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 (e.g. 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. 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. Here we only address the high alkalinity issue in irrigation water. In Ontario, many greenhouses and
nurseries are use water from wells, rivers, and other sources which have 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 you
are using, and the target alkalinity and pH you want to achieve. There are a number of useful sources to sue for guidance. The
following are a few free web based calculation tools you can use to easily achieve your goal.
You can choose whichever method you prefer, but the first thing you need to know is the alkalinity of your water. You can either
measure the alkalinity using some of the simple, inexpensive kits available, or you can 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 for you
to use for converting one unit to another:
Also, to convert from ppm CaCO3 to ppm HCO3-, multiply the CaCO3 value by 1.22. The other thing you will need to know for links
above is the pH of your water. You can determine this yourself by using a pH meter or pH test kit, or you can send a sample to a laboratory.
In terms of which acid to use, the following table may give you some idea. If you are using sulphuric acid (H2SO4), phosphoric acid
(H3PO4), or nitric acid (HNO3), you need to 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 you use HNO3 to acidifying
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 your irrigation water, then H2SO4 is not recommended, because
SO4- will eventually accumulate in your water to an unacceptable level.
It is important to ensure that the injection equipment is appropriate for use with acids. It is important to 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 in order to avoid salt precipitation. Agitation of the
pipes may be required to prevent the acid from resting and wearing down the equipment and all metal irrigation pipes should be well rinsed
at the end of each acid injection treatment.
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 be providing ongoing updates on
this and other aspects of our research program at the University of Guelph.
If you have any question2, please contact Dr. Youbin Zheng (519 824 4120 ext 52741, firstname.lastname@example.org).