Uptake and export of nutrients:
When the plants are producing fruit, the amount of fertilizers required should be based not only on soil and leaf analysis, but also the nutrients removed in the harvested fruits.
In fact the nutrients removed in the fruit are an excellent guide to establish a fertilization programme for any fruit tree in is production phase (Mengel and Kirkby, 1987; Torres and Sánchez López, 1992; Hermoso and Farré, 1997). The quantity of each nutrient removed in the fruit varies between varities but, on average 10 mt fruits remove 27 kg N (Table 11.2).
The amount of nutrients in soursop fruits produced in Venezuela and in Brazil differs greatly in K and Ca but is similar for the other macro-nutrients (Table 11.3). In Paraiba state, Brazil, the quantity of micro-nutrients per tonne of soursop fruit is: Fe, 8.03 g; Cu, 1.65 g; Mn, 2.71 g; Zn, 3.71 g; and B, 2.75 g (Silva et al., 1984), with Fe being the largest amount.
Functions and importance of macro-nutrients:
Nitrogen (N):
Deficiency of N causes an intense yellowing in the oldest leaves because if the supply of N is limited it is mobilised and transported to the youngest tissues, principally for growth. Symptoms of deficiency in young plants (seedlings) appear in the first 30-40 days after germination. Generally, plants of the genus Annona when N is deficient show a visual progression in the symptoms to intense yellowing and loss of leaves. Other than yellowing of the leaves, young plants are markedly smaller in height and have a very premature loss of leaves.
Phosphorus (P):
Deficiency of P is seen by an irregular chlorosis of the basal leaves with many showing a dark green colouring. With increasing deficiency, the leaves become small and take on irregular shapes. Deficient plants grow very slowly and have brown spots on the leaves with necrosis on the lamina margins, followed by the leaves falling off the plant.
Potassium (K):
In general, K deficient plants do not have the ability to transport sugars produced by photosynthesis in the leaves to the other organs, principally the fruit. Potassium can move from the oldest organs, primarily the leaves, to the youngest or to growth. Brownish spots start at the apex and the basal portion of the leaf lamina and gradually merge. In seedlings these symptoms first appear eight months after sowing the seed when the size of the leaves is reduced and they become yellow and fall off. Deficient plants produce fewer flowers but there is no loss of yield.
Calcium (Ca):
Symptoms of Ca deficiency are manifest 30 days after sowing and appear first at the points of growth, like buds and the young leaves because Ca is not mobile within the plant. About 70 days after sowing, the leaves show interveinal chlorosis, stop growing and become curled.
Magnesium (Mg):
Unlike Ca, Mg is mobile in plants so that initially deficiency symptoms occur in the oldest leaves. In nursery conditions, an interveinal chlorosis starts in the leaves about 50 days after sowing and progressively the leaves become totally necrotic. The adequate ratio of Ca: Mg is 3: 1 because a greater proportion than this of Ca induces Mg deficiency. In the same way, elevated ratios of K induce deficiencies of Mg and Zn.
Sulphur (S):
Like calcium, the first symptoms of S deficiency occur in the youngest leaves because S is largely immobile in the plant. In nursery conditions, young S deficient plants are intensely yellow and atrophied after about 75 days.
Functions and importance of micro-nutrients:
Boron (B):
Like Ca, B is immobile in phloem and for this reason the first symptoms of its deficiency occur in the young leaves. In seedlings still in the nursery, deficiency symptoms appear around 70 days after sowing, when the seedling leaves have an intense green colour with chlorosis of the lamina. By 140 days after sowing the plants have atrophied. Maintaining adequate amounts of plant-available B and Ca during flowering and the first stages of fruit production reduces the possibility of internal darkening of the pulp, which is common in annonas.
Iron (Fe):
Like Ca and B, the redistribution of Fe in the plant is practically zero. Thus the initial symptoms of Fe deficiency occur in the young leaves and are characterized by partial chlorosis with yellowish-green colouration of the lamina, which with time becomes totally yellow except in the region around the veins.
Zinc (Zn):
Plants with Zn deficiency frequently show interveinal chlorosis in the leaf lamina area and appear pale-green in colour. Deficient plants have irregularly distributed, small, branched and hardened leaves at the apex of new branches, and this is known as leaf rosette.
The observation and identification of the nutrient deficiency symptoms in the field can be done quickly but it requires very experienced people. Therefore, not only field observation, but also soil, fruit and leaf analysis are very important to determine the nutritional state of the plant.
To aid the determination of macro- and micro-nutrient deficiencies in plants, including the annonas, the symptoms have been described by many authors (Avilan, 1975; Navia and Valenzuela, 1978; Mengel and Kirkby, 1987; Torres and Sánchez, 1992; Silva and Silva, 1997). Finally, there is evidence that well nourished plants are more resistant to pests and diseases, producing a larger yield of good quality fruits.
Author:
Alberto Carlos de Queiroz Pinto
