Fertigation, Water Quality, and Fertilizer Selection for Specialty Crops
Fertigation Water Quality for Specialty Crops
Water quality has a major influence on any specialty crop 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 applied to your crop (Table 1).
Nutrients in the Irrigation Water
Commercial specialty crop cultivators often choose reverse osmosis filtration for their source water, but irrigation water can itself be a valuable source of secondary mineral nutrients. Some irrigation waters contain all or a substantial portion of the calcium (Ca), magnesium (Mg), and sulfur (S) needed for normal growth. Concentrations of soluble mineral nutrients vary considerably depending on the water source and local geology. Surface water from streams and ponds typically has 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 costs $49 USD, with results emailed in approximately one week. The Source Water Analysis covers:
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 | ||
|---|---|---|---|---|
| ppm CaCO₃/L | mEq CaCO₃ | ppm CaCO₃/L | mEq CaCO₃ | |
| Plugs | 60–100 | 1.2–2.0 | <40 or >120 | <0.8 or >2.4 |
| Small pots / shallow flats | 80–120 | 1.6–2.4 | <40 or >140 | <0.8 or >2.8 |
| 4″–5″ pots / deep flats | 100–140 | 2.0–2.8 | <40 or >160 | <0.8 or >3.2 |
| Pots 6″+ / long-term crops | 120–180 | 1.6–3.6 | <60 or >200 | <1.2 or >4.0 |
Low Alkalinity: Use a low acidifying or basifying feed program, provide adequate calcium and magnesium in the constant liquid feed, and ensure growing medium lime rate is adequate. Contact us for further advice.
High Alkalinity: Use an acidifying feed program and/or add mineral acids to irrigation water such as phosphoric or sulfuric acid. Exercise appropriate precautions when handling concentrated acids and use acid-safe injectors. Contact us for further advice.
Table 2. General Water Quality Guidelines for Soilless Growing Media
| Parameter | Normal Range | Low | High |
|---|---|---|---|
| Parts per million except where noted | |||
| Soluble Salts (mmhos/cm) | 0.3–1.0 | <0.2 | >1.3 |
| Major Nutrients | |||
| Nitrate Nitrogen (NO₃-N) | — | — | >10 |
| Ammonium Nitrogen (NH₄-N) | — | — | >10 |
| Phosphorus (P) | — | — | >10 |
| Potassium (K) | — | — | >10 |
| Calcium (Ca) | 40–75 | <25 | >100 |
| Magnesium (Mg) | 30–50 | <15 | >50 |
| Sulfur (S) | 10–80 | <10 | >80 |
| Trace Nutrients | |||
| Manganese (Mn) | — | — | >1.50 |
| Iron (Fe) | — | — | >2.00 |
| Copper (Cu) | — | — | >0.20 |
| Boron (B) | — | — | >0.50 * |
| Zinc (Zn) | — | — | >0.40 |
| Molybdenum (Mo) | — | — | >0.20 |
| Other Elements | |||
| Sodium (Na) | — | — | >50 |
| Chlorides (Cl) | — | — | >70 |
| Fluorides (F) | — | — | >1.0 |
| Aluminum (Al) | — | — | >1.0 |
* With boron-sensitive crops (e.g. poinsettia), 0.25 ppm may be considered high.
Sulfur reported as elemental sulfur. To calculate as sulfate ion, multiply by 3. Small amounts of sulfur can be added via magnesium sulfate (Epsom salt) — 1 oz per 100 gallons delivers 7.5 ppm Mg and 30 ppm SO₄.
Adjusting pH & Alkalinity
The pH gives an indication of how much acid will be required to reach the desired 5.5 level, and EC reflects total dissolved salts. Once the base nutrient level of the water is known, its buffering capacity should be determined by acid titration — a process in which small increments of acid are added to a known quantity of irrigation water (1 liter) to determine the amount needed to lower pH to the target level (pH 5.5).
ALKCALC — a spreadsheet tool from the University of New Hampshire floriculture department — calculates the acid requirement for fertigation and hydroponics pH adjustment. Users can specify sulfuric, nitric, or phosphoric acid and set their target pH based on results from the Source Water Test.
Also available: ICL-developed Excel acid/alkalinity calculator (includes citric acid calculations).
Have questions about your water quality or fertigation program? Contact us — we're happy to help.