annual leaf analysis as a fertilisation guide and for the point that misreading it leads to fertilisation even when nutritional status is adequate.
• [8] Fernández-Escobar, R. et al. (2008). Nitrogen status influence on olive tree flower quality and ovule longevity. Environmental and Experimental Botany 64:113-119. Source for the effect of nitrogen status on flower quality and ovule longevity.
• [9] Restrepo-Díaz, H. et al. (2008). Potassium fertilization of rainfed olive orchards. Scientia Horticulturae 116:399-403. Source for the effectiveness of foliar potassium fertilisation in rainfed groves and for the appearance of potassium deficiency even on soils with high total potassium.
• [10] Effect of foliar boron application on olive fruit set and yield. Acta Horticulturae 868. Source for the improvement in fruit set from foliar boron applied before bloom.
• [11] Elevated fruit nitrogen impairs oil biosynthesis in olive (Olea europaea L.) (2023). Frontiers in Plant Science 14:1180391. Source for the direct impairment of oil biosynthesis by elevated nitrogen within the fruit.
• [12] Olive Nutritional Status and Tolerance to Biotic and Abiotic Stresses (2019). Frontiers in Plant Science 10:1151. Source for the role of nutritional status, and of potassium in particular, in the tolerance of the olive to water stress.
• [13] Therios, I. (2016). Nutrition and fertilizers. Gartaganis Publications, Thessaloniki. In Greek. Greek reference source for an uptake of 15-35 kg nitrogen, 8 phosphorus, up to 50 potassium and 20-50 calcium per hectare, and for the timing of fertilisation in relation to bud differentiation and soil temperature.
• [14] Türktaş, M. et al. (2013). Nutrition Metabolism Plays an Important Role in the Alternate Bearing of the Olive Tree (Olea europaea L.). PLOS ONE 8(3):e59876. Source for the involvement of nutrition metabolism, hormonal control and flowering processes in alternate bearing, and for starch accumulation in the "off" year.
• [15] Expression and function of FT genes in olive (2024). Frontiers in Plant Science 15:1323087. Source for the OeFT1 and OeFT2 genes and for the rise in their expression from December to February, before inflorescence initiation.
• [16] Plant Protection Products Database, Greek Ministry of Rural Development and Food. In Greek. The official source of approvals for every product referred to or implied in this article.
• [17] Mahmoud, T.S. et al. (2017). Influence of Foliar Application with Potassium and Magnesium on Growth, Yield and Oil Quality of Koroneiki Olive Trees. American Journal of Food Technology 12(3):209-220. Source for the application of 3% and 5% potassium sulphate solutions to olive with no foliage damage reported. Full text used. The work was carried out in Egypt and is not a recommendation for Greek conditions.
• [18] Grieve, C.M., Grattan, S.R. and Maas, E.V. Plant Salt Tolerance, chapter 13 of Agricultural Salinity Assessment and Management, ASCE. Source for the classification of the olive among the species moderately tolerant to boron, at 2 to 4 milligrams per litre in the soil solution, against 0.5 to 1.0 for stone fruit, grape and walnut.
• [19] Fernández-Escobar, R. Salinity, Module 2 of the World Catalogue of Olive Varieties, International Olive Council. Source for the olive tolerating 1 to 2 milligrams of boron per litre in the irrigation water and for boron becoming toxic in excess, mainly in young trees.
• [20] Liakopoulos, G. and Karabourniotis, G. (2005). Boron remobilization at low boron supply in olive (Olea europaea) in relation to leaf and phloem mannitol concentrations. Tree Physiology 25(2):157-165. Source for the redistribution of boron from mature leaves to new growth through complexes with mannitol, that is, for the phloem mobility of boron in the olive. The abstract and the principal findings were used.
• [21] Deliboran, A. et al. (2020). Determination of Nutritional Status of Olive Trees Grown in Izmir and Mugla Province in Terms of Boron and The Other Microelements. Journal of Tekirdag Agricultural Faculty 17(3):392-405. Source for leaf boron of 8.66 to 29.33 ppm in commercial groves, with 85% of the samples deficient and none in excess. Full text used.