1/21/2010 - By Dr. Brian Kunkel

By Dr. Bill McElhannon

A working knowledge of water quality is indispensable for greenhouse growers and nurserymen. Water quality can be extremely variable location to location; change over time, and the various properties of irrigation water can have significant impact on plant growth. Every grower and nurseryman should be able to quickly scan a water analysis and understand the consequences of its use.

Reading the dataWater analysis are reported in one of three units: ppm - parts per million; mg/l - milligrams per liter; and meq/l - milliequivalents per liter.

Conversions: ppm = mg/l; mg/l = ppm; meq x meq wt (mg/l) = ppm

Water characteristics that affect crop performance:

• EC (soluble salts) – pH - Alkalinity
• Concentration of plant nutrients (Ca, Mg, S, B, Cl)
• Concentration of potentially toxic elements (Na, Cl, Li, F, B)

EC: Electrical ConductivityEC is a measurement of the conductance of electricity through water and is directly proportional to concentration of dissolved salts in water. Pure water is a poor conductor electricity. Water containing high salts has high conductance and waters with low salt content have low conductance. The preferred method of reporting EC is dS/m (desiSiemens per meter) but this value can easily be converted to other units using the table below. The most common terms, dS/m, mS/cm and mmho/cm provide exactly the same value.

Units for expressing electrical conductivity

Unit	Abbreviaton	Units	Example
Millisiemens	mS/cm	EC x 10-3/cm	2.25 mS/cm
Decisiemens*	dS/m	EC x 10-1/m	2.25 dS/m
Millimhos	mmho/cm	EC x 10-3/cm	2.25 mmho/cm
Micromhos	micromhos/cm	EC x 10-6/cm	2250 micromho/cm

pH:The pH of most irrigation waters range from 4.6 to 8.5 . Considered alone, pH is not a major factor affecting irrigation suitability. However, water pH will affect the efficacy of may pesticides and other chemicals, and solution pH is a primary factor influencing nutrient availability. The micronutrients Fe, Mn, B, Cu, and Zn become less available as the media solution pH increases and become more available (in some cases toxic) as the pH decreases. The reverse is true for Mo, Ca, and Mg with availability decreasing as the pH decreases. Risk of increased PO₄ leaching and Al toxicity are also increased at low pH.

Alkalinity:Alkalinity is a measurement of water’s ability to resist change; it buffers water against changes in pH. Carbonate and bicarbonate account for 90% of the alkalinity in most waters, and are the only two variables that need to be considered. Carbonate (CO₃^-2) and bicarbonate (HCO₃^-) are salts of carbonic acid (H₂CO₃) which are formed when carbon dioxide dissolves in water. The carbonate ion exists only at very high pH values, and generally, the major factor influencing the alkalinity of irrigation water is bicarbonate. Bicarbonate (or carbonate) reacts with the hydrogen ions in water, forming carbonic acid, which then dissociates into carbon dioxide and water. Waters that have high levels of carbonates or bicarbonates can therefore remove hydrogen ions from the solution and buffer water against changes in pH. Growers irrigating with waters that have high alkalinity often find that the pH of the growing substrate increases over time and plants may eventually develop micronutrient deficiencies. Growers using low alkalinity water may see the reverse, with the growing substrate developing very low pH and micronutrient toxicities are frequently observed. High water alkalinity can be adjusted by use of acidic fertilizers or by acid injection. The NC State Alkalinity Calculator is a useful excel program that calculates acid additions to irrigation water. It is available free at http://www.ces.ncsu.edu/depts/hort/floriculture/software/alk.html.

How is alkalinity reported?Laboratories may report alkalinity as mg/l or meq/l CaCO₃ or as ppm carbonate and bicarbonate.Parts per million carbonate, bicarbonate and total alkalinity are related as follows:

1 meq bicarbonate (HCO₃^-) = 61 mg/l or 61 ppm1 meq carbonate (CO₃^-2) = 30 mg/l or 30 ppm1 meq CaCO₃ = 50mg/l or 50 ppmFrom a reactivity perspective, 1 meq of bicarbonate = 1 meq carbonate = 1 meq calcium carbonate.

To convert ppm bicarbonate to meq/l CaCO₃ divide ppm bicarbonate by 61.To convert ppm carbonate to meq/l CaCO₃ divide ppm carbonate by 30

Calcium and Magnesium (Ca⁺² & Mg⁺²):Ca and Mg are present in most waters. Both are plant nutrients, and in many cases irrigation water may provide part or all of the plants requirements for these elements. It is rare when Ca and Mg levels are high enough to cause salt damage to crops. However, in most cases, supplemental Ca and Mg must be supplied to fulfill the crop’s needs.

Sodium (Na⁺):Na is present in most waters, and high levels of Na can contribute to salinity problems. Sodium can interfere with the uptake of Ca and Mg and cause foliar burns associated with poor water uptake or accumulation in plant tissue. If the concentration of Na is less than 69 ppm (3 meq/l), sodium is rarely a plant growth problem. The potential for sodium toxicity from root assimilation is not a concern if the AdjR_Na is less than 3 or the SAR is less than 4. Mineral soils with an AdjR_Na greater than 6 may have permeability or drainage problems which can often be mitigated with supplemental Ca. With greenhouse and nursery production, waters containing less than 1.5 meq Na (35 ppm) are most desirable.

Chloride (Cl^-):Chloride is often found in irrigation water in association Na. Foliar damage can occur with some crops if the Cl content is greater than 2 meq/l (71 ppm). Plant roots are less sensitive to Cl than plant foliage and normally toxicity by root assimilation is not observed if the concentration is less than 4 meq/l (144 ppm).

Sulfate (SO₄^-2):Sulfate in water is often a primary source of S in the growers fertility program. If the irrigation water contains less than 48 ppm (1 meq ) of SO₄, the water will not supply all of the S requirements of the crop. Sulfate levels of 100 – 150 ppm normally do not cause problems with plant growth.

Boron (B):Growers and nurserymen should consider the additive concentration of B in irrigation water, growing substrate and fertilizer when determining the B hazard of a fertility program. Total concentration of B from all sources should be less than 1 ppm with sensitive crops. If B levels in irrigation water are more than 0.50 ppm , growers should consider using B free media and fertilizers.

Fluoride (F)and Lithium (Li):Fluoride can be excessive in some irrigation waters and in municipal waters treated with this element. Foliage plants or Easter lilies may develop tip burn if F levels in water exceed 1.0 ppm. With most crops, lithium content in water should be less than 2.5 ppm. Citrus and poinsettias are very sensitive to lithium and Li levels should be 0.075 ppm.