Water Quality
All hydroponic/soilless culture systems require sizable quantities of relatively
pure water. The best domestic water supplies and/or water for agricultural
use frequently contain substances and elements that can affect (positively or
negatively) plant growth. Even rainwater collected from the greenhouse covering
may contain both inorganic and organic substances that can affect plant
growth. In many parts of the United States and indeed throughout the world,
water quality can be a major problem for hydroponic/soilless culture use due
to contamination by various inorganic and organic substances.
Therefore, a complete analysis of the water to be used for any type of
hydroponic/soilless culture system is essential. The analysis should include
inorganic and organic components if the water is being taken from a river,
shallow well, or other surface source. When taken from sources other than
these, an inorganic elemental assay will be sufficient to determine elemental
composition and concentration.
Natural water supplies can contain sizable concentrations of some of the
essential elements required by plants, particularly Ca and Mg. In areas where
water is being taken from limestone-based aquifers, it is not unusual for
concentrations of Ca and Mg to be as high as 100 and 30 mg/L (ppm),
respectively. Some natural waters will contain sizable concentrations of Na
and anions such as bicarbonate (HCO3
–), carbonate (CO2
3–), sulfate (SO4
2–),
and chloride (Cl–). In some areas, B may be found in fairly high concentrations.
The Nutrient Solution 73
Sulfide (S–), primarily as iron sulfide, which gives a “rotten egg” smell to water,
is found in some natural waters.
Suggested composition characteristics of waters suitable for use hydroponically
as well as for irrigation have been published. Verwer and Wellman (1980)
established what the maximum mineral concentration would be for water used
in rockwool culture, as shown in Table 7.1. Farnhand et al. (1985) have
established criteria for irrigation water based on salinity, electric conducivity
(EC), total dissolved solids (TDS), and ion content (Table 7.2). Waters et al.
(1972) have set the suitability of water for irrigating pot plants; their data are
given in Table 7.3. Smith (1999) elemental maximums for water for hydroponic
use (Table 7.4). Ideal water-quality guidelines for plug culture and characteristics
of high-quality irrigation water are given in Table 7.5 and Table 7.6.
Table 7.1 Maximum Mineral Concentrations for Irrigation Water Used in
Rockwool Culture
Element/Ion Maximum Concentration (mg/L, ppm)
Chloride (Cl) 50 to 100
Sodium (Na) 30 to 50
Carbonate (CO3) 4.0
Boron (B) 0.7
Iron (Fe) 1.0
Manganese (Mn) 1.0
Zinc (Zn) 1.0
Source: Verwer, F.L. and Wellman, J.J.C., 1980, in Fifth International Congress on Soilless
Culture, International Society for Soilless Culture, Wageningen, The Netherlands.
Table 7.2 Water Quality Guidelines for Irrigation
Degree of Problem
Characteristic None Increasing Severe
EC, dS/ma <0.75 0.75 to 3.0 >3.0
TDS, mg/Lb <480 480 to 1920 >1920
Sodium (Na) sodium absorption ratio
(SAR) value
<3 3 to 9 >9
Chloride (Cl) mg/L <70 70 to 345 >345
Boron (B), mg/L 1.0 1.0 to 2.0 2.0 to
10.0
Ammonium (NH4) and nitrate (NO3), mg/L <5 5 to 30 >30
Bicarbonate (HCO3) mg/L <40 40 to 520 >520
a Electrical conductance.
b Total dissolved solids.
Source: Farnhand, D.S., Hasek, R.F., and Paul, J.L., 1985, Water Quality, Leaflet 2995. Division
of Agriculture Science, University of California, Davis, CA.
74 Hydroponics: A Practical Guide for the Soilless Grower
Surface or pond water may contain disease organisms or algae, which can
pose problems. Algae grows extraordinarily well in most hydroponic culture
systems, plugging pipes and fouling valves. Filtering and/or other forms of
pretreatment are required to ensure that the water used to prepare the nutrient
solution is free from these undesirable organisms and suspended matter.
In most cases, some form of water treatment will be necessary to make
and maintain suitable nutrient solutions. Depending on what an analysis of
the water supply indicates, no special treatment or filtering may be required
to remove suspended matter. However, the grower may at one end of the
quality scale simply have to filter out debris using sand beds or fine-pore
filters; at the other extreme, sophisticated systems dedicated to ion removal
by means of ion exchange or reverse osmosis may be required (Anon., 1997a).
In hard-water areas, there may be sufficient Ca and Mg in the water to
provide a portion or all of the plant requirements. In addition, the micronutrient
element concentration could be sufficient to preclude the need to add this
group of elements to the nutrient solution. These determinations should be
made only on the basis of an elemental analysis of the water (see pages 314).
Table 7.3 Suitability of Water for Irrigating Potted Plants
Water
Classification
Electric
Conductance
(mmho/cm)
Total Dissolved Solids
(Salts), mg/L, ppm
Sodium (% of
Total Solids)
Boron (mg/L,
ppm)
Excellent <0.25 <175 <20 <0.33
Good 0.25 to 0.75 175 to 525 20 to 40 0.33 to 0.67
Permissible 0.75 to 2.0 525 to 1400 40 to 60 0.67 to 1.00
Doubtful 2.0 to 3.0 1400 to 2100 60 to 80 1.00 to 1.25
Unsuitable >3.0 >210 >80 >1.25
Source: Waters, W.E., Geraldson, C.M., and Woltz, S.S., 1972, The Interpretation of Soluble
Salt Tests and Soil Analysis by Different Procedures, AREC Mimeo Report GC-1972,
Bradenton, FL.
Table 7.4 Common Compounds and Elements and the Maximum Levels
Allowable in Water for General Hydroponic Use
Element Concentration, mg/L (ppm)
Boron (B) <1
Calcium (Ca) <200
Carbonates (CO3) <60
Chloride (Cl) <70
Magnesium (Mg) <60
Sodium (Na) <180
Zinc (Zn) <1
Source: Smith, R., 1999, The Growing Edge 11(1):14–16.
The Nutrient Solution 75
Organic chemicals, such as pesticides and herbicides, many of which are
water soluble, can significantly affect plant growth if present even in low
concentrations. Water from shallow wells or from surface water sources in
intensively cropped agricultural areas should be tested for the presence of
these types of chemicals.
Treatment should be employed only if the chemical and/or physical composition
of the water warrants. Obviously, financial and managerial planning
must incorporate the costs of producing nutrient-pure water in a grower’s
specified environment. For example, it may be financially prudent to accept
some crop loss from the use of impure water rather than attempting to recover
the cost of water treatment. Treatment may be as simple and inexpensive a
task as acidifying the water to remove bicarbonates (HCO3) and carbonates
(CO3) or as expensive as complete ion removal by reverse osmosis.
Water samples should be submitted to a testing laboratory for a complete
analysis before use, and the analysis should be repeated whenever a change
in the water source is made. It is also advisable to have the initial nutrient
Table 7.5 Ideal Water Quality Guidelines for Plug Culture
Element/Constituent Optimum Range, mg/L (ppm)
Boron (B) <0.5a
Iron (Fe), Copper (Cu), Zinc (Zn), Manganese (Mn) <1.0a
Calcium (Ca) 50 to 125b
Chloride (Cl) <100
Fluoride (F) <1.0
Magnesium (Mg) 5 to 25b
Phosphate (P) <5
Sodium (Na) <50
Bicarbonate alkalinity <120c
Alkalinity as calcium carbonate <100c
Total soluble salts 650 to 1050
pH 5.8 to 6.4d
Electrical conductivity (EC) 1.0 to 1.5 μS/cm or preferably lesse
a Commercial water-soluble fertilizers usually contain adequate levels of micronutrients. If
your water contains high levels (1 to 5 mg/L) of any of the micronutrients, consult a water
specialist.
b Calcium and magnesium at these levels can be beneficial nutritionally. Use these values
in determining which micronutrients to feed seedlings.
c Water analysis may report bicarbonate alkalinity or express the total alkalinity in terms of
calcium carbonate.
d Water pH range at which alkalinity is low and manageable.
e Electrical conductivity (EC) is now measured in milli-Siemens (mS). Some reports may use
the older expression of millimho (mmho/cm). They are the same. Labs that report “total
soluble salts” or TSS derive this number from the EC. One μS/cm is equal to about 650 to
700 ppm total salts.
Source: Faulkner, S.P., 1998b, The Growing Edge 10(1):87–88.
76 Hydroponics: A Practical Guide for the Soilless Grower
solution assayed to be sure that its composition is as intended before its use.
Instrumental devices and analysis kits can be used when monitoring water
and nutrient solutions (see Chapter 13).
