Almost half of every plant lives
underground. Because we don't see the roots,
it is easy to underestimate the importance
of the below ground environment. To achieve the high
levels of production essential in modern
production, close attention must be paid to
the soil. Compared with many nations, New
Zealand enjoys excellent soils and a climate
that engenders rapid growth. Together these
translate into record-breaking production.
New Zealand's rural industry has seen a
recent shift in emphasis from extensive,
land uses such as pastoral agriculture, to intensive
ones such as perennial vine/ treecrop
horticulture (kiwifruit, winegrapes,
pipfruit, stonefruit etc). If this change is
to be economic, significant productivity
gains (biomass/hectare/year) must be
achieved. In most cases this requires capital
improvement to the soil. This upgrade
remedies any shortcomings so as to lift
soil properties closer to those identified
as ideal for the new crop.
Many soils tend gradually to lose
condition. This process is greatly speeded
under intensive production. Their physical
condition is degraded by tillage, the use of
machinery etc; their chemical condition is
degraded by the crop's removal of minerals,
by leaching etc; and their moisture content
is depleted by transpiration - plants remove
water from the soil at a rate proportional
to their growth. To achieve and maintain
high rates of production, it is basic that a
soil will require ongoing maintenance of
its: (1) physical condition (structure,
organic content, aeration/ compaction); (2)
chemical condition (pH, mineral
excesses/deficits/ balances) and (3) moisture
content (water excesses/ deficits).
It is usual to manage these separately
with practices such as manuring (to raise
soil organic content) and tillage/ ripping (to
reduce weed competition/ increase soil
aeration/ break up pans). Fertilisers are
broadcast (to raise soil-nutrient levels/
adjust soil-nutrient balance/ pH). And,
drainage is installed and irrigation applied
(to remove excess water/ minimise water
deficit). Sometimes, however, these
activities are combined.
Aspects of the soils (1) physical
condition can be addressed at the same
time as aspects of its (2) chemical
condition. An example is in the use of
gypsum - both a remedy for soil-structure
problems and a calcium/ sulphur fertiliser.
More recently, (2) mineral nutrients have
been added to (3) irrigation water. Fertigation
is on the increase as it offers a number of
advantages (plus a few disadvantages). Less
common, but now being considered by some, is
the combination of a practice that addresses
aspects of a soil's (1) physical condition
with ones that addresses its (2) chemical
condition and also its (3) moisture
condition - namely the addition of mineral
gypsum to irrigation water.
This article discusses the waterborne
distribution of gypsum to tree/ vinecrops
via an irrigation/ fertigation system. The
practice had its origins a good many years
ago, overseas (mostly in the USA and
Australia) where much of the relevant research
has been done. We are not aware of New
Zealand research.
Rates
Horticultural gypsum serves both as a
fertiliser
and as a soil
soil
conditioner. At low rates (200-1,200
kg/ha) it is used, along with other
mineral fertilisers, to support the
calcium/ sulphur requirements of the crop.
At higher rates (2,000-4,000 kg/ha) it is
used to remedy soil-texture, aeration and
drainage problems in heavy (high clay)
soils where it flocculates small particles
into larger aggregates. The first usage is
an annual one needed to replace minerals
continuously lost from the site (removed
with the crop or leached below the root
zone). The second usage is repeated every
few years to develop and maintain good
soil structure.
Irrigation water is applied to most
fruitcrops during the summer months to
supplement rainfall. The amount of water
applied, and the frequency of its
application is very dependant on the soil
type/ the crop/ rooting depth/ the
weather. For many fruitcrops it would be
common to apply around 400 mm (4,000 m3/ha)
of water each year. This amount accounts,
roughly, for the excess of evapotranspiration
(Et) over precipitation (P) during
the summer months (see Fig. 1).
Figure. 1. Averaged values for evapotranspiration
(Et) and precipitation (P) in
New Zealand. During the five cooler months
(April-August) P>Et and approximately
200 mm of excess water is lost due run
off/ infiltration. During the seven warmer
months (September-March) Et >P and this
creates a summer water deficit of
approximately 400 mm. To a variable, but
always rather limited extent, storage of
water in the soil buffers the seasonal
water excesses and deficits.
Texts give the solubility of CaSO4·2H2O
(the dihydrate of calcium sulphate) as
0.241 g /100 cm3 of cold water.
This solubility equates to 2.41 kg /m3.
Winstones's
gypsum is almost pure CaSO4·2H2O.
This should allow the
application of about 10,000 kg gypsum
/ha/year via an irrigation system (4,000 m3
x 2.41 kg /m3 = 10,000 kg).
However, the practical upper limit for
gypsum's solubility is much lower than the
saturated value - it takes a long time and
a careful laboratory procedure to make up
a saturated solution. For all practical
purposes, you will have either a true
solution of gypsum that is quite
dilute or you will have a more
concentrated solution but that also
contains, in suspension, a
significant proportion of solid gypsum
particles.
In principle it should be possible to
make up, and to distribute, a watery
mixture (a slurry) that contains a very
substantial proportion of fine gypsum
particles in suspension. In this way it
would be possible, greatly to increase the
irrigation water's gypsum content to a
level far above that of a true saturated
solution. However, the likelihood of the
gypsum particles settling out and causing
blockages in less-turbulent parts of a
distribution system are extreme. These
blockages would be difficult and expensive
to removed. So far, research has not
developed a practicable system for this, so
we are left with the use of true
solutions.
Because true solutions, close to
saturation are difficult to make up and to
manage, practical recommendations for
horticultural dissolutions of gypsum in
irrigation water commonly suggest rates
lying between 15 and 30% of the saturated
value. This makes dissolution faster and
more straightforward and also reduces the
risk of blockage from settlement of
undissolved solid in pipelines and
outlets. Unfortunately, the use of a
dilute solution (say, 20% of saturation)
reduces the amount of gypsum that can be
applied via an irrigation system to about
2,000 kg/ha/year (20% of 10,000
kg/ha/year).
Clearly there is a little flexibility
to raise this maximum - either by
increasing the volume of irrigation
water applied or by increasing the concentration
of gypsum closer to the saturated value.
Conversely, for a fruitcrop in which
significantly less irrigation water
is usually applied (e.g. winegrapes
commonly receive only about ¼ of this
amount, say, 100 mm per year) the amount
of gypsum able to be distributed is
correspondingly reduced.
These maxima also assume that all irrigations
are carried out with water having a
significant dissolved-gypsum content. This
requirement will almost certainly create
difficulties of incompatibility
(precipitation) with other nutrient
materials that must be distributed via the
same system. Therefore, a practical upper
limit of 1,000 kg per year is suggested
(allowing for half the irrigations to be
gypsum ones).
This amount of gypsum (1,000
kg/ha/year) compares with the usual
dry-application rate for gypsum when used
as a fertiliser (200-1,200 kg/ha/year). It
is definitely low when compared with the
gypsum amounts required to achieve a
significant improvement in soil structure
(2,000-4,000 kg/ha).
We may fairly conclude that waterborne
applications of gypsum at rates sufficient
to ameliorate a soil's physical properties
are not feasible - the amounts of gypsum
able to be applied falling well below
those usually required to obtain
significant benefits.
On the other hand, waterborne
applications of gypsum for fertilisation
purposes are feasible. The practice
requires only the acquisition of suitable
equipment to create the capability. That
is, an agitated mixing tank to form a
gypsum slurry, and injectors, filters etc
to introduce and intermix the slurry at
accurately metered rates into the
irrigation/ fertigation system. Ideally,
the system should incorporate high-flow
lines able to handle the resulting
solution so as to minimise the risk of
settlement and blockage from any stray
particles of undissolved gypsum entering
it. These technologies are available and used overseas.
Feasibility is not advisability,
however, and we must consider all aspects
of the technology and, especially, its
relevance to the New Zealand situation.
Some (promotional) literature claims a
number of benefits for waterborne gypsum
applications. It is important to recognise
that these do not necessarily apply
to the same extent, or even at all, in New
Zealand because of marked differences
(compared with the USA, the Mediterranean
basin, Australia etc) in our:
- Politico/ economic environment (no
subsidies/ tax breaks to distort the
economics of capital-intensive
technologies)
- Climate (mild, maritime, temperate)
- Rainfall (about 800 mm per annum and
distributed fairly uniformly
throughout the year. Our rainfall is
also lighter and more frequent - more
wet days)
- Soils (often heavy, rarely sodic)
- Irrigation water (generally high
quality/ plentiful)
Some proponents of waterborne gypsum
application include in their lists of
benefits, ones that are generic to all applications
of gypsum. That is, they include those also
obtainable from dry-broadcast gypsum
applications. While in a sense this is
valid, it is important not to imagine that
all the benefits claimed as associated
with waterborne applications are
necessarily peculiar to them.
Some issues to consider...
Fertiliser
Published material promoting proprietary
gypsum products for waterborne
distribution tends to focus on their
benefits as calcium/ sulphur fertilisers
rather than on their benefits as soil
improvers.
This emphasis is fair, in view of
our conclusion regarding the maximum
application rates for waterborne gypsum.
However, some promotional material seems
to claim that the 'fertiliser effect' is
in some way better achieved with their
waterborne product than with a dry-broadcast
one. This idea could be misleading,
especially in the New Zealand's climate.
Once in the soil where they have their
effect, calcium and sulphate ions are,
after all, just calcium and sulphate ions!
Sodicity
In Australia, and in some low-rainfall
areas of the US, it is not uncommon for
waterborne gypsum to be used in the
reclamation of sodic soils (characterised
by high exchangeable sodium and high pH).
New Zealand has few areas with sodic
soils, so this application is largely
irrelevant here.
Water quality (salinity)
The cation balance of a soil tends to
equilibrate with that of the irrigation
water. For example, irrigation water
having a high Na/Ca ratio tends to degrade
the soil. Gypsum applied along with
poor-quality irrigation water can be
beneficial.
By and large in New Zealand, we are
not forced to irrigate with poor quality
water, so this is less relevant.
Root zone
When gypsum is applied along with the
irrigation water, deposition will be
localised to the root zone, therefore (it
is argued) less gypsum is required. This
benefit is most relevant in low-rainfall
climates where roots congregate around the
dripper/ sprinkler outlets and are
relatively scarce elsewhere in the soil
volume - infrequent rain = dry soil = few
roots. There is little point in applying
gypsum where there are no roots!
This is less of an advantage in New
Zealand where, because of a relatively
high rainfall, crops are less dependent
upon irrigation water and their roots are
not as localised around the irrigation
outlets.
Dissolution
A 'super-fine' gypsum product is claimed
to offer the advantage of rapid
dissolution.
This idea has more relevance to
broadcast applications of a 'coarse'
mineral gypsum in 'low-rainfall' climates.
The standard Winstone
products are substantially composed of
fine-milled material. Also, the New
Zealand climate is not a low-rainfall one.
Gypsum dissolution is relatively rapid
here, with surface applications of 4,000
kg/ha usually disappearing within about
three months (the actual period depends on
time of year and weather).
'Enhanced' uptake
Some promotional material implies that
very tiny, undissolved, particles of
'super-fine' gypsum (i.e. solid CaSO4·2H2O)
are small enough to be somehow taken up by
the plant more directly than are the
component Ca2+ and SO42-
ions when dissolved in the soil water.
It is difficult to identify a
scientific basis for the claim.
Soil water movement
A downward flow of water into the soil
(infiltration) is required to carry the
dissolved gypsum from the soil surface
down to the root-zone (generally from 50
to 750 mm below the surface) where it will
have its main beneficial effect. Under
low-rainfall conditions, there is reduced
tendency for surface-applied gypsum to
move into, and down through, the soil and
thus a potential benefit for waterborne
gypsum applications.
This benefit of waterborne
applications is less relevant in New
Zealand where there is sufficient rainfall
to dissolve and move up to 16,000 kg
/ha/year of gypsum down into the soil (a
calculated value based on the solubility
of gypsum and an annual rainfall of 800
mm). The rate of movement of a dissolved
solute in the soil is very much affected
by the soil's ion-exchange properties.
Water quality (hardness)
Gypsum when dissolved in hard water (water
containing significant levels of HCO3-)
can cause a precipitation of lime scale
(CaCO3) in the irrigation pipes
and nozzles, leading to their eventual
blockage.
Much of our apple and winegrape
production in the Hawkes Bay is irrigated
with 'hard' water.
Purity
For waterborne distribution, the gypsum
product must be of high purity (possibly
at higher cost) to avoid the possibility
of line/ emitter blockage by insoluble
minor contaminants.
This is probably not a problem with Winstone's
product, which is derived from a
high-purity mineral, mined in Australia.
Cost
Waterborne applications of gypsum require
the installation of expensive equipment
(extra capital cost) and this equipment
requires careful attention to maintain
cleanliness, to clear filters and to
attend to minor blockages (extra
operational cost).
In some overseas
situations conventional broadcast
applications of gypsum every 4-5 years
have been found to be less expensive and
to require less management time than
waterborne applications. This is true,
even in a politico/ economic environment
having subsidies and tax breaks that often
distort the economics of a
capital-intensive technology.
Risk
Any undissolved particles of gypsum
escaping into the reticulation system1
will tend to settle out wherever water
flow is slow or less turbulent (e.g. in
the main lines or in the emitters).
Dissolution of these particles will be
slow2.
A solid deposit, building up with
time will gradually impair system
performance (reduced/ unbalanced flows).
If the gypsum solution is substituted for
pure water from time to time the deposits
will, presumably, dissolve away but if a
line were to become completely blocked,
correction would be much more difficult.
