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Water Quality
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.
INTENDED USE | NORMAL RANGE | LEVEL OF CONCERN (U) | ||
---|---|---|---|---|
ppm-mg CaCO3 /L |
Milliequivalents CaCO3 (V) |
ppm=mgCaCO3 /L |
Milliequivalents CaCO3 |
|
Plugs | 60 to 100 | 1.2 to 2.0 | <40. > 120 | <0.8. > 2.4 |
Small pots/shallow flats | 80 to 120 | 1.6 to 2.4 | <40. > 140 | <0.8. > 2.8 |
4" to 5" posts/deep flats | 100 to 140 | 2.0 to 2.8 | <40. > 160 | <0.8. > 3.2 |
Pots: 6" or more/long term crops | 120 to 180 | 1.6 to 3.6 | <60. > 200 | <1.2. > 4.0 |
(U) | Highest level a grower can manage depends on plant type grown, media type, potential acidity of feed program and watering practices. Levels listed in this table are guidelines only! |
(V) | Milliequivalents = ppm total alkalinity expressed as milligrams Calcium Carbonate per liter divided by 50. |
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.
PARAMETER | NORMAL RANGE | LOW | HIGH |
---|---|---|---|
(Parts per million except where noted) | |||
Soluble Salts (mmhos/cm) | 0.3 to 1.0 | < 0.2 | > 1.3 |
MAJOR NUTRIENTS (w) | |||
Nitrate Nitrogen (NO3-N) | ----------- | ----------- | > 10 |
Ammonium Nitrogen (NH4-N) | ----------- | ----------- | > 10 |
Phosphorus (P) | ----------- | ----------- | > 10 |
Potassium (K) | ----------- | ----------- | > 10 |
Calcium (Ca) | 40 to 75 | < 25 | >100 |
Magnesium (Mg) | 30-50 | <15 | >50 |
Sulfur (S) (x) | 10 to 80 | < 10 | > 80 |
TRACE NUTRIENTS (y) | |||
Manganese (Mn) | ----------- | ----------- | > 1.50 |
Iron (Fe) | ----------- | ----------- | > 2.00 |
Copper (Cu) | ----------- | ----------- | > 0.20 |
Boron (B) | ---------- | ----------- | > 0.50(z) |
Zinc (Zn) | ---------- | ----------- | > 0.40 |
Molybdenum (Mo) | ----------- | ----------- | > 0.20 |
OTHER ELEMENTS | |||
Sodium (Na) | ----------- | ----------- | > 50 |
Chlorides (Cl) | ----------- | ----------- | > 70 |
Fluorides (F) | ----------- | ----------- | > 1.0 |
Aluminum (Al) | ----------- | ----------- | > 1.0 |
(w) | N, P and K levels usually low. Levels greater than 10 to 20 ppm may indicate nutrient runoff into water source. |
(x) | Sulfur reported as elemental Sulfur. To calculate sulfur as the sulfate ion, the form in which most sulfur is likely to be pre- sent, multiply by 3. Small amounts of sulfur can be added through the addition of epsom salts, magnesium sulfate, to non- calcium fertilizer formulations. One ounce per 100 gallons of water will deliver 7.5 ppm Mg and 30 ppm SO4. |
(y) | Concern with trace nutrient levels that are two times the level found in a PETERS PEAT-LITE fertilizer solution of 200 ppm N |
(z) | With Boron-sensitive crops (like poinsettia), 0.25 ppm may be considered high. |
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.
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