The Importance of High Quality Irrigation Water in Your Crop
The Tomato Magazine
August 2006
By Mary Peet, Brian Whipker,
John Dole and Ted Bilderback– North Carolina State University
Plants need water, but high quality irrigation water is getting harder
and harder to come by and keep. In some cases, existing wells or water
supplies have changed in pH or alkalinity, resulting in problems with
equipment or difficulty in controlling substrate pH and nutrient availability.
With water resources shrinking and the demand for water increasing worldwide,
neither water quality or water quantity can be guaranteed. The web-based
and print resources listed at the end of this article will help you make
decisions on whether and how to treat your water to grow successfully
now and in the years to come!
The Basics:
All types of water supply (pond, cistern, municipal, well, lake, stream)
can have water quality problems, and all require different types of
treatment. At a minimum, irrigation water should be CLEAN! It should
not contain plant or human pathogens, herbicides, heavy metals or industrial
pollutants, such as gasoline, at levels that could damage plants or
people. The less particulate matter (such as sand or organics) in the
water, the less filtration that is needed to keep the drip irrigation
system from clogging. Fig. 1 shows a media filter of a type frequently
used in the field for drip and sprinkler irrigation. The media bed
captures a wide variety of organic and inorganic materials and filtered
water flows out.
With this type of system, material captured by the
media filters builds up over time and is periodically removed from
the system by reversing the flow of water (backflushing).
Even when no contaminants or particulate matter are present, certain
water characteristics make it difficult to manage your fertility program
and maintain emitters, filters, evaporative pads and fogging systems.
This article discusses these characteristics: pH, alka
linity, EC, salinity and macro and microelements. Everyone knows that
before starting a greenhouse, the water should be tested. But how do
you interpret those water tests? And what do you do if water doesn’t
meet one or more quality specifications? There are a lot of water treatment
options out there, but which is the best for your particular situation?
pH
The most important reason to treat water is to maintain rootzone pH in
a range that optimizes availability of nutrients. As shown in Fig.
2, if the pH level is too low, plants take up more iron, manganese,
zinc and copper, but less molybdenum, calcium and magnesium. When the
pH is too high, plants may take up too much molybdenum and too little
iron, manganese, zinc, copper and boron. A pH of 5.6-5.8 is optimal
for tomatoes. Although people sometimes think of pH 7 water as being ‘neutral’,
it is too high for good tomato production in soilless media. Researchers
in the UK reported 25 percent declines in tomato yield at a pH level
of 7.0 in rockwool systems, for example.
Factors in addition to the irrigation water pH that affect rootzone
pH include fertilizers added to the irrigation water or present in the
substrate, and plant uptake. The fertilizer bag should list the potential
acidity on the label as pounds of calcium carbonate equivalent per ton.
For example 20-8-20, like most blends, is an acidic fertilizer with a
potential acidity of 400 pounds of calcium carbonate equivalent per ton.
As a general rule, materials containing ammonium or phosphate, such as
diammonium phosphate, are acidity forming in the substrate (i.e. they
lower pH). Those containing nitrate or calcium are generally alkalinity
forming (i.e. they raise pH). Alkalinity forming fertilizers include
calcium nitrate, potassium nitrate and sodium nitrate. Ammonium nitrate,
however, is acidity forming, rather than alkalinity forming, so it will
reduce the pH. Potassium sulfate, superphosphate, calcium sulfate and
potassium chloride are generally neutral and do not change pH very much.
Thus, the pH of the irrigation water should also be checked after the
fertilizer is injected.
Finally, the leachate from the containers should
be checked to ensure the rootzone is not becoming too acid or alkaline
from salt buildup, plant uptake or other causes. Use the standard recommendations
as described in the references at the end of this article and the alkalinity
calculator described below to establish baseline levels and adjust based
on regular monitoring of the rootzone using the pour-through technique.
Resources on using the pour-through technique are also listed at the
end of this article. Often the amount of acid or base to inject has to
be determined somewhat empirically. Generally in trying to correct and
maintain rootzone pH, it is better to monitor closely and make small
changes over time rather than suddenly making a drastic change.
Alkalinity
Water pH and alkalinity levels must be considered together. The various
carbonate species, also called the hardness minerals, contributing
to alkalinity of the water (carbonates, bicarbonates and carbonic acid)
tend to buffer water from pH changes, which is good if they are present
only in small amounts. If your water has excess alkalinity, however,
the pH will tend to rise over time in the rootzone and more acid is
necessary to bring the irrigation water pH down to the desired range.
As discussed under pH changes, high pH reduces nutrient availability.
At a high pH, minerals and iron also tend to come out of solution and
are deposited as white scale or red stains on equipment or plants.
They will also cause scale buildup inside of pipes, plug misters, foggers,
emitters and irrigation lines. Publications listed at the end of this
article provide guidelines on maximum levels of the various forms
of carbonates and recommendations
on the amount and types of acid needed to bring pH down at various levels
of alkalinity. To make these calculations easier, you can download an
Excel spreadsheet, alkalinity.xls. The spreadsheet allows you to enter
your alkalinity levels and desired pH. It then calculates the amount
of acid needed to bring the pH down. The buffering capacity can be hard
to predict, so levels of acid addition to correct alkalinity may need
to be determined somewhat empirically. It is better to start with low
levels of acid addition and make adjustments as needed. This is a good
job for the start of the season, before crops are present in the greenhouse!
However, even after plants are growing well, weekly monitoring of rootzone
conditions will allow the grower to correct problems with small changes
before they damage the plant. Examples of monitoring charts are given
in the Plant Root Zone Management guide listed at the end of the article.
Soluble Salts (EC)
Soluble salts are the total dissolved salts in the water as measured
by their electrical conductivity. This water quality attribute is somewhat
confusing to new growers because it describes both the nutrient level
of the fertigation water and the sodium (salinity) level of the water.
If the EC of the water supply is high, it may contain high concentrations
of calcium, magnesium, sulfates, sodium or bicarbonate, not all of
which contribute to plant growth. Monitoring EC in the rootzone is
important because at high EC, both shoot and root growth are poor.
A high EC makes it hard for plants to take up water, and in extreme
cases, the plant will die. Fig. 3
shows a tomato leaf after the plant was ‘burned’ by the
high salt level of added fertilizers. Seedlings are particularly susceptible
to high EC. High rootzone EC can also result when there is not enough
irrigation during leaching, when too much fertilizer is being applied
or when the irrigation water itself is high in soluble salts. Generally,
pour-though EC levels should be in the range of 2.0 to 3.5 mS/cm for
actively growing crops, but only 0.5 mS/cm for seed germination and early
seedling growth. Reverse osmosis systems are the most common option to
reduce the EC of irrigation water, but these systems are somewhat complex
and require pre-filtration and maintenance, as discussed. To convert
EC units, you can use the conversion tools on the Greenhouse Vegetable
Website: www.ces.ncsu.edu/greenhouse_veg/. An alkalinity and a fertilizer
calculator are also available on this site.
Water Treatment Options
Should you treat your water? Most large greenhouses, especially those
with recirculation systems, have elaborate water treatment systems,
as shown in Fig. 4, Even small, non-recirculating systems require at
least acid injection for pH control and filtration to remove particulates.
Depending on the specific water quality problem and the overall operation,
a number of water treatment technologies may be required to produce
consistently high quality water. Manufacturers of water treatment equipment
will analyze your water, and then often recommend that you need to
purchase their systems. Should you believe them and invest money in
a water treatment system? Before investing, have your water tested
by an independent commercial laboratory or your state water testing
facility and consult the water quality guidelines in HIL 557 (see below).
If your water is above the suggested limits, here are some questions
you can ask yourself before making an expensive investment:
Can I use acid injection to remove carbonates and bicarbonates?
As described above, and in HILS 557 and 558, injecting acids, most commonly
sulfuric, is the most common way to eliminate moderate levels of carbonates
and bicarbonates and can be the most cost-effective. Unless the alkalinity
levels are extreme or iron, manganese, hydrogen sulfide or other contaminants
are present in the water, acid addition may be all that is necessary.
Do I have an alternative water source available?
Both high alkalinity and high EC can potentially be reduced by substitution
of another water source or dilution with higher quality water. Well
water is the most likely water source to contain iron, manganese, calcium,
magnesium and organics. Municipal water, surface water or a different
well may be cost-effective alternatives to expensive water treatment
systems for alkalinity in some cases. Municipal water prices and chlorine
levels vary widely, however, so using municipal
water may not be practical in some areas. Tomatoes are fairly tolerant
of the chlorine injected into municipal water but levels over 70 ppm
Cl- are not recommended for any crops. With municipal water, it is still
necessary to control algae because polyphosphates are added to prevent
lime deposits by keeping the lime in solution. Over time phosphates break
down sufficiently to support algae growth. If municipal water is used,
other means, such as copper ionization, ozone or various algaecides must
be used to control algae growth. Blending municipal water or rainwater
with well water may be practical in some cases to reduce alkalinity or
EC. It may also be possible to blend in pond water which usually does
not contain excessive levels of alkalinity or nutrients. However, herbicide
runoff is a potential problem, and, in any case, pond water will require
filtration before use, as shown in Fig. 1, and algae control.
What is reverse osmosis (RO) and how will it improve my water?
Reverse osmosis is the most common method for reducing high EC in irrigation
water. With this system, dissolved and suspended substances are separated
from water and discarded. Reverse osmosis can remove up to 99 percent
of salts by forcing water through a semi-permeable membrane. It also
removes organic material, particulates and bacteria. The water is completely
pure at this point and will have EC and total dissolved solids readings
of zero. It is easy to make fertilizer calculations using RO water,
because the amount injected is exactly what the plant experiences.
Dr. Moyhuddin Mirza in Alberta recommends correcting high EC water
with reverse osmosis when: sodium is over 200 ppm, SAR (sodium adsorption
ratio) is over 5 or the water is not usable for some other reason.
It is generally not a good idea to use only reverse osmosis water for
irrigation. Dr. Mirza recommends adding only enough reverse osmosis
water to keep the sodium concentration below 50 ppm. One problem with
using only reverse osmosis water is that it has no buffering capacity.
For this reason, Dr. Mirza recommends adding enough potassium bicarbonate
to raise the bicarbonate level to 50 ppm if only reverse osmosis water
is used. Reverse osmosis systems also require proper pre-treatment
filtration, regular maintenance and safe disposal of the briney wastewater.
Large industrial water treatment systems can be quite complex, especially
when utilized as part of a recirculation system. There are many decisions
to be made in selecting a reverse osmosis system, however, including
the type of membrane and the system capacity. While small systems for
homeowners are available for under $1,000, they can only produce up
to 150 gallons per day of RO water. The system also requires large
amounts of water: up to 4 gallons of water to produce 1 gallon of RO
water. Large storage areas are also required for the processed water.
Can I solve my water quality problems by changing cultural and water
management practices?
Increasing the amount of extra water applied to the containers (leaching
fraction) will reduce buildup in the media, although it wastes more water.
When salts start to build up in the substrate, applying plain water to
the containers will help remove accumulated salt, if the irrigation water
itself is not too salty. In some cases, running the irrigation system
once weekly without injecting any fertilizer will also reduce salt buildup.
Seedlings are more sensitive to high EC than established plants, so purchasing
transplants or using municipal water or RO water when establishing plants
may also be a useful practice. Reducing the amount of added fertilizers,
especially calcium and magnesium, if these are already high in the irrigation
water, will also reduce alkalinity and EC problems. If micronutrients
are high in the irrigation water, be especially careful about adding
any additional micronutrients to avoid toxicities. If pH is too high,
acidity-forming fertilizers, such as ammonium nitrogen forms, can be
chosen. However with tomatoes, ammonium should not represent more than
25 percent of the total nitrogen.
If the main water quality problem is buildup of mineral deposits on the
evaporative cooler pads from high carbonates, increasing the bleed-off
rate using the bleed-off valve, will reduce buildup, though again at
the cost of extra water usage. Information on managing evaporative pad
systems is also included at the end of the article.
What if I just need to remove iron, manganese and hydrogen sulfide?
Subsurface water high in certain forms of iron and manganese will oxidize
on contact with air to form insoluble, rust-colored deposits which
stain and clog equipment, including drip emitters. There are several
ways to treat this type of water. Commercially available oxidizing
filters precipitate out iron, manganese and hydrogen sulfide, then
remove them from the water by mechanical filtration. Some of these
systems also reduce pathogens and bacteria through ozone and filtration.
Using a similar approach, iron and manganese can be removed from well
water prior to adding fertilizers by spraying into a holding tank or
pond. Aeration causes the iron and manganese to oxidize and form an
insoluble precipitate. The precipitate is then allowed to settle to
the bottom before water is withdrawn for irrigation.
Can I get by just removing calcium and magnesium with water softeners?
‘
Hard’ water contains high levels of calcium and magnesium left
as white deposits after water evaporates. In home water softening systems,
the calcium and magnesium are replaced with sodium to avoid clogging
of pipes and water heaters, unsightly deposits of ‘lime scale’ and
to help soap and detergents dissolve. Iron may also be removed with the
proper softeners. Water softening is basically an ion exchange process
in which the calcium and magnesium ions are exchanged for either sodium
or potassium ions (NaCl or KCl). Most home units use sodium and are not
appropriate for greenhouse use. Softening systems using potassium can
be used, however. Water softeners will not reduce the EC, but the potassium
added by the softener can be subtracted from the amount of fertilizer
potassium added. Removing the calcium and magnesium also avoids competition
with potassium. However the plant will still need recommended levels
of calcium and magnesium for good growth.
What about the other types of water treatment systems I see advertised?
There are a lot of different water treatment systems out there in addition
to those listed here. Some only remove particular types of water impurities,
such as iron, manganese and hydrogen sulfide. Others claim to increase
yields dramatically, but may only be effective if there is a problem
with high calcium in the system or some other imbalance. As with any
large purchase, the best policy is to research the product, avoid expensive
systems if there are cheaper alternatives, and check on where the products
were tested and are currently being used. Check also on the return
policies if the unit does not function as promised.
Resources
Water Quality Information leaflets
Two excellent bulletins on water quality and a very useful alkalinity
calculator are available online in either an .html or .pdf format at:
http://www.ces.ncsu.edu/depts/hort/hil/flowers-index.html. The site has
lots of useful information, but for water quality information, consult
HIL 558 Alkalinity Control for Irrigation Water used in Nurseries and
Greenhouses and HIL 557 Water Considerations for Container Production
of Plants.
Water quality publications
Two excellent print publications: Plant Root Zone Management and Pour-Through
Management. These can be ordered online at: http://www.nccfga.org/publications.html
for $35 and $15, respectively. These water quality topics are discussed
in much more detail in the Root Zone Management Guide, and monitoring
is illustrated in the Pour-through guide. The publications also provide
guidance on long-term strategies for monitoring and correcting pH and
EC problems in the rootzone.
Maintaining Kool-Cel pads: http://www.cooling-pads.usgr.com/cooling-pads.html
Greenhouse Product News
Water Quality Zone
Go to www.gpmmag.com and select water quality on the list of zones. You
can view articles in the archives, look at water treatment systems and
supplies, view frequently asked questions and email your own questions.
Editor’s note: Mary Peet, Brian Whipker, John Dole and Ted Bilderback are all with the Department of Horticulture, North Carolina State University, Raleigh, N.C. Peet, who submitted this material, is a professor and can be contacted by phone at (919) 515-5362 or by e-mail at mary_peet@ncsu.edu.
© 2006 Columbia Publishing
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