Why is the pH level of the water at the plants different than the pH level of the water coming out of my water system?

Introduction

When mixing a recipe in an irrigation unit, you may have noticed that it is relatively easy to maintain desired EC levels but it is sometimes difficult to have the irrigation unit maintain a desired pH level.  In some cases, the pH level may fluctuate at the irrigation unit, or the pH level at the drippers will be higher than the measured pH level at the irrigation unit. It may appear that the irrigation unit is not working quite right but the actual symptoms, problems and solutions are much more complicated. The root cause of this is the bicarbonate levels of the irrigation water.

What are bicarbonates?

Bicarbonates are an essential component of the chemistry of irrigation water. It influences both the pH stability and the availability of nutrients in irrigation water. Chemically, bicarbonates (HCO₃⁻) are a form of dissolved carbon dioxide that exist in equilibrium with carbonic acid (H₂CO₃) and carbonate ions (CO₃²⁻).

Bicarbonates function as a buffering agent, helping to regulate pH by neutralizing acids in the water. Understanding how bicarbonate levels impact pH, particularly in drip irrigation systems, is crucial for effective water and nutrient management. This article explores the effects of high and low bicarbonate concentrations in irrigation water and their impact on pH levels at different points in the irrigation system.

What is the ideal bicarbonate level?

The ideal bicarbonate concentration depends on the crop type, crop age and soil conditions, but generally, the bicarbonate concentration going into the irrigation unit should be 0.2 - 0.5 mmol/l. This range helps maintain a stable pH level between 5.2 and 6.6.

Why is stable pH control important?

The ability of crops to absorb elements is dependant on the pH level of the water. Different elements are best absorbed within different pH ranges. The image below illustrates the uptake of various elements by plants at different pH levels.

On average, plants absorb nutrients most effectively at a pH level between 5.5 and 6.5. When the pH is higher, plants struggle to absorb elements like iron, manganese, boron, copper, and zinc. Conversely, when the pH is lower, elements such as nitrogen, phosphorus, potassium, sulphur, calcium, and magnesium are not easily absorbed.

Why are my bicarbonates too high or too low?

The amount of bicarbonates in your irrigation water depend on where you are getting your water from. Different sources contain varying bicarbonate concentrations, depending on geological and environmental factors:

• Groundwater (Wells): Often has high bicarbonate levels, ranging anywhere from 1 to 10 mmol/L, due to prolonged contact with carbonate-rich rocks and minerals.
• Surface Water (Rivers or Lakes): Typically has moderate bicarbonate levels, ranging from 0.5 to 2.5 mmol/L. These levels can vary significantly from season to season or based on the amount of recent rainfall.
• Rainwater: Usually has ideal bicarbonate levels (0.2 mmol/l to 0.5 mmol/L) because it lacks significant mineral content.
• Reverse Osmosis (RO) Water: Has extremely low bicarbonate levels (<0.5 mmol/L) because the RO process removes most dissolved minerals, including bicarbonates. This makes RO water highly susceptible to pH fluctuations.

Understanding the bicarbonate levels in different water sources helps in selecting appropriate water treatment strategies to optimize pH control and nutrient management.

What happens if the bicarbonates are too high?

When irrigation water contains high levels of bicarbonates (HCO₃⁻), it acts as a buffering agent that resists changes in pH. However, as the water travels through the irrigation system, certain chemical reactions occur that cause the pH at the drippers to be higher than the pH at the irrigation unit.

1. Creation of carbonic acid: In the water system, acid is added to the water. As the acid mixes with the bicarbonates in the water, carbonic acid is created. (HCO₃⁻) + (H⁺)  ⇌  (H₂CO₃⁻)
2. Degassing of carbonic acid: The carbonic acid is created in the irrigation pipes. When mixed with air as it comes out of the dripper, CO₂ is degassed. (HCO₃⁻) ⇌ (CO₂) + (H₂O)
3.  Increased pH: Since some of the acid added to the irrigation water reacted with the bicarbonates and is no longer present in the irrigation water, the resulting pH of the water being delivered to the plants is higher than anticipated.

What happens if the bicarbonates are too low?

While high bicarbonate levels make it difficult to achieve desired pH level, very low bicarbonate levels can cause erratic pH levels for several reasons:

1. Lack of Buffering Capacity: Bicarbonates help maintain pH stability by neutralizing acids or bases that enter the irrigation system. When bicarbonate levels are low, even small additions of acids can cause large pH swings, making pH control more difficult.
2. Influence of Acidic or Alkaline Inputs: Water with low bicarbonate concentrations is highly sensitive to external influences, such as acidic fertilizers or acidic or alkaline growing media. These inputs can rapidly alter pH levels at different points in the system, leading to unpredictable conditions for plant roots.
3. Microbial and Chemical Reactions: In irrigation systems, biological activity (such as microbial growth) and chemical processes can contribute to pH changes. Without sufficient bicarbonates to moderate these reactions, pH variations can be extreme and inconsistent.

Will high or low bicarbonate levels affect my plants?

The presence of bicarbonates in irrigation water directly affects plant health, nutrient availability, and soil conditions.

• High Bicarbonate Levels:
  • Can lead to nutrient imbalances by reducing the availability of essential micronutrients like iron, zinc, and manganese.
  • Causes soil alkalization over time, negatively impacting root function and nutrient uptake.
  • May result in poor plant growth, chlorosis (yellowing of leaves), and reduced crop yields due to nutrient deficiencies.
• Low Bicarbonate Levels:
  • Increases the risk of rapid pH fluctuations, which can stress plants and disrupt nutrient absorption.
  • May cause acidification of the root zone, leading to toxicity from elements such as aluminium or excessive solubilization of certain nutrients.
  • Can negatively impact beneficial microbial activity in the soil, affecting overall plant health.

What can I do about bicarbonate levels?

To maintain stable pH levels and ensure optimal nutrient availability, irrigation water should have a balanced bicarbonate concentration. Here are some practical management strategies:

• For High Bicarbonate Water:
  • Use a Priva Neutralizer to precondition your irrigation stock tanks by removing bicarbonates from your water.
  • Use acid injection (e.g., sulfuric, phosphoric, or nitric acid) to lower pH and reduce bicarbonate buffering capacity.
  • Periodically clean irrigation lines to prevent scale build up.
  • Monitor pH levels at the drippers to detect significant changes.
• For Low Bicarbonate Water:
  • Add a buffering agent, such as potassium bicarbonate, to stabilize pH fluctuations.
  • Regularly test water pH to anticipate and correct sudden variations.
  • Adjust fertilization practices to minimize pH swings caused by nutrient applications.

Conclusion

Bicarbonates in irrigation water play a crucial role in determining pH behaviour throughout the irrigation system. High bicarbonate levels cause pH elevation at drippers due to CO₂ loss, carbonate precipitation, and evaporation effects, while low bicarbonate levels result in erratic pH variations due to a lack of buffering capacity. Additionally, both high and low bicarbonate levels can have significant effects on plant health by influencing nutrient availability and soil conditions. Proper water management, including bicarbonate adjustments and regular monitoring, is essential for maintaining a stable and efficient irrigation system, ultimately supporting healthy plant growth and productivity.