The application of balanced nutrition aims to ensure adequate aerial and root growth to store as much carbohydrate in specialized organs. Adequate nutrition of the crop is a key factor in obtaining good harvests.
To achieve an appropriate nutrition plan in avocado is necessary to know the nutrient demand in quantity and type of nutrient. It is also important to know the role of each nutrient for crop growth, yield and quality of production.
Nitrogen is a component of a great number of plant compounds being part of the structural chlorophyll molecule which is present in all proteins. This is the key element for controlling vegetative and reproductive growing balances; in addition, it is the most limiting factor for avocado production.
Nitrogen is the most important mineral nutrient since it determines the avocado production; then management unbalances of this element can generate large vegetative growing in detriment of production.
Nitrogen deficiency symptoms
• Slow plant growing, small leaves of green pale or yellow pale color.
• Initial symptoms on old leave; due to nitrogen is very mobile within the plant.
Nitrogen deficiency on avocado trees.
It is found in organic and inorganic forms in the soil. Inorganic phosphorus comprises chemical available forms in the soil solution. Ionic forms depend on the pH, thus, roots prefer the H2PO4- ion which is predominant in 4 to 6 pH rages; a larger pH value and up to approximately 7.5 is important in presence of HPO42- form. It is estimated that only 20% of the applied phosphorus as fertilizer, is possible to be directly used by the roots, while the rest remain in less available forms, which depend on the pH.
) plays an important role in the activation of enzymes (there are more than 60 of them activated by this cation), which act on divers metabolic processes such as photosynthesis, protein and carbohydrate synthesis. Also, potassium participates in the water balance; besides of having a positive effect by directly influencing the pumping of this element from the soil and on the meristem growing (Mengel y Kirby, 1987). Potassium, by participating in these metabolic processes, acts favoring the vegetative growing, maturity and fruit quality.
Figure: Potassium promotes the photosynthates translocation from the leaf to the fruit, thus in deficiency potassium trees the fruit is smaller, with an opaque color and mort susceptible to sun stroke.
Potassium accelerates the flow of assimilate products to the fruit
Potassium deficiency symptoms
The main K deficiency symptom is a marginal necrosis of the leave. This necrosis reaches up to the leaf center from the margins which tends to be bent up ward. Potassium promotes photosynthates´ translocation from the leaf to the fruit, thus the potassium deficient trees have smaller fruits, with opaque color and more susceptible to sun stroke.
differs from the majority of other nutrients due to that generally deficiency symptoms are developed in specific plant parts, in stead of affecting the total plant growth. This reinforces the Ca characteristic of having a limited mobility within the plants, which are not able to easily transport this element from the tissues with adequate Ca to those with deficient levels (fruits). To obtain quality fruit is based on two components: internal quality (Ca level and post-harvest fruit life) and external quality (color, size, shape, friction and spots present on the fruit) determined by physiological disorder susceptibility, disease resistance, softening, among others.
Most orchards have a great variation in Ca levels from one year to the other. This means that the fruit farmer must be always concern to Ca deficiencies; he never should think that there is no problem. Low Ca level in fruits should not be a supply problem and/or a Ca deficit absorption from the soil, but it is a distribution problem within the plan. Therefore, Ca level measurements in the soil or in the vegetative parts of the plant (foliar Ca) are not adequate indicators of Ca present in fruits.
Young fruits (cellular division period): Ca is absorbed by roots and transported by the transpiration flow (Xylem via).
Grown fruits (cellular expansion period): Ca is transported by the respiration flow via toward the high transpiration organs (leave) by the xylem via. But Ca cannot be re-transported to the fruit since it is not mobile within the phloem. Even though, an option in this stage is externally applying Ca to be absorbed by the fruit skin.
Calcium concentration in the fruit significantly varies according to the variety and tree’s vigor. For avocado Hass variety, Ca ranges between 1,300 to 1,650 ppm, from fruits of no vigorous and vigorous trees, respectively.
Fruit position on the tree, also affects the Ca distribution. The fruit that grows under sunny situation can easily reaches pulp temperatures from 35 to 45° C, during three or four months during the growing period, generating high transpiration rates for the fruit. Thus, a greater supply of Calcium, Magnesium and Potassium is favored, which are minerals translocated from the root absorption by the xylem flow. In addition, an adequate summer pruning, allows a greater light incidence and heat within the canopy.
The companion ion has also influence in the Calcium absorption. NH4+, K+ and Mg+2 cations depress the Ca absorption, while NO3 anion, followed by Cl- and SO4-2 favor its absorption; a balance between these elements allow a good Ca accumulation.
Ca is one of the most determinant minerals of fruit quality. In avocado, it has been observed that the Ca concentration is positively related with a greater resistance to degradation of cellular membranes, which implies a larger post-harvest life.
This macroelement is the central component of the chlorophyll molecule, thus, it is essential for the photosynthetic process. From the total Mg content within the plant, between 25 to 30% form part of the chlorophyll molecule.
Mg present a positive charge (cation), therefore it can fixed in clay soils, the same than potassium and calcium, reducing the supply toward the plan. Acid pH soils also present Mg deficiencies.
Mg deficiency symptoms:
Mg deficiency is characterized by an internal chlorosis (chlorophyll disintegration), from a bronze aspect which take a V shape. This is initiated in the older leave base, advancing toward the center and covering the margins.
B moves from the roots with the transpiration flow through the xylem. Once in the leaves, B re-translocation is restricted and it remains forming part of the cellular walls, providing elasticity. Furthermore, B concentrations into the phloem would be low, existing continuous B accumulation in the leave and presenting symptoms in the plant meristem tissues. As a result, it has been concluded that boron is a immobile element in the plants´ phloem; even though, studies with marked B have proven that in divers fruit trees of the Rosaceous family, foliar post-harvest applications have increased the concentration of this element in the leaves and that part of B was mobilized toward the bark of the sprouts. Then, during spring, B was translocated through the phloem from the bark toward the flower and increased the fruit set. This mobility of B in fruit trees would be possible since these species translocate carbohydrates in the form of sugar-alcohol (sorbitol), which is able to be associated with boron and facilitates its transport.
Deficiency symptoms in fruits can be appreciated as necrosis and deformations in the fruit shoulders.
Boron deficiencies in fruits of avocado trees.