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Crop Guide: Olive Trees Nutrition

  1. Summary of main plant nutrient functions
  2. The three tools for optimal nutrient management
  3. Seasonal nutrient requirements of olive trees
  4. Main plant nutrients, their rates in olive leaves, deficiency symptoms and application rates and methods
  5. Foliar feeding of nitrogen
  6. Phosphorus (P)
  7. Potassium (K)
  8. Magnesium (Mg)
  9. Sulfur (S)
  10. Calcium (Ca)
  11. Boron (B)
  12. Zinc (Zn)
  13. Iron (Fe)
  14. Manganese (Mn)
  15. Copper (Cu)
  16. Chloride (Cl)
Although the olive tree has relatively modest mineral nutrition requirements, it will respond to fertilizers with healthy vegetative growth and bountiful yield. It is important therefore to keep close, continuous track of the mineral condition of the tree to avoid periods of under-nutrition, which would jeopardize the entire year-long efforts. Moreover, as mentioned earlier, it is important to maintain a balanced mineral nutrition regime, with special focus on the correct amounts of nitrogen and potassium, in order to reduce the amplitude of alternate bearing.
3.1 Summary of main plant nutrient functions
Nitrogen (N)
Synthesis of proteins (growth and yield).
Phosphorus (P)
Cellular division and formation of energetic structures.
Potassium (K)
Transport of sugars, stomata control, cofactor of many enzymes, reduces susceptibility to plant diseases.
Calcium (Ca)
A major building block in cell wall and reduces susceptibility to diseases.
Sulfur (S)
Synthesis of essential amino acids cystin and methionine.
Magnesium (Mg)
Central part of chlorophyll molecule.
Iron (Fe)
Chlorophyll synthesis.
Manganese (Mn)
Necessary in the photosynthesis process.
Boron (B)
Formation of cell wall. Germination and elongation of pollen tube.
Participates in the metabolism and transport of sugars.
Zinc (Zn)
Auxins synthesis; enzymes activation.
Copper (Cu)
Influences the metabolism of nitrogen and carbohydrates.
Molybdenum (Mo)
Component of nitrate-reductase and nitrogenase enzymes.
3.2 The three tools for optimal nutrient management
The three tools for optimal nutrient management are:
  1. Observation of trees and environmental conditions.
  2. Soil and water analysis.
  3. Leaf analysis.
1. Observation
Visual symptoms should be used as an aid to interpreting soil and leaf analyses:
  • Look for abnormal symptoms in foliage or growth.
  • Look for significant variations in yield.
  • Observation can suggest deficiencies of nitrogen, potassium and boron.
2. Soil analysis
Soil analysis is not accurate enough to be used to diagnose fertility needs in olives, but it is useful for determining pH and to diagnose salt problems (excesses or imbalances).
In spite of many discrepancies between soil tests and mineral leaf contents, some empirical relationships were found:
  • A minimum P leaves content of 0.058 % corresponds to a soil-available phosphorus value of 4.35 ppm.
  • P leaf content of 0.07 % (in dry matter) is suitable for rain-fed olive orchards, whereby exchangeable P in soil is ~8 ppm.
  • When a leaf potassium content value of 0.43% is found (quite low!), it may correspond to soil-K availability of 80 ppm, where soil clay is less than 15%, but for clayey soils (> 15% clay), the minimal value of available potassium should be 110 ppm.
Soil analysis is performed to assess the need for soil amendment applications, e.g., lime application to adjust low soil pH, and gypsum application to adjust Ca:Mg ratio or to reclaim alkali soil. Ideally,3 – 10 spots in a site should be sampled. Because soils differ in composition at different depths, the top 15 – 30 cm (6 – 12") should be a separate sample, as well as each subsequent 30 cm downward. Samples taken at different distances from the trunk may be combined, but different soil depths should be separate. Samples should represent the effective rooting zone. A soil auger may be used to obtain samples. Generally, about 1 liter (1 quart) of soil per sample is adequate. The testing lab will often provide an interpretation of the results, as well as suggestions for corrective action.
3. Leaf analysis
Initial leaf analysis should be undertaken when trees are two years old, and then on a regular 1 – 2 year basis. Ideally, a sample should be taken from similar trees. Different varieties or parts of the orchard with different soils, microclimates, or irrigation systems should be sampled separately. Samples should consist of a few leaves of as many similar trees as possible, selected at random throughout the orchard. Avoid sampling leaves from abnormal trees, unless this is the specific problem to be solved. In this case, the abnormal leaves or trees should become a separate sample.
Summer, and mainly July (Northern hemisphere) or at least 5 – 8 weeks after full bloom, is the best time to perform leaf analysis because the levels of most nutrients stabilize in the olive leaf during that time.
Leaf collection
  • Remove 4 mature and healthy-looking leaves per tree from the middle of current season, non-fruit-bearing shoots.
  • Pick these leaves from about 20 – 25 trees representing a homogenous plot of up to 10 ha.
  • Wrap the leaves in paper bags or newspaper, but NOT in plastic, glass or other material which will cause humidity build-up.
  • If testing for boron, mature fruit samples may be more reliable than leaf samples.
Interpretation of leaf analysis results is based on the relationship between leaf nutrient concentration and growth or yield. Comparing actual leaf nutrient concentration to reference values allows the diagnosis of nutrient deficiency, sufficiency or excess. Optimum tree nutrition could be achieved by combining this information with soil and environmental factors that affect tree growth, and symptoms of nutrient deficiency or excess.
Leaf analysis standards
Leaf analysis interpreted as indicated in Mediterranean countries (Table 3.1) is a useful guide for fertilizer management of olive plantations, and may promote more environmentally responsible use of fertilizers in olive orchards.
Table 3.1: Important nutrient level ranges in olive leaves from tissue analysis (dry weight basis)
< 1.4%
1.5 – 2.0%
> 2.55%
< 0.05%
0.1 – 0.3%
> 0.34%
< 0.4%
0.8 – 1.0%
> 1.65%
< 0.6%
1.0 – 1.43%
> 3.15%
< 0.08%
0.1 – 0.16%
> 0.69%
< 0.02%
0.08 – 0.16%
> 0.32%
< 40 ppm
90 – 124 ppm
> 460 ppm
< 8 ppm
10 - 24 ppm
> 84 ppm
< 14 ppm
19 – 150 ppm
>185 ppm
< 5 ppm
20 – 36 ppm
> 164 ppm
< 1.5 ppm
4 - 9 ppm
> 78 ppm
> 0.20%
100 ppm
> 0.50%
Sources: Connell & Vossen, 2007; Producing Table Olives, by Stan Kailis, David Harris, 2007
3.3 Seasonal nutrient requirements of olive trees
Figure 3.1: Seasonal nutrient requirements of olive trees
Figure 3.2: Seasonal changes in requirements of olive trees for nutrients, by organ
3.4 Main plant nutrients, their rates in olive leaves, deficiency symptoms and application rates and methods
Nitrogen (N)
Nitrogen is one of the essential nutrients needed by plants, mainly as a building block of all proteins in the cytoplasm and the enzymes of the organism, and for chlorophyll buildup associated with the photosynthetic activity. Nitrogen uptake and metabolism is a key factor for olive roots to change the pH of their surrounding solution, which facilitates nutrient uptake by increasing their availability to the plant.
Nitrogen is one of the primary nutrients absorbed by olive roots, preferably in the form of the nitrate (NO3 -) ion. Nitrogen is a constituent of amino acids, amides, proteins, nucleic acids, nucleotides and coenzymes, hexosamines, etc. This nutrient is equally essential for good cell division, growth and respiration.
Because olives usually require larger amounts of nitrogen than other mineral nutrients, this is the most commonly applied fertilizing element in olive orchards.
The composition of virgin olive oil is affected by cultivar, fruit ripeness, agro-climatic conditions and growing techniques. Several studies have shown the effects of nitrogen fertilization on oil composition. Annual applications of nitrogen influence olive oil quality, especially fatty acid composition and antioxidant compounds.
Timing of side dressing
For highest uptake of nitrogen by the tree, and optimal effect on floral induction, nitrogen should be in the root zone just before the period of greatest uptake, i.e., just ahead of shoot growth and bloom in the early spring to early summer.
In dry farming orchards, nitrogen fertilizer is added to the soil in the fall to mid-winter, in order to have available nitrogen during the critical period. Where low rainfall prevails, nitrogen should be applied at the beginning of the floral induction period, while in regions enjoying higher rainfall, it is a common practice to apply nitrogen at the end of this period.
The total annual rate in bearing orchards is 0.5 – 1.0 kg / tree, (1 – 2 lbs. / tree).
  • If only broadcasting is possible, it is best to apply half in January and half in October, to moderate alternate bearing.
  • If fertigation systems are available, it is best to apply some 25% of the annual rate after fruit-set in order to contribute to vegetation and high yield in the next year.
Excessive amounts of nitrogen before fruit-setting may lead to high fruit load, resulting in small size fruits and alternate bearing (see Table 3.2).
Table 3.2: The effect of N rates on yield & size at a heavy crop year, cv. Mission, Palermo, Ca.
N application rate
Share of yield fit for canning
lbs. / tree
kg / tree
lbs. / tree
kg / tree
1/2 lb.
1 lb.
3 lbs.
Source: H.T. Hartmann, UC Davis
Over-fertilization of nitrogen causes the accumulation of nitrogen in fruit, which negatively affects some components. Excessive nitrogen reduces polyphenol content, thereby lowering the main natural antioxidants, the oxidative stability of the oil and its bitterness.
Foliar feeding of nitrogen
If nitrogen application was not done by soil application ahead of time, it can also be applied in the critical stages by foliar fertilization. Urea gives good results at a concentration of up to 3% – 4%. Foliar fertilization is effective in dry farming orchards where the absorption of nitrogen through the root system is very restricted.
Nitrogen-containing fertilizers:
% N
% N
Potassium nitrate
Ammonium nitrate
Magnesium nitrate
Ammonium sulfate
Calcium nitrate
Symptoms of nitrogen deficiency
  • Small, yellowish leaves
  • Poor shoot growth
  • Sporadic bloom
  • Poor fruit-set





Figure 3.3: N deficiency symptoms: pale color, lack of new growth


Phosphorus (P)
Phosphorus is one of the three primary nutrients and is absorbed by olive roots mainly in the form of orthophosphate (H2PO4-). An adequate supply of phosphorous at early growth stages is important for producing healthy rhizome and a strong root system, root growth and development of reproductive parts. It plays a key role in reactions involving ADP & ATP, essential for energy storage and transfer for subsequent use in growth and reproductive processes. In fact, almost every metabolic reaction of any significance in plants proceeds via phosphate derivates. Phosphorus is also an important structural component, as it is a component of sugar phosphates, nucleic acids, nucleotides, coenzymes, phospholipids, phytic acid, and more.
This element is necessary for many life processes such as photosynthesis and metabolism of carbohydrates. It helps plants, speeds-up the maturity process, and increases disease- and drought-stress resistance. It also influences flower setting and general vegetative growth.
Phosphate fertilization is especially necessary in acid soils and soils containing high amounts of calcium carbonate. The same applies to orchards planted in shallow, infertile soils or in new, irrigated olive orchards (1 – 10 years old) in which ample nitrogen is used every year.
Some P deficiency symptoms are similar to those of nitrogen deficiency, such as small leaf size, but without: leaf deformity, red leaf, light green leaf tips or dark green color (Figures 3.4 & 3.5).





Figure 3.4: Gradual P deficiency stages







Figure 3.5: Severe P deficiency


The characteristic visual symptom of phosphate deficiency is widespread chlorosis of the leaves. However, it is not a safe diagnostic criterion because it is often confused with other causes (e.g., nitrogen deficiency). Safe diagnosis can be done by chemical leaf analysis.
When phosphate fertilization is necessary, it should not exceed 20% – 30% of the amount of nitrogen added. So, if 1 kg / tree of N is added (e.g., 5 kg ammonium sulfate), the corresponding amount of phosphate should not exceed 200 – 350 g / tree of Ρ2Ο5 (e.g., 1.0 – 1.7 kg of SSP, 0-20-0). As a rule of thumb, it is suggested to add 500 g / tree of Ρ2Ο5 (e.g., 2.5 kg of SSP) in a two-year period.
In a case of severe phosphate deficiency, an amount of 4 – 5 kg / tree of Ρ2Ο5 (e.g., 20 – 25 kg SSP) is added in trees at the full production stage. For younger trees, smaller amounts (1 – 8 kg SSP) are added, depending on age and development stage.
Source: 2009-2010
Specialized laboratories can test for soil-P in three different ways – Mehlich III, Bray II, and Olsen – to optimize soil management. Thus, total P, the immediately available P, the amount in reserve that can become available, and the amount in the soil that is unavailable can be assessed. Hence, it is advisable to perform a soil test every other year, and apply P to the soil only if it is needed, avoiding excessive P build-up in the soil. It is perfectly fine to apply P fertilizer only once every two - three years, depending on testing results.
For soil application it is suggested to use SSP, Single superphosphate (0-20-0), or TSP, Triple superphosphate (0-46-0).
For application by nutrigation, fully soluble fertilizers are suggested, such as:
Potassium (K)
Potassium is not concentrated and does not form a constitutional part of any unique tissue or organ in the plant, but plays an important role in a multitude of physiological activities within the plant cell as well as in the coordination between tissues and organs in the whole plant.
Potassium is required as a cofactor for over 40 enzymes. It has a role in stomatal movements by maintaining electro-neutrality in plant cells. It is required for many other physiological functions, such as formation of sugars and starches, synthesis of proteins, normal cell division and growth, neutralization of organic acids and involvement in enzymatic reactions. Potassium affects transpiration rate by regulating stomata opening and closure, where a high transpiration rate increases nutrient absorption. Regulating stomata opening and closure also regulates carbon dioxide supply and improves efficiency of sugar use, increases water uptake and is consequently helpful in cell expansion. It also increases plant resistance to biotic and abiotic stresses such as frost tolerance, by decreasing the osmotic potential of cell sap due to higher ratio of unsaturated / saturated fatty acids. In addition, it assists in drought resistance, regulation of internal water balance and turgidity, regulation of Na influx and / or efflux at the plasmalema of root cells, chloride exclusion by rough selectivity of fibrous roots for K over Na and imparting salt tolerance to cells by increasing K holding capacity in the vacuole against leakage when Na is incurred in an external medium. Sub-optimal potassium status reduces nitrogen uptake.
A few instances of the major role of potassium in the water management systems of the olive tree are cited below.


Figure 3.6: Potassium deficiency increases stomatal conductance in olive trees 
Cultivar: ‘Chemalali de Sfax’. Source: Arqero, Barranco & Benlloch (2006)




Figure 3.7: Potassium deficiency increases transpiration rate of olive treesCultivar: ‘‘Lechin de Granada’.
Source: Benlloch-Gonzalez, Arqero, Fournier, Barranco & Benlloch (2008)




Figure 3.8: Potassium deficiency reduces water-use-efficiency in olive trees
Cultivar: ‘Chemalali de Sfax’. Source: Arqero, Barranco & Benlloch (2006)


Olive trees demand this nutrient. High amounts of potassium are removed from the soil with fruit harvest and pruning, particularly in high yield seasons. Regular potassium fertilization is required in order to maximize yield and quality (see Table 3.3), especially in orchards where no potassium fertilizer has been added for several years.
Potassium is a mobile nutrient and thus deficiency is most clearly shown in older leaves. They present pale chlorotic patches with the appearance of "burns" (necrosis) at the leaf tips and edges. These areas of dead tissue progress from the tip to the base, and from the leaf margin towards the intervein area. The leaf tip tends to curve downwards.
Potassium deficiency diagnosis is not safe on the basis of these symptoms, and must be further confirmed by leaf analysis. Deficient leaves contain about 0.1% – 0.3% potassium (on a dry basis), whereas the content of well-supplied leaves ranges from 0.4% – 1.7%.
Table 3.3: Tree response to potassium fertilizer
N application rate
Yields(kg / tree)
% Canning fruit
4 year mean
1st year
2nd year
3rd year
K+ mass dose
Source: H.T. Hartmann, UC Davis
Potassium deficient leaves (Figures 3.9 – 3.12) are light green and, in cases of severe deficiency, show tip burn as well as dead areas in the leaves. Continuous deficiency will cause twig die-back, reduced fruit number and smaller fruit size.





Figures 3.9 - 3.12: Potassium deficiency symptoms in olive leaves in order of severity, including twig die-back


The following potassic fertilizers are available:
  • Potassium chloride (Muriate of potash), 0-0-60. Highest in potash content, but also carries high chloride that may accumulate in the soil and bring about chloride toxicity. See more on chloride toxicity at the end of this chapter.
  • Potassium sulfate, 0-0-50. Non-chloride potassic fertilizer, with relatively low solubility.
  • Potassium nitrate, 13-0-46. Highly soluble, carries with it a considerable amount of nitrogen, in the nitrate form, which is highly available and nutritious for the tree in most stages. This fertilizer is also the preferred one where soil pH is somewhat low. The absorption of the nitrate component will increase soil pH.
  • Mono-potassium phosphate (MKP) 0-32-54. Highly soluble, carries with it a considerable amount of phosphate that is nutritious to the tree in several growth stages.
The amount of applied potassium should be determined in combination with nitrogen. In olive orchards in which no potassium has been used in the past, it is preferable to add twice as much potassium as nitrogen. For example, if 0.5 kg / tree of N (i.e., 2.5 kg ammonium sulfate) is applied, then 1 kg / tree of K (i.e., 2 kg potassium sulfate) must be added. Later on, potassium dosage is adjusted to be equal to nitrogen. After high yield seasons, it is preferable to increase potassium to supplement the amount that is being removed. Leaf analysis, wherever possible, may give better direction for potassium fertilization.
Many times, potassium deficiency is due to low soil moisture (drought); potassium is fixed by clay minerals in the soil and thus trees cannot take it up from the soil. The problem can be relieved by selecting cultivating techniques that enhance the growth of the root system and ensure adequate soil moisture. In this case, larger amounts of fertilizer are added, usually 10 – 15 kg of potassium per tree. Alternatively, half of the above-mentioned amount can be added in the winter in the form of potassium sulfate, and the remaining amount in the form of potassium nitrate through the irrigation system. Potassium nitrate is applied through the irrigation system at a dose of 300 – 500 g / tree after fruit-setting.
Potassium should be applied to the soil at the rate of 2.3 – 4.6 kg (5 – 10 pounds) of pure potassium per tree, or about 280 – 560 kg / ha (250 – 500 pounds / acre). The smaller rates correspond to sandy, or light-textured soils, while the greater rates apply to heavier soils. Where there is no fertigation system it is recommended to apply the potassium fertilizer once a year between December to January (northern hemisphere) to be washed into the soil by winter rainfall. In such cases, the potassium fertilizer should be banded alongside the row of trees or in a circle around the tree, where it would be absorbed by the soil by the drippers / sprinklers/ jet emitters. Broadcasting of the fertilizers where there is no active root zone will be of no avail. In drip-irrigated plots, application can be made under the emitters. The best would be to acquire a fertigation system and the fertilizer should be dissolved and distributed via the irrigation system (fertigated). Regular K inputs maximize yield and quality.
When potassium is notably deficient, a foliar spray of 1.2% (e.g., 10 pounds per 100 gallons) potassium nitrate, such as Multi-K, can quickly correct the deficiency. The new vegetation in the spring will absorb it very quickly and results will start to be obvious within a week.
Potassic fertilizers (e.g., Multi-K), should be applied in irrigated orchards in the spring and during the entire growing season.
Maintaining soil acidity at the right pH level (in the region of 6.5) is critical for facilitating the optimal uptake of other nutrients. Multi-K, due to the presence of nitrate-nitrogen, increases soil pH of acidic soils in the root zone.
Magnesium (Mg)
Magnesium is a secondary plant-nutrient, absorbed as Mg2+. Magnesium is a crucial constituent of the chlorophyll molecule. It is required, nonspecifically, by a large number of enzymes involved in phosphate transfer. It is involved in photosynthesis, carbohydrate metabolism, synthesis of nucleic acids, related to movement of carbohydrates from leaves to upper parts, and stimulates P uptake and transport in addition to being an activator of several enzymes.
The main symptom of magnesium deficiency is the chlorosis of leaves that begins from the top or the edges of the leaf and spreads gradually to the whole leaf area. Other symptoms include severe leaf shedding and a poor vegetative cycle.
Magnesium deficiency is best controlled by soil application, or foliar spray of magnesium sulfate ("Epsom salt") or magnesium nitrate (11-0-0-16MgO), such as Magnisa™l.
Sulfur (S)
Sulfur, also a secondary plant nutrient, is essential for protein formation as a constituent of the three amino-acids cystine, cysteine and methionine. Sulfur is required for the formation of chlorophyll and for the activity of ATP-sulfurylase. These essential functions enable the production of healthy and productive plants, which are preconditions for high yields and superior quality.
Sulfur is best supplied by ammonium sulfate and potassium sulfate.
Source: Producing Table Olives, By Stan Kailis, David Harris
Calcium (Ca)
Calcium is also one of the secondary plant nutrients, absorbed by plant roots as Ca2+. Calcium is a constituent of the middle lamella of cell walls as Ca-pectate. Calcium is required as a cofactor by some enzymes involved in the hydrolysis of ATP and phospholipids. It is an important element for root development and functioning, a constituent of cell walls and is required for chromosome flexibility and cell division.
Calcium deficiencies take place only in soils lacking this element, e.g., washed soils in tropical regions. The main symptom of calcium deficiency is the chlorosis starting at the tips of the leaves, like in boron deficiency, but in this case the veins in the chlorotic area of older leaves become white. Other deficiency symptoms are general poor growth, especially in the roots and shoots. Unlike boron deficiency, there is a lack of young shoots (Figure 3.13).
Figure 3.13: Calcium deficiency
Calcium deficiency is corrected rather easily by adding 5 – 10 kg of calcium oxide per tree. To avoid calcium deficiency, soil pH must be determined before planting a new orchard. The amount of calcium added must be determined after soil analysis.
Calcium nitrate (15.5-0-0-26.5), e.g., Multi-Cal, is an optional calcium fertilizer that, due to its excellent solubility, can be used in fertigation systems.
Trace elements
Major sources: Producing Table Olives, by Stan Kailis; David Harris, 2007.
The olive tree requires small amounts of boron, zinc, manganese, copper and molybdenum. A deficiency in any of these elements can reduce growth and fruiting. Deficiencies of trace elements are commonly associated with alkaline, lime-rich (calcareous) soils, where they are retained in an oxide form. Lowering soil pH by adding elemental sulfur, which is converted to an acid form by microorganisms, can overcome this problem. Sulfur in the form of sulfate is not an acidifying material.
Boron (B)
Boron plays a role in cell wall development and is important in pollination, fruit development and the translocation of sugars. An adequate supply of boron is important for flowering. The quality of olive fruit is affected if boron is deficient.
Within plants, boron is relatively immobile. It is not readily relocated from old to young plant tissue. Plants are therefore dependent on a continuous uptake of boron during the growing season. In this respect, its behavior in plants is very similar to calcium (both are immobile) and deficiency symptoms can be confused.
Boron deficiency symptoms
Boron deficiency occurs more commonly in dry weather. Microbial activity in the soil is reduced, and the movement of boron in the soil solution to plant roots is restricted. Boron is not very mobile, so deficiency appears in the young leaves.
The main symptoms are:
  • Leaves with deficiency contain less than 20 ppm boron, while those from healthy trees have more than 20 ppm (on a dry basis).
  • Leaves around the terminal bud turn light green at their tip and eventually fall off.
  • Gradually, the same symptom appears on leaves near the base of the shoots, which appear dry at their edges.
  • Later growth shows small and distorted leaves that are stunted, fragile and finally drop off. If a small piece of the stem is cut off with a sharp knife, a brown discoloration shows due to necrosis of the cambium.
  • Chlorosis (yellowing) and death of the growing points.
  • Trees suffering from boron deficiency appear chlorotic from a distance and delay entering the vegetative stage.
  • Distortion, thickening and cracking of stems. The stems may be hollow or brittle.
  • Formation of rosettes, growth of auxiliary buds (side shooting), bushy growth and multiple branching. Shortened internodes and secondary shoot production at the tree base.
  • Thickening, twisting and failure of roots to spread out or develop properly. In some cases the roots may show excessive branching.
  • Dropping of buds or blossom. Poor fruit-set.
  • Fruits and seeds may also be affected. Brown sunken areas may develop in fruit, in a symptom called "Monkey-face".
See Figures 3.14 – 3.17.





Figure 3.14: Boron deficiency - leaves with dead tips and a yellow band, but still green at the base, and with a rosette form






Figures 3.15 & 3.16: Boron deficiency symptoms on olive leaves


Figure 3.17: Boron deficiency effect on fruits: the "monkey face" symptom and premature fruit drop
Correcting boron deficiency
Boron is taken up by plants as undissociated boric acid H3BO3.
Boron deficiency is corrected by broadcasting 113 – 225 g (0.25 – 0.5 pound) of a 14% – 20% boron fertilizer per tree, or 28 – 56 kg / ha (25 – 50 pounds / acre) on the soil surface within the drip line during winter. One treatment will last for several years, but because of its mobility in the soil and susceptibility to leaching, annual applications of boron are recommended in most situations. Frequent applications at low rates also minimize the risk of toxicity.
BE VERY CAREFUL not to apply too much since toxicity may occur.
A fast correction of boron deficiency in a specific season can be achieved by a foliar spray of 0.05% – 0.1%, (i.e., 7 – 14 ounces per 100 gallon of water) of Borax. Spray should be applied until runoff is achieved. Such applications prior to flower bud initiation or immediately prior to flowering significantly improve fruit-set, even in trees with no visible symptoms and low, but not deficient leaf boron levels.
Boron fertilizers
Borax, (11% B) is a fine crystalline product for dry soil application, or by application in solution to the soil or foliage.
Granubor, (15 % B) and Borate Granular, (14.3 % B) are granulated fertilizers, which makes them more suitable for dry application by machine to the soil. They can be used on their own or in blends with other fertilizers. Granubor and Borate Granular do not dissolve, hence are unsuitable for foliar application.
Solubor, (20.5 % B) is a fine, soluble powder for application in solution through a boom-spray to the soil or foliage. Application rate is 1 – 1.5 g / L. Solubor is more soluble than Borax, especially in cold water, and is the recommended choice for foliar applications or ground applications in solution.
The marketplace for boron products also offers high analysis Boron solutions, designed to rapidly correct boron deficiencies in all crops in both soil and foliar applications. Some of them allow enhanced foliar and root uptake due to formulation with organic acids, which assist with assimilating the boron in the plant. They can be applied as a foliar spray, or by fertigation, .e.g., AgroDex Boron, (10%). Application rate: 1 – 2 L / ha.
Boron toxicity symptoms
Olives are classified as "somewhat tolerant" of boron in irrigation water, accepting water levels of boron of 1 to 2 mg / liter (roughly equivalent to 1 – 2 ppm). Water with 12 ppm will cause problems for olives that are not tolerant to high levels of boron. A soil analysis would be the only way to determine if there is a soil problem. One of the most common causes is over-fertilization with or poor placement of boron fertilizer.
Toxicity symptoms
In the early stages, the symptoms of boron toxicity are normally expressed as marginal and tip chlorosis of the older leaves. Moderate to severe toxicity produces progressive leaf necrosis, beginning at the tip or margins and gradually covering the whole leaf, resulting in premature leaf drop.
Zinc (Zn)
Zinc activates a number of enzymes and is important in the biosynthesis of auxins, such as IAA.
Zinc levels are adequate in the olive tree if zinc concentration is higher than 10 ppm on a dry weight basis.
Zinc deficiency symptoms
Yellow spots may appear on adult leaves, small pale-green leaves, with interveinal chlorosis. Otherwise, signs are similar to iron- and manganese-deficiency – reduced shoot growth resulting in rosette formation (see Figure 3.18).
Figure 3.18: Zn deficiency in olive leaves
Correcting zinc deficiency
Replenishment of Zn is especially important in early spring. Correction of zinc deficiency can be done by foliar spray with 0.1% zinc sulfate or by zinc-containing fungicides if these are planned for actual fungal diseases.
Iron (Fe)
Iron, a micronutrient, is a constituent of cytochromes and non-haeme iron proteins. It is involved in photosynthesis and N2 fixation and respiratory linked dehydrogenases. Iron is also involved in the reduction of nitrates and sulfates and in reduction processes by peroxidase and adolase.
Iron deficiency can occur even though the soil has an abundant amount of iron, but it is unavailable due to a high pH of the soil or irrigation water. Competition with other ions, such as manganese, zinc and potassium, can also contribute to iron deficiency by displacing iron from chelating agents in the soil.
Iron deficiency symptoms include yellowing of immature leaves, with the mid-rib and veins greener than inter-vein areas. Fruits tend to be pale-yellow rather than green-yellow.
Correcting iron deficiency
Iron deficiency can be corrected by foliar spray of iron chelates, e.g., EDTA-Fe, which contains 12% Fe, and should be applied at 50 g / L of water. Foliar sprays of iron are quick acting, but are not long lasting. The same product can be applied by fertigation to the soil for a longer effect. Other treatments are drenching the soil with iron sulfate (20% iron), and should be applied at 20 g / m2 in water, or injecting iron sulfate or iron citrate directly into the tree trunks. Other commercial iron chelates are EDDHA-Fe, (e.g., Multi-micro EDDHA Fe 6%), and DTPA.
Manganese (Mn)
Manganese is required for the activity of dehydrogenases, decarboxylases, kinases, oxidases, peroxidases, and non-specifically by other divalent cation-activated enzymes. It is required for photosynthetic evolution of O2, besides involvement in production of amino acids and proteins. Manganese has equally essential roles in photosynthesis, chlorophyll formation and nitrate reduction.
The concentration of metallo-enzyme peroxidase in the leaf is considered to be the best marker of Mn deficiency.
Manganese deficiency symptoms
Starts with interveinal chlorotic mottling of immature leaves, similar to that seen in iron deficiency. Flower buds often do not fully develop, turn yellow and abort.
In severe deficiency, new growth is yellow in color but, in contrast to iron deficiency, necrotic spots usually appear in the interveinal tissue.
Correcting manganese deficiency
Manganese is absorbed by the plant roots in the form of Mn2+. Mn deficiency can be corrected by:
  • Application of acidifying fertilizers such as elemental sulfur and ammonium sulfate.
  • Foliar spraying with manganese sulfate at 0.2%, or manganese-containing fungicides.
  • Waterlogging of the soil that depletes soil oxygen, releasing high amounts of soluble ferrous and manganese cations, could be considered, but excessive concentrations may be toxic to the roots. Also, olive trees are intolerant to waterlogged soil, so this method should be ruled out!
Copper (Cu)
Copper plays an active role in some enzymes performing key functions like respiration and photosynthesis, e.g., cytochrome oxidase, diamine oxidase, ascorbate oxidate, phenolase, leccase, plastocyanin (a protein having ribulose biphosphate carboxylase activity), ribulose biophosphate oxygenase activity, superoxide dismutase, plant acyanin and quinol oxidase. Copper is also a constituent of cytochrome oxidase and heme in equal proportions. Cu-proteins have been implicated in lignification, anaerobic metabolism, cellular defense mechanism, and hormonal metabolism. Copper proteins exhibit electron transfer and oxidase activity. It also acts as a terminal electron acceptor of the mitochondrial oxidative pathway.
Copper deficiency symptoms
Copper deficiency symptoms are often found in sandy soils. This problem is exacerbated if excessive amounts of phosphorus fertilizers are used. Copper deficiency symptoms are stunted growth, distorted leaves, leaf rosettes and pale yellow-white leaves.
Correcting copper deficiency
Application of copper sulfate at 0.25 – 0.5 kg / tree to the soil, or foliar sprays as Bordeau mixture, or copper sulfate at 0.05%.
Beware of over-application of copper. It can be toxic to the tree and to soil microorganisms.
Molybdenum (Mo)
Deficiencies are rare but more likely in acid soils due to low bioavailability. Symptoms often consist of interveinal chlorosis in older leaves. Young leaves may be severely twisted.
Chloride (Cl)
Chloride is required by all plants in very minute amounts, similar to iron, whose normal concentration is about 100 ppm.
  • Chloride is essential for the proper function of the plant stomata, thus controlling internal water balance.
  • It also functions in photosynthesis, specifically the water splitting system.
  • It functions in cation balance and transport within the plant.
  • Research has demonstrated that chloride diminishes the effects of fungal infections in an undefined way.
  • It is well documented that chloride competes with nitrate uptake, tending to promote the use of ammonium N. This may be a factor in its role in disease suppression, since high plant nitrates have been associated with disease severity.
Although chloride is classified as a micronutrient, it is generally applied in very high rates by irrigation water. Additionally it is often supplied by commodity fertilizers, e.g., potassium chloride (MOP) and calcium chloride, resulting in a very marked uptake that will bring its concentration to the level of a secondary element such as sulfur, namely ~0.5%, which is about 5,000-fold of the required rate!
It was found in an experiment that increasing the salinity of soil solution results in accumulation of Na and Cl in leaf, shoot and root tissues of olive trees. Simultaneously, K and Ca concentration are decreased, but Mg content is not affected by the salinity stress.
Source: Al-Absi, Qrunfleh, & Abu-Sharar, 2002.
Chloride toxicity
Chloride accumulation to these high rates can develop into a serious problem. The chloride anion markedly reduces plant vigor and tends to accumulate in the leaf margins, producing leaf-edge scorching and necrosis (tissue death), that stems from concentration levels of up to 3%! Such leaves are prone to premature leaf abscission and reduced photosynthesis activity.
Therefore, the use of high chloride water, especially when there is a Ca / Cl ratio of less than 2:1 in the irrigation water, is highly risky. A water test is important. For obvious reasons, don't use fertilizers high in chloride or that contain much muriate of potash (MOP; potassium chloride) or calcium chloride.
Chloride toxicity symptoms
Typical salt toxicity symptoms are dead leaf edge, leaf drop and necrosis of stem tip. Toxicity symptoms appear above 50 mM of NaCl, and become more severe at high salinity levels.
Figure 3.19: Typical salt toxicity symptoms in the olive tree are dead leaf edge, leaf drop and necrosis of stem tip
Source: Chartzoulakis, 2005
Need more information about growing olives? You can always return to the olive tree fertilizer & olive crop guide table of contents