Three Factors That Can Impact The pH Of Growth Media

The pH of water used in plant production is important when it comes to mixing with pesticides and dissolving micronutrients in water. However, according to Jose Chen Lopez, a horticulture specialist with Premier Tech Horticulture, the water pH has little influence on the pH of the growing medium. Other factors impact the pH of the growing medium.

Water Alkalinity Influence On Growing Media pH
This chart shows that the higher the water alkalinity, the more the pH of the growing medium rises, regardless of the water pH.

Water Alkalinity

The pH of the irrigation water is not as important as its alkalinity (CaCO3) when dealing with non-inert growing media, such as peat moss. Alkalinity is a measurement of the amount of carbonates and/or bicarbonates, or the amount of limestone dissolved in the irrigation water.

When water has high alkalinity, it means that the water has high limestone. Every time watering occurs, more limestone is added to the growing medium, resulting in an increase in its pH over time. In addition, water with high alkalinity has a large buffer capacity compared to water with low alkalinity. Buffer capacity refers to the ability to resist change; therefore, the higher the buffer capacity, the greater the resistance to change and the ability to lower the pH of the water.

There are ranges for the optimum alkalinity for different container sizes:
• Plugs: 60 to 100 ppm
• Small pots (4 inches and smaller) and bedding plant flats: 80 to 140 ppm
• Large pots (more than 4 inches) and long-term crops: 120 to 180 ppm

The main difference between these ranges is the relation to the fertilizer application rate and the size of the plant for each container size. The effect of an acidic fertilizer in a small plug is going to be too little since the rate would be low and the plant is very small compared to the rate of a large pot.


As plants take up fertilizer, they emit hydrogen and hydroxyl/bicarbonate ions into the growing medium, effectively changing the growing medium pH. Generally, the larger the plant, the more fertilizer is used, and therefore the faster the plant can change the pH of the growing medium. All fertilizer elements, especially for nitrogen, have the effect of either lowering or raising growing medium pH.

Nitrogen is available in three different forms: nitrate, ammonium, and urea. When plant roots take up nitrate (negatively charged), they release OH- or bicarbonate (HCO3-) to maintain electrical balance. When plants take up ammonium (positively charged), the roots release H+ to balance the charges within the plant. And finally, urea has to be transformed into ammonium in order to be taken up by the plant. Urea could be further transformed into ammonia and then into nitrate with the help of bacteria. The process of transforming ammonium into nitrate is called nitrification.

By selecting potentially acidic and basic fertilizers based on the type of nitrogen source, a growing medium pH can be maintained within optimum ranges. However, it is extremely important to monitor the fertilization management when using water with low alkalinity to water plants. Water with low alkalinity has a limited buffering capacity; therefore, the application of potentially acidic fertilizers can have a huge impact on the pH of the growing medium. In this case, potentially acidic fertilizers can drop growing medium pH to unacceptable levels. The same principle applies to reverse osmosis water since the alkalinity of the treated water can be extremely low.

The pH of the growing medium is important because it determines the availability of the nutrients. For example, the availability of micronutrients like iron, manganese, zinc, and copper increases when the pH is below 5.5. On the other hand, the availability of molybdenum, calcium, and magnesium decreases. The opposite occurs at pH levels higher than 6.5.

Plant Species

Although not as significant, different plant species also influence the pH of the growing medium. For example, zinnia and vinca tend to increase the pH of the growing medium over time, pansies do not have any effect on the pH of the growing medium, and tomato, dianthus, and celosia tend to lower the pH of the growing medium.

It is very important to verify the alkalinity of the water before the beginning of each season. By knowing the alkalinity of the water, growers can make better decisions regarding the fertilizer management to maintain the pH of the growing medium within optimum ranges.


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