December 15, 2011
By Dr. Youbin Zheng, Katherine Vinson, and Dr. Mike Dixon
University of Guelph

The majority of the Ontario potted nursery crops (perennials, shrubs, and trees) are grown in organic-based growing substrates containing components such as pine bark, peat moss, and other types of composts.

Most plants grow best at a pH of 5.4 to 6.4 in organic substrates. Of course, some crops prefer an even lower root zone pH. For example, the best pH for azalea is 4.5-5.8. A substrate pH above 6.2 can lead to Iron (Fe), Manganese (Mn), Copper (Cu), Zinc (Zn), Boron (B), and even Phosphorus (P) deficiency in some crops (blueberry, pin oak, red maple, hydrangeas) and cause symptoms like leaf chlorosis. Also, when the substrate pH is above the optimum range, the applied fertilizer can be wasted.  For example, when the substrate pH is above 6.5, most of the phosphorus is in insoluble forms, which are difficult if not impossible for plants to uptake.  

Low pH rare in Ontario

When there is P limitation, even if there is plenty of nitrogen, potassium, and other nutrient elements, plants still cannot utilize these nutrients to best effect. Therefore, the fertilizer applied will be wasted and may also move out of the system to harm our environment. A substrate pH that is too low can cause Fe, Mn, Zn, and Cu toxicity, and Calcium (Ca) and Magnesium (Mg) deficiency. However, in Ontario, too low of a pH is rarely seen. Managing substrate pH may prove to be the single, most cost effective practice in potted nursery production.

Substrate pH is mainly controlled by the materials used in mixing the substrate, the fertilizer applied, irrigation water, and the plants themselves. In this article, we will address the water issue only.

In Ontario, many nurseries use water from wells, rivers, and other sources with a high alkalinity and a high pH. A high alkalinity in irrigation water is usually caused by high concentrations of carbonates, usually calcium carbonate (CaCO3), magnesium carbonate (MgCO3) and bicarbonates (HCO3-).  pH is a measure of the acidity and basicity of irrigation water. Using irrigation water with a high alkalinity can lead to an increased pH in the growing substrates or soils. To reduce water alkalinity and pH, acid can be injected into the irrigation water. How much to inject depends on the alkalinity of the irrigation water, the type of acid used, and the target alkalinity and pH you want to achieve. There are a number of useful sources to use for guidance. The following are a few free web-based calculation tools to easily achieve your goal.  

Calculate alkalinity

Choose whichever method you prefer, but the first thing is to know the alkalinity of your water. Either measure the alkalinity using some of the simple, inexpensive kits available, or send a sample of your water to a commercial laboratory. When you do the calculation, please pay special attention to the alkalinity unit, since different laboratories may give you different units and different calculators may require different unit inputs. Measured as either HCO3- or CaCO3, the unit can be milliequivalents per litre of water (meq/L), parts per million (ppm), or mg/L.

Here are three simple equations to use for converting one unit to another: 1 meq/L  CaCO3 = 1 meq/L HCO3-, 1 meq/L CaCO3 = 50 ppm = 50 mg/L, and 1 meq/L HCO3- = 61 ppm = 61 mg/L.  Also, to convert from ppm CaCO3 to ppm HCO3-, multiply the CaCO3 value by 1.22. The other thing to know for links above is the pH of your water.  Determine this yourself by using a pH meter, or pH test kit, or send a sample to a laboratory.

If using sulphuric acid (H2SO4), phosphoric acid (H3PO4), or nitric acid (HNO3) to lower the pH of your irrigation water, calculate how much S, P, or N you are adding to your pots, so you can reduce the fertilizer application of that element, otherwise you may run into some nutrient imbalance issues. For example, when using HNO3 to acidify irrigation water, you are adding more N into the system. If you are still using the same amount and type of fertilizer as you did before, then your plants may grow more rapidly, but less hardy. If you do not want to change your type of fertilizer and application rate, then H2SO4 may be a better acid to use, since plant uptake of SO4- is much slower than the uptake of NO3-; therefore in the short term, you may not run into any big nutrient imbalance issues. However, if you are recycling irrigation water, then H2SO4 is not recommended, as SO4- will eventually accumulate in your water to an unacceptable level. For more information on the benefits and drawbacks of each acid, see the table at

Systems vary

It is important to ensure that the injection equipment is appropriate for use with acids. Read product descriptions carefully as one company may provide multiple injection systems, but each system may vary in terms of its suitability for strong acids. It is also suggested that separate injectors be used for fertilizer applications to avoid salt precipitation. Agitation of the pipes may be required to prevent the acid from resting and wearing down the equipment. All metal irrigation pipes should be well rinsed at the end of each acid injection treatment.

Ongoing updates

Currently, our group is testing whether acidifying irrigation water alone can keep growing substrates within an optimum pH range and improve the performance of several plant species, some of which require a lower substrate pH. We will provide ongoing updates on this and other aspects of our research program at the University of Guelph. 
If you have any questions, contact Dr. Youbin Zheng, 519-824-4120, ext. 52741,  Dr. Zheng is a researcher in horticulture,