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.
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.
