Blueberries are distinguished from other crops in that they thrive on acid peats or sandy mineral soils where the pH is between 4-5. There are several reasons for this requirement. The efficiency with which blueberries take up Fe is low. Consequently, acid conditions are required to increase the quantity of Fe available in soil solution. In additional, acid conditions also ensure that N is able to exist in the form of ammonium (NH₄⁺) which is more readily utilised by blueberries than other N sources, such as nitrate (NO₃^-).

This review concentrates on highbush Vaccinium corymbosum species which are currently the most widely grown commercially.

PLANT ANALYSIS
Samples for leaf analysis should be collected in the three week period immediately prior to harvest, and subsequently, during the first week of harvest. The youngest mature leaves on fruiting shoots should be selected, with each sample consisting of about 50 leaves taken at random from 10 or more bushes. Large plantings should be divided into about 2 ha units with one or two samples being taken from each area. Interpretive standards for nutrient concentrations are listed in Table 1.

SOIL TESTING
The most important information to be obtained from soil testing is pH which should lie between 4-5. Suggested MAF soil quick test levels for the macronutrients are P, 15-25; K, 6-10; Mg, 10-12; and Ca < 8.

FERTILISER REQUIREMENTS
Maintenance fertiliser applications are listed in Table 2. These rates should be adjusted to meet local requirements, eg, soils with volcanic ash components may require higher P rates to compensate for P fixation.

The need for N applications should be based on the concentration of N in the leaves and the type of soil. Peat soils require a single N application in spring only (September 15 - October 15).

These rates (30-40 kg N/ha) should also be applied to mineral soils. However, to ensure that N is available throughout growth on mineral soils where N is more likely to be leached, a further 20 kg N/ha, either as a single or split application, should be applied in summer (early December). Ammonium sulphate, diammonium phosphate or urea should be used for N applications rather than fertilisers which supply N in the form of nitrate.

Foliar N applications are not recommended overseas.

Of the other nutrients, potassium sulphate is preferred to muriate of potash (KCL) to prevent possible plant injury from chloride.

NUTRIENT DISORDERS
The most common nutrient disorders affecting production are those associated with N and Fe. Nitrogen deficiency (N <1.7%) is associated with reduced vigour and cessation of growth before the end of the season. Unlike other crops, the colouration of the entire leaf gradually changes to red as the deficiency advances. Young shoots (whips) arising form the base of N deficient plants have a distinct pink colour. These turn pale green however when growth ceases. In severe instances of deficiency, defoliation of the older leaves may occur, while those that remain may have necrotic spots on part of, or over the entire leaf surface. Death of the growing tip also occurs. Flower number, fruit set, fruit size, and yield are all reduced by N deficiency.

Iron deficiency (Fe < 60 ppm) is likely to be induced by high pH which makes Fe less available. Young leaves at the shoot tip exhibit an interveinal chlorosis with a fine lace like network of green veins. The internodes are not shortened, nor is the leaf size or shape markedly affected as is the case with Zn deficiency which also affects these leaves. The symptoms of Fe deficiency may be confused with those of Mn deficiency; however, the two can be differentiated in that leaves with Mn deficiency tend to have a broader band of green tissue either side of the major veins. Small basal leaves and yellow shoots are symptomatic of severe Fe deficiencies.

Other nutrient deficiencies are less likely to occur, although instances of B (B <20 ppm) and Zn deficiency (Zn <8 ppm) have been observed in New Zealand. Shoot tip leaves of B deficient plants are usually small, bluish-green in colour and distorted in appearance. Chlorotic spotting of young leaves is likely and auxilary buds turn brown. Shoot die back is quite prominent.

CORRECTION OF NUTRIENT DISORDERS
Correction of Fe deficiency can be achieved either by reducing the pH of the soil with acidifying compounds such as elemental S, ammonium sulphate, and aluminum sulphate, or by addition of iron-containing compounds if the total Fe content in soil is inadequate in the correct pH range. Rates of 38 and 126 kg S/ha are required for 0.1 pH unit decreases on sandy and loamy soils respectively, where pH exceeds 4.5. Aluminium sulphate, which reacts more rapidly than S, is required at six times the respective rates recommended for S.

Where the pH is less than 4, agricultural lime or dolomite can be used to bring the pH into the correct range. If it is necessary to provide Ca without appreciably altering the pH, up to 1.25 tonne/ha of agricultural lime or dolomite can be used on peat soils. For soils with pH above 5 however, gypsum or Ca-containing fertilizers such as superphosphate should be used. Autumn application of these materials is best to allow activation before the following season.

To avoid possible toxicity problems arising from foliar application of trace metals, care should be taken when recommending this method of correction.

FURTHER READING
Ballinger, W.E. (1966). Soil management, nutrition and fertiliser practices. In: Blueberry Culture. (Eck, P. and Childers, N.F., Eds), Rutgers University Press, USA, pp. 132-178.

Cain, J.C. and Eck, P (1966) Blueberry and cranberry. In: Temperate to Tropical Fruit Nutrition. (Childers, N.F., Ed), Horticultural Publications, Rutgers - The State University, USA, pp. 101-129.

Doughty, C.C., Adams, E.B and Martin, L.W. (1981). High bush Blueberry Production in Washington and Oregon. Washington State University, USA. pp. 25