California Crop Fertilization Guidelines

Soil samples for nitrate should be taken in spring, at which point the soil available N tends to be high and trees are starting to rapidly take up N ^[N21,N32]. Samples should be taken from the major rooting zone, and within the wetting area of the irrigation system. In irrigated olives most root activity generally occurs in the top 18 inches of soil ^[N4,N12,N35]. For more information on soil sampling, see Soil Sampling in Orchards.

Soil nitrate-N present in spring before leaf-out contributes to the tree's N nutrition. One ppm of NO₃-N per foot of soil corresponds to roughly 3.5-4 lbs N/acre. For example, a NO₃-N concentration of 5 ppm in the top foot of the profile corresponds to about 17.5-20 lbs N/acre.

When well water is used for irrigation, it may contribute a significant amount of N ^[N37]. Well water samples should be taken after the pump has been run for several hours, to make sure that the water sampled is representative of what the orchard will receive ^[N27]. Nitrate in well water can be calculated as the ppm of nitrate-N x 2.72 = lbs N applied in one acre foot of water. For example, a test value of 5 ppm nitrate-N in an orchard receiving two acre-feet of water per year would mean that about 27 lbs N/acre are added annually with the irrigation water. If the concentration of nitrate rather than nitrate-N is given, multiply by 0.614 rather than 2.72. To estimate N credits, only the nitrate-N in the water taken up by the trees, which corresponds to the evapotranspiration (ET) rate, should be counted ^[N27].

Irrigation water quality should be tested prior to orchard establishment. If the water supplies N in excess of crop demand, it can cause excess vegetative growth that will limit production throughout the life of the orchard. A general recommendation is to avoid sites where the irrigation sources contain nitrate-N concentrations exceeding 5 ppm ^[N39].

Leaf samples are taken in July, at which point nutrient concentrations are most stable. If a healthy area is being compared with an unhealthy one, a comparative leaf sample may be taken at any time ^[N20]. Leaf sampling should be done as follows ^[N2,N20,N30]:

• The orchard should be divided into uniform blocks with respect to soil type, tree age, variety, location, and management.
• Trees that have received foliar N and K should not be sampled for at least one week after the application. Leaves that have been sprayed with B should not be sampled for B analysis.
• Samples should be mature leaves from the middle of non-bearing, current-season shoots. A sample of 80-100 leaves, a few leaves from each tree, should be taken. Leaves should be sampled from different points around the tree canopy, at around the same height.
• Unhealthy, stunted or injured trees and trees from non- representative areas are avoided or sampled separately.
• If only macronutrients (NPK) will be tested, leaves do not need to be washed. If B is tested, leaves should be washed in tap water with a little detergent and rinsed twice in distilled water.
• Samples are submitted to a lab as soon as possible. They should be refrigerated until they can be sent if a delay is anticipated.

It's important to bring a notepad along to record tree appearance and vigor in the different blocks ^[N7].

For more information, see Tissue Sampling in Orchards and Vineyards.

Critical nutrient levels in July olive leaf samples ^[N7].

Some studies suggest that 1.7% may be a better upper threshold for sufficient N ^[N13], as decreases in fruit set ^[N9] and oil quality ^[N14] have been observed above that level.

Leaf N may not always indicate when excess N is present ^[N16], so leaf N tests should be interpreted along with observations of tree vigor and performance, and oil quality where appropriate ^[N15]. Shoot growth of 8 to 20 inches per year, with healthy bloom and fruit set, is considered optimal ^[N7,N20]. When N is adequate, no fertilizer N should be applied the following year ^[N37].

Rates for young trees can be calculated by multiplying the rate for a mature tree by the percent canopy cover. For example, an orchard with 10% canopy coverage would receive 10% of the fertilizer required by a mature orchard ^[N7]. July leaf analysis should be performed every year, and rates adjusted to keep them in an adequate range ^[N43]. See Leaf Analysis for more details.

It's not recommended to add N fertilizer or compost to the planting hole, as this may potentially damage the developing root system (Joseph Connell, personal communication). Nitrogen fertilizer needs to be carried to the roots with water. In drip-irrigated orchards N fertilizer can either be injected into irrigation water or placed under the emitters. For dry-farmed trees it should be applied just before a rain ^[N42]. Fertigation is generally recommended, as it allows N supply to be tailored to tree demand throughout the growing season ^[N44].

Normally, the type of N fertilizer is not important ^[N5,N41]. However, a greenhouse experiment with 'Koroneiki' olive seedlings suggests that where irrigation water is high in B, ammonium-based fertilizers may be better than nitrate-based fertilizers, which are associated with greater B accumulation in the leaves ^[N5].

More information on different types of N fertilizer may be found at the International Plant Nutrition Institute website.

For newly planted trees, light N applications should only be started once trees have begun to push new growth in spring ^[N43].

Nitrogen may be fertigated in low doses throughout the growing season, starting around February ^[N37]. Applications before January are not recommended, as they are more at risk of being leached beyond the root zone. ^[N4,N32]. In a trial of irrigated 'Coratina' olives in southern Italy, 4-year old trees took up about 40% of their annual N demand at the beginning of vegetative growth and during fruit set, 30% from fruit set until pit hardening, and the remaining 30% between pit hardening and harvest ^[N4].

Applying N too late can cause winter flushing that can lead to frost damage ^[N28]. Therefore, the last nitrogen applicaitons for the season should be completed by the end of September (Joseph Connell, personal communication).

No fertilizer should be applied if leaf N is adequate or new shoot growth is greater than 20 inches per year ^[N20].

Sufficient N should be available for bloom development, new vegetative growth, and fruit growth. Olives contain 4 to 8 lbs of N in every ton of harvested fruit, or about 20-40 lbs N/acre for a 5 ton/acre crop ^{[N3,N18,N32,N33]}. The N required for new growth depends on many factors including fruit load, variety, orchard spacing and pruning ^[N3,N44]. For California oil varieties, which are usually grown in irrigated, super-high density systems, Vossen ^[N41] suggests an annual N application of 40-100 lbs N/acre. This is in line with values measured for a super-high density 'Arbequina' orchard in Spain, where about 80 lbs N/acre/year were contained in the harvested fruit and prunings in both 'on' and 'off' years ^[N1,N34]. For olives which are grown at a lower density (~ 85 trees/acre), a starting rate of 0.5-1 lb N/tree is recommended ^[N6,N37]. Rates should be adjusted so that new shoot growth remains between 8-20 inches per year and July leaf N between 1.5-2.0% ^[N41].

Application rates should be reduced to take into account N available from non-fertilizer sources. About 60-70% of the N contained in residues from pruning and topping may be released back to the soil within two years if the material is shredded and left in the orchard ^[N29]. For a discussion on estimating N which will become available over the season from the soil organic matter, cover crops and other organic inputs, see Field Specific Nitrogen Fertilizer Adjustments. See Soil Test N for more information on testing for soil nitrate from non-fertilizer sources.

Nitrogen fertilizer is carried to the root system with water. In drip-irrigated orchards N fertilizer can either be injected into irrigation water or placed under the emitters. A general recommendation for orchard crops is to inject fertilizer into the irrigation water in the middle third of the irrigation set. For example, in an 18-hour irrigation set, fertilizer is injected from hour 6 through hour 12. This prevents urea and nitrate from moving below the root zone but still ensures that the N is distributed well in the wetting zone and not overly concentrated, causing root burn ^[N27].

Dry N fertilizer can also be broadcast or spread in the tree row. For dry-farmed trees it should be applied just before a rain ^[N6,N41].

The type of fertilizer does not appear to be important, and choice can be based on price ^[N5,N41].

More information on different types of N fertilizer may be found at the International Plant Nutrition Institute website.

It's inefficient to apply N in late fall or early winter. Olives are semi-dormant in winter and do not take up much N before growth is initiated in early spring, so N applied during dormancy is at risk of leaching ^[N4]. In dry-farmed orchards, or if pelletized N is spread in the orchard, it's recommended to apply N fertilizer ahead of a storm in January, so it can be carried by rain into the root zone but is at less risk of being leached than if it were applied earlier in winter ^[N22,N41].

Nitrogen applications to fertigated olives should start in February, to ensure sufficient N is available for peak demand in March and April ^[N22,N37]. Fertigation may continue throughout the season. Different application timings may be appropriate for "on" and "off" years (see Fertilizing 'on' and 'off' year olives).

If leaf N is low or the fruit load that year was very heavy, it may be beneficial to apply postharvest N. Fall-applied N is stored in the leaves, from which it will be remobilized to support new growth the following spring ^[N31]. Postharvest N may be applied foliarly or to the soil, as long as the weather is warm enough and there is sufficient time for it to be taken up before steady rains begin ^[N31]. However, since non-deficient plants take up little N ^[N17] and unused N may be lost over winter, this technique isn't likely to be efficient unless tree N status is low ^[N8]. Little is known about the relative efficiency of postharvest or early spring N applications.

Nitrogen may not need to be applied every year, especially on fertile or heavy clay soils ^{[N23,N15,N41]}. On sandy soils, frequent light applications are best ^[N23]. If an orchard has been heavily pruned, N fertilizer should be withheld until the trees start bearing again, at which point normal fertilization can resume ^[N36].

The main mechanism for alternate bearing in olive appears to be that the current year's fruit inhibits the vegetative growth and flower induction that would support the following year's crop ^[N11]. Thus, it doesn't seem alternate bearing can be mitigated by adding extra fertilizer ^[N3]. Extra N in 'off' years may exacerbate alternate bearing, and lead to undesirably high N concentrations in the fruit ^[N3,N38]. Thinning heavy crop loads is a better strategy for reducing alternate bearing ^[N19].

While yield is lower in 'off' years, the N demand for new growth is greater. Thus, total N demand may not differ much between 'on' and 'off' years, depending on how much vegetative growth is desired ^[N1,N3]. In California, greater N application rates in 'on' years and lighter rates in 'off' years are generally recommended ^[N47].

To avoid the negative effects of excess fruit set, in 'on' years it's safest to apply relatively more of the N requirement after fruit set ^{[N6, N23]}. In 'off' years, if N is needed, a greater proportion of the total N should be applied early in the season to support spring vegetative growth ^{[N3, N6]}.

The University of California Olive Production Manual recommends applying a 2% solution of low-biuret urea at a rate of approximately 0.4 lbs N/tree ^[N8,N20]. Concentrations as high as 4% urea may be safely used ^[N25].

Urea is quickly absorbed by olive leaves ^[N25], and is the most commonly used foliar N source. Low-biuret urea is recommended. Ammonium nitrate and potassium nitrate are also effective ^[N31].

Foliar N is most likely to benefit the developing crop if sprayed during the early stages of fruit growth ^[N20]. Foliar N sprayed in mid-June or later is not translocated and remains in the leaves for that season, and so doesn't affect the fruit ^[N8,N20].

Postharvest foliar N sprays on table olives may be a good option for raising tree N status when it's too late in the year for soil applications to be efficient. Postharvest sprays on oil olives are too late in the year to be effective (Joseph Connell, personal communication).

Soil P does not vary much during the year, so soil samples may be taken at any time ^[P11]. Soil samples should be taken from the active rooting zone and in the wetting area of the irrigation system. Most root activity in mature irrigated olive trees is concentrated in the top 18 inches, and in the wetting zone of the irrigation system ^[P6,P18]. For more information on soil sampling, see Soil Sampling in Orchards.

The University of California has not developed critical values for soil analysis in olive orchards ^[P11]. For orchard crops in general, an Olsen-P value of less than 10 ppm is considered low ^[P9]. however, as this number hasn't been validated for olives in California any suspected deficiency should be confirmed by leaf analysis and observation of the growing environment.

In California, low leaf P levels are usually caused by by poor soil drainage. In this case, improving drainage is more appropriate than applying P fertilizer ^[P8].

Leaf samples are taken in July, at which point nutrient concentrations are most stable. If a healthy tree is being compared with an unhealthy one, a comparative leaf sample may be taken ^[P8].

Leaf sampling should be done as follows ^[P2,P8,P14]:

• The orchard should be divided into uniform blocks with respect to soil type, tree age, variety, location, and management.
• Trees that have received foliar N and K should not be sampled for at least one week after the application. Leaves that have been sprayed with B should not be sampled for B analysis.
• Samples should be mature leaves from the middle of non-bearing, current-season shoots. A sample of 80-100 leaves, a few leaves from each tree, should be taken. Leaves should be sampled from different points around the tree canopy, at around the same height.
• Unhealthy, stunted or injured trees and non- representative areas are avoided or sampled separately.
• If only macronutrients (NPK) will be tested, leaves do not need to be washed. If B is tested, leaves should be washed in tap water with a little detergent and rinsed twice in distilled water.
• Samples are submitted to a lab as soon as possible. They should be refrigerated until they can be sent if a delay is anticipated.

It's important to bring a notepad along to record tree appearance and vigor in the different blocks ^[P4].For more information, see Tissue Sampling in Orchards and Vineyards.

Critical nutrient levels in July olive leaf samples ^[P4].

In California, low leaf P levels are usually caused by poor soil drainage. In this case, improving drainage is more appropriate than applying P fertilizer ^[P8].

The vast majority of California soils are capable of supplying the amount of P removed by the crop. P fertilization will therefore not normally improve yields; however, to maintain soil P levels over time fertilizer may be added to replace the P removed with the harvested fruits. On average, about 2-4 lbs of P₂O₅ are removed with each ton of harvested olives ^{[P3,P7,P15,P16]}. That is, for a 5 ton/acre crop, 10-20 lbs P₂O₅ are exported from the orchard. For the major California olive oil olive varieties, a nutrient removal calculator is available here.

Additional P is removed with pruning and topping. However, if residues are shredded and returned to the soil about 80% of the P is released within two years ^[P13].

In the rare instances where P application is indicated, applying solid P fertilizers in a strip or ring at the drip line is more effective than broadcast applications ^[P2].

Phosphorus fertilizers can be fertigated. However, care must be taken to prevent the formation of calcium phosphates which can plug the emitters.

Several foliar products also contain P. See Foliar P for more information.

Facts sheets about the most common fertilizers can be found on the International Plant Nutrition Institute website.

Olive trees take up most P in spring, during active shoot growth ^[P6]. However, since P is not very mobile in the soil, application timing is not important.

Soil K does not vary much during the year, so soil samples may be taken at any time ^[K14]. Soil samples should be taken from the active rooting zone and in the wetting area of the irrigation system. Most root activity in mature irrigated olive trees is concentrated in the top 18 inches, and in the wetting zone of the irrigation system ^[K8,K22]. For more information on soil sampling, see Soil Sampling in Orchards.

Vossen ^[K24] recommends that new orchards should be planted in soils with a concentration of at least 125 ppm K.

Leaf samples are taken in July, at which point nutrient concentrations are most stable. If a healthy area is being compared with an unhealthy one, leaves may be sampled at any time ^[K10].

Leaf sampling should be done as follows ^[K2,K10,K16]:

• The orchard should be divided into uniform blocks with respect to soil type, tree age, variety, location, and management.
• Trees that have received foliar N and K should not be sampled for at least one week after the application. Leaves that have been sprayed with B should not be sampled for B analysis.
• Samples should be mature leaves from the middle of non-bearing, current-season shoots. A sample of 80-100 leaves, a few leaves from each tree, should be taken. Leaves should be sampled from different points around the tree canopy, at around the same height.
• Unhealthy, stunted or injured trees and non- representative areas are avoided or sampled separately.
• If only macronutrients (NPK) will be tested, leaves do not need to be washed. If B is tested, leaves should be washed in tap water with a little detergent and rinsed twice in distilled water.
• Samples are submitted to a lab as soon as possible. They should be refrigerated until they can be sent if a delay is anticipated.

It's important to bring a notepad along to record tree appearance and vigor in the different blocks ^[K7].

For more information, see Tissue Sampling in Orchards and Vineyards.

Critical nutrient levels in July olive leaf samples ^[K7].

Potassium fertilizer is only likely to increase yields in K deficient trees. However,to maintain soil K levels over time fertilizer may be added to replace the K removed with the harvested fruits ^[K21]. Fruits from irrigated orchards tend to accumulate more K than those from rainfed orchards. Rainfed olives from Spain and Portugal contained about 7-10 lbs K₂O per ton ^[K9,K19], while irrigated olives from California and Israel contained between 15 and 20 lbs K₂O per ton ^[K4,K20]. Thus, about 75-100 lbs K₂O/acre would be required to replace the K removed with a 5 ton/acre harvest from an irrigated orchard. For the major olive oil varieties in California, a nutrient removal calculator is available here.

Additional K is removed with pruning and topping. However, if residues are shredded and left in the orchard almost all of the K is released back to the soil within two years ^[K15].

When a leaf analysis suggests that K is deficient, larger amounts than what is removed by the crop must be applied to overcome the soil's ability to tie up K. A banded application of 500-1000 lbs potassium sulfate (K₂SO₄) per acre will move K in to the root zone and can correct a K deficiency for several years. When K is fertigated through drip systems, 200-300 lbs/acre K₂SO₄ may be used ^[K6].

Dry K fertilizer can be surface-applied in a concentrated band just inside the drip line. This method is very effective in olive orchards under non-tilled orchard floor management as roots are close to the surface (Joseph Connell, personal communication). On heavier soils, in which K is less mobile, it may be more effective to shank it in below the surface, especially in cultivated orchards ^[K10]. In drip-irrigated orchards K fertilizer may be applied directly under the emitters. Liquid K fertilizers may also be injected into the irrigation system ^[K10,K23].

Foliar sprays can correct K deficiency more quickly, but the effect does not last as long ^[K23]. Soil application, especially by fertigation through drip irrigation, is preferred ^[K6]. See Foliar K for more information.

Many types of K fertilizers can be used. Olives sometimes suffer from chloride toxicity when potassium chloride (KCl) is applied on sandy or sandy loam soils, or soils with poor drainage ^[K10]. Chloride toxicity results in leaf tip burn, and leaf chloride levels greater that 0.5%. Potassium sulfate (K₂SO₄) is safer to use than KCl on light or saline soils, on soils with high perched water tables, on soils with restrictive layers that impede leaching, and on chloride-sensitive varieties ^[K10]. Contact your local farm advisor for more information.

More information on common fertilizer types can be found at the website of the International Plant Nutrition Institute.

Heavy applications of K fertilizer to correct a K deficiency are best made in early winter ^[K10]. In this way, salts and chloride can be leached out of the soil by winter rains. Fertigated potassium sulfate can be applied throughout the growing season ^[K6].

Since K is not very mobile in most soils and will not leach, K applications may not have to be made every year. However, smaller, more frequent applications may be useful on light-textured soils.

Potassium nitrate (KNO₃) can be sprayed at a rate of 10 lbs KNO₃ per 100 gallons of water ^[K7]. A severe deficiency can be corrected by 2-5 sprays ^[K6].

Potassium nitrate (KNO₃), potassium chloride (KCl), potassium sulfate (K₂SO₄), and potassium phosphate (KH₂PO₄) are all effective sources of foliar K. Potassium carbonate (K₂CO₃) may cause leaf burn because of its high pH ^[K18]. Potassium nitrate is the most commonly used material in California and is normally recommended. If KCl is used, leaf chloride should be monitored to ensure that it doesn't become toxic (>0.5%).

Young leaves take up K more efficiently than older leaves, suggesting that in California, foliar K is most efficiently applied during the spring flush ^[K7,K18]. Trials on two drip-irrigated commercial 'Manzanillo' table olive orchards in Israel found that spraying a 3% solution of an 8-16-40 foliar product two weeks after full bloom increased the percentage of large fruits without reducing yields in both 'on' and 'off' years; the same benefit was not observed for a spray at pit hardening ^[K3].

Boron deficiency first appears as chlorotic tips on the youngest leaves. As the deficiency progresses tips become necrotic, followed by a band of yellow and a green base. In contrast, K deficient leaves don't normally have any transition between the dead and healthy tissue. Late in the season, new leaves are small and distorted. In severe cases, upper twigs suffer from dieback.

Boron is important to pollen tube formation, and deficiencies reduce fruit set ^[B13]. Immature fruits may drop early, in July-August. If fruits don't drop, they can sometimes be misshapen ("monkey-faced") with the pit protruding from the bottom of the fruit in severe cases. Young trees may die in 2 to 3 years ^[B6,B12].

In California, B deficiency is more often observed in the foothills, or on very sandy soils ^[B6].

Boron toxicity first appears as marginal chlorosis on the mature leaves, which extends to the limb. Leaves fall early, and shoot growth is reduced ^[B1].

Olives are more tolerant than most other tree crops of high B levels in soil and irrigation water ^[B12]. However, B toxicity can occur if fertilizer is misapplied ^[B6] or when irrigation water is high in B. See Soil Test B for more information.

Soil samples for B analysis can be taken at any time. Normally only the top 6 inches are analyzed ^[B4]. For detailed sampling instructions see Soil Sampling in Orchards. Hot water extracts are usually used to detect B deficiency, while a saturated soil paste is used to assess B toxicity ^{href="#B">[B7]}.

A survey of olive orchards in Greece found that B-deficiency symptoms and low leaf B generally corresponded with a hot-water-extractable soil B concentration of 0.33 ppm or less. This threshold is slightly lower than that of 0.5 ppm traditionally used for olives ^[B2]. However, in the Greek study deficiencies were sometimes observed in very shallow soils with much higher B concentrations (up to 0.96 ppm). Conversely, in deep soils low surface B concentrations were not always associated with low leaf B.

Soil and irrigation water should be tested prior to planting the orchard. Olive is relatively B tolerant; however, in a few areas in California B toxicity may occur. Boron concentrations of over 2 ppm in either the soil (saturated paste method) or the irrigation water indicates that excess B may become a problem ^[B8].

Leaf samples are taken in July, at which point nutrient concentrations are most stable. If a healthy tree is being compared with an unhealthy one, leaves may be sampled at any time ^[B7].

Leaf sampling should be done as follows ^[B3,B7,B10]:

• The orchard should be divided into uniform blocks with respect to soil type, tree age, variety, location, and management.
• Trees that have received foliar N and K should not be sampled for at least one week after the application. Leaves that have been sprayed with B should not be sampled for B analysis.
• Samples should be mature leaves from the middle of non-bearing, current-season shoots. A sample of 80-100 leaves, a few leaves from each tree, should be taken. Leaves should be sampled from different points around the tree canopy, at around the same height.
• Unhealthy, stunted or injured trees and non- representative areas are avoided or sampled separately.
• If only macronutrients (NPK) will be tested, leaves do not need to be washed. If B is tested, leaves should be washed in tap water with a little detergent and rinsed twice in distilled water.
• Samples are submitted to a lab as soon as possible. They should be refrigerated until they can be sent if a delay is anticipated.

It's important to bring a notepad along to record tree appearance and vigor in the different blocks ^[B7].

For more information, see Tissue Sampling in Orchards and Vineyards.

Critical nutrient levels in July olive leaf samples ^[B7].

Between 14-19 ppm B, olive trees may not show deficiency symptoms but they may respond to B application ^[B11].

Boron deficiency can be corrected by applying about 5-10 lbs/acre actual B, which corresponds to 45-90 lbs/acre borax (11% B). This should correct deficiency for several years ^[B6].

Borax is the most cost-effective solid B fertilizer. Solid B fertilizer can be broadcast on the soil surface within the drip line, in winter. Solubor®, a solid 20.5% B material, is more expensive but is commonly available, readily dissolves in water, and is moved into the root zone by rainfall.

Boron can also be applied as a foliar spray, for correcting deficiencies or increasing fruit set. Soil applications are a more long-lasting solution to deficiencies ^[B8] (See Foliar B).

Solid B applications will move into the soil with rainfall and may be applied near the end of winter ^[B5].

A spray of 7 oz B/100 gallons of water can temporarily correct B deficiency ^[B6]. This is equivalent to about 2.1 lbs Solubor® (20.5% B)/100 gallons.

For increasing fruit set, good results are most consistently obtained with about 3-4 oz B/100 gallons of water, sprayed at a rate of 100 gallons/acre ^[B9,B11,B13]. This is equivalent to about 1 lb Solubor® (20.5% B) per 100 gallons of water.

Foliar B is most commonly sprayed as Solubor® (20-21% B) ^[B5]. More information about common B fertilizers may be found here.

Foliar B should be sprayed about two weeks before flowering. Fruit set improvement is normally only seen in 'off' years ^[B9,B14].