Where apples are produced in warmer climates than in the UK, there is much concern about the overuse of nitrogen fertiliser. In North America, at harvest, high nitrogen fruit tend to be larger, greener, softer, more subject to pre-harvest drop, and more likely to be affected by cork spot and bitter pit. After storage they develop greater amounts of scald, bitter pit, internal browning and internal breakdown.
In the UK no such dramatic adverse effects on fruit quality have been associated with nitrogen application. Contrary to the results of Bramlage et al., (1980) there are reports that the application of nitrogen fertiliser reduces the susceptibility of Cox apples to bitter pit. However, Richardson (1986) reported softer fruit and a greener background colour in Cox apples following additional application of nitrogen fertiliser.
Upper limits for leaf and fruit N of 2.6% (dry weight) and 70 mg 100g-1 (fresh weight) respectively were set prudently by Sharples (1980) to combine adequate cropping and good storage quality in Cox.
It is suspected that where nitrogen application is associated with poorer storage quality the effects may be due indirectly to the stimulation of vegetative growth and an increase in fruit size. These effects can alter the concentration of other nutrients in the fruit such as calcium and phosphorus that are more causally linked with the development of physiological disorders during storage.
For Cox and other dessert apple cultivars the judicious use of nitrogen fertiliser should be considered in order to maximise red colour development.
Evidence has suggested a positive role for nitrogen in the development of firm Cox apples. Sprays containing calcium nitrate or urea applied during the cell enlargement phase of fruit growth (late June – harvest) have had a positive influence on the texture of Cox apples. Growers should consider using calcium nitrate sprays in preference to calcium chloride for Cox apples (see section on calcium below).
Bramley produces higher yields and more vegetative growth than Cox and consequently has a higher requirement for nitrogen fertiliser (MAFF, 2000). The satisfactory range in leaf nitrogen for Bramley is 2.4-2.8 % (dry weight) and the upper limit for nitrogen in the fruit at harvest is 60 mg 100g-1 (fresh weight) (MAFF, 2000). For Gala and Braeburn, Nutrient Management Guide RB209 suggests leaf nitrogen for Gala is 2.3% (dry weight) and Braeburn should follow Cox recommendation of 2.6-2.8% N in dry weight samples.
The method of soil management in Bramley orchards is particularly important. Complete grass cover is intensely competitive and dramatically reduces the availability of water to the tree and the uptake of nitrogen. It is difficult to overcome the adverse effects of grass competition on fruit yields by nitrogen application alone.
Where leaf N is below the recommended minimum of 2.4 % (dry weight) the yield and size of fruits is likely to be reduced. In addition the increased yellowing of the background colour and increased redness detracts significantly from the market value.
Daily irrigation in Gala orchards has been reported to decrease leaf N concentrations, suggesting that nitrogen is being leached out of the soil.
Where effects of grass competition are reduced or removed by the use of herbicide, applied in the tree row or to the entire soil surface, there was little effect of N application on storage quality. To achieve optimum yields and quality in terms of greenness and least red coloration a minimum leaf N of 2.6% is suggested.
There have been no trials to evaluate the effect of nitrogen application on other dessert cultivars important in UK production such as Gala and Jonagold. However, in recent studies in commercial orchards average nitrogen levels in the leaves (2.3%) of these cultivars were well below the minimum level recommended for Cox (2.6%).
Cox nutrient standards should not necessarily be applied to other dessert cultivars. However, commercial experience (FAST Ltd) with Jonagold indicates a high requirement for nitrogen and that routine top dressings are required to maintain high yields.
Further research requirements
Recent work in AHDB Project TF 214 investigating the improvement of nitrogen use efficiency, sustainability and fruit quality in high density orchards has reported different rates of nitrate leaching from the soil profile between Gala and Braeburn trees grown on M9 rootstocks.
Further work is required into the effects of nitrogen fertiliser application on the quality of dessert apples under UK growing conditions. These studies are required particularly for Gala and Braeburn where production continues to increase.
It is important to be able to establish guide-lines for nitrogen nutrition in order to maximise cropping potential and profitability of commercial orchards but at the same time safeguarding storage and eating quality.