Soil and Water pH - Part 5

Soil and Water pH

Part I – What is pH?

Part II – How is nutrient availability affected by pH

Part III – How do fertilizers affect pH?

Part IV – How to adjust pH of soils

Part V – How to adjust pH of irrigation waters

The process of adjusting irrigation water pH is the process of changing the amount of H⁺ or OH^- ions that are dissolved in the water.  In most cases where water pH modifications are required, it is to lower pH rather than increase pH. 

There are actually two factors to consider when adjusting water pH:

1) the pH of the water.

2) the alkalinity of the water.

pH - the free H⁺ and OH^- ions in the water ranges from a scale of 1-14.  The optimum pH for most crops ranges from 5.0 to 7.0, mostly due to the effect on pH on nutrient availability to plants.

Alkalinity - the measure of the concentration of bicarbonates (HCO₃), carbonates (CO₃) and other akali compounds dissolved in the water, which can neutralize acids that are added to the water.  These are similar to liming materials that can be added to soils to increase the soil pH.  However, in the case of irrigation water, these materials are in solution.  The most common bicarbonates and carbonates are those of calcium bicarbonate [Ca (HCO₃)₂] and calcium carbonate [Ca(CO₃)].  However, other carbonate complexes include those of magnesium (Mg) and sodium (Na). These chemicals act as buffers in the water to prevent acidification.  Therefore, acidifying a source of irrigation water with high alkalinity may require more acid than a water source of similar or lower pH, but with lower alkalinity.  If your irrigation water is derived from water sources around limestone - i.e. underground limestone caverns, then there is a greater probability of higher bicarbonates and carbonates in the water.  The relative amount of these chemicals dissolved in most irrigation waters usually ranges from 0 to 10 meq Ca(CO₃)/L (Figure 1).

Steps to adjust irrigation water pH:

1. Know the recommended pH for the crop in question.
2. GET A WATER TEST DONE - CAUTION:  Follow recommended procedures for sampling.  Most labs will provide guidelines.
3. Follow recommendations of the lab.  Most laboratories will measure pH, alkalinity and then conduct a titration, a process where they determine the amount of acid required to lower the water pH to a recommended set point. 

Labs may report alkalinity in one of 3 ways: 1) meq CaCO₃/L, 2) mg CaCO₃/L, 3) ppm CaCO₃.  These values can be converted to each other as follows:

1 meq CaCO₃/L = 50 mg CaCO₃/L

1 ppm CaCO₃     = 1 mg CaCO₃/L

1 ppm HCO₃     = 0.017 meq CaCO₃/L

1 ppm HCO₃    =  0.83 mg CaCO₃/L 

Methods of lowering water pH and decreasing alkalinity:

All methods involve the injection of acids.  The most common acids used are: sulfuric acid, urea sulfuric acid, phosphoric acid, hydrochloric acid (muriatic acid), and citric acid.

Methods of increasing alkalinity

In some cases, it may be necessary to increase alkalinity to prevent rapid acidification in certain systems caused by acidic media or fertilizers.  In these cases, chemicals such as potassium bicarbonate may be required.

With this article, and the four preceding it, you now have the complete series of articles related to pH. Next article in ‘Get Cultured’ will review chelates and their use.

1 mg CaCO₃/L  = 0.02 meq CaCO₃/L

1 meq CaCO₃/L = 60 ppm HCO₃