Analyzing water quality is needed to determine its efficacy for cannabis production.
Before planning a greenhouse or indoor facility, determination of water quality is essential. Typical water analysis parameters include pH, electrical conductivity (EC), sodium adsorption ratio (SAR), nitrogen (NO3, NH4), phosphorus, potassium, calcium, magnesium, sodium, sulphates, chlorides, iron, manganese, copper, zinc, molybdenum and silicon. In addition to the above, the labs also provide total dissolved solids (TDS), biological oxygen demand (BOD), anion and cation balance, as well as information on carbonates and bicarbonates.
Cannabis growers must understand the role of each nutrient and the values they provide to make proper assessment and decisions.
Rule of thumb
For cannabis production, water with an EC of below 0.8 mS/cm or 800 uS/cm and a SAR value of below 4.0 is good for irrigation and should not present any management problem. For aeroponic or other growing systems where roots grow in water, my suggestion is to go for an EC value of below 0.5 mS/cm and a SAR value of less than 3.0.
SAR is a measure of the amount of sodium (Na) relative to calcium (Ca) and magnesium (Mg) in the water or water extract from saturated soil paste. It is the ratio of the Na concentration divided by the square root of one-half of the Ca + Mg concentration. With soilless growing media, higher than 4 SAR value means that sodium will accumulate enough over a period of time that it will cause “salinity” and affect plant growth.
Hard vs. soft water
In hard water, calcium is higher than 100 mg/L or parts per million (ppm) and carbonates are also above 100 ppm. In soft water, sodium is more than 100 mg/L. Soft water or chemically softened water is not suitable for cannabis cultivation. Hard water with higher calcium and magnesium can be used for cannabis production. It is important to note that calcium and magnesium in water exist as carbonates or bicarbonates, and pH has to be adjusted to make these ions available for plant growth.
If the water quality is good based on EC value of less than 0.8 mS and a SAR value of less than 4, then there is no need to invest in costly reverse osmosis (RO) units. In situations where sodium is high, RO is a good choice. Base your decision on water quality parameters described above.
RO water presents its own challenges, particularly when bicarbonates and carbonates are removed. These two ingredients provide the buffering capacity in the water and in the growing medium. I have seen serious root and plant issues arise because of lack of bicarbonates in the water.
There may be other reasons for using RO systems. If boron levels are over 1 ppm, or when water supplies contain high iron as Ferric and iron bacteria can be an issue, then RO systems should be considered.
Alkalinity and pH
pH is the term used to describe “hydrogen ion” concentration or “power of hydrogen,” while alkalinity is the sum of carbonates and bicarbonates in water. But there is a relationship. When an acid is added, the bicarbonates are neutralized resulting in hydrogen (H), and bicarbonates as carbon dioxide and oxygen. This hydrogen ion translates into pH. If all the bicarbonates are neutralized then there is no buffering left in water and pH drops to very acidic levels. Growers must know how much acid should be added. As a rule, 30 to 50 ppm of bicarbonates should be left in water to provide buffering.
The photos shown here depict some water quality issues that have not been addressed.
In the left photo, water pH was adjusted by adding acid without calculating the amount of bicarbonates that must be neutralized. In this case, the pH of the growing medium also was not taken into consideration. Coir (coco-peat) was the growing medium and pH was already around 5.5. The young clone started rooting and very quickly magnesium deficiency was evident. In soilless growing media, magnesium uptake is reduced when pH starts going below 5.0. On the other hand, uptake of iron and manganese increases when pH starts going below 5.0, resulting in toxicity.
In the right photo, no magnesium was added as part of the fertilizer program thinking that there is 50 ppm of magnesium present in water. But this magnesium is not 100 per cent available for plant use. Since the problem was not corrected, magnesium deficiency continued and photosynthesis efficiency was reduced because this element is key in the chlorophyll molecule.
These two pictures exemplify poor quality buds due to water quality issues that resulted in unbalanced nutrient delivery to plants. The pH was not monitored and adjusted during the growth cycle of the plant. Bud maturity was significantly delayed as well. There was not enough food in leaves to feed the buds. Such yellow leaves cannot manufacture food for the plant’s needs.
Moyhuddin Mirza, PhD, is chief scientist with the Cannabis Nature Company in Edmonton, and a consultant with the cannabis industry. Email him at firstname.lastname@example.org
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