Minimal pruning is generally carried out on citrus grown in New Zealand, thus the quantity of nutrient which needs to come from soil reserves or fertiliser applications in a cropping orchard, depends on that removed in harvested fruit. Typical fruit yields under New Zealand conditions for Satsuma mandarins and navel oranges range from 20 to 30 tonnes/ha, while yields for valencia oranges and tangelos are slightly higher, 25 to 40 tonnes/ha.

Most citrus grown in New Zealand has a tendency to biennial bearing. During an off year the amount of fertiliser applied should be reduced. In cases of extreme biennial bearing with almost no crop in the off year, as is often the case with Wheeny grapefruit, fertiliser could be eliminated altogether, provided that there were no obvious nutritional disorders apparent.

PLANT ANALYSIS
Samples for leaf analysis should be collected in late February to mid March. The youngest mature leaves on non-fruiting terminals should be selected with each sample consisting of at least 24 leaves taken at random from trees throughout the block. Interpretive standards for nutrient concentrations in the leaves are listed in Table 1.

SOIL TESTING
A soil test should be carried out prior to planting. Suggested target MAF levels for adequate nutrition are as follows:

The most common commercial rootstock for citrus in New Zealand, Poncirus trifoliata does not perform well where the soil pH exceeds 6.5. Under these conditions the uptake of Fe, Mn, and Zn is seriously impaired. Once the orchard is established a routine soil analysis should be taken every year.

FERTILISER REQUIREMENTS

Young trees

Young trees are usually fed on the little and often principle. The approximate quantities of nutrients required by young trees are listed in Table 2.

Fertiliser is usually applied on four separate, but equally spaced occasions beginning in September, and ending in February to coincide with the autumn flush.

Cropping trees

In a mature cropping orchard the minimum quantities which need to be replaced each year from soil reserves or from fertilisers depend largely on the amount removed in harvested fruit. An estimate of these quantities is given in Table 3.

Fertilisers are usually broadcast throughout the orchard. Phosphorus and K can be applied in a single application in late winter-early spring. The best time to apply N is to coincide with the start of a growth flush. Most citrus have two growth flushes, one in spring and the other in autumn. As there can be undesirable effects on fruit quality from heavy dressingsl ate in the season, at least two thirds of the annual requirement should be applied in early spring with the remainder in February to coincide with the autumn growth flush. Seminole tangelos are an exception however. Spring N applications should be withheld from this variety until after harvest to prevent excess N from adversely affecting storage quality of fruit.

For lemons, three equal applications to coincide with each growth flush ( around September, November and February-March) are recommended.

NUTRIENT DISORDERS
The most common nutrient disorders of citrus in New Zealand are those associated with deficiencies of N, P, Mg, Mn and Zn.

The critical time for N deficiency to occur is just prior to and during flowering, fruit set and December leaf drop. Symptoms of this disorder include yellow coloured foliage, poor flowering, poor yield, and stunted growth. Like most trees however, citrus store N in the woody parts and can utilise this reserve in spring, so a deficiency expressed at that stage could be the result of a shortage created during the previous season. Care should be taken to avoid applying too much N as this can result in por fruit quality. In particular, it affects fruit colour, delays maturity, reduces juice content, and results in thick skins.

Visual symptoms of P deficiency are not usually seen in New Zealand but the effects on fruit quality are common in most districts. These include low juice content, thick skins, and acid juice.

Magnesium deficiency is particularly common in years of heavy crops. Symptoms include the yellowing of leaves with an inverted v of green tissue at the base of the leaf. Magnesium deficiency can be corrected by applying 200-400 kg/ha of Mg as Causmag. In severe cases, where an immediate repsonse is required foliar applications containing 1 kg magnesium sulphate plus 1 kg calcium nitrate/100 l can be used.

Manganese deficiency occurs in all citrus growing areas of New Zealand. Symptoms include the development of an interveinal yellowing with a band of darker green along the midrib and veins. Zinc deficiency produces symptoms which are similar to Mn deficiency but the interveinal yellowing is less blotchy and more clearly defined. In extreme cases leaves can be small, narrow, pointed and rosetted in the classical little leaf symptom. Zinc deficiency can also affect fruit shape. Manganese and Zn deficiency often occur together and can be corrected together or singly. The most effective means of correcting both disorders is by foliar sprays. A typical spray mixture consists of 100 g MnSO₄, 100 g ZNSO₄ and 750 g urea/100 l plus a suitable wetting agent. The urea is helpful for increasing the uptake of Mn and Zn.

Foliar sprays should be applied on their own without the addition of insecticides or fungicides. They should be applied at a time when there is plenty of young foliage, as older leaves are less efficient at absorbing nutrients. Early November, or an equivalent stage in autumn, when the growth flush is about two thirds completed appears to be the most suitable time. Sprays should also be applied in the early morning or in the evening to avoid intense day time temperatures.

FURTHER READING
Embleton, T., Jones, W. and Reitz, H. (1967). Citrus fertilisation. In: The Citrus Industry, Vol. 3 (Reuther, W., Webber, H.J. and Batchelor, L.D., Eds), University of California, USA. pp 122-182.

Embleton, T., Jones, W., Labanauskas, C and Reuther, W. (1967). Leaf analysis as a diagnostic tool and guide to fertilisation. In: The Citrus Industry, Vol. 3 (Reuther, W., Webber, H.J. and Batchelor, L.D., Eds), University of California, USA. pp 183-210.

Jorgensen, K.R. and Price, G.H. (1978). The citrus leaf and soil analysis system in queensland. Proceedings of the International Society of Citriculture, 297-299.

Sale, P.R. (1983). Citrus Nutrition. Nutrient function and determining requirements. New Zealand Ministry of Agriculture and Fisheries AgLink, HPP 293.

Smith, P.F. (1966). Citrus nutrition. In: Temperate to Tropical Fruit Nutrition (Childers, N.F., Ed), Horticultural Publications, Rutgers - The State University, USA. pp 174-207.

Smith, P.F. (1966). Leaf analysis of citrus. In: Temperate to Tropical Fruit Nutrition (Childers, N.F., Ed), Horticultural Publications, Rutgers - The State University, USA. pp 208-228.