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 Fertigation Water Quality for Cannabis

Water quality has a major influence on any cannabis fertigation program or hydroponic system. The most important considerations are the total salt level, as measured by electrical conductivity (EC), and the mineral nutrient concentrations in the water that will be applied to your crop (Table 1).

Nutrients in the irrigation water

Commercial cannabis cultivators often choose reverse osmosis filtration for their source water, but irrigation water can be a valuable source of secondary mineral nutrients. In fact, some irrigation waters can contain all or a substantial portion of the (Ca), magnesium (Mg), and sulfur (S) needed for normal growth. The concentrations of soluble mineral nutrients in irrigation water vary considerably from grow to grow depending on the water source and the local geology. Because it has had less time to dissolve soluble minerals in the soil, irrigation water from surface sources such as streams and ponds will usually have lower soluble salt levels than well water. Water quality can also vary seasonally, especially if different wells are used. 

TABLE 1. ALKALINITY GUIDELINES FOR ICL LAB ANALYSIS. POT DIAMETER/SIZE IMPACTS THE EFFECT OF ALKALINITY.

LOW ALKALINITY: Use a low acidifying or basifying feed program, provide for adequate calcium and magnesium in the constant liquid feed and assure that growing medium lime rate is adequate. Call 1-866-3006 ext 3 for further advice.

HIGH ALKALINITY: Use an acidifying feed program and/or add mineral acids to irrigation water such as phosphoric, sulfuric acids. Exercise appropriate precautions when handling concentrated acids and use acid-safe injectors. Call 1-866-3006 ext 3 for further advice.

TABLE 2. GENERAL WATER QUALITY GUIDELINES FOR GROWING IN SOILLESS GROWING MEDIA.

The pH gives an indication of how much acid will be required to reach the desired 5.5 level, and the EC reflects the total dissolved salts. Acidify irrigation water pH to target level — Once the base nutrient level of the water is known, its buffering capacity should be determined by acid titration. Titration is a process in which small increments of an acid are added to a known quantity of irrigation water (1 liter) to determine the amount of acid that will be required to lower the pH to the desired level (pH 5.5). Titrations can be done by any water testing lab or by nursery personnel using a pH meter and a burette or pipette. Any acid can be used for titrating as long as its normality is known so that conversions between different acids can be made.

ALKCALC calculates acid requirement for fertigation and hydroponics solution pH adjustment.