Plant Nutrients
To successfully produce blackcurrants, growers implement a nutrient programme designed to promote growth and yield: nitrogen is directly related to strong plant growth; phosphates for growth, fruit setting and crop yield; potassium promotes growth in individual shoots and increases the weight of individual fruits; magnesium, being a constituent of chlorophyll in leaves, helps increase yields through interaction with potassium; calcium is required for cell division and enlargement, therefore essential to young plants and buds.
Soil samples and leaf analysis throughout the season determine the levels of nutrients available to the plant and where there is insufficient, they can be added either as foliar feed or to the soil during the winter.
Nitrogen
Traditionally, blackcurrants have been looked upon as gross feeders requiring large quantities of bulky manure’s rich in nitrogen. Present day practice is based on a more restricted use of nitrogen, often applied in the form of split dressings to provide a sustained level of nutrition. The availability of nitrogen is the principal factor governing the amount of vegetative growth produced. Nitrogen also plays an important role by increasing flower numbers in proportion to its availability within certain defined limits.
In the soil, the chief source of nitrogen is from the breakdown of organic matter. Maintaining the soil organic matter content therefore contributes equally to the availability of nitrogen & to the soil moisture retaining capacity of the plantation. To help this, growers plant grass or clover mixes in-between rows of bushes. Not only does this practice improve the soil organic content and structure, but it also helps prevent soil erosion, and encourages beetles and other insects.
Recommendations on the amount of nitrogen to be applied to plantations are not easy to formulate, as many factors have to be taken into account. Perennial fruit crops utilise nutrients in developing a system of roots and branches, which increases every year. For blackcurrants, the parts being harvested comprise only the fruit, which contains a moderate level of nitrogen. All other plant material including leaves & prunings remain on site to be recycled, thus returning nitrogen to the crop.
Successful establishment requires the bushes to be grown to a harvestable size as quickly as possible, in order that it is able to produce consistent, and good quality, crops. One of the grower’s objectives should be to ensure that the nitrogen will be available as and when required by the crop.
The requirements of young bushes under establishment are best dealt with by a manure or green organic crop, if available, ploughed under to provide slow release nitrogen, potash & phosphate. Failing this, a good compound fertiliser can be substituted. A suitable analysis would be 15:15:20.
Plantations in their second or third growing years, require nitrogen to build up big strong bushes to provide the type of framework required to sustain future cropping. Nitrogen is essential from mid-April until mid-July when the growth rate diminishes in line with the decreasing daylight hours.
For the mature plantation, the bush framework is fully developed and applied nitrogen requirements are less than in the formative years due to the substantial quantities returned in the form of fallen leaves and prunings. The nitrogen content of the year’s crop of leaves and prunings could be up to double that removed in the harvested fruit. Much of this nitrogen will be returned following the breakdown of organic material within the plantation - the resultant recycling of nutrients enables bush vigour to be maintained by the use of a reduced input of fertiliser compared with younger bush requirements.
Phosphates
The availability of phosphorus is essential in establishing young bushes and it is of critical importance in respect of shoot growth & overall length, fruit setting and crop yield. Phosphates require adequate soil moisture for mobility and this encourages root growth and uptake of nutrients. Soils with a high organic content and those to which animal manures have been applied, will benefit from the increase in water holding capacity as well as the phosphates liberated following the mineralisation of the soil organic material.
Fertilisers containing water-soluble phosphates are quickly available to plants, whereas ground mineral (rock) phosphates are not water-soluble and whilst cheaper, are of value only on fairly acid soils where phosphate requirements are limited. Phosphate levels are maintained in plantations by having soil and leaves analysed on a regular basis, and applying phosphate in line with current recommendations.
Potassium
Potassium, or Potash as it is sometimes known, has a very significant effect on shoot length, which is independent of the availability of nitrogen & magnesium. This nutrient also plays a critical part in increasing the weight of individual fruits and therefore of overall yields. Any deficiency is likely to lead to reduced flower numbers.
Potassium is also of very great importance in the water status of plants which have a lower transpiration rate when they are adequately supplied, leading to increased cell size & water content. High applications of potassium fertilisers are required to support continued intensive cropping. Due to its importance in metabolism & the fact that its content in blackcurrant fruit is around 2%, the availability of potash to the bushes must be ensured otherwise crop yields will suffer.
Clay soils are generally rich in potassium, whereas organic soils, peat’s and free draining acid sandy soils will be deficient due to their lack of potash bearing minerals. Potash may be lost due to fixation in certain soils containing mica, and very large amounts of potassium fertilisers would then be required to rectify the shortage. As with phosphates, levels are maintained by using leaf and soil analysis.
Magnesium
Magnesium is a constituent of chlorophyll in the leaves of plants and it interacts closely with potash in nutrition. In blackcurrants the response to magnesium increases as the availability of potash increases. By increasing the availability of potash, increased yields can be obtained if the level of magnesium is sufficiently high. Like potash, the amount of magnesium in the soil is highest in clays, whilst on sandy soils leaching may result in low availability.
Magnesium appears to be an important nutrient for fruit of all kinds, although blackcurrants can tolerate small deficits without serious effects. Deficiencies are most likely to arise on acid organic soils subject to leaching as well as on very free draining soils, which have been well limed. Excessive potassium levels may give rise to a magnesium deficiency.
Calcium and Lime
Soils which are well supplied with calcium have a higher pH level and contain more carbonates, nutrients and nitrifying bacteria compared with peats & sands which may be deficient. In these calcareous soils, heavy metals are less soluble so that iron deficiency - chlorosis - is more likely to occur.
In plants, calcium is required in cell division and enlargement, being essential to the growth of root tips & the growing points of young shoots. Pollen tube growth also depends on the availability of calcium. This nutrient is not easily transported in plant tissues - the youngest leaves & shoots may show deficiency symptoms even though older tissues contain adequate levels of calcium.
Applications of lime will increase the pH & improve the structure of non-calcareous soils. Liming is important on these soils to replace calcium, which might be lost as a result of using acid fertilisers, and through nitrification & leaching. Lime leaches out more rapidly in high rainfall areas.
