The mineral nutrition of apple trees has a major influence on their growth and cropping and on the storage behaviour of the fruit. The latter aspect is dealt with in detail in Part 2 of this Guide.
Fertiliser and irrigation practice in orchards is likely to have a major influence on the uptake of minerals and water by the trees and on the mineral composition of the fruit. The choice of soil management system can profoundly affect the availability of soil nutrients to orchard trees.
The fertiliser requirement of trees in terms of their growth and cropping and assessment of the adequacy of fertiliser programmes to optimise these effects are beyond the scope of this section of the Guide, which deals only with implications for storage quality.
The paper by Greenham (1976) provides an important overview of the fertiliser requirements for fruit trees in England, although most of the experience on apple relates to Cox’s Orange Pippin in less intensive systems of production than are generally used today.
More recently, work in AHDB Project TF 214 has investigated nitrogen management of high intensity Gala and Braeburn plantings and a recent update to Nutrient Management Guide RB209 on fruit production has provided more recommendations.
Although application of nitrogen and the use of herbicides to reduce competition from grass and weeds may increase the yield and size of Cox apples, they may also produce softer fruit with a greener background colour and a lower content of alcohol-insoluble solids.
Whilst from the production view point increased area of herbicide use in orchards may be regarded as beneficial, it is important to recognise possible deleterious effects on eating quality.
The advice contained in the remainder of this Guide should enable growers to take advantage of practices that increase yield and fruit size whilst maintaining high quality in the market place.
Soil management has important effects on the concentration of phosphorus in apples. Inadequate levels of phosphorus in Cox and Bramley apples increases the risk of low temperature breakdown (LTB) and may be associated with more rapid softening during storage.
The effect of grass sward in increasing phosphorus concentrations in Cox apples has been clearly established and consequently the use of herbicides to remove sward cover is associated with reduced phosphorus uptake into leaves and fruits.
Application of nitrogen fertiliser has similar depressing effects on phosphorus uptake. Growers should take every advantage from the higher productivity and lower management costs associated with herbicide management of the soil in orchards. However, the phosphorus levels in fruit should be monitored annually and remedial treatments such as foliar sprays containing phosphorus should be applied where fruit phosphorus levels are persistently low.
Although some other dessert cultivars such as Gala, Jonagold, Red Pippin and Spartan have an innately lower concentration of phosphorus in the fruit they are more resistant to LTB than Cox. Consequently the negative effects of herbicide use on phosphorus nutrition may not compromise the storage potential of these cultivars.
Effects of soil management treatments on susceptibility of Cox apples to bitter pit and senescent breakdown have been inconsistent. Effects of soil treatments on bitter pit development are generally related to effects on cropping and fruit size in any particular season.
More recently, it has been found that fruits’ ability to accommodate the stresses during fruit expansion have been related to the development of external bitter pit.
Effects of soil management systems on the storage quality of Bramley apples deserves special mention as the cultivar is highly susceptible to disorders such as superficial scald, bitter pit and LTB and overall grass is intensely competitive, reducing tree growth and yield and greatly decreasing leaf nitrogen.
Although fruits from trees grown in grass were higher in phosphorus and sometimes firmer and developed less bitter pit during storage, other quality characteristics were compromised including fruit size and background colour.
It is important to ensure that Bramley trees are supplied with sufficient nitrogen, by reduced competition and fertiliser application, to maximise yield, fruit size and greenness. It is hoped that any increased bitter pit potential due to improved size will be countered by a full programme of calcium sprays and the use of stringent CA storage conditions.
Similarly the reductions in phosphorus content of fruits due to herbicide use and nitrogen application will need to be countered by orchard sprays containing phosphorus.