ARCHIVE: Fertiliser Manual (RB209)

Apple Fruit Analysis

Analysis of fruit sampled within three weeks of picking is a useful indicator of the risk of some physiological disorders in stored apples. Results can also be used to rank orchards for potential storage quality. During the period between sampling for analysis and harvesting the fruit, the concentration of calcium falls mainly due to dilution as fruit size increases. Analysing fruit too far in advance of harvest may over-estimate the storage potential.

Fruit samples should be taken as near to harvest as possible but within 2 weeks of picking. In each orchard randomly select 30 trees of the same age and variety. Take one apple at random from each tree, alternating from side to side and at different heights but ignore abnormally large or small fruits. Try to make the sample representative of the side of trees where most fruit is growing.

If fruit analysis produces consistently high or low concentrations of a particular nutrient over two to three years, modification of fertiliser application should be considered. The most likely change will be a reduction in nitrogen or potash use. Fruit analysis may also show deficiencies of calcium or phosphorus which can reduce fruit storage quality. These deficiencies can be corrected by foliar sprays of calcium and phosphorus or by post-harvest calcium treatments.

Apple Fruit analysis – satisfactory nutrient concentrations for storage (sampled at harvest)

Crop

Nitrogen (N)

Phosphorus (P)

Potassium (K)

Magnesium (Mg)

Calcium (ca)

 

mg/100g fresh weight

Cox

50-70

11.0 minimum

130-150

5.0

4.5 minimuma
5.0 minimumb
Bramley

60 minimum

9.0 maximum

105-115

5.0

4.5 minimuma
5.0 minimumb

a. For controlled atmosphere storage (Cox in 2% oxygen until late February or 1.2% oxygen until late March; Bramley in 8–10% carbon dioxide until June or 5.0% carbon dioxide plus 1.0% oxygen until July).
b. For storage in air at recommended temperature (Cox until mid October; Bramley until November).

Satisfactory nutrient concentrations have not been established for other varieties, but the calcium requirements are likely to be similar to those given in the table. The standards given for calcium and potassium concentrations in Cox also apply to Egremont Russet. Average values across many orchards for Gala and Braeburn apples are given below.

Average nutrient concentrations in Gala and Braeburn apples 

Crop

Nitrogen (N)

Phosphorus (P)

Potassium (K)

Magnesium (Mg)

Calcium (ca)

 

mg/100g fresh weight

Gala 2 wk < harvest

45

10.2

106

5.3

9.6

Gala harvest

42

9.3

122

5.0

7.4

Braeburn 2 wk < harvest

52

11.9

102

5.1

6.3

Notes:

  • Gala is naturally low in P compared to Cox, but this does not affect storage potential
  • Gala is naturally high in Ca and does not suffer from Ca-dependent storage disorders
  • Large concentrations of K in Gala fruit may increase the risk of breakdown
  • Gala fruits higher in Ca and lower in K may be firmer ex-store
  • Braeburn has a similar composition to Cox except that K concentration appears lower
  • Suggest using Cox calcium threshold concentrations to judge storage potential of Braeburn apples

Nitrogen (N): As the nitrogen content increases, fruit becomes more susceptible to rotting, loss of firmness, poor skin finish and a lack of red colour. Above 80 mg N/100g the risk of disorders in Cox is high. In Bramley, a large nitrogen concentration leads to more green colour, but risk of lower firmness.
Phosphorus (P): At phosphorus contents below 11 mg P/100g in Cox, there is an increased risk of fruit losing firmness and developing breakdown, particularly if calcium is also low. In Bramley, the phosphorus threshold for breakdown is lower at 9 mg P/100g.
Potassium (K): A high potassium content will increase the risk of bitter pit, Gloeosporium rotting and core flush. The risk of bitter pit is also greater if the calcium level is low in relation to potassium. Generally, fruit flavour and acidity increases with increasing levels of potassium. Thus, if calcium levels are adequate (over 5.5 mg Ca/100g), large concentrations of potassium may be advantageous in terms of fruit quality.
Magnesium (Mg): A high magnesium content will increase the risk of bitter pit, especially when calcium levels are marginal.
Calcium (Ca): Calcium levels of 5.0 mg Ca/100g and above are necessary to maintain high quality throughout long-term storage. However, the storage potential will be modified depending on the content of other elements especially potassium and phosphorus. Disorders associated with low calcium (bitter pit in particular) are more likely to occur in apples of a given calcium content which are stored in air rather than controlled atmosphere. Thus, fruit which meets the standards for nitrogen, phosphorus, potassium and magnesium and has a calcium
concentration in the range of 4 to 5 mg Ca/100g should not be rejected for mid-term controlled atmosphere storage as it is unlikely to develop commercially significant levels of bitter pit or breakdown. This much lower risk is reflected by the dual standards for calcium given in the table.

The risk of bitter pit and susceptibility to Gloeosporium will depend on the ratio of K: Ca. If the K: Ca ratio is over 30:1 in air stored Cox or Bramley stored in controlled atmosphere, or over 25:1 in air-stored Bramley, commercially important losses due to bitter pit are likely. Where calcium contents are marginal (3.5–4.0 mg Ca/100g) and fruit phosphorus is also less than 9.0 mg P/100g, both Cox and Bramley are more susceptible to breakdown. In such cases, Cox should be marketed early. Bramley should be stored at a higher temperature (average 4.5°C) and sold earlier than fruit with optimum levels of calcium and phosphorus.

Gloeosporium risk is dependent on the level of inoculum in the orchard and is influenced by skin finish, fungicide programme and rainfall prior to harvest. Fruit analysis is a measure of fruit susceptibility to Gloeosporium and not necessarily the eventual amount of rotting.

The incidence of senescent breakdown is greatest in late picked fruit for any given content of phosphorus and calcium.

Susceptibility to low temperature breakdown in Cox stored at 3.0°C (air) and 3.5°C (controlled atmosphere), although less common in Bramley at 4.0°C, is also partly due to low calcium and phosphorus contents.

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