Torres and Sánchez López (1992) recommend different quantities of nutrients depending on the region, i.e. the InterAndean Valley, the Atlantic Coast and Eastern Plains of Columbia.
These authors suggest that the amount of N fertilizer for plants between three and six years old should be based on the amount of soil organic matter, whilst for P and K they should be based on the level of plant available P and K in the soil.
Because N and K are the nutrients in greatest demand by soursop, the amounts should increase proportionally with the age of the plant and its level of production. However, care should be taken to give excess N because this causes the plants to grow too quickly and produce less fruit.
Soursop’s large demand for K means that the concentration of K in the leaf should not be less than 10 g/kg to ensure that K is not limiting growth. On sandy soils and others where there is a risk of N and K being lost by leaching, N and K fertilizers should be applied six times during the growing season.
For adult plants fertilizers should be applied beneath the crown in an area including two thirds of the radius beneath the crown and extending one more quarter from the edge of the crown’s projection (Fig. 11.1).
In general, soursop is sensitive to Zn and B deficiency. To prevent deficiency, 2 g m- 2 B can be incorporated monthly into the 10 cm of soil beneath the plant’s crown before irrigation and a 0.1% solution of zinc sulphate applied as a foliar spray. Galrão (2004) recommends the following quantities of micro-nutrients during the production phase of the adult plant: 2.0 g B, 3.0 g Cu, 4.0 g Mn and 5.0 g Zn, all incorporated into the soil below the crown projection together with other fertilizers, at the start of fruit production.
There has been little research on foliar applications of both macro- and micronutrients for soursop. When the fruit is mature the absorption of nutrients diminishes and foliar applications of nutrients are more effective at this time. Today producing fruit by organic systems of production is an excellent method for increasing the value of the fruit.
However, there is serious lack of information regarding appropriate methods of growing annonas, especially soursop, by organic methods. One of the few exceptions is that of Bonaventure (1999) for cherimoya (Annona cherimola Mill.). He recommends using microorganisms and algae, as well as a bio-activator, which accelerates the metabolism and increases the production of this important annona.
Currently, the use of organic compost and mulching with organic material in soursop plantations has been recommended because the plants respond both in growth and yield. Organic compost and mulching facilitates not only the development of vigorous and abundant roots, but also improves moisture retention in the soil and minimises the risk of soil erosion.
As with other perennial fruit trees, chemical soil and plant tissue analysis are the techniques most used to evaluate the nutritional state of the plants. In some cases, for example, leaf analysis may indicate deficiency of Mg but the cause may be in the soil where there may be too little Mg or an excess of Ca.
Currently, some researchers have also tested the analysis of fruit tissue (Stassen 1997) to complement soil and leaf analysis. Soil sample collection in soursop orchards in the production phase is the same as that recommended for other crops, but the soil should come from within the crown projection.
The recommended method for leaf sampling for nutrient content depends on the age of the plant, the position of the leaf within the crown, the variety and whether the branches are with or without fruit and the period of sampling.
Laprode (1991) suggests that the leaves should be taken from the third and fourth pairs of intermediate branches in the crown and of the four cardinal points. Pinto and Silva (1994) recommend that the leaves should be 8 to 9 months old, taken from healthy plants free from residues from any foliar sprays. In general, the sample should consist of 100 leaves for every five hectares by taking four leaves per plant from a group of 25 plants randomly selected in the orchard.
For a more uniform sample it is recommended to divide the orchard into sections of similar soil characteristics, and in each section separate the plants by chronological age. Collect only healthy leaves from plants that have not been recently fertilized, avoiding the period of flowering and periods of intense rain.
Soil and leaf analysis data are interpreted using calibration curves for each nutrient, based on the correlation between the composition of each nutrient and the productivity of the fruit tree (Silva et al., 2002).
Isolated leaf analysis is not sufficient for precise interpretation and diagnosis of the nutrient status of the plant because of the many factors that cause variation in leaf nutrient status. In general, the composition of N is about ten times that of P and twice that of K. Gazel Filho et al. (1994), analysed leaves from a range of one-year-old soursop varieties growing in Cerrado do Amapá, Brazil.
The varieties were: Blanca, Lisa, Morada, Soursop A, Soursop B, FAO II and Matriz CPATU 415. The content of macro-nutrients in g/kg ranged from: 19.6 to 20.4 for N; 1.2 to 1.4 for P; 14.9 to 17.2 for K; 12.0 to 15.2 for Ca and 1.9 to 2.2 for Mg. The authors only found significant differences for Ca and Fe and the largest concentrations were in cv. Morada with 15.2 g/kg Ca and 215.8 mg/kg Fe.
This result seems to contradict the belief that the nutrient composition of the leaves is genetically controlled and this may vary with variety. However, in this case many of these analyses were made on different leaves, whether they showed deficiency or not and came from branches with or without fruit. Some authors point out that the comparison of leaf composition between macro- and micro-nutrients is important in leaves with and without visible deficiency.
Avilan (1975) in Venezuela and Silva et al. (1984) in Brazil (Table 11.5) made these comparisons. Normal leaf concentrations for N and K in soursop grown in Brazil are 1.6 to 2.0 times greater than those in deficient leaves. The difference in N composition between normal and deficient leaves in Venezuela was much greater than that in Brazil but the difference in K was small.
Author:
Alberto Carlos de Queiroz Pinto
