The genus Annona includes, for the most part, tropical and subtropical plants, although a few species develop in temperate climates. Many species grow at low altitudes and those with a wide adaptation to altitude are also the species most adapted to variations in latitude. The optimal range of latitude is between 27ºN and 22.5ºS (Nakasone and Paull, 1998).
Soursop is the most tropical of the Annona species and is considered a plant of low altitudes and a hot and humid climate. It is grown primarily at altitudes lower than 900 m above sea level (Zayas, 1966). However, good productive orchards are found at altitudes of up to 1100 m (Pinto and Silva, 1994).
Soursop adapts well to Savanas and tropical humid regions, i.e. A and Aw type climates, where the annual precipitation generally exceeds evapo-transpiration (Ayoade, 1991). Two important climatic factors are rain, principally when out of season, and strong winds. Both, when they occur in great intensity and during flowering, greatly reduce pollination (Nakasone and Paull, 1998).
Although the photoperiod is not an important physiological factor for the annonas, excessive shading induces poor setting of the fruit. Therefore, pruning, plant spacing and fertilization are some of the very important practices in orchard management.
Soursop is very demanding of light and shading the plants greatly reduces the production of fruit (Villachica et al., 1996). Soursop grows and produces well at 21 to 30ºC, being very sensitive to severe changes in temperature, especially if the limit of 12ºC is reached (Pinto and Silva, 1994). Nakasone and Paull (1998) considered that the best temperature range was between 15 and 25ºC.
Soursop will grow in a wide variety of soils, from sandy to clay loams, but it prefers deep soils with good aeration (Melo et al., 1993; Ledo, 1992). Good drainage is necessary for good root development, and, especially, to avoid problems of root diseases. Soil pH should be between 6.0 and 6.5 (Pinto and Silva, 1994).
Soil and crop management:
Soil preparation for a soursop orchard includes land clearing, aeration, ploughing, application of lime, if necessary, and appropriate fertilization. Soil sampling and analysis are precede aeration and ploughing. The amounts of lime and fertilizer to apply are decided on the basis of the analytical data.
Where the soil is acidic, which is very common in Brazil, liming is extremely important not only to adjust the pH to 6.0 to 6.5, the best range for soursop, but also to achieve a base saturation of between 60 and 70% (Pinto et al., 2001). Liming is also recommended when the subsoil to a depth of 60 cm is acidic, i.e. Al saturation >20% and/or Ca <0.5 cmolc/dm3 (Andrade, 2002).
Fertilization is generally recommended for soils deficient in phosphorus (P) and potassium (K), and the fertilizers are broadcast onto the soil around the plant, followed by incorporation into the soil (Andrade, 2004). The recommendation for P fertilizer is based on the amount of clay and the quantity of plant-available P determined by soil analysis (Table 11.1).
Table 11.1. Phosphorus fertilizer application according to percent clay in the soil and level of plant-available phosphorus in the soil. Correcting soil fertility where an orchard is to be planted has a cost, which must be considered. If fertilizer is broadcast over the whole area of the orchard annual plants should be grown between the rows of soursop to provide some economic return to the grower before the soursop starts to produce harvestable fruits three years after planting the trees.
Soursop can be propagated from seed (“loam tree”) and by budding. The height of the plant is not greatly affected by budding and the majority of producers prefer using grafted seedlings rather than seeded seedlings. Propagation by seed or graft is done in plastic bags in a growth medium that varies from region to region.
The constituents in the growth medium in the nursery phase are very important. Depending on the material and quantity used, there is the possibility of interfering with seed germination and of phytotoxicity burning the young leaves and causing the death of the seedlings (Pinto and Silva, 1994).
Although there is variation in the recommended use of nutrients in different regions, Pinto (1996) recommends the following constituents for each m3 of growth medium (about 700 kg): 300-350 kg of local soil, 300-350 kg cured bovine manure, 300-500 g lime and 400-600 g of single superphosphate. After preparing the mixture it should be exposed to sunlight to eliminate diseases.
Rego (1992) studied the effect of cured bovine manure applied at 0, 5, 10, 15, and 20% of the growth medium on the growth of seedlings over a period of four months. The author concluded that 15% manure was the most effective.
After germination and during seedling growth, nitrogen (N) should be applied every 21 days as a solution of ammonium sulphate at 5 g/L water. After the fourth month the seedlings should receive micro-nutrients as a foliar spray bimonthly using a commercial product in a 1-2% solution (Pinto and Silva, 1994).
Alberto Carlos de Queiroz Pinto