Like many valuable horticultural crops (avocados are priced around NZ$3 /kg fob), avocados are extremely fussy when it comes to the soil conditions under which they will flourish.
For instance, they are very prone to ‘wet-feet’ fungal disease problems, they are very susceptible to calcium deficiency and they require a low soil pH between 5.5 and 6.5 (so lime should not be applied to correct a soil calcium deficiency).
Avocados are litter feeders, with most of their ‘feeder’ roots in the top 10 cm of soil and in the natural organic mulch under the tree. The slow mineralisation of the organic component of this mulch serves as an important source of slow-release nutrient. The use of gypsum as a source of calcium also helps to give stability to the organic components of the soil. Meanwhile, the sulphur content of gypsum helps to prevent the loss of valuable nitrogen from this organic layer by trapping the ammonia (released by bacteria) and converting this to ammonium sulphate – a fertiliser.
Avocado trees are acid loving plants. Although they will tolerate a slightly greater pH range they grow best in soils with pH values falling in the very narrow band between 5.8 and 6.3. Gypsum applications are a good way in which to obtain the high calcium levels required by this crop without raising soil pH.
Avocado trees are very susceptible to root rot caused by Phytophthora sp. This disease is exacerbated by poor drainage and by anaerobic soil conditions. The physical improvements that gypsum can make to soil structure (where the underlying soil is heavy) and, particularly to drainage and aeration, will assist in reducing the severity and incidence ofPhytophthora infections. In addition, a low soil pH has a suppressive effect on P. cinnamomi.
Calcium plays an important role in avocado nutrition. It is involved structurally in the plant and is an important component of both cell walls and cell membranes. Fruit calcium status has long been recognised as involved in determining fruit quality, particularly affecting long-term storage. Good keeping properties are essential if avocado fruit are to survive shipping to distant markets.
Symptoms of calcium deficiency in avocado include:
• Young leaves have distorted or irregular margins
• Older leaves are sensitive to sunburn and fading
• Leaves develop spotted or necrotic areas
• Leaves show necrosis at the tips
• Defoliation in cases of extreme calcium deficiency
Healthy high yielding trees typically have leaf calcium levels of 1.6% or more, levels over 2% are desirable. Typically, gypsum applications for avocados vary from 200 to 1,000 kg/ha twice a year. Actual rates will depend on soil type and thus on the goal of application. Lime is generally unsuitable for avocados except where the soil is extremely acid.