Tomato Water Quality

A considerable quantity of high quality water free from suspended and dissolved substances is needed for all hydroponic growing systems. An abundant supply of domestic drinking water is not a guarantee as a quality source since it too may contain substances that can affect plants adversely. Tests to determine the content of elements and organic substances present in the water supply are required. When necessary, a method(s) for removing undesirable substances in water will need to be selected.

Elements or ions found in many water sources that are undesirable are boron (B), sodium (Na), chloride (Cl-), sulfide (S-), fluoride (F-), carbonate (CO32-), and bicarbonate (HCO-). If these elements and/or ions are present at substantial concentrations, they must be removed or diluted by the addition of pure water to such a level that they will not affect plants. What maximum concentration ranges these elements or ions can be in irrigation water to be used in a rockwool-drip irrigation system, for example, are:

Element/Ion
Maximum Concentration
(mg/L, ppm)
Chloride (Cl)
50-100
Sodium (Na)
30-50
Carbonate (CO3)
40
Boron (B)
0.7
Iron (Fe)
1.0
Manganese (Mn)
1.0
Zinc (Zn)
1.0

Guidelines for irrigation water quality characteristics in terms of the degree of a problem they pose are:

Characteristic
Degree of Problem
None
Increasing
Severe
EC, dS/m <0.75 0.75-3.0 >3.0
TDS, mg/L <480 480-1920 >1920
Sodium (Na), SAR value <3 3-9 >9
Chloride (Cl), mg/L <70 70-345 >345
Boron (B), mg/L 1.0 1.0-2.0 2.0-10.0
Ammonium (NH4) and Nitrate (NO3), mg/L <5 5-30 >30
Bicarbonate (HCO3), mg/L <40 40-520 >520

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Note: EC = electrical conductivity, TDS = total dissolved solids,
SAR = sodium absorption ratio.

Calcium (Ca) and magnesium (Mg), two essential elements for plants are commonly found in many water supplies, and when present, they can provide a portion or most of that needed to sustain plant growth. For those using a complete nutrient solution, these elements may be in sufficient concentration in the water to reduce or eliminate the need to add reagents for supplying Ca and Mg, resulting in substantial savings in chemical costs. However, when present in substantial concentrations in the water supply, failure to compensate for their presence can lead to elemental imbalances and stressed plants.

With the continuous use of water containing a substantial concentration [>30 mg/L (ppm)] of Ca (frequently referred to as hard water), the pH of the growing media will increase and eventually become alkaline (pH 8.3) due the accumulation of calcium carbonate (CaCO3).

Organic substances, such as pest chemicals, petrochemicals, and suspended organic substances must be removed before the selected water supply would be suitable for irrigating plants and making a nutrient solution.

Water treatment to remove undesirable substances can be expensive, requiring filtering to remove suspended substances, carbon-type filtering to remove organic chemicals and either ion exchange or reverse osmosis treatment to remove inorganic ions. The treatment system must be of sufficient size and capacity to provide the volume of water needed within the time frame required.

The release of spent water or nutrient solution from the greenhouse may require control measures depending on local environmental laws and regulations because these effluents will contain nitrate (NO3) and phosphate (PO4) ions, ions that can contribute to both surface and groundwater pollution.