Citrus species can thrive in a wide range of soil and climatic conditions. Citrus
is grown from sea level up to an altitude of 2100 m but for optimal growth a temperature range from 2° to 30° C is ideal. Long periods below 0° C are injurious to the trees and at 13° C growth diminishes. However, individual species and varieties decrease in susceptibility to low temperatures in the following sequence: grapefruit, sweet orange, mandarin, lemon/lime and trifoliate orange as most hardy.
Temperature plays an important role in the production of high quality fruit. Typical coloring of fruit takes place if night temperatures are about 14° C coupled with low humidity during ripening time. Exposure to strong winds and temperatures above 38° C may cause fruit drop, scarring and scorching of fruits. In the tropics the high lands provide the best night weather for orange color and flavor.
Depending on the scion/ rootstock combination, citrus trees grow on a wide range of soils varying from sandy soils to those high in clay. Soils that are good for growing are well-drained, medium-textured, deep and fertile. Waterlogged or saline soils are not suitable and a pH range of 5.5 to 6.0 is ideal. In acidic soil, citrus roots do not grow well, and may lead to copper toxicity. On the other hand at pH above 6, fixation of trace elements take place (especially zinc and iron) and trees develop deficiency symptoms. A low pH may be corrected by adding dolomite lime (containing both calcium and magnesium)
A citrus orchard needs continuous soil moisture to develop and produce, and water requirement reaches a peak between flowering and ripening. However, many factors such as temperature, soil type, location, plant density and crop age influence the quantity of water required. Well-distributed annual rainfall of not less than 1000 mm is needed for fair crop. In most cases, due to dry spells, irrigation is necessary. Under rain-fed conditions, flowering is seasonal.
There is a positive correlation between the onset of a rainy season and flower break. With irrigation flowering and picking season could be controlled by water application during dry seasons. Irrigation systems involving mini sprinklers irrigating only soil next to citrus trees have been developed as an efficient and water conserving irrigation method.
PLANTING GUIDE OF CITRUS
- Select seeds from healthy mother trees for root stocks
- Hot water treat seeds at 50° C for 10 minutes
- Seeds perform better when planted soon after they are extracted
- Sow seeds in seedbeds or in polythene bags. Seeds germinate in 2 to 3 weeks
- Water the seeds regularly, preferably twice a day until they germinate
- Seedlings are normally ready for budding when reaching pencil thickness or 6 to 8 months after germination
HOW TRANSPLANTING IS DONE
- Transplant in the field at onset of rains.
- Clear the field and dig planting holes 60 x 60 x 60 cm well before the onset of rains.
- At transplanting use well-rotted manure with topsoil.
- Spacing varies widely, depending on elevation, rootstock and variety. Generally, trees need a wider spacing at sea level than those transplanted at higher altitudes. Usually the plant density varies from 150 to 500 trees per ha, which means distances of 4 x 5 m (limes and lemons), 5 x 6 m (oranges, grapefruits and mandarins) or 7 x 8 m (oranges, grapefruits and mandarins). In some countries citrus is planted in hedge rows.
- It is very important to ensure that seedlings are not transplanted too deep.
- After transplanting, the seedlings ought to be at the same height or preferably, somewhat higher than in the nursery.
- Under no circumstances must the graft union ever be in contact with the soil or with mulching material if used.
CITRUS MANAGEMENT AND MAINTENANCE
- Keep the trees free of weeds.
- Maintain a single stem up to a height of 80-100 cm.
- Remove all side branches / rootstock suckers.
- Pinch or break the top branch at a height of 100 cm to encourage side branching.
- Allow 3-4 scaffold branches to form the framework of the tree.
- Remove side branches including those growing inwards.
- Ensure all diseased and dead branches are removed regularly.
- Careful use of hand tools is necessary in order to avoid injuring tree trunks and roots. Such injuries may become entry points for diseases.
- As a general rule, if dry spells last longer than 3 months, irrigation is necessary to maintain high yields and fruit quality. Irrigation could be done with buckets or a hose pipe but installation of some kind of irrigation system would be ideal.
For normal growth development (high yield and quality fruits), citrus trees require a sufficient supply of fertilizer and manuring. No general recommendation regarding the amounts of nutrients can be given because this depends on the fertility of the specific soil. Professional, combined soil and leaf analyses would provide right information on nutrient requirements.
In most cases tropical soils are low in organic matter. To improve them at least 20 kg (1 bucket) of well-rotted cattle manure or compost should be applied per tree per year as well as a handful of rock phosphate. On acid soils 1-2 kg of agricultural lime can be applied per tree spread evenly over the soil covering the root system. Application of manure or compost makes (especially grape-) fruits sweeter (farmer experience).
Nitrogen can be supplied by inter cropping citrus trees with legume crops such as mucuna, cowpeas, clover or dolichos beans, and incorporating the plant material into the soil once a year. Mature trees need much more compost/well rotted manure than young trees to cater for more production of fruit.
Conventional fertilization depend on soil types as well.
There are a large number of citrus diseases caused by bacteria, mycoplasma, fungi and viruses. The organic citrus disease management consists in a 3-step system:
- Use of disease-free planting material to avoid disease problems
- Choosing root stocks and cultivars that are tolerant or resistant to prevalent diseases
- Application of fungicides such as copper, sulphur, clay powder and fennel oil. Copper can control several disease problems. However, it must not be forgotten that high Copper accumulations in the soil is toxic for soil microbial life and reduce the cation exchange capacity