Nutrition And Fertilizing

Citrus trees need a certain amount of minerals related to their foliar surface and productivity. The soil must supply elements to the roots in an adequate quantity and quality. So a chemical analysis of the soil is the first thing to do before planting.

Mineral intake is influenced by several factors, not least the capacity of the rootstock to capture elements from a certain chemical complex. There are two techniques to check chemical fertility: soil and leaf analysis.

Although soil analysis is the only procedure to find out the mineral composition of the root-explored layer, it has limitation, since the presence of nutrients may not correspond to their suitability for citrus trees. Some stable chemical compounds are not easily absorbed by roots. Sampling must be done in a uniform surface of soil. The sample must be taken at a depth of 10—60 cm (the main root-explored layer). Even in cases of apparent uniformity, several samples should be taken for separate analysis. Each single analysis is usually the result of mixing four soil samples taken at various points in each unit surface (one hectare or more). Analysis must be carried out for at least the main elements. The determination of the 'cation—exchange capacity' is also a good method of checking soil suitability for citrus trees. Humus-rich soils as well as clay soils have a high exchange capacity (>10 milliequivalents). Deficiencies must be corrected by an overdosage of fertilizer. Excess cannot be corrected, except in the case of soluble elements (nitrogen, chloride, etc.). Leaching is rapid in heavy rainy regions. In such cases soil permeability is very important. Water irrigation may carry salts which must be considered for the chemical soil balance.

Leaf analysis is the most efficient guide to checking elements in citrus trees. Plants accumulate nutrients in the leaves, so leaf analysis is the best tool for detecting the nutritive conditions of a citrus plant. Not every leaf is suitable for mineral analysis. Leaves must be taken from the periferic and equatorial part of mature trees scattered here and there in the orchard. They must be five—seven months old, coming from the main flush (spring cycle in the northern hemisphere) and not fruiting shoots. Leaves should be gathered early in the morning, kept in thermic bags and brought to the laboratory as soon as possible. The chemical data obtained must be compared with the standard data (Table 3.1). When mineral leaf content is satisfactory fertilization must replace the amount of nutrients lost to fruit or by leaching and vaporization.

Loss of minerals varies greatly in relation to the amount of production, soil, climate, agricultural practices. As an approximate estimation, in the Mediterranean countries a yearly loss of the main elements per hectare, in a mature orchard producing 30 tons of fruit, is as follows: N= 180-220 kg; P2Os = 30-40 kg; K20 = 90-120kg. These are the amounts per hectare that must be replaced by yearly fertilizing where there is a satisfactory presence of elements in leaves. These amounts have to be increased for

Table 5.1 Percentage of different elements in 4—/ month-old orange leaves from non-fruiting terminal shoots (spring cycle) (Cohen, Israel)

Element

Unit

Deficient less than:

how

Optimum

High

Excess more than:

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