A common component of a vineyard irrigation system in the western U.S is the
"gypsum machine," which consists of a tank with agitation paddles and
an injection pump. Finely ground gypsum (calcium sulfate) is mixed with water
to form a slurry within the machine, and is then injected into the irrigation
system. Ideally, when the slurry mixes with the irrigation water, most of the
gypsum dissolves into calcium and sulfate ions, which pass through the
irrigation system and into the soil with the irrigation water. That is the
idea, and the idea is quite sound. It is the practice that presents the
There are three generally recognized agronomic reasons for injecting gypsum
into an irrigation system:
1) Gypsum contains calcium, and calcium is a secondary plant
macro-nutrient. It plays an important metabolic role in cell elongation and
division. Some crops require high levels of calcium during the growing season,
and supplying sufficient quantities sometimes becomes a management challenge,
depending on the Ca:Mg ratio in the soil. Magnesium ions may interfere with
calcium uptake into plants. Calcium-magnesium ratios (optimal Ca:Mg ratio is
2:1 to 5:1) can also be favorably influenced with gypsum applications.
2) In irrigated viticulture, calcium plays an important role in the
maintenance of soil structure. The soil colloids (clay and humus) possess
negatively charged sites, which attract and hold positively charged ions
(cations) in an exchangeable form. The structure of the soil is determined by
organic matter, microbiological processes, and the relative ratio of calcium,
magnesium, and sodium cations that are bonded to the negatively charged sites.
If too many "cation exchange sites" are occupied by sodium ions, the
soil colloids tend to disperse and pack tightly together. This reduces the size
and number of pore spaces between the aggregates, making it difficult for water
and air to infiltrate the soil. Replacing most of the sodium ions with calcium
ions tends to aggregate the soil colloids, thus providing larger pore spaces,
better water penetration, and improved soil structure.
3) The cation balance of the soil tends to come into equilibrium
with the cation balance of the irrigation water, so irrigating with high-sodium
water, Colorado River water for example, can lead to "sodic soils"
(high levels of sodium in the soil causing de-flocculation of the soil
colloids) and poor water infiltration. The cation balance of the irrigation
water is expressed as the sodium adsorption ratio (SAR) and is a common
laboratory analysis. Irrigation water high in sodium (SAR above 6.0) can be
modified through the addition of calcium, either through the irrigation water
or by direct application to the soil.
Irrigation water that is very low in total salts can also be a problem
because its low infiltration rate produces more run-off under the emitters.
This is due to the high surface tension of low-salinity water, such as Sierra
snow melt. The addition of any salt will tend to reduce this surface tension
and increase water infiltration rates. Gypsum is a very common choice for this
Dealing with the
The practical challenges of injecting gypsum relate to plugging up the drip
system. The most immediate incidence of gypsum plugging occurs either when more
gypsum is injected than can be dissolved into the irrigation stream or when not
enough time is allowed between the injection of the slurry and its arrival at
the system filter. In either case undissolved gypsum coats the filter and plugs
it. This can be overcome by reducing the concentration rate of the injection
and, when possible, by moving the injection point farther upstream from the
filter. In some cases growers move the injection point below the filter, but if
the injection stream is not adequately pre-filtered, severe problems with
plugged emitters can result.
If the gypsum is ground finer than the mesh size of the irrigation filters,
the insoluble (non-gypsum) fractions can pass into the system, which presents
another challenge. In many cases, these particles will pass through the emitter
without causing any plugging problems. But often, because of their density,
they settle out in areas of the irrigation system where the velocity of the
water is low. This can occur in large mainlines as well as in the emitters
themselves. Over time these silt-like deposits build up and affect the system
performance. The deposits become difficult to remove, especially if allowed to
remain over long periods of time. Because of this, it is recommended to
frequently flush all mainlines and hose laterals with sufficient velocity to
prevent these deposits from building up into a chronic plugging problem.
Perhaps the most common plugging problem associated with gypsum injections
is lime scale formation. Lime scale (calcium carbonate) is formed when calcium
ions (either naturally present in the source water or added as calcium sulfate
in gypsum) combine with naturally occurring bicarbonates in the water. As
described in "Formation of Lime Scale in Drip Irrigation Systems,"
(PWV March/ April 1998), calcium ions are often the missing element in a source
waters ability to deposit lime scale.
When we dissolve gypsum into water that contains appreciable (100 mg/L or
greater) amounts of bicarbonates and has a pH of 7.0 or greater, we are setting
up a system to actively precipitate calcium carbonate (lime scale). When
dealing with waters with high potential for lime scale formation (high total
alkalinity), it may be more cost effective to seek methods of applying calcium
to the vineyard other than through irrigation injection.
If the irrigation water has a high (>6.0) sodium adsorption ratio (SAR),
you may need to supply calcium to the soil to keep sodic soils from developing.
The product you use depends upon the pH of the soil, and whether or not there
is any limestone (calcium carbonate) already present in the soil. If the soil
is acidic, the addition of limestone will supply calcium and tend to raise the
pH of the soil. If the soil is neutral or basic, the addition of gypsum
(calcium sulfate) will supply the needed calcium with little effect on the soil
Applying bands of gypsum to the soil beneath the emitters will be as
effective as injecting the material into the water, without the plugging
potential. Soil applications also have the advantage of using natural rainfall
to carry calcium into the soil. If the soil contains limestone (also called
"free lime"), then soil applications of elemental sulfur or sulfuric
acid will react with the calcium carbonate to release calcium ions. Sulfur and
sulfuric acid will lower the pH of the soil as well.
If the irrigation water is very low in total dissolved solids (TDS) or
electro-conductivity (EC), making water infiltration a problem, applying a band
of gypsum underneath the emitters may be a better choice than injecting gypsum
into the irrigation system. Another option is the use of "water
penetrants" or "surfactants," which decrease the surface tension
of the water, allowing for better infiltration. There are many products on the
market formulated specifically for this application. Promoting cultural
practices that increase the organic content of the soils will also help improve
soil texture and water infiltration.
Another common occurrence on irrigated soils in the arid western U.S. is
surface crusting beneath drip emitters. In many cases, this is caused by the
formation of lime scale where evaporation causes the calcium salts in the water
to concentrate beyond their solubility point, and calcium carbonate is
deposited as a thin layer of concrete. Increasing the calcium content through
the injection of gypsum will only make this situation worse. This surface scale
can be removed by adding sulfuric acid through the irrigation water. Elemental
sulfur may be added to the soil. The microbial oxidation of soil sulfur by
bacteria digests elemental sulfur, freeing hydrogen protons. The protons react
with the calcium carbonate, forming carbon dioxide, water, and calcium ions.
How to avoid plugging
If the water source is high in bicarbonates and the pH is high (conditions
which lead to lime scale precipitation), how do we avoid the potential plugging
caused by chronic lime scale formation?
Gypsum may be injected without the worry of lime scale formation if the pH
and the bicarbonate (often expressed as total alkalinity) levels of the water
are low enough. If the natural water is too high in either of these two
factors, it can be modified with the injection of sulfuric acid prior to the
gypsum injection point, effectively reducing the total alkalinity and pH.
To determine how much acid you will need, obtain a valid water quality
analysis from a reputable independent water laboratory. Before obtaining the
sample, contact the lab and tell them that you want to know what quantity of
gypsum you can add to the water before lime scale starts to precipitate. Also
tell them how much gypsum you would like to add to the water. If you need to
add enough gypsum to make lime scale precipitate, ask them to determine how
much acid you need to add to prevent the lime scale from forming. They will be
able to instruct you on how to sample and preserve the water, so their test
will be valid. These analyses should be relatively inexpensive.
One alternative to acid injections is the use of "lime scale
inhibitors." These compounds have been around for years in
"spotless" dishwasher detergents and the steam-generating industry,
and several have been registered for use in agricultural irrigation systems.
Because of their cost effectiveness and inherent safety factors, lime scale
inhibitors are becoming quite popular alternatives to acid injections.
Another potential plugging problem may be avoided by refraining from
injecting phosphorus fertilizers into irrigation systems that have had gypsum
injections until the mainlines and hose laterals have been completely flushed.
The calcium remaining in the system can react with the phosphorus to form
insoluble precipitates of calcium phosphate, which are very difficult to flush
from the system.
Addition of soluble calcium to the vineyard has many agronomic benefits, but
there is a high potential that adding calcium to the irrigation water will
cause emitter-plugging lime scale to form, which will affect the uniformity of
the water and nutrient applications. There are many viable alternatives to
gypsum injections, and there are ways to modify the irrigation water to prevent
lime scale formation during gypsum injections. Many vineyard managers are
finding that soil applications every four or five years take less management
time than irrigation applications, which require constantly dealing with gypsum
machines and acid injectors.
Lizanne E. Wheeler and Patrick D. Brown are independent irrigation design
consultants, specializing in developing bilingual training and management
programs for micro-irrigation systems. They may be contacted through Vineyard
Water Systems, 521 Lucerne Road, Cayucos, CA 93430, tel: 805/995-0587. Reprints
of their recent publications are available at
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