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Understanding pH management and plant nutrition Part 2: Water quality

Understanding pH management and plant nutrition  Part 2: Water quality

Bill Argo, Ph.D.
Blackmore Company, Tel: 800-874-8660, Int’l 734-483-8661, E-mail: bargo@blackmoreco.com
Originally printed in 2003 in the Journal of the International Phalaenopsis Alliance, Vol. 13 (1).

Water quality is a key factor affecting pH and
nutritional management in any container-grown crops,
including orchids. One challenge is that the water
quality in your operation can differ dramatically from
that of your neighbor, and certainly from greenhouses in
other locations both inside and outside the U.S. For
example, the range of water qualities used by
commercial greenhouses in the U.S. can be found in
Table 1. For those of you using rain water or reverse
osmosis purified water exclusively, the pH will range
from 4.0 to 5.5 (if measured correctly), the alkalinity
will be less than 10 ppm, and the concentration of other
ions will be very low to nonexistent.
Understanding a few technical details about
water quality will help you improve nutrient
management appropriate for your own greenhouse.
Always remember that the success or failure of any
fertilizer will always depend on the water quality
because it is the combination of the two that affect your
plants. In Part 2 of this series, we will discuss how
water quality affects pH and nutritional management of
the substrate.

pH and Alkalinity are two different aspects of
water quality
There is a great deal of confusion when it
comes to understanding the definition of water pH and
water alkalinity, and why they are important to the
health of your plants.
The term pH is a direct measurement of the
balance between acidic hydrogen ions (H+
) and basic
hydroxide ions (OH-), and can be measured with a pH
meter. The pH of a solution can range between 0 (very
acidic) and 14 (very basic). At a pH of 7.0, the
concentrations of H+
and OH- are equal, and the
solution is said to be neutral. When the pH is above
7.0, the concentration of OH- is higher than H+
, and the
solution is said to be basic or alkaline (not to be
confused with alkalinity). When the solution is below
7.0, the concentration of H+
is higher than OH-, and the
solution is said to be acidic.
Alkalinity is a measure of how much acid it
takes to lower the pH below a certain level, also called
acid-buffering capacity. Alkalinity is usually measured
with a test kit where dilute acid is added until a color
change occurs at a specific pH. Alkalinity is not a
specific ion, but rather includes the concentration of
several ions that affect acid-buffering capacity. Under
most conditions, the ions that have the greatest effect on
alkalinity are bicarbonates like calcium, magnesium, or
sodium bicarbonate and, to a lesser extent, carbonates
like calcium or sodium. Several other ions including
hydroxides, phosphates, ammonium, silicates, sulfides,
borates, and arsenate also can contribute to alkalinity,
but their concentration is usually so low that they can be
ignored.
In a water sample, the concentration of all of
the ions that makes up the alkalinity term are combined

and reported as equivalents of calcium carbonate
(CaCO3, which is the main component of lime).
Alkalinity can therefore be thought of as the “liming
content” of the water. The units used to report
alkalinity can be parts per million (ppm), mg/liter, or
millequivalents (meq.).

Water alkalinity has a big effect on substrate-pH.
When it comes to managing the pH of a
substrate, the alkalinity concentration has a much
greater effect than does water pH. Alkalinity (calcium
bicarbonate, magnesium bicarbonate, and sodium
bicarbonate) and limestone (calcium and magnesium
carbonate) react very similarly when added to a
substrate. And just like too much limestone, the use of
irrigation water containing high levels of alkalinity can
cause the pH of the substrate to increase above
acceptable levels for healthy plant growth.
For example, a limestone incorporation rate of
5 pounds per cubic yard will supply approximately 100
meq of limestone per 6 inch (15-cm) pot. Applying 16
fluid ounces (0.5 liters) of water containing 250 ppm
alkalinity to that 6 inch pot will supply about 2.5 meq of
lime. That does not sound like much until you consider
that after 10 irrigations you have effectively increased
the limestone incorporation rate by 25%. Even if you
are using a completely inert substrate, the liming effect
that high alkalinity water has will cause your substrate
pH to increase to unacceptable levels.

Units of measure for alkainity
The concentration of alkalinity (or any other plant
nutrient) can be expressed a number of different ways.
1) Parts per million (ppm or mg/liter). The term ppm is a
weight per weight ratio. One part per million is
equivalent to 1 unit of something dissolved in a
million units of something else. In the case of
anything dissolved in water, 1 ppm is equal to 1 mg
per 1,000,000 mg (or 1 Kg = 1 liter) of water. So, 1
ppm is equal to 1 mg/liter. A 1% solution (1 unit in
100 units) is equivalent to 10,000 ppm.
2) Milliequivalent (mEq./liter). The term mEq./liter is a
chemistry term that is not only dependent on a
materials concentration, but also on its molecular
weight and charge. In the case of alkalinity, 50 ppm
(or mg/liter) CaCO3 equals 1 meq/liter CaCO3.
Sometimes the concentration of bicarbonates is also
reported on a water test from a commercial laboratory.
In most cases, bicarbonate makes up most of the
alkalinity. The relationship is 61 ppm bicarbonate
equals 1 meq alkalinity.
3) Grains per gallon (gpg): An outdated term for
expressing concentration. 1 gpg = 17.1 ppm

To compare the effect of water pH or alkalinity
on the ability to raise pH (or neutralize acid) in a
medium, 50 ppm alkalinity (which is a low alkalinity)
would be similar to having a water with pH 11 (i.e. an
extremely high pH). A water with a pH of 8.0 would
have the same effect on substrate pH as an alkalinity
concentration of only 0.05 ppm (i.e., almost nothing).

Don’t ignore water pH.
Water pH is still important for crop
management. Even though it has little impact on the
substrate, water-pH does affect the solubility of
fertilizers, and the efficacy of insecticides and
fungicides before you apply it to the crop. Generally,
the higher the water pH, the lower the solubility of these
materials.

Minimizing the effects of high alkalinity
The common problems associated with high
alkalinity result from its tendency to increase substrate-pH. High substrate-pH can causes micronutrient
deficiency in container grown crops because
micronutrient solubility decrease as the substrate pH
increases.
In commercial greenhouses, the most common
method for minimizing the “liming effect” of high
alkalinity is to add a strong mineral acid (usually
sulfuric acid or phosphoric acid) directly to the
irrigation water. As the pH of the water decreases,
some of the alkalinity is neutralized. The ideal
alkalinity concentration will depend on the type of
fertilizer being used (to be covered in Part 3). All of the
alkalinity has been neutralized when the pH of the water
reaches 4.5. For more information on injecting strong
mineral acids into irrigation water, you can download
the “acid addition calculator” from Purdue University
and North Carolina State University at
www.ces.ncsu.edu/depts/hort/floriculture/software/alk.h
tml.
For small greenhouse operations and hobbyists,
strong mineral acids are very difficult and dangerous to
use. Difficult because these acids are highly
concentrated and therefore are difficult to add to a small
volume of water, and dangerous because small
greenhouses and hobbyists typically lack the specialize
equipment needed to safely add acid to water. Some
acids should never be considered, like anhydrous
hydrochloric acid or anhydrous acetic acid because they
not only are caustic, but are also fuming acids, which
make them extremely dangerous to handle. Nitric acid
is especially dangerous and should never be considered.
There are alternatives to adding mineral acids
for alkalinity control. The first is using a weaker,
organic acid, like citric acid. Citric acid is available in a

pure granular form. A rate would be about 0.2 grams
per gallon to remove 50 ppm alkalinity. Pre-mixed
citric acid solutions (Seplex, GreenCare Fertilizer (815-936-0096)) are also available for alkalinity control.
Other organic acids like vinegar and lemon juice will
also work, but because the concentration of acid in
these materials is variable, for example, the acetic acid
content in vinegar can range from 4% to 8% by weight,
that the results that you get will not be consistent.
Another option for alkalinity control is to use
acidic fertilizers (to be covered in greater depth in Part
3). Fertilizers high in ammoniacal nitrogen produce an
acidic reaction when added to the substrate, which can
be used to neutralize the affect of water alkalinity. For
example, 20-20-20 (69% NH4-N) has enough acidity to
be used with water containing around 200 ppm
alkalinity water without further acidification.
There are several drawbacks to using fertilizer
for alkalinity control. Fertilizers high in ammoniacal
nitrogen may cause excessive growth and are not
effective when the temperature of the substrate is less
than 60o
F. In addition, you lose flexibility because you
can only choose commercial fertilizers based on
ammonium content. For example, high ammonium
fertilizers available to you may lack calcium or other
key nutrients.
Another option for alkalinity control is to
change water sources. There are a number of sources,
such as rain water or reverse osmosis purified water,
that contain little if any alkalinity. Drawbacks to using
alternative water sources include cost and storage
problems. Changing water sources will also change the
composition of the fertilizer solution applied to the
crop.

Low alkalinity Effects
Not everybody in the world has irrigation water
with high alkalinity. In the United States alone, there
are a large number of growers in states like AL, AR,
CA, CO, GA, HI, NC, NJ, NY, VA, and New England
states that have alkalinity levels below 40 ppm without
any acidification. Even in areas were high alkalinity is
considered the norm, some growers have switched to
low alkalinity sources such as reverse osmosis purified
water or rain water.
The primary problem associated with low
alkalinity water is a tendency for substrate-pH to drop
over time, which can cause micronutrient toxicity
problems. Usually, low pH problems are a result of
fertilizer selection. Fertilizers high in ammoniacal
nitrogen are acidic, and without any alkalinity in the
water to balance the reaction (resist lowering of pH),
acidic fertilizers will tend to drive the substrate-pH
down over time.

 
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