In general, because soursop grows in the humid tropics it needs extra water by irrigation to guarantee adequate growth and fruit yield especially during periods of drought.
George et al. (1987) described a common disorder in annonas, especially in soursop and sweetsop, which is a hardening of the pulp with brownish lumps.
It is suspected that this symptom is caused by sudden movements of water within the pulp that, together with the deficiency of B, produces a more serious manifestation of the symptom.
This disorder is very common in north-eastern Brazil, especially in dryland orchards or irrigated orchards with limited water supplies.
The occurrence of any water stress generally slows the growth of young plants, preventing vegetative growth and decreasing the size of the fruit, highlighting the importance of irrigation for soursop.
Irrigation methods:
The selection of the most appropriate irrigation method is directly associated with three factors: technical, economical and human (Silva et al., 1996).
Soil factors related to water infiltration, water management, with respect to its quality, quantity and availability, climatic factors and plant features are important technical factors.
For example, the spray method is recommended only in areas where water is not limiting and the gradient of the surface is not greater than 16% (Nunes, 1997). The price of the system and its installation and maintenance costs are very important economic factors.
Finally, for success in the installation and operation of the system, the quality of the available labour should not be disregarded. Simple irrigation methods are used in many regions in north-east Brazil, depending on economic conditions, quantity of water available and the type of soil.
At Fazenda Bom in the municipality of Trairi, in Ceará state, the soil is a Quartz Neosol (Sandy Quartz). There they use a simple yet effective installation comprised of a flexible hose at the end of which is a hard PVC ring with 0.5 cm diameter holes giving a flow of water of about 20 to 30 L/h. Each plant is watered for 1-2 hours at 7 to 14 day intervals.
Currently, localized drip and jet irrigation are recommended especially where there is a shortage of water. In these systems, water is applied only to the soil in the region of the root system, thus reducing water loss by evaporation and limiting the area infested by weeds.
Overall, the great disadvantage of drip irrigation is the need to use filters to avoid blockages in the system where water quality is poor and insoluble fertilizers are used. Therefore, for success with drip irrigation, the system should be installed adequately following the method proposed by Bucks and Davis (1986).
Water requirements:
The amount of water required varies from one area to the next and depends on climatic conditions, soil type and growth stage of the plant. For example, the dry period in the Cerrados of Brasilia coincides with lower solar radiation and low temperature that create a comparatively smaller evapo-transpiration demand than in the north-east of Brazil. Consequently, water deficiency in the Cerrados is less serious than that in the north-east.
An adult plant requires much more water than a young plant and Latosol soils retain much more water than Quartz Neosols. Soursop orchard of adult plants with a 3 m radius crown and a density of 204 plants/ha (spacing 7 x 7 m), using jet irrigation to cover 60% of the crown area with water, requires a daily input of approximately 63 L/plant, or 3.72 mm/plant, which is equivalent to 1,000-1,200 mm of yearly rainfall (Pinto et al., 2001).
It is estimated that the annual water needs of soursop for growth and production are about 1,000 to 1,200 mm/plant. In regions where the annual precipitation is equal to or greater that 1,600 mm, the cv ‘Morada’ produces up to 10 kg fruit/plant.
In the Brazilian semi-arid regions, where annual precipitation is about 500 mm, this same variety only produces more than 3 kg fruit/plant if it is adequately irrigated.
Fertigation:
Compared to applying fertilizers to the soil, the application of nutrients in the irrigation water allows a faster response by the crop, better control of the quantity of nutrients used and larger yields of better quality fruits.
Water quality is just as important as the quantity to be applied and the period of application, because elements like calcium could precipitate with phosphates and cause blockage of the emitters (Pinto and Silva, 1994).
Equally, producers of soursop should be concerned about the presence of sodium in the water, because it accumulates in the soil, especially in shallow and poorly drained soils.
It goes without saying that salting severely damages growth and production of soursop. Mansour (1997) commented that sodium chloride, calcium chloride and calcium carbonate all adversely affect the growth of annonas and drastically reduce the total dry weight of the plants, especially at a leaf concentration of 0.3% Na.
Elevated levels of any of these salts causes leaves to burn and fall. Also, elevated levels of B and Cl in irrigation water induce phytotoxicity and injuries to the leaves and fruit that are difficult to control (Pinto and Silva, 1994).
Author:
Alberto Carlos de Queiroz Pinto
