Gypsum mixes well with other dry fertilisers - particularly granulated forms. Gypsum is also useful as a carrier to
assist in the uniform application of small quantities of zinc, manganese, boron and the other trace elements. Conveniently, 1 kg
of gypsum has a volume of approximately 1 litre (1 tonne has a volume of 1 m
3). Quick inter-conversion between mass
and volume simplifies accurate measurement.
Gypsum is easy and safe to apply using conventional dry-spreading equipment. Recent interest has been
shown in waterborne applications via an irrigation/fertigation system. All is not straightforward with this
technology, however, and the benefits reported overseas often do not apply in New Zealand.
Most production fruit crops require between 250 and 500 kg/ha of calcium each year. A substantial part of this can
be supplied in the form of gypsum with other calcium-containing fertilisers accounting for the balance. Commonly,
nitrate is applied in the form of calcium ammonium nitrate (containing 27 % of nitrogen and 8 % of calcium) at the
rate of around 300 kg/ha. This supplies only 24 kg/ha of calcium so that most of the required calcium can be supplied
in the form of gypsum (300 kg/ha of calcium requires 1,200 kg/ha of gypsum).
Most horticultural crops need about 120 kg/ha of sulphur annually. About one third of this will come from the organic
sulphur cycle within the soil, one third from a base dressing of gypsum (40 kg/ha of sulphur requires 220 kg/ha of
gypsum) and the balance from other sulphur containing fertilisers.
To obtain significant improvements in the structure of clay soils requires much higher rates of gypsum
application (2,000-4,000 kg/ha of gypsum).
Gypsum relies upon rainfall to solubilise it and so move it into the soil profile where it has its effect, it is
therefore best applied in early spring or after harvest when rainfall can do its work. It can, however, be applied
at any time because gypsum does not damage plant tissues - even at high application rates.
Calcium transport into fruit occurs mostly during spring. Where gypsum is being used to remedy a soil calcium deficiency,
it is best to apply it well before bloom to ensure that the calcium reaches the feeder roots in good time.
Ground gypsum is quite fine and even the pulverised product contains a proportion of fine material. It should, therefore,
be applied only in calm or at the most 'light breeze' (1-2 knots) conditions. Otherwise, substantial amounts of the
product will be blown away from the target area and may cause offence to the neighbours.
See Gypsum Material Safety Data Sheet (PDF 158Kb).
