Apples and pears yield best on well drained soils that are not prone to water logging or drying out too quickly in the summer. Regular applications of gypsum for orchards located on heavier, poorly drained soil 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, a reduction in the incidence of root diseases, such as phytophthora.
Fruit Calcium
An important problem with apples is that they are susceptible to the low-calcium disorder ‘bitter pit’, eg. the Braeburn cultivar is particularly susceptible. To increase the calcium status in apples, and reduce the problem, requires as many as 12 foliar calcium sprays each season.
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). Rootstock selection, orchard floor management, root growth, tree vigour and irrigation practice must be taken into account. Also some on-orchard management interventions including pollination, fertilisation, fruit thinning and crop load may need to be taken into account.
The underlying issue with fruit calcium deficiency is not the soils, or the trees themselves, but that the fruit contains only 1/10th of the calcium of 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).
Gypsum applications to soils that are already calcium-sufficient is unlikely to improve fruit calcium levels – simply putting more calcium on to the ground will not necessarily raise calcium levels in the fruit. Recent research suggests 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 fruit storage quality.
Soil Calcium
Certain soils where pipfruit is grown are deficient in calcium, e.g. areas of Nelson, Canterbury and Otago regions. 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 nutritional deficiency. Gypsum can be applied at any time and with other fertiliser applications. However, because calcium uptake by the fruit occurs during the first 6-8 weeks after full bloom, it is best to apply gypsum in late winter to early spring to ensure that the calcium reaches the roots before flowering commences. Gypsum should be applied in calm conditions.
Applications of gypsum will depend on soil type and results of a recent soil test.