Apples and pears have been grown successfully on a wide range of soils but they yield best on well-drained soils that are prone neither to water-logging nor to drying out too quickly in the summer. The application of gypsum to orchards that are located on the heavier, poorly drained soils will generally improve soil structure, reduce compaction and improve drainage. This should result in improvements in the general health of the trees and, in particular, in a reduction in the incidence of ‘wet feet’ problems.
An important problem with apples is that they are susceptible to the low-calcium disorder ‘bitter pit’. Braeburn and Cox’s Orange Pippin are both very susceptible and together these cultivars account for almost half of New Zealand’s apple production. To increase the calcium status of their fruit, and thus to reduce the problem, it is conventional to apply as many as 12 foliar calcium sprays each season.
The underlying issue with fruit calcium deficiency is not that the soils, or indeed the trees themselves, are calcium deficient, but that the fruit contain very much (x1/10) less calcium than the rest of the tree – the tree may be calcium rich while the fruit are calcium deficient! Low fruit calcium status is the result of a physiological limitation in the transport of calcium from the tree into the fruit (calcium is immobile in the phloem and fruit are mostly phloem fed) and there is not much that can be done about this. Low fruit calcium status is also a problem with most other fruit crop species including kiwifruit, tomatoes, and capsicum etc and the fruit of each species suffers a particular calcium deficit disorder.
The incidence of bitter pit in apples varies from season to season (implying a weather effect) and also from orchard to orchard (implying a soil or a management effect). Not much can be done about the weather of course but some on-orchard factors may be amenable to management intervention. The latter include managing factors such as pollination, fertilisation, fruit thinning and crop load. Also, rootstock selection, orchard floor management, root growth, tree vigour and irrigation practice.
It is thought unlikely that improved fruit-calcium will result from gypsum applications to soils that are already calcium-sufficient – simply putting more calcium on to the ground will not necessarily raise calcium levels in the fruit. Recent research suggests that it may be possible to raise tree calcium to luxury levels by applying gypsum in conjunction with certain other soil treatments that influence the uptake properties of the roots. This, then, can impact favourably on fruit calcium status and thus fruit storage quality.
Certain soils where pipfruit is grown are deficient in calcium (e.g. areas of Nelson, Canterbury and Otago). In these regions, fruit quality will respond to applications of gypsum as a calcium fertiliser. In this case, gypsum is being used to correct a specific soil nutrition deficiency. Gypsum can be applied at any time and with other fertiliser dressings. However, because calcium uptake by apple fruit occurs during the first 6-8 weeks after full bloom, it is best to apply gypsum in early spring to ensure that the calcium reaches the roots before bloom. Gypsum should be applied in calm conditions.