Hydroponics uses a mineral-based nutrient solution to nourish plants in a soilless environment. In an active hydroponics system, this nutrient solution is re-circulated, passing over the roots several times. This is the primary reason why plants grown in hydroponics use 80-90% less water than plants grow using conventional techniques. Given that fresh water is arguably the most precious resource on our planet, this statistic seems to lend hydroponics some serious environmental credentials.
However, let’s not forget all those minerals that have been mined and refined to create that nutrient solution in the first place! How we manage our nutrient solution, especially when it comes to run-off / disposing of spent nutrients, is absolutely key is we’re to take our stewardship of the planet’s resources seriously.
Is zero-runoff possible in hydroponics? We asked hydroponics consultant, Harley N. Smith, to explore this challenge, starting with the stuff that makes up the bulk of your nutrient solution: WATER!

If you're serious about minimizing the waste your hydroponics
garden produces, you need to get serious about your water quality.
No matter which hydroponic system you choose, the first step to
nutrient solution management is obtaining clean water. Plants
use a tremendous amount of water to support life. In fact,
for every carbon molecule assimilated through photosynthesis, about
500 water molecules are lost through transpiration! So
topping off the reservoir from a clean water source is very
important in order to prevent trace elements in the water from
accumulating to toxic levels over time. The goal of zero
runoff is to replace all of the water lost through transpiration,
while providing only enough mineral salts to replace the specific
elements removed by the plant. Nothing should go down the
drain.
In the Netherlands rain water is collected from the greenhouse
roof. Rain sensors on the greenhouse roof detect rainfall,
and the computer starts the collection process about 15 minutes
after the rain begins. The short delay allows enough time for
most of the dust and debris to be washed from the roof before the
rainwater is collected. If there is not enough rainfall to
support all of the needs of the plants, supplemental water sources
must be tapped, especially during the dry season. In the
Netherlands, where zero runoff is the national standard, any well
water removed from the ground must be carefully metered, and it
must be replaced by law during the rainy season.

Different water sources have different levels of minerals,
measured in parts per million (ppm). If you are considering
commercial hydroponic production, a water analysis should be
performed before the first crop is planted. A water analysis
should test for all essential elements, pH, EC, sodium and
bicarbonate levels. If all mineral elements are within
tolerance, a custom nutrient formula can be easily created for a
specific crop and climatic conditions. But if even one
element is at toxic levels, the water should be filtered and
treated first.
City water can also be a good source of raw water for
hydroponics since it is already pretreated, but not all city water
is alike. Some sources have innate problems that may be
detrimental to plant growth. Chlorine levels are a well-known
example. Chlorine in the form of chloride is an essential
element for plant growth in trace amounts, but high levels of
chlorine can be toxic to plants, especially if high levels of
sodium are also present. Occasionally, water treatment plants
may have to shock the water supply with excessively high levels of
chlorine to treat biological contaminates. So it's a good
idea to remove excess chlorine from your source water before
irrigating your plants. To remove excess chlorine, simply let the
water sit over night or place an air stone in the reservoir.
The chlorine gas will dissipate quickly and become harmless to
plants. Chloramine, on the other hand, is a more recalcitrant
form of chlorine and it stays in the water longer. The US
Environmental Protection Agency claims that the normal level of
chloramines in drinking water is safe for plants, but it can be
detrimental to fish or to compost teas. If you are concerned,
passing water through an activated charcoal filter will adequately
remove chloramines from city water.
Well water may also be used in hydroponic systems as long as no
mineral is at toxic levels. Some well water is classified as
"hard water", meaning that it has high levels of bicarbonates,
usually as calcium carbonate. When phosphoric acid is used to
lower the pH of well water, the bicarbonates are burned off as
carbon dioxide and water, but some of the acid reacts with the
calcium to form calcium phosphate. Calcium phosphate is what
your bones are made of. It is 95% water insoluble and it
precipitates out of solution as lime scale. Once
precipitated, the calcium and the phosphate become unavailable to
the plant, and it can start to clog up drip irrigation lines and
submersible pumps. Also, because of the "buffering capacity"
associated with high levels of bicarbonates, it could easily take
ten times as much acid to lower the pH of the nutrient solution to
target levels compared to city water, with a danger of phosphorus
toxicity.
So if you have hard water, make sure to use a nutrient solution
specifically formulated for hard water. A hard water nutrient
is formulated to compensate for the higher levels of calcium,
magnesium and sulfate usually found in hard water, and it is more
acidic than a normal nutrient formula. Dilute nitric acid is
the preferred acid for treating hard water since nitric acid
doesn't form lime scale, but it isn't recommended for
hobbyists. CAUTION! Nitric acid is an extremely
aggressive acid that can cause severe burns and other safety risks
if handled improperly! So it is recommended that you use a
good hard water nutrient, instead. A premixed hard water
nutrient should include some of its nitrogen as nitric acid, but at
levels safe for handling. It will also have proportionately
higher levels of ammonium-N. When ammonium ions are taken up
by the plant, hydrogen ions are exuded from the roots, helping to
naturally lower the pH of hard water.
Don't use water from a water softener. Water softeners
remove calcium and magnesium ions from the water, but replace them
with sodium ions. Sodium is not an essential element for
plant growth. It can accumulate in the plant tissue to toxic
levels. If you must use water from a water softener, use
potassium chloride salt instead of sodium chloride in your water
softener. Potassium ions will be substituted for the calcium
and magnesium ions instead of sodium. Since potassium is
required by plants in relatively high quantities, it will be much
less detrimental for plant growth. Also, try to avoid using
water from a hot water heater. Hot water heaters may contain
toxic levels of iron and other heavy metals.

The best way to remove excess minerals from your water is to use
a reverse osmosis (RO) filter. An RO filter has a membrane
that only allows pure water to flow through it, removing all of the
mineral ions from the raw water. Therefore, when using RO
water all of the minerals in your nutrient solution will come from
your fertilizers, not from the water supply. In this way, a
nearly perfect nutrient balance can be achieved.
The only down side to RO water, besides the cost of producing
it, is that all of the bicarbonates are removed. With no
bicarbonates, there is no buffering capacity, so pH can swing
wildly during periods of rapid plant growth. During the
vegetative growth stage, plants take up proportionately more
nitrate-N, a negatively charged ion. To maintain internal
balance, the plant must exude hydroxyl ions (OH-) from the roots,
causing the pH of the nutrient solution to spike upwards.
Conversely, during heavy fruiting and flowering, plants take up
proportionally more potassium in the form of positively charged
ions. As a result, the plant roots release hydrogen ions (H+)
back into the water, sometimes causing the pH to crash.
Therefore, if you want to use RO water, it's a good idea to add at
least 10% tap water back into the mix to help buffer changes in
pH. Or better yet, condition the RO water with a potassium
bicarbonate-based buffering solution at about 60 ppm before adding
your fertilizer.
To maximize quality and yield, it's always best to take periodic
water samples and discuss the results with a qualified hydroponics
consultant. If you start with clean water and top off with
clean water, you are well on your way to growing bumper crops of
nutritious fruits and vegetables in the most
environmentally-responsible way possible!
(To learn more about hydroponics and nutrient solution
management, please refer toThe Science of Hydroponicse-book series
written by Sue and Harley Smith. Send e-mail inquiries to prohydotech@gmail.com)