Citrus

 

Citrus – rich in vitamins

The citrus fruits, comprising the species oranges, mandarins, grapefruits and lemons, are currently collectively the most important group in terms of production worldwide. Citrus fruit is grown mainly in tropical and subtropical regions of the world. Today, Brazil is the largest producer of oranges and Japan of mandarins. Spain, Italy and Mexico lead in the production of lemon and limes, while the USA produce the largest quantity of grapefruit.

 
Citrus fruits are rich in vitamins, especially vitamin C. Its mild acid and bitter taste is good for the digestion and blood circulation. Citrus fruits are either consumed fresh or processed. The citrus peel is rich in pectin which is used in the production of jellies, marmalades, sweets, jams and pharmaceutical preparations.

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Potassium – for optimal yield and quality

Citrus crops have a high demand for potassium. The potassium requirement in high-yielding fruit orchards often equals or even exceeds that of nitrogen.

• potassium plays an important role in most metabolic processes, e.g. photosynthesis and is essential for osmotic turgor regulation, protein and carbohydrate synthesis, translocation of assimilates (sugar and starch) and enzyme activation
• potassium is needed for the development of a good root system and promotes general tree growth
• adequate potassium supply improves the plants resistance to diseases and adverse weather conditions, particularly drought and cold
• potassium improves fruit number and size as well as quality parameters, such as the acidity of the fruit juice and the vitamin C content 

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Magnesium – to cover peak demand at fruit formation

• magnesium is the central atom of the chlorophyll molecule and traps the light energy during photosynthesis and converts it into sugar
• an improved photosynthesis improves the production rate of sugars, starches and proteins, which are essential to fruit yield and quality
• magnesium is required for an appealing skin colour and ensures an optimal content of soluble solids, acidity and vitamin C
• magnesium promotes the drought and cold tolerance of citrus trees, reducing frost damage significantly

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Sulphur – the growth control

• sulphur is a constituent of amino acids (methionine and cystine) and therefore it is needed for the formation of proteins and chlorophyll
• sulphur has an important impact on the photosynthesis rate as an optimal photosynthesis rate is necessary to obtain high yields

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Micronutrients – to secure yields and quality

In citrus, a wide range of deficiencies may occur, so that it is advisable to evaluate the crop’s nutritional requirements locally, especially with regard to micronutrients, as their impact on yield and quality is often underestimated.

 

Threshold levels of micronutrient concentrations in 4-6 month old spring cycles leaves from non-fruiting terminals

 

Range	mg kg-1 in dry matter
	Fe	Mn	Zn	Cu	B	Mo
Deficient	<35	<17	<17	<3	<20	<0.05
Low	36 - 59	18 - 24	18 - 24	3 - 4	21 - 35	0,06 - 0,09
Optimum	60 - 120	25 - 100	25 - 100	5 - 16	36 - 100	0,10 - 1,0
High	121 - 200	101 - 300	101 - 300	17 - 20	101 - 200	2,0 - 5,0
Excess	>200	>500	>500	>20	>250	>5,0

 

Sources: Smith, 1966; Koo, 1984: Malavolta, 1989

 

• Manganese is very important for the photosynthesis, root growth, sugar production and the disease resistance
• Zinc deficiency causes stunted trees; leaf and stem size is reduced
• Iron is needed in enzyme systems and in the electron transport during photosynthesis

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Fertiliser recommendations

The quantity of nutrients needed to adequately supply the requirement of productive citrus orchards depends on species, variety, soil, climate and yield expectation. It is recommended to take soil samples during planting and repeat at regular 3 - 5 year intervals. Annual leaf analysis is a suitable tool to determine the nutritional status and adjust the fertiliser rates accordingly. A general guideline is given in the table below where the recommendation is based on tree age.

 

Age (years)	K (g tree-1)	Sop (g tree-1)	Mg (g tree-1)	ESTA Kieserita (g tree-1)	Patentkali (g tree-1) *
1	350 - 450	700 - 900	35 - 105	200 - 600	120 - 1500
2 - 3	600 - 750	1200 - 1500	60 - 160	350 - 900	2000 - 2500
4 - 6	800 - 1200	1600 - 2400	80 - 260	450 - 1500	2700 - 4000
7 - 8	1000 - 1500	2000 - 3000	105 - 290	600 - 1800	3300 - 5000
> 8	1200 - 1800	2400 - 3600	120 - 320	750 - 2000	4000 - 6000

 

* The preferred application to replace SOP and Kieserite application where Patentkali is available in the market.

 

Micronutrients can be applied through both soil and foliar application although the latter is usually recommended, especially for the alleviation of acute deficiency or as a means of tree health insurance.

 

EPSO Microtop and EPSO Combitop are foliar fertilisers which can be used in citrus production to satisfy the manganese needs of citrus. The application is generally made during the time when the new shoots are fully developed.