“I know your enchantment, oh father olive! Giving us the blood you extract from the soil”

Federico Garcia Lorca

Apparently the olive tree was domesticated in the eastern Mediterranean where it was grown already 5,500 years ago. This species is perfectly adapted to the climate of the region, so that it was grown on drylands for thousands of years.

This adaptation is thanks to the tree’s ability to explore large volumes of soil, with its roots reaching dozens of meters into the soil and growing horizontally far beyond its canopy.

Moreover, the leaves have anatomical and physiological mechanisms to reduce the loss of water by transpiration, they are small, thick and their few stomata are to be found in small hollows on their lower side, protected by hairs that maintain the humidity of the air surrounding them.

Even so, when olive trees undergo water stress, the fruit does not develop fully and the water in its tissues migrates to other plant organs, so that it remains small in size and shriveled.

The profitability of the orchard can be increased considerably and the years of alternative bearing reduced with a well managed fertigation program.

With the implementation of fertigation the characteristics of the crop and its requirements have changed completely.

olive-with-leaf

We could argue that this is a wholly different crop. The management of the orchard must adapt to these changes, including the spacing of the trees in the plantation, the pruning and harvesting techniques, the application of fertilizers, and more. Thus, per example, pruning too heavily reduces the number of leaves and fruits.

Excessive fruit thinning does not allow the tree to achieve the maximum potential achievable yield with fertigation.

olives-in-boal

Nutrient requirements of olive trees

The guiding principle of fertigation is to supply the crop with the required nutrients at the rate it can use them. There is no magic formula allowing us to make and exact recommendation; but the nearer we are to this aim, the efficiency of the fertilizer will be higher and the better we will protect our environment. Modern olive orchard fertilization does not aim at feeding only the yield developing during the current year, but also to enhance the accumulation of reserves in the tree’s roots, trunk and the branches.

As with other wooden crops, the olive tree does not respond immediately to the application of a fertilizer, since it has large reserves of most of them.

Nitrogen

is a constituent of many organs of the olive tree, being vital for photosynthesis and growth metabolism. Nitrogen deficit leads to poor vegetative growth and uniformly pale leaves. Excess Nitrogen leads to overly vegetative growth, delaying fruit maturity and increased sensibility to frosts, pests and disease. Even physiological disorders may negatively affect fruit quality.

Phosphorous

is an important component of DNA and RNA, it takes part in all energy transforming processes; is a constituent of the phospholipids in cellular membranes and has important functions in the growth of roots, on flowering and fruit set. The symptoms of Phosphorus deficit are small, very dark leaves, which fall off prematurely and reduced branch growth.

Potassium

takes part in many important processes, regulating the opening and closing of stomata, the transport of organic and inorganic ions within the plant; promoting the maturity, yield, size and quality of the fruit. Per example, Potassium is the element with the highest incidence of deficit symptoms in Span’s olive orchards.

This frequently is as a consequence of an excess of nitrogen applications, leading to an imbalance among the nutrients. The leaves of trees suffering of potassium deficiency show yellowing or reddening at the leaf tips. Under severe deficiency the tip and the sides of the leaves die (necrosis) and later fall off the tree.

This leads to reduced new growth, less flowering and fruit setting.

olive-tree

Calcium

is essential for the development and functioning of roots and increases their resistance to diseases and adverse environmental conditions, enhances fruit quality, takes part in photosynthesis and other enzymatic activities. Calcium deficient trees stop developing; their leaves start yellowing at their tips, remain small and narrow.

Magnesium

is a component of the chlorophyll molecule and is required for the activity of some enzymes. Olive trees with Magnesium deficiency appear chlorotic, with depressed growth, mainly in the autumn.

Micro-Elements

Depending on the conditions they grow in, olive trees may also show microelement deficiency of zinc, iron, manganese, copper, molybdenum and boron. Iron deficiency is frequent on calcareous soil with a high pH. The trees show a typical iron induced chlorosis on young leaves and poor development of the shoots. Trees suffering of Iron deficiency have low yields because of poor fruit set. The fruits don’t reach normal size and have a yellowish. With extreme deficiency trees may fail to produce at all and may not recover anymore.

Boron

is very important for flowering, pollinization, and olive fruit set. It also takes part in the synthesis of fats. Leaves of trees with boron deficiency show tip-chlorosis which develops into necrosis. Under severe deficiency the symptoms may appear on young branches and may affect yield till it becomes nil. Sometimes shoots develop as “witch brooms” and the fruit is deformed.

Boron requirements are highest during flowering. Sometimes even foliar applications are required during this stage. Boron applications should be performed with great care since at relatively low concentrations it is toxic and more so on young trees. As long as fertigation equipment is available, the best method for applying boron is together with the irrigation water.

onlive-plantationOlive trees are moderately tolerant of salinity so that they may be grown were other woody plants can not grow. The fact that some varieties and stocks are more resistant to salinity than others should be taken into consideration under these conditions.

The accumulation of salts in the soil solution and in the plants must be followed along the seasons so as to take the necessary measures, like applying excess irrigation water to leach them from the wetted soil volume.

Characteristic symptoms of salinity in olive trees is the reduction of growth of the shoots, the shortening of internodes, smaller leaves and fruits, physiological changes that lead to diminishing yields.

Foliar analysis

Foliar analysis is required in order to avoid deficiencies before they affect yield or fruit quality. The best way to monitor the nutrient status of olive trees is to make a yearly foliar analysis during the month of July, starting with the third year of the plantation.

 
In order to have a representative leaf sample it should be taken very carefully and in accordance with established standards.
The leaves are sampled from the middle of the current year’s young shoots (spring shoots) not bearing fruit.
The leaves should be healthy and representative.
The plot must be uniform and samples must be taken separately from each variety.
From each tree 4 leaves, together with their petioles (leaf stalks) are taken, one each from the N, E, S and W sides of the trees, at eye-height.
In each plot, 30 to 40 representative trees, taken at random, are sampled so that each sample consists of 120 to 160 leaves.
No leaves with abnormal deficiency symptoms should be included.
Trees with obvious deficiency symptoms should be sampled separately.
The sample leaves must be kept in a fresh place, in paper bags on which the pertinent data is clearly written. The samples should be sent to a laboratory as soon as possible.
If the bagged samples must be kept overnight they should be kept in a refrigerator.
The interpretation of the analysis should be made most carefully, by a person who knows the plantation, its present condition and its past history, preferably with the support by a professional with both field experience and knowledge in the interpretation of foliar analysis. Also the values published in the professional literature (and which differ widely among different authors), the fruit load, the physiological status of the plantation, its age, its development, soil solution salinity and foliar applications (if made), etc. should be taken into consideration.

olive-labBelow we present a table with representative values for adequate nutrient levels in olive leaves sampled in July.

The iron (Fe) content is not included since its value fluctuates widely.

Sufficiency level of nutrients in olive leaves sampled from the mid-length of current year’s young shoots that do not bear fruit.

Element Sufficiency level*
N (%)   1.7 - 2.1
P (%)   0.1 - 0.2
K (%)   0.8 - 1.3
Ca (%)   2.0 - 2.6
Mg (%)   0.25 - 0.30
Mn (ppm)   30 - 50
Cu (ppm)   7 - 20
Zn (ppm)   15 - 50
B (ppm)   20 - 50

* Taken from diverse sources.

Fertigation of an olive orchard

One of the basics of drip irrigation is the continuous application of a nutritive solution together with the irrigation water as to provide the trees with the required nutrients at the rate they can make best use of them. Taking into consideration that the wetted volume is exposed to constant leaching, impoverishing the rooting volume whenever nutrients are not steadily supplied. There is no need to apply more fertilizer, but to split it into as many applications as the number of irrigations per season. This is the way to make the best use of the fertilizer.

The right thing to do is to ensure that the correct nutrient balance is injected into each drop of water. In comparison to other methods of application of fertilizers, with this method the nutrients are made better use of. This is one of the main advantages of drip irrigation.

The higher efficiency of the fertilizers with fertigation is also due to the fact that the fertilizer is applied to the volume of soil where the most active roots are found, the dosage may be fitted to each phonological stage of the tree, boosting the availability of nutrients. This is most remarkable with elements such as Phosphorus, which once in the soil looses its availability at short notice, so that applying it continuously has the advantage of increases its availability to the plant.

Fertilization programs for Olive plantations

Some of the nutrients required by olive plantation are contributed by the soil and others by the irrigation water; and all of the deficit must be supplied by fertilizers. For programming purposes we have to take into consideration the spacing, the age and the fraction of the ground covered (shaded) by the trees; also, the actual fruit load; the information obtained from foliar analysis and the properties of the soil and the irrigation water are important. The soil’s texture and any factor that may reduce the availability of some of the nutrients should be investigated. As for the irrigation water, information about its nutrient content, its salinity and the concentration of bi-carbonates are important. Therefore it is difficult to make generalized recommendations. The fertilization program must be adapted to each plantation.

Ferreira et. al., state that the extraction of nutrients by each ton of harvested olives is: 15 kg N/ha; 4 kg P2O5/ha and 25 kg of K2O/ha; (Ferreira y col., 1986; Olea 17:141-152). Therefore a yield of 8 ton olives/ha extracts: 120 kg N/ha, 32 kg P2O5/ha and 200 kg K2O/ha. For any specific fertilization program we have to take into consideration all the above mentioned factors as well as the method by which it is applied to the plot and the efficiency of the fertilizer.

As a general rule we may consider that an orchard carrying a high yield, with 50% coverage (shading) of the ground and foliar analysis with sufficiency levels for all nutrients, a yearly application of 120-140 kg N/ha; 50-60 kg P2O5 /ha and 160-180 kg K/ha would be required.

The easiest and safest method for applying the fertilizers is the use of soluble solid fertilizers which may be applied to the orchard in the irrigation water (fertigation) as required.

The following table presents an example for the programming of a fully developed olive plantation with GAT solid soluble fertilizers:

Expected yield: 8 ton per hectare  
Month Product Composition kg/ha of product   kg/ha Nutrients
    N P2O5 K2O     N P2O5 K2O
mar - apr   Gatit   28 - 8 - 12   200   56 - 16 - 24
may - jun   Gatit   20 - 9 - 23   175   35 - 16 - 40
jul - aug   Gatit   15 - 9 - 30   200   30 - 18 - 60
sep - oct   Gatit   15 - 9 - 30   167   25 - 15 - 50
                    Total   146 - 65 - 174

In order to make up the fertilizer solution to be injected into the irrigation system each Kg of GATIT solid-soluble fertilizer should be completely dissolved in 4 liters of water (per example: dissolve 100 Kg GATIT in 400 Lt water).

This is a preliminary recommendation only, to be modified according to the conditions prevailing in the orchard. As stated above, for each plot a specific program should be developed while taking into consideration the nutrients available in the soil and in the irrigation water. Also, the soil’s properties, the latest foliar analysis and the experience the grower has acquired in years past. The requirements of Calcium (Ca), Magnesium (Mg) and/or microelements should be taken into consideration as well.

A product that encourages the development of the volume of the canopy as fast as possible should be applied to young olive plantations. According to the conditions prevailing in the plot, a product with a high content of Nitrogen and Phosphorus and some Potassium should be used. As necessary, also Calcium and/or Magnesium should be applied.

Using soluble solid fertilizers makes application much easier and GAT is able to prepare such fertilizers with the combination of nutrients required for each plantation, (within the limitations imposed by their chemical properties). The farmer should be aware of the properties of the fertilizer he applies. The most relevant properties are: the nutrient content (%) and the origin of the nutrients (meaning the raw materials used in the formulation).

Since olive trees are quite tolerant of salinity, it may be possible to use formulations made with muriate of potash (potassium chloride, KCl) as in the GATIT-C family of products, except when a high level of salinity is present in the soil or in the irrigation water. Under salinity conditions reduced chloride GATIT-R fertilizer is recommended. GATIT may be supplied with added micro-nutrients as required by the crop. According to the circumstances, Nitrogen may be applied as nitrate, ammonium or urea.

The fertilizer should be injected into the irrigation system nearly continuously, starting when the system has reached operational pressure and finishing about half an hour earlier than the irrigation, so as no fertilizer residues remain in the irrigation system. In order to obtain the required concentration of nutrients in the irrigation water, about one liter of fertilizer solution should be injected into each cubic meter of irrigation water. However to calculate the optimum concentration our agronomist should be consulted.

Irrigation programming

Irrigating adult olive trees

Drip and micro-sprinkler are the methods best suited for the irrigation of olive orchards. With drip irrigation more water and fertilizer can be saved, but it requires both thorough design and management. Both methods allow for the injection of fertilizers into the irrigation water using specially designed injectors so that a fertilizer solution is applied. This method, combining irrigation and fertilization is called fertigation.

The most critical stages for olive water consumption are during the rainless period. During the spring, water stress can cause the abortion of the ovary, reduce pollination and fruit set. During June and July fruit growth is hampered by reducing the number of cells in the fruit. From August to harvest, yield is affected by reduced growth of cell size.

Water deficit during the summer affects the initiation of flowering buds for the next year’s harvest. This happens while the embryo of the current year’s fruit is developing and its seed hardening. Therefore water stress during this stage affects not only the yield of the current season, but also that of the following year.

In order to evaluate the volume of water that should be applied to the olive grove each day the daily reference evapotranspiration (ETo) should be considered. ETo is multiplied by the crop coefficient (Kc) in order to obtain the daily irrigation requirement of the crop: Crop Evapotranspiration (ETc).

 
ETc = ETo x Kc [1]

Kc is an empiric parameter for different growing conditions and it changes along the seasons. In the absence of a local study the following table may serve as a first approximation.

Kc coefficients for the calculation of water requirements of an olive grove
April May June July August September October
0.3-0.5*   0.4-0.6*   0.6   0.6   0.65   0.65   0.65

* Depending on residual soil humidity

Equation (1) is used for trees covering (shading) at least 50% of the ground. For trees with a smaller coverage a correction factor (Kr) is used which estimates the effect of the percentage of the ground covered (shaded) by the tree’s canopy (Sc) calculated with equation (2).

 
Sc = π x D2 x N [2]
  400  

π = 3,1416

D: canopy diameter, N: trees per hectare

Knowing Sc, the value of Kr is calculated with equation (3)

 
Kr = 2 x Sc [3]
  100  

The daily water supply is calculated with equation (4)

 
ETc = ETo x Kc x Kr [4]

The following table shows Kr coefficients calculated for different planting densities and canopy diameters. When the canopy diameter reaches 50% (5,000 m²/ha) the coefficient is considered to be: Kr = 1.

Surface covered (shaded) in groves with different planting densities and the Kr coefficient to evaluate irrigation requirements of an adult olive orchard when less than 50% of the ground surface shaded.

Density (trees/ha) Canopy diameter Surface shaded Kr
* (m) (m² / ha)  
200   4.0   2513   0.50
    5.0   3927   0.79
    5.6   4926   1.00
250   3.0   1767   0.35
    4.0   3142   0.63
    5.0   4909   1.00
300   3.0   2121   0.42
    4.0   3770   0.75
    4.6   4986   1.00
Example
Tree density:   250 trees/hectare
Canopy diameter:   4 m
Irrigation date:   July, 6th
ET o on this date:   7 mm/day
Kc for July:   0.6
Kr for this density and canopy diameter:   0.63

How many daily hours are required to irrigate the plantation?

Assuming that each tree has 4 drippers discharging 4 Lt/hour each = 16 Lt/h per tree. 26,460 Lt/ha must be applied each day.

Each one of the 250 trees per heactare must receive: 26460 / 250 = 105.84 Lt/tree per day

Dividing this quantity by the hourly discharge (16 Lt/hr per tree)105.84/16 = 6.615~6.6 hours/day

Or all in one step 26,460 Lt / 250 trees / 16 LT/ h =6.6 hours / day = 6h 35 min / day.

Irrigating a young olive orchard

As for a young olive grove, (up to 2 m canopy diameter) the evaluation is made separately. Since we are interested in the speedy development of the young tree to reach maximum yields, we must avoid any water stress. Example:

Irrigation requirements for young olive trees, in liters per tree and week as a function of canopy diameter
Month Canopy diameter
0.5 m 1.0 m 1.5 m 2.0 m
April   52   63   80   108
May   60   73   95   130
June   69   84   108   149
July   74   91   118   163
August   69   85   110   151
September   57   69   89   121
October   42   48   60   78

This text has been translated from the Spanish edition “Fertirrigación del Olivo” published by GAT Spain.

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