Citrus fruits – cash crops of the Americas

CITRUS FRUIT FERTILIZATION

Citrus fruits – cash crops of the Americas

Citrus fruit growers are an attractive end-market for fertilizer suppliers due to the high K and N requirement of this widely-cultivated cash crop and the efficacy of fertigation and foliar spraying. We examine the nutrient needs of citrus trees and how balanced application of fertilizers helps maximise citrus fruit quality and yield.

Oranges ripening on a tree.

PHOTO: JANAPH/SHUTTERSTOCK.COM

Large volumes, modest growth

Citrus are the second most widely traded fruits globally, after bananas, and remain lucrative cash crops for the countries of the Americas. With worldwide fresh fruit exports of more than 17 million tonnes annually – valued at around $4.1 billion – they generate significant export earnings for the United States ($903 million), Mexico ($752 million), Chile ($439 million), Peru ($287 million), Argentina ($264 million) and Brazil ($138 million).

World citrus production has undergone a major expansion in recent decades, having more than doubled in the last 30 years to exceed 160 million tonnes per annum currently (Figure 1). Consequently, more than 10 million hectares of land is now devoted to citrus cultivation globally.

Although citrus is a large volume market, accounting for about one-fifth of the global fruit trade, growth rates have stagnated. The 10-year average for citrus is 1.5 percent, for example, versus growth rates of 3.3 percent for banana, 12-13 percent for avocado and blueberries, and 2.1 percent for the fruit market overall.

“Citrus fruits, while remaining a leading fruit and vegetable category, will face significant challenges to maintain their position in an increasingly globalised, complex, and competitive market,” comments trade body the World Citrus Organisation. “This will be especially due to strong growth in mango, kiwi, table grape, and avocado, as well as the reduction of fruit consumption due to lifestyle changes and growing competition from other food products.”

Fig 1: Citrus fruits – world production, growing area and average yields, 2021

Regionally important

Latin America countries, together with California and Florida in the United States, are responsible for around one third of the world’s citrus production. Brazil, the US, Mexico and Argentina are all top 10 citrus fruit producers globally (Figure 2) – with the Americas having a particular focus on orange, lemon and lime cultivation. Notably:

Fig 3: World’s top 10 orange producers (‘000 tonnes), 2019

• Brazil is the world’s largest producer of oranges – harvesting almost as much as China and India, the two next largest producers, combined – while the US and Mexico are the fifth and sixth largest orange producing countries, respectively (Figure 3)
• Brazil is also the world’s largest producer and exporter of orange juice, according to the USDA, exporting almost all of the 1.1 million tonnes it produced domestically in 2022/23
• Mexico, Argentina and Brazil, respectively, are also the world’s second, fourth and fifth largest producers of lemons and limes (Figure 4)
• Brazil is a top 10 producer of mandarins (Figure 5) while Mexico is a top 10 grapefruit producing nation (Figure 6).

Leading producers

Oranges remain the most popular commercially grown citrus fruit, accounting for almost half of world citrus production. Lemons and limes provide a further one-quarter of global citrus fruit output and are followed by tangerines and grapefruit in order of production volume (Figure 1). Geographically, citrus production is distributed as follows1 :

• Oranges. Growing is concentrated in Brazil, China, the US and Mexico, together with Spain, Italy and Greece in the Mediterranean region (Figure 3)
• Lemons and limes. Asia (India, and China), the Americas (Mexico, Argentina, Brazil and the US) and the Mediterranean countries of the EU (Spain and Italy) are the main global growing regions (Figure 4)
•  Tangerines and grapefruit. China is the leading global grower of grapefruit and easy-to-peel citrus varieties, such as tangerines, mandarins, clementines and satsumas, and has an impressive one-quarter share of the world citrus market (Figures 2, 5 & 6).

According to USDA figures, almost two-thirds of world orange production (47.8 million tonnes) was consumed as fresh fruit (30.2 million tonnes) while just over one-third (17.2 million tonnes) went for processing in 2022/23, yielding 1.5 million tonnes of orange juice. Of these amounts, 4.6 million tonnes of fresh fruit and 1.4 million tonnes of fruit juice were exported from the major producing countries2 .

Citrus production trends

The latest USDA figures show 6.5 million tonne annual declines in global orange production over the last five years, partly offset by four million tonne per annum growth in tangerine/mandarin production over this period (Table 1).

USDA estimates that US orange production will fall by more than 25 percent to 2.3 million tonnes in 2022/23 its lowest level in over 56 years. Production in Florida has fallen precipitously over the last decade, from 8 million tonnes in 2011/12 to the less than one million tonne estimate for 2022/23. Recent production declines are linked to extreme weather and falling yields resulting from ‘citrus greening’ disease afflicting the state’s groves.

Fig 5: World’s top 10 tangerine producers (‘000 tonnes), 2019

Fig 6: World’s top 10 grapefruit producers (‘000 tonnes), 2019

“Yields continue to decline in Florida due to fruit drop caused by citrus greening, reduced area harvested and high winds from hurricanes,” USDA said2 . “California is estimated to produce over twice as many oranges as Florida in 2022/23.”

Hurricanes have been a particular problem for Florida’s citrus farmers in recent years. The high winds and damaging rains from Hurricane Irma that hit key citrus-producing regions in 2017 resulted in the smallest Florida crop yield in over 70 years. More recently, Hurricane Ian’s landfall in southwest Florida in September 2022 is estimated to have caused up to $675 million of damage to citrus crops and grower infrastructure in the state.

Table 1: Fresh citrus fruit production (‘000 tonnes), 2018/19 to 2022/23

California’s citrus output has been more stable. The state accounted for 79 percent of total US citrus production in 2022/23, while Florida totalled 17 percent, and Texas and Arizona produced the remaining 4 percent, according to USDA. Although the 2022/23 US citrus crop is valued at $2.58 billion (at the packinghouse door), this represents a 13 percent fall on the last season.

Varieties, climate and soil

The genus Citrus is a type of evergreen tree native Israel to tropical and subtropical Asia. Belonging to the family Rutaceae, it Turkey numbers around 1,500 species, the main commercial, fruit-producing varieties being: Sudan

• Orange (Citrus sinensis [L.] Osbeck)
• Mandarin (Citrus reticulata Blanco) Thailand
• Lemon (Citrus limon [L.] Burm.)l Lime (Citrus Indiaaurantifolia [Christ.] Swing.)
• Grapefruit (Citrus paradisi Macf.)
• Pomelo South (Citrus Africagrandis [L.] Osbeck)

Due to their Mexico intolerance to low temperatures and frost, major citrus growing regions are generally to USbe found in two bands between 25-35 degrees either side of the equator.

Citrus growing in semi-tropical and Mediterranean climates is most favourable as it China results in smooth skinned, bright coloured fruit with an ideal balance of sweetness and acidity. Sweet oranges and mandarins thrive in sub-tropical regions, where the hot humid summers and mild winters yield large, good quality, sweet fruits with a high juice content, making them ideal for either processing or fresh consumption. Navel oranges, blood oranges and lemons, in contrast, are almost exclusively grown in Mediterranean-type climates.

Citrus trees prefer well-drained, low salinity soils with a pH of 5.5-7.0. Although they can be grown on a range of soil types, a well-structured soil ensures root aeration and helps avoid root disease. Acid soil conditions are avoided as citrus yields almost half below pH 4.5.

Citrus trees develop shallow, near-surface root systems in the area underneath their canopy. These require careful water management to avoid root damage. Irrigation is common in many citrus growing areas outside the tropics as higher fruit yields are typically obtained from irrigated groves compared to those in rain-fed areas.

Nutrients and citrus quality

Nutrient and water availability both have a major influence on citrus fruit quality and yield. Fruit size, colour, juice content, sugar content (expressed as total soluble solids, TSS) and acid concentration are some of the most important quality factors for citrus growers, processors and packers.

Distinctly different qualities are required for the fresh fruit and processing markets. Size, shape, colour and maturity date are valued in fresh fruit, whereas high juice and

soluble solids contents are the main quality factors in fruit processing. Importantly, the external characteristics favoured by fresh citrus produce, such as fruit size and rind coarseness, require lower N and higher K applications than are necessary for fruits grown for processing.

Potassium followed by nitrogen and then calcium are the nutrients removed in greatest quantity by citrus trees (Table 2). The high level of potassium removal is linked to the high K content of citrus juice.

Citrus trees are able to store significant amounts of nutrients in the roots and trunk and later redistribute these to developing leaves and fruits. The nutrient requirements of citrus trees also vary at different growth stage (Table 3).

Leaf analysis is used to monitor the nutrient status of citrus trees, identify deficiencies and tailor fertiliser rates to ensure the correct ratio of plant nutrients. This is supplemented by soil analysis of pH and available N, P, K, Ca and Mg.

Nitrogen for juice and colour

Nitrogen is critically important in citrus production as it has more influence on tree growth, appearance, fruit production, and fruit quality than any other nutrient3 . Fruit yield is largely regulated by N due to its contribution to photosynthesis, carbohydrate production, leaf weight and carbon allocation within trees.

Table 2: Major nutrient and micronutrient removal by citrus fruits

Table 3: Nutrient effects at different citrus growth stages

Mature trees require N at around 100300 kg/ha, depending on environmental factors, the irrigation system and target yield. A fruit yield of 40 t/ha, for example, removes about 50 kg of N from soil. Although fruit yields generally correlate with N application, 200 kg/ha applied annually is thought to be sufficient to sustain good yields and tree development. In fertigation, however, yields continue to increase at N applications of up to 300 kg/ha4 .

‘Luxury consumption’ from excess N can reduce the commercial value of harvested fruits by affecting fruit quality and shortening storage life. The fruit becomes large and puffy, skin thickens and coarsens, and the percentage and quality of juice also declines5 . These adverse effects become exacerbated when P is low.

Visible signs of nitrogen deficiency are rare but citrus trees will show symptoms – leaves turning light green to yellow and dropping early is one sign – when leaf N content falls below two percent. However, large falls in N over a prolonged period need to occur before citrus yields are affected.

This is because citrus trees have the capacity to adjust to inadequate nitrogen application by recycling stores of N, usually from older leaves into newer ones.

Increasing N application results in the following changes to fruit yield and quality6 :

• Higher juice content, total soluble solids (TSS) and acid concentration and improved colour
• Increases in TSS per hectare, although excessive N reverses this trend, particularly with inadequate irrigation
• Decreases in fruit size and weight l Increases in peel thickness and numbers of green fruit at harvest
• Increasing incidence of creasing and scab but decreasing incidence of peel blemishes.

Potassium for size

The large amounts of potassium removed by citrus fruit make the application of K fertilizers essential for maintaining soil productivity. The production of one tonne of oranges, for example, takes up around 2.5 kg of K2 O, corresponding to a soil removal rate of 125-250 kg/ha.

“Nutrient availability is a major influence on citrus fruit quality and yield”

Potassium improves the external characteristics of citrus fruit but can reduce juice yield and quality. In Florida, Brazil and Australia, potassium application has been found to increase fruit production until leaf K content reaches 1.5–1.7 percent.

Insufficient K typically produces small fruit with thin rinds which are prone to creasing or splitting, making them unsuitable for the fresh fruit market and export. Excessive levels of K, in contrast, results in large fruits with coarse, thick rinds and poor colour.

Higher applications of K are associated with:

• Larger fruit size, weight, green fruit and peel thickness
• Reduced incidence of creasing and fruit plugging
• Less stem-end rot in stored fruit
• Higher fruit production and TSS per hectare
• Reduced juice content, TSS and juice colour
• Increased acidity.

The effectiveness of K applications varies widely with soil type. Potassium uptake by citrus trees is highest in acid, sandy soils in humid regions such as Florida.

Phosphorus for growth

Phosphorus application is particularly important for early tree growth in new groves and maintaining fruit yield and quality in mature groves. However, application rates are relatively low, with the exception of South Africa and parts of Florida, as citrus trees have a limited P requirement compared to N and K4 . The production of one tonne of citrus fruit requires only 0.2 kg of P, for example, and a fruit yield of 40 t/ha removes just 8 kg of P per hectare.

In P deficient trees, leaves become a dull bronze-green colour and are shed from young shoots, and tree growth and fruiting are also reduced. Low P produces misshapen fruit with coarse, thick rinds and acid juice. Ensuring a balanced supply of nitrogen and phosphorus is important as excess N can exacerbate P deficiency.

The application of P is associated with the following changes to fruit quality:

• No change or a slight decrease in fruit size
• A decrease in rind thickness
• No change or slight increase in TSS
• Reduction in juice acidity
• Lower juice vitamin C content.

Ameliorating soil acidity

Citrus trees contain more calcium than any other metal and Ca plays a vital role in regulating the uptake of other nutrients such as potassium and magnesium. Under normal conditions, however, soil Ca levels and general fertilisztion practice are usually enough to satisfy citrus growing requirements4 . Irrigation water can also supply Ca in appreciable amounts.

As well as its role as a nutrient, adding calcium can improve the physical properties of heavy soils and reduce soil acidity. Citrus trees are particularly sensitive to acidity and stop growing below pH 5.0 due to root system damage. Dressings of dolomitic or calcareous limestone to correct pH are known to increase yields by up to 200 percent4 .

Magnesium, a constituent of chlorophyll, is found in the leaves and shoots of citrus trees and deficiency is very common on highly acid, low Mg soils. This results in leaf chlorosis and bronzing in older leaves.

Fertilizer recommendations

Fertilizer recommendations for young trees vary with soil type, climate and intensity of cultivation. The N, P and K applications rates for establishing trees on loamy, organic-rich Brazilian soils, for example, are lower than those for sandy, low organic matter Floridian soils. Recommendations in the two countries also vary because Brazilian groves are mostly unirrigated and begin producing in year three, whereas citrus fruits in Florida are more intensively grown and groves are irrigated because of low water retention in soils.

Fertilizer rates during the first three years in citrus groves are calculated on a per tree basis. A minimum of 4-6 annual applications of dry fertilizer is recommended for young trees, whereas 10 or more yearly fertigation applications are usual at this stage.

Citrus trees are commonly harvested for fruit from year four onwards. N and K are key nutrients at this stage whilst P is less critical because of the much smaller quantities removed by fruit. Mature trees are therefore generally fertilized at a N:P2 O5 :K2 O ratio of about 1.0:0.2:1.0. Around 3-6 kg of nitrogen is required to produce a single tonne of fruit and N applications rates are often used as the basis of recommendations for other nutrients.

In Florida, N is applied to oranges at a rate of 135-225 kg/ha (120-200 lbs/acre) in years 4-7 and then at 160280 g/ha (140-250 lbs/acre) from year eight onwards. These amounts of N are supplied by at least one application for controlled-release fertilizers, by 3-6 separate applications in field dressings and 10 fertigation applications each year. The generally accepted rule for citrus K application is to follow exactly the same application rate as N for both young and mature tree. Identical N and K application rates, for example, are recommended by Haifa in its citrus guide.

Phosphorus application rates are determined on the basis of leaf analysis and soil testing results. Application of P is generally only recommended when soil P is insufficient and leaf P falls below optimum3 .

Magnesium fertilizers are applied, either to soils or as a foliar spray, at 20 percent of the N application rate, If leaf Mg is below optimum and soil tests show medium or low Mg levels. Liming the soil to regulate pH at 6.0-6.5 usually supplies sufficient Ca. If soil pH is maintained, there is no need to apply gypsum or soluble calcium fertilizers unless soil test and leaf analysis show levels are insufficient and below optimum.

Micronutrients are applied by foliar spraying or directly to soil on an ‘as-needed’ basis in response to low leaf analyses or visible signs of leaf deficiency. Foliar applications of Mn, Zn, Cu, B and Mo are generally much more effective and economically practical than soil applications. Foliar spraying usually takes place after the full leaf expansion of new growth. Copper is not applied separately if Cu fungicides are used. Molybdenum deficiency occurs in very acid soils and is a potential indicator of aluminium toxicity. Iron deficiency can be corrected using an Fe chelate.

Cash crop opportunities

The status of citrus fruit as a profitable cash crop makes it a lucrative target market for leading fertilizer suppliers. Numerous bespoke and speciality products are now available for citrus growers, backed by detailed recommendations and extensive agronomic research. Although not exhaustive, a snapshot of currently-available citrus fertilizer products is provided below.

Haifa Chemicals’ Multi-K potassium nitrate (NOP) and Solu-Potasse potassium sulphate (SOP) from Tessenderlo Kerley International provide citrus growers with chloride-free, water-soluble sources of major nutrients and are suitable for fertigation. Tessenderlo’s K-Leaf SOP product is also suitable for foliar application to citrus. SQM offers formulations that are well-suited to the nutrient needs of citrus trees in its field-applied (QDrop), fertigation (Ultrasol) and foliar (Speedfol) NOP product ranges.

Monoammonium phosphate (MAP) products for citrus fertigation include SQM’s UltrasolMAP, Haifa’s Multi-MAP and ICL’s NovaMAP. Citrus micronutrient deficiencies can be addressed using Yara International’s YaraVita range, for Mn, Zn and B deficiency, for example, and SQM’s Ultrasolmicro iron chelate products for Fe deficiency. Both Yara (YaraLiva Tropicote) and SQM (QDrop Calcium) also market calcium ammonium nitrate (CAN) fertilizers for citrus groves.

References

1. FAO, 2020. Citrus Fruit, Fresh And Processed, Statistics Bulletin 2020. United Nations Food and Agriculture Organisation, Rome.

2. USDA, 2023. Citrus: World Markets and Trade. United States. Department of Agriculture Foreign Agricultural Service, July 2023.

3. Obreza, T. & Morgan, K., 2011. Nutrition of Florida Citrus Trees, Second Edition. University of Florida IFAS Extension.

4. Erner, Y. et al., 1999. Fertilizing for High Yield Citrus. IPI Bulletin No 4, 2nd Revised Edition. International Potash Institute (IPI), Switzerland. 5. NSW, 2002. Citrus Nutrition. Agfact Series H2.3.11, Second edition. NSW Department of Primary Industries.

6. Zekri, M. et al., 2003. Irrigation, Nutrition, and Citrus Fruit Quality. Document SL 207. Revised 2009, reviewed 2015. University of Florida, Soil and Water Science Department.

CITRUS – LOOKING FOR BALANCED NUTRITION

Tessenderlo Kerley International share some insights on the nutrients needs of citrus trees and combatting citrus greening – a devastating bacterial disease spread by the Asian citrus psyllid insect.

Fertilization aims and application rates

Citrus fertilization prior to planting is aimed at promoting good rooting and rapid development, whereas the annual fertilization programme for established citrus trees targets balanced nutrition to optimise yield and quality. Fertilization choices will be largely guided by soil analysis complemented by foliar diagnosis.

Citrus cultivation requires an NPK ratio of 1-0.2-1.3. As a rule of thumb, fertilization rates of 130 kg/ha N, 25 kg/ ha P2 O5 , 170 kg/ha K2 O and 20 kg/ha S are recommended to balance nutrient removal. This is based on a fresh fruit target yield of 60 t/ha in a fertile soil.

The efficiency of fertilizers, as well as irrigation, greatly depends on root location. The roots of citrus trees are generally shallow and located just beneath the canopy. Fertilizer efficiency is also sharply reduced when the tree canopy lacks space and sun. If branches are too close, a programme of pruning should be implemented as a priority, with severe pruning necessary in some cases.

Chloride sensitivity and sulphur supply

Citrus fruit are very sensitive to chloride. According to the variety, the maximum permissible chloride level in soil water to avoid leaf injury is 20-50 mol/m3 (equivalent to an EC of 2-5 mmhos/cm). Fertilizers with a high chloride content, such as potassium chloride (MOP), should therefore be avoided. Potassium sulphate (SOP) has the lowest salt index of all potash fertilizers – which is why it is recommended for citrus crops instead.

Sulphur is an important nutrient for plants and plays a key role in protein synthesis. For citrus fruit, the normal sulphur content in the leaves is 0.20– 0.40 percent of dry matter, with 70 percent of S-proteins contained in the chloroplasts (photosynthesis). The sulphur content of SOP (18 percent) largely covers citrus requirements.

Blossoming orange tree bearing fruit

PHOTO: ELLEN LEVY FINCH

The benefits of SOP are not limited to yield improvements. Potassium sulphate also has a positive effect on the size of fruits, sugar content, juice production, fruit colour and the edible part of the fruit.

Fertigation and foliar application

The method of fertilizer application plays a key role in its efficiency, especially for potassium which is not mobile except in sandy soils.

Since the uptake of nutrients occurs at root level, fertilizers must be applied as close to these as possible, either on the soil surface or ‘side dressed’ along the line of the trees, ideally at a depth of 15-20 centimetres in clay soils, or at a distance of around 50 centimetres from the trunk when using fertigation drippers.

The application of fertilizers via irrigation water – fertigation – is made according to the age of the trees, their productivity and the individual requirements of each vegetative growth stage. Tessenderlo’s SoluPotasse® is a completely water-soluble grade of SOP specially prepared for fertigation, while the company’s K-Leaf® water-soluble SOP product is ideal for foliar application.

Foliar sprays are an efficient way of supplying nutrients to citrus trees. They can complement other nutrient sources or, when severe deficiency is present, supply nutrients while the soil’s nutrient content is being replenished.

Foliar applications of SOP have a positive effect on citrus production and quality. In trials on Clementines in Egypt, for example, two sprays of K-Leak (application rate of 21 kg K2 O/ha), applied at the beginning of fruit growth and mid-growth improved fruit yields by nine percent versus the control.

Tackling citrus greening

University of Florida research has shown that fields irrigated with alkaline water raises the pH of soils above the optimum for combatting citrus greening. Ammonium thiosulphate solution (Thio-Sul® ) has been shown to be more effective than other acidifiers at lowering soil pH more uniformly throughout the top two feet of soil.

For greening-affected citrus trees, Thio-Sul® should be applied at a rate of up to 12 gallons per acre (112 l/ha) via micro sprinkler irrigation and repeated every 14 days, as necessary. This fertilizer recommendation has been tested on citrus groves in Florida and found to be valid in situations where alkaline irrigation water is used.

Further reading

Marchand, M., 2020. Sulfate of Potash: More than 100 years of experience. Tessenderlo Kerley International. Available at: https://mailchi.mp/cropvitality/ sopbook [Accessed 6/10/23]