Fertilizer International 517 Nov-Dec 2023
30 November 2023
The road to high yielding and sustainable citrus production
CITRUS FRUIT FERTILIZATION
The road to high yielding and sustainable citrus production
A proper fertilization programme is necessary to sustain both citrus productivity and soil fertility over the longer term. ICL agronomists Fabio Vale, William Wang, Patricia Imas and Francisco Morell outline the nutrient needs of citrus fruit – a regionally important crop in the Americas, Europe and China.
Citrus nutrient requirements
The citrus fruit family comprises of several tree species, most notably oranges (Citrus sinesis), lemons (C. limonum), grapefruits (C. paradisi ) and mandarins (C. reticulata), with each type of citrus fruit having several dozen varieties. Although the nutrient needs of citrus trees are broadly similar, their exact requirements will vary according to the species cultivated and/or plantation growing conditions.
In general, high yielding citrus trees will take up large quantities of the major mineral nutrients – nitrogen (N), phosphorus (P), potassium (K), and calcium (Ca). These macronutrients need to be supplemented by minor quantities of magnesium (Mg), sulphur (S) and micronutrients.
Most of the nutrients removed by citrus trees during the growing season are subsequently exported with the fruit at harvest. N, P and K, for example, are generally exported at a ratio of 3:1:5. More precisely, one tonne of harvested oranges removes1 :
- 1.18-1.90 kg of N
- 0.17-0.27 kg of P
- 1.48-2.61 kg of K
- 0.36-1.04 kg of Ca
- 0.16-0.19 kg of Mg
As with most crops and farming systems, fertilization management is essential to properly regulate the uptake of nutrients by citrus plants and maintain nutrient availability during the growing season in response to demand. A proper fertilization programme is also necessary to maintain soil fertility and citrus productivity over the longer term.
ICL’s fertilization programmes for citrus
The fertilization programme recommended for citrus varies according to the climatic conditions and production system (irrigated or rainfed). In general, irrigated production systems are found in the Mediterranean climates of both the northern and southern hemispheres – including California, Chile, parts of Australia, South Africa, and the growing areas that fringe the Mediterranean basin in Israel, north Africa, and southern Europe. Rainfed production systems, meanwhile, predominate in tropical and wet subtropical climates that are typical of the main citrus producing regions of China, India, and South America.
For citrus plantations in full production, the fertilization programme needs to consider the nutrients status of soils and crops, the yield target, and the end-market for the produce.
Orange and mandarin plantations growing fruit for fresh consumption, for example, need around 160, 100 and 180 kg/ha of N, P2 O5 and K2 O, respectively, for a yield of 40-50 t/ha, assuming a fertile soil and a well-nourished plantation. These application rates will, however, need to be adjusted according to the nutrient status of the plantation, the end-market and the yield target.
For oranges destined for the juice industry, the fertilization plan requires a higher nitrogen application rate together with a slightly lower application rate for potassium – although, again, these rates need to be adjusted for the yield target and nutrient status of the plantation.
In rainfed systems, fertilizers are applied to the soil surface with two-thirds applied under the canopy and one-third outside. The total fertilizer supply is usually split between five or six applications across the growing season. The use of controlled-release fertilizers (CRFs), such as ICL’s Agromaster product, is advantageous, as this allows the number of applications to be reduced while, by avoiding ‘plant hunger’, improving yields as well as nutrient use efficiency (NUE) in most cases.
Citrus fertilization programmes can also include ICL’s Polysulphate fertilizer, either applied together in combination with Agromaster or as a separate application. This multi-nutrient (K, Mg, Ca, and S) polyhalite product continues to fertilize citrus plants following rainfall events, as its special solubility pattern prolongs nutrient availability.
In irrigated systems, crop fertilization is performed via fertigation. ICL’s Agrolution family of water-soluble fertilizers (WSFs) are ideal for the fertigation of citrus trees These products, as well as components such as PeKacid, provide a range of formulations with different nutrient balances for each development stage of the crop.
A fertilization programme that combines fertigation (e.g., Agrolution) with soil application (e.g., Agromaster and Polysulphate) can be suitable for citrus growing in areas with meaningful spring rainfall (>200-300 mm). Fertigation can avoid periods of plant nutrient hunger, while the application of Polysulphate ensures the supply of secondary macronutrients to the crop during wet periods.
Fertilization management for organic citrus farming
Organic farming practice involves maintaining citrus productivity levels by building up soil fertility over the medium- and long-term. In both irrigated and rainfed systems, organic citrus growers rely on base fertilization of the soil to provide an increased proportion of the total nutrient supply. Soil fertilization can include both recycled nutrient sources, such as compost and organic fertilizers, and natural mineral fertilizers, such as Polysulphate. Comprising of the naturally-occurring mineral polyhalite, Polysulphate is certified as a suitable organic input for farming systems in the main citrus producing countries.
It is also now possible for organic citrus growers to complete their nutrient supply through fertigation – using this to activate soil nutrient mineralisation and improve nutrient uptake by the plant. In countries such as Spain, for example, ICL offers the Flecotec range, a whole family of organic and mineral fertigation products, for organic farming. These can be used in combination with Polysulphate, which is also organic and, uniquely, the world’s only polyhalite fertilizer.
Evidence from global trials
Polyhalite (Polysulphate) has been shown to improve the productivity and nutrients status of citrus trees across trial results from different regions.
For example, improvements to the yield of sweet oranges (fruit counts and size) and plant nutrient status were observed in Polysulphate trials in Brazil2 . Combining the use of Polysulphate, at an application rate of 400 kg/ha, with potassium chloride increased orange yields by 30 percent, versus the control in which potassium chloride was applied alone as the sole potassium source (Figure 1).
ICL experiments with Polysulphate in China have also shown increased grapefruit yield and quality. The application of Polysulphate also reduced N, P and K inputs by 44 percent, 38 percent and 17 percent, respectively, versus standard NPK practice. This lowered fertilizer costs by 11 percent while raising the income of growers by seven percent, compared to conventional NPK fertilization.
Citrus trials with the CRF product Agromaster have also demonstrated proven benefits. When applied in mandarin plantations at a rate of 4 kg/tree, Agromaster (18-1018) delivered a yield increase of 23 percent and – by improving fruit size – raised the percentage of commercial fruit, versus growers practice that applied conventional fertilizers at similar rates (Figure 2).
Deficiencies and foliar analyses
It is usually possible to visually recognise severe deficiency symptoms in citrus plants – as these are clearly visible in either old or young leaves, depending on the nutrient. In general, severe deficiencies of N, P, K, Mg show in old leaves, while severe Ca, S, Fe, Mn, and Zn deficiencies show in young leaves.
Calcium deficiency notably occurs in fruits and young leaves. Unlike certain other nutrients, Ca cannot be mobilized from older tissues and redistributed via phloem. Instead, developing plant tissues rely on the immediate supply of Ca in the xylem – which is dependent on transpiration – and, consequently, Ca deficiency always shows up in young leaves and fruits first.
Leaf analyses enable more precise measurement of nutrient deficiencies and can therefore be used to guide the fertilization strategy at citrus plantations. These analyses are particularly useful when assessing crop nutrient uptake and then adjusting crop management accordingly.
Leaf sampling for nutrient analysis is typically performed on the 4–7 months old leaves of non-fruiting twigs or fruiting terminals1,3 . On fruiting terminals, samples are collected on the third and fourth leaves away from the fruit towards the plant, with sampling carried out when fruit reaches the size of a table tennis ball. Leaf nutrient contents are then checked against reference values1,4 , as well as locally developed technical guidelines.
Foliar nutrition and biostimulants
Foliar nutrition programmes can be widely applied in all kinds of citrus production systems, being suitable for both irrigated and rainfed systems and cultivation in all kind of climates.
During the establishment of new plantations, citrus plants usually require intensive crop protection programmes during their first 2–3 years. This is necessary to enable plant growth and control leaf miners (Phyllocnistis citrella) and other pests. The pressure from pests, as well as the growth and nutrient needs of citrus plants, is higher during warm periods. Crop protection is therefore practiced during these periods, typically by applying foliar sprays. Spraying also offers the opportunity for an intensive foliar nutrition programme. Citrus varieties with extended vegetation periods, for example, are sprayed as frequently as once every 10 days.
A full foliar nutrition programme for citrus plantations can be designed using ICL’s Agroleaf and Nutrivant product ranges, as these offer a wide range of formulations for each development stage. Agroleaf Power improves the assimilation of nutrients by the plant – thanks to its DPI and M-77 growth enhancer technologies. Nutrivant Booster, meanwhile, comes in macro- and micro-nutrient formulations designed for citrus crops.
Nutrient products also incorporate Ferti-Vant technology to improve the spread of nutrients over the leaf surface. This can boost the uptake of applied nutrients for a period of more than three weeks, helping to deliver better fruit yields, higher quality fruit, and improve grower profits.
Additionally, in certain citrus growing regions, such as Brazil, ICL offers a complete biostimulant programme to improve productivity and resilience to biotic and abiotic stresses. These products can be applied within foliar sprays alongside nutrient and crop protection components.
New citrus plantations
When starting a new plantation, the CRF Agroblen can be added to the planting hole (50-100 grams per tree) during transplanting operations – in either rainfed or irrigated production systems. Agroblen, by fulfilling nutrients needs throughout the first year, helps young plants to become well-established, thereby reducing the numbers that will require replanting.
During the establishment of new plantations, the soil fertilization programme can be complemented with an intensive foliar fertilization programme, as described previously. When additional nutrient supply is required, these can provided by applying fertilizers to the soil surface, in circular strips around the tree canopy, or via irrigation systems using WSFs.
About the ICL authors
Fabio Vale, Agronomy Lead, Brazil and LATAM; William Wang, Agronomist, Southern China; Patricia Imas, Agronomy content manager; Francisco Morell, Agronomy Lead, Europe.
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