Irrigation Water Quality for the Nursery and Landscape

As water levels start to decline and the concentration of salts and particles start to rise, concerns regarding water quality for irrigation of outdoor ornamental crops develop. Water quality is at its worst in high summer and probably its best in early spring and perhaps late fall. That’s why it’s important to test your irrigation water sources in the spring, summer (at least monthly) and the fall, so you can get a sense of overall water quality. Test all of your irrigation water sources: ponds, wells, cisterns, canals etc. You will be in a better position to gauge just how significant the changes are during the growing season, and be able to correlate water test results with changes in plant health. Common symptoms of high salt levels include foliar burning (edge and interveinal) as well as root tip burning. Once you can correlate water test results with adverse affects on the plants that you grow, you can avoid problems in the future. This is where the real key lies…monitoring and record keeping.

Have you ever had your irrigation water tested at an accredited lab and then wondered what the results mean? Let’s review some of the terminology used in water quality tests. “pH” is the concentration of hydrogen ions, the number value is used to denote “acidity” or “alkalinity” of the solution (where <7 is acidic, 7 is neutral and >7 is alkaline). One thing worth mentioning here is a common misunderstanding regarding pH and alkalinity. The pH of water does not give a true sense of alkalinity. Alkalinity actually refers to the water’s ability to resist changes in pH and is influenced by the concentration of calcium carbonate (CaCO3) or bicarbonate (HCO3). Carbonates are what you really need to look for if you are worried about high pH and nutrient availability. Exceedingly high levels of carbonates cause the water to resist changes in pH and will contribute to the rise in pH of the container media. This can lead to nutrient deficiencies of metals (e.g. Fe, Mg, Zn) and also reduced efficacy of pesticides (e.g. Cygon, Dipel, Sevin). Although acidifying fertilizers (those that contain ammonium nitrogen and sulphur) can help, acid injection may be the only effective solution for alkaline water. Southern Ontario is quite famous for its high levels of calcium and magnesium in sub soil and underlying parent material. That’s why well water sources often show higher levels of carbonates than surface water sources.

“E.C.” represents “electrical conductivity” and is a measure of the ions in the solution, it is often referred to as the “total soluble salts”. Where EC and elemental salt levels (e.g. Na, Cl, B) are higher than the suggested ranges (see Table 1), you can make arrangements for alternative water sources to be used on your most sensitive plants (e.g. herbaceous material and young woody material). Dilution of existing water sources can also help bring salt levels down to acceptable levels. The ranges provided in the table are for general reference. It is difficult to make strict guidelines for ornamentals given the broad range of plant material grown and the range of production conditions and plant sizes. The combined use of plant health monitoring and regular water analysis (at least 2-3 times a year) are a good way of catching problems early. Foliar analysis may help confirm suspicions about water quality problems (e.g. levels of salt accumulation in leaves).

Note: These ranges are a general guideline, ornamentals vary greatly in their sensitivity to salts and water chemical properties.

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