Understanding pH

Managing the chemical properties of nutrient solutions and root zones is one of the primary activities in hydroponic crop production. Specifically, nutrient solution and substrate EC and pH are the two chemical properties most frequently monitored and adjusted. Understanding the fundamentals of pH — what it is, why it is important and how it changes — will improve the ability to manage it.

What is pH?

The definition of pH is the negative decimal logarithm of the hydrogen ion concentration. To simplify, pH is a measurement of hydrogen ions (H⁺). The negative decimal part of the definition relates to the counter-intuitive values. As the pH value goes up from, say 5.5 to 6.5, the concentration of H⁺ is decreasing, not increasing. Alternatively, as the pH drops from 5.5 to 4.8, the H⁺ concentration is increasing. The log-scale portion of the definition relates to the H⁺ concentration, where a pH of 5.0 has 10 times the H⁺ concentration compared to a pH of 6.0, and 100 times the H⁺ concentration compared to a pH of 7.0.

Above 7.0, the pH is considered “alkaline” or “basic” and the concentration of OH_- is greater than H+. Alternatively, below 7.0, solutions are “acidic” and the concentration of H⁺ is greater than OH-. A pH value of 7.0 is neutral, where the concentration of H+ is equal to OH_-. Although 7.0 may be considered “neutral”, a pH of 6.0 serves as a better “center” for pH values for horticultural purposes, including hydroponics. A pH of 7.0, while neutral, is too high and undesirable. Broadly speaking, pH ranges from 5.8 to 6.2 or 5.5 to 6.0 are desirable for hydroponic nutrient solutions and substrates. That applies across most crops and production systems as well.

Why is pH important?

The pH is important for effectively managing mineral nutrition for plants. Nutrient availability changes as pH changes differently for the 12 essential macro- and micronutrients. The most pronounced effect of pH on availability is for micronutrients. Starting at a pH of 6.0, the availability of micronutrients decreases as the pH increases. Alternatively, micronutrient availability increases as the pH decreases below 6.0. This is the reason many plants develop interveinal chlorosis when the pH is too high (Fig. 1).

By maintaining nutrient solutions and substrate within the target pH ranges recommended for various crops, and specifically avoiding high pH, nutrients are kept available to plants and deficiencies are avoided. In order to maintain target pH ranges, recirculating nutrient solutions are constantly treated to adjust their pH. Acids are used to reduce pH, while alkali or bases are added to increase pH. Usually, substrate pH is maintained by adjusting nutrient solution pH for hydroponic production systems since nutrient solutions are applied so frequently and have a strong influence on substrate pH.

How does pH change?

There are several reasons why pH can change in recirculating nutrient solutions and hydroponic substrates. One of the causes of pH change is nutrient uptake. Roots need to maintain an electrochemical balance to properly function. Therefore, when a negatively charged ion is taken up, an OH- is released, whereas when a positively charged ion is taken up, an H⁺ is released. A good example of this is the nitrogen in fertilizers. When positively charged ammonium (NH₄⁺) is taken up and an H+ is released, the pH decreases due to the increasing H+ concentration. Alternatively, when negatively charged nitrate (NO₃^-) is taken up and an OH^- is released, the pH increases due to the decreasing H⁺ relative to the increasing OH^- concentration.

In addition to nutrient uptake and conversion, root respiration also affects pH. Although roots of hydroponic crops don’t photosynthesize, they do respire. As roots respire, oxygen is consumed and carbon dioxide and water are given off. The CO₂ given off reacts with water to form carbonic acid and this, in turn, decreases pH.

While several factors can change nutrient solution and/or substrate pH, alkalinity in water also affects pH. Alkalinity is a measurement of the carbonate and bicarbonate in water. Ultimately, alkalinity acts as a buffer against pH change. Water with low alkalinity will result in nutrient solutions and substrates where pH can change easier. Alternatively, water with high alkalinity will inhibit nutrient solutions and substrates from readily changing pH.

The take-home message

One of the biggest benefits to hydroponic crop production is the precise control of water and mineral nutrition. The pH of your water, nutrient solution and substrate can affect crop quality. By understanding and anticipating potential pH effects, combined with proactive management, pH-related disorders can be avoided and productivity can be maximized.

The author is an associate professor in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu