There has been considerable work on blackcurrants (Ribes nigrum) by British and East European scientists. Few New Zealand results are currently available.

PLANT ANALYSIS
Samples for leaf analysis should be collected 2-3 weeks before fruit maturity. The youngest mature leaves on new season's growth should be selected and the leaf, plus the petiole, taken for chemical analysis. Each sample should consist of about 50 leaves taken at random from 10 or more bushes. Interpretive standards for nutrient concentrations are listed in Table 1.

FERTILISER REQUIREMENTS
It is clear from the results of overseas trials, and from monitoring blocks within New Zealand, that the fertiliser requirements of blackcurrants are not well understood. Annual NPK recommendations for example vary between 40-140 kg N/ha, 0-120 kg K/ha and 0-50 kg P/ha. The following recommendations are guidelines only and have been based on crop removal factors (Table 2) with allowances for leaching, pruning removal and new growth.

To minimise leaching losses and to coincide with periods of maximum crop demand it is suggested that the N requirement (5 kg N/tonne of fruit anticipated/ha) is split; half being applied immediately prior to growth commencement in spring, with a second application before fruit swelling in early December.

Soils tests (taken in May) are suggested as the basis for determining K and P requirements. Potash should be applied at rates of 5 kg K/tonne of fruit anticipated/ha and 3 kg k/tonne/ha where MAF quick test soil levels are between 5-10, and 11-20, respectively. Additional K will be required for test levels below 5. Phosphorus at a rate of 1 kg P/tonne/ha is recommended for soil test levels of between 5-15. Soil reserves are sufficient to meet crop requirements where test levels for K and P respectively exceed 20 and 15.

While there is insufficient information to suggest that more that one application is necessary, potash could be applied with N in spring and early December as a split dressing. As with other soft berries, blackcurrants are sensitive to excess chloride, hence sources of K other than muriate of potash (KCI) need to be used for quantities in excess of 100 kg K/ha.

The Ca and Mg requirements of blackcurrants are small, consequently, regular dressings of these elements are not normally applied. When foliar analysis or soil tests indicate the need, 15-20 kg Mg/ha should suffice. Blackcurrants perform better at pH levels above 5.8. Use of lime or dolomite every 2-3 years to maintain pH will ensure that the Ca and/or Mg requirements of this crop are adequately met.

NUTRIENT DISORDERS
While responses in growth and subsequent yield to various fertiliser treatments have been recorded, foliar symptoms attributable to nutrient deficiencies have rarely been observed in New Zealand. Nitrogen deficiency (N < 2.6%) where the leaves develop bright orange, red or purplish red colourings, or potassium deficiency (K < 1.0%), where leaves that first show reddish-purple tints gradually develop a necrotic marginal scorch, are likely to be the most common disorders encountered.

FURTHER READING
Langford, G.I. (1982). Blackcurrants. Management techniques. New Zealand Ministry of Agriculture and Fisheries AgLink, HPP 257.

Langford, G. and Mavromatis, G. (1981). A Review of the New Zealand Blackcurrant Industry. New Zealand Ministry of Agriculture and Fisheries, Christchurch, pp. 51.

Ljones, B. (1966). Bush fruits nutrition. In: Temperate to Tropical Fruit Nutrition. (Childers, N.F., Ed), Horticultural Publications, Rutgers - The State University, USA, pp. 130-157.