Optimal pH for hydroponics

Solving the hydroponics pH puzzle

In hydroponics, pH is of paramount importance.

The solution pH (potential hydrogen) is defined as the concentration of H+ ions in the nutrient solution. A pH value of 7 indicates neutral pH. When the pH drops below 7 nutrient solution becomes acidic and when the pH value rises above 7 the solution pH becomes alkaline.

Fluctuation of pH and root exposure to highly acidic or highly alkaline pH have been associated with reduced growth rate and toxicity. For example, highly acidic pH has been reported to enhance the solubilization of metals such as Aluminium (Al) (Delhaize and Ryan, 1995) leading to Al toxicity, unless plants have Al tolerance. Root injuries were observed in tomatoes grown at pH 3 in early studies by Arnon and collaborators (1942) at the University of California, Berkeley.

Hence, pH may regulate the bioavailability and absorption of minerals.

It is commonly believed that the optimum pH for nutrient uptake in hydroponic systems is between pH 5.5-6.5. However, many studies that suggest a pH range between 5.5 - 6.5 in hydroponics often reference just a single source: Hochmuth (1990, revised June 2001 & April 2022).

Below is the exact quote from the author:

“The pH of the media refers to the concentration of hydrogen ions (H+) in the media solution. The concentration is determined by a pH electrode or can be approximated by a pH color-strip paper dipped into the solution. The pH of the media solution is important because certain plant nutrition aspects are influenced by pH such as solubility of essential elements. Most elements are absorbed best from a media with a pH of 5.5 to 6.5. Media pH above 7.0 results in reduced micronutrient and phosphorus solubility. Extremely acidic pH can lead to micronutrient toxicities especially on soil-based media if manganese and aluminum are present.” - Hochmuth, 1990 Fertilizer Management for Greenhouse Vegetables - Florida Greenhouse Vegetable Production Handbook, Vol 3.

Nevertheless, the origin of the recommendation to use pH 5.5 - 6.5 in hydroponics remains a mystery, since Hochmuth (1990) did not reference publications that support this recommendation.

Swedish scientist Fabricius (2023) reviewed over 93 publications examining the effects of nutrient solution pH on biomass production but was able to shortlist only 4 studies that were relevant and quantified the effects of pH on plant biomass and yield. In studies by Bress & Weston (1992) pH levels 5.0, 5.5, 6.0, 6.5 did not affect the yield of lettuce grown in the nutrient film technique (NFT). Gillespie et al. (2020) reported no effect on the biomass of basil grown in a deep water culture (DWC) system at pH 4.0, 4.5, 5.0, 5.5. Interestingly enough, the author noted that plant growth was just fine at a pH as low as 4.0. Later, when experimenting with spinach in a DWC system, Gillespie et al. (2021) found that reducing the pH of the nutrient solution from pH 5.5 to pH 4.5 resulted in a 62% reduction in aboveground biomass, stunted overall growth and severely inhibited root development. Studies by Samarakoon et al. (2020) on lettuce grown in an NFT system found that the maximum yield was obtained between pH 6.0 and 6.2. Therefore, considering the above, it may be safe to say that the optimal pH for hydroponic production is crop-specific.

One may consider, that plants grow equally well between pH 4 and 7. Although, this statement could only be true if nutrients do not become a limiting factor. At higher pH levels availability of Mn, Cu, Zn and Fe decreases. At lower pH levels decline in availability of P, K. Ca and Mg may be observed. Reduced availability could lead to reduced nutrient uptake, which could result in nutritional disorders.

Bugbee (2003) recommended pH for hydroponic culture between 5.5 to 5.8 because of the overall availability of nutrients being optimized at a slightly acid pH. It is probable, that this range may not be the “sweet spot” for all plants. Nevertheless, it could at least be regarded as a safe range for plant growth and development in hydroponics.

Further Reading:

Arnon, D. I., Fratzke W.E., and Johnson, C.M. (1942) Hydrogen ion concentration in relation to absorption of inorganic nutrients by higher plants

Bugbee, B. (2003) Nutrient Management in Recirculating Hydroponic Culture, Utah State University.

Dehaize, E. and Ryan, P. R. (1995) Aluminum Toxicity and Tolerance in Plants. Plant Physiol. 107:315-321.

Fabricius, E (2023) Review of pH recommendations for soulless cultivation systems Swedish University of Agricultural Sciences.

Hochmuth, G.J. (1990) Fertilizer Management for Greenhouse Vegetables - Florida Greenhouse Vegetable Production Handbook, Vol 3.