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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. 

Custom Hydro offers mineral nutrient analyses of irrigation water performed by our partner laboratory, the ICL Testing Lab. A complete water analysis for both nutrients and quality cost USD $49, and we E-mail you the results in around a week. The Custom Hydro Source Water Analysis will tell you all you need to know about your water.
pH, Alkalinity, Soluble Salts, Total Nitrogen, Nitrate–N, Ammoniacal–N, Phosphorus, Potassium, Calcium, Magnesium, Sulfur, Boron, Copper, Iron, Manganese, Molybdenum, Zinc, Sodium, Chloride, Fluoride, Aluminum

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.

The floriculture department at University of New Hampshire has posted ALKCALC, a spreadsheet application on their website that allows growers to calculate the amount of acid to inject to neutralize the alkalinty in their irrigation water based on the results from the Source Water Test. Users can specify their choice of sulfuric, nitric, and phosphoric acid as well as their target pH at the following website: https://extension.unh.edu/Agric/AGGHFL/alk_calc.cfm

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