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Interpreting Water Test Results

As we become more deeply involved with orchid growing, we may become interested in the quality of our water supplies, so will need a guide at interpreting water test results.  Considering that orchid plants have evolved with their water supply being very pure rainwater, it makes sense that collected rain water, reverse osmosis, or distilled water are the best options available.  However, in many cases, out tap water – whether from a well, or a municipal supply – might be just fine, and not mandate the investment in other sources.

Another thing to consider is that often, water supplies contain some essential nutrients – calcium and magnesium, primarily – so it’s good to know if the supply in the water is sufficient, so you can adjust accordingly through selection of the proper fertilizer or supplement.

[table caption=”A simplified summary of water quality parameters important for irrigation water sources, with epiphytic orchid growing in mind.” width=”800″ colwidth=”150|200|400″ colalign=”left|left|left”]
Parameter,Levels of Concern,Comments
pH,<5.0 & >7.0,Must be interpreted along with alkalinity level.  A small excursion outside of that range is not a major concern.
Total Alkalinity (as CaCO3),Above 100 mg/L,Acid injection used to treat high alkalinity.
Hardness (Ca and Mg),Above 150 mg/L,Treatment with water softening is discouraged as it may result in increased sodium.
Calcium (Ca),Below 40 mg/L (plant deficiency) – above 100 (may cause P and Mg deficiency),See hardness above.
Magnesium (Mg),Below 25 mg/L (plant deficiency).,Additions may be necessary in fertilizers to prevent deficiency.
Electrical Conductivity (EC),Above 500µS/cm.,Typically caused by high salts or hardness.
Total Dissolved Solids (TDS),Above 250 mg/L,Closely related to water conductivity.
Boron (B),Above 1.0 mg/L.,Anion exchange or reverse osmosis treatment.
Chloride (Cl),Above 30 mg/L,Dilution or reverse osmosis/distillation treatment.
Sodium (Na),Above 50 mg/L,See chloride for treatment options.
Sodium Adsorption Ratio (SAR),Above 2.0,Elevated levels are most important if sodium is also elevated above 50 mg/L.
Nitrate-Nitrogen (NO3-N),No concern for plant growth.,Levels above 5.0 mg/L indicate potential contamination that may affect other water uses. Can vary significantly throughout the year. Nitrates above 5 mg/L may indicate broader contamination problems.
Ammonium-Nitrogen (NH4-N),No concern for plant growth., Consider levels in overall fertilization program. Plant problems often related to fertilization rates. Toxicity symptoms include stunting/root death/ leaf yellowing and distortion of growing points. Correct by switching to nitrate fertilizer.
Phosphorus (P),Above 5.0 mg/L may cause nutrient deficiencies,Most often reduced through dilution with other water sources.
Potassium (K),Not usually an issue but should be kept low,Consider total with that of fertilizer- keep total below 100-150 mg/L; Low levels or low availability due to high pH may limit production.
Sulfur (S),Below 10 mg/L may require addition in fertilizer in rare cases.,Rarely a parameter of concern. Addition in fertilizer may be needed for some plants,
Iron (Fe),Above 5.0 mg/L toxic.,Large scale removal is most efficient using a settling pond. Various oxidizing filters can also be used depending on other chemistry.
Manganese (Mn),Above 2.0 mg/L may be toxic to some sensitive plants.,See iron notes above. Manganese removal is more difficult and may require additional pH adjustment.
Copper (Cu),Above 0.20 mg/L toxic to some plants,Corrosion of pipes is a likely source.
Molybdenum (Mo),Above 0.05 mg/L toxic to some plants,Large scale removal of molybdenum is generally not cost effective. Use dilution or alternative water supplies.
Zinc (Zn),Above 0.30 mg/L,Most likely from corrosion of galvanized pipe. Plant toxicity most likely where low pH occurs in growth media.
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