In general, mature cropping avocado trees (Persea americana) have a relatively low demand for nutrients. N, Zn and B are considered to be the minerals most limiting to production. N is an important management tool that can be used to alleviate the effects of biennial bearing, when a heavy "on" year crop depresses yields in the following "off" year. Adjusting fertiliser inputs to higher rates prior to an 'on' year and restricting elements such as nitrogen in an 'off' year can minimise these fluctuations. Avocados are very susceptible to the root rot fungus, Phytophthora cinnamomi. The activity of this fungus can be suppressed by increasing the calcium status of the soil. Heavy applications of gypsum are standard practice to reduce root rot rather than to improve the nutrient status of the soil. Most of the information included in this section is from research carried out in California, Australia, and South Africa.

PLANT ANALYSIS
Samples for leaf analysis should be collected in March/April and consist of terminal spring flush leaves (5-6 months of age) from non-fruiting and non-flushing shoots. At least 50 leaves (blades plus petioles) from the outside of the canopy should be taken at random from 10 or more trees, excluding border trees. Interpretive standards for nutrient concentrations are listed in Table 1.

SOIL TESTING
The ideal soil pH for avocado trees is 5.5 to 6.5. Soil pH values above 7.0 should be avoided as avocado trees are susceptible to Fe deficiency. Deficiencies under these conditions are due to immobilisation rather than inherent deficiency of Fe in the soil. Target soil test values to maintain high production have not been defined for avocado trees grown in New Zealand. Results from a survey of well producing orchards in the Bay of Plenty make it clear that avocado trees tolerate a wide range of macronutrient concentrations in the soil; the ranges in the soil test values were as follows: pH, 5.4-6.3; P, 10-135; MAF quick test units:- K, 7-12; Mg, 14-29; Ca, 4-10.

FERTILISER REQUIREMENTS
Maintenance fertiliser applications are listed in Table 2. These rates should be adjusted to meet local requirements and according to the values obtained from regular plant analysis.

Compared to other horticultural crops, the quantities of nutrient removed in fruit are low, reflecting the relative low demand of the avocado tree for nutrients. Under New Zealand conditions a yield of between 10-12 tonnes/ha can be expected by the seventh year. The suggested annual fertiliser applications in Table 2 should be adjusted for higher producing orchards.

The amount of fertiliser applied depends to a large extent on the age of the tree as the roots are sensitive to excess nutrients, particularly N. Thus for N, only small quantities are required for vigorous growth of young trees, but the quantity increases as the tree begins to bear fruit (Table 3).

For elements such as P larger quantities may be needed at planting to increase the basic level of fertility than when the tree matures.

Fertiliser should be applied in early spring prior to flowering, as the demand for nutrients in the tree is highest during flowering and fruit set. However, because of the competition between fruiting and non-fruiting tissues, fertiliser, particularly N, should not be applied any later than six weeks before fruit set.

NUTRIENT DISORDERS
The nutrient disorders which are most likely to affect production in New Zealand are those associated with excess N, and deficiencies of Zn and B.

Results of surveys of New Zealand orchards have shown the N status of many trees to be well above the optimum range. Under these circumstances N should not be applied until the concentration in the leaves has fallen to more reasonable levels. Excess N can result in reduced yields as a result of excessive vegetative growth at the expense of flowers and fruit set, and poor fruit quality.

There is some evidence that Zn and B are low in some avocado orchards in New Zealand. Plant analysis is a very accurate method of diagnosing deficiencies of these two elements. Symptoms of Zn deficiency include, small narrow leaves, interveinal chlorosis, multiple bud development, retention of a rosette of terminal leaves with the remainder dropping giving a 'feather duster effect', twig dieback, and rounded fruit shape. Foliar sprays of Zn (100g zinc sulphate/100l) have proved to be effective in the short term for correcting deficiencies. In the longer term a combination of foliar sprays plus a soil application of zinc sulphate or zinc chelate is required. In California, soil applications of 1-5 kg/tree have proved to be effective for mature trees.

Symptoms of B deficiency include the gradual death of both the apical and axillary growing points. The leaves are distorted and often have necrotic patches to produce a "shot-hole" effect. The midrib and main veins on the lower surface of the leaves frequently split and become corky. An application rate of 5 kg/ha of borax is generally sufficient to overcome a deficiency.

FURTHER READING
Embleton, T.W. and Jones W.W. (1966). Avocado and mango nutrition. In: Temperate to Tropical Fruit Nutrition; (Childers, N.F., Ed), Horticultural Publications, Rutgers - The State University, USA, pp. 51-76.

Godall, G.E. (1983). Fertiliser requirements for avocados. Avocado Grower; 7: 10-14.

Sale, P.R. (1983). Avocados. Orchard management. New Zealand Ministry of Agriculture and Fisheries AgLink, HPP 76