A Guide to pH in Ornamental Plant Production and Its Effects

Effective management of the pH of irrigation water and the growing media—or, more precisely, the nutrient solution circulating within the substrate—is essential for successful ornamental plant growth.

Maintaining pH within the crop‑specific optimal range helps prevent nutritional imbalances, physiological stress, and growth disorders that negatively affect plant quality. This guide explains the role of pH in ornamental horticulture, its influence on nutrient availability, and the practical strategies used to measure and manage pH in order to achieve consistent, high‑quality production results.

Why pH Is Important for Plant Health

On its own, pH has limited relevance in crop production. It becomes a critical parameter when elevated pH values (> 7) are linked to high alkalinity or to increased concentrations of calcium, magnesium, and bicarbonates.

pH directly influences nutrient availability and uptake by plants. When the pH (of irrigation water or of substrate) is not aligned with the requirements of a specific crop, nutrient absorption may be restricted—even in well‑fertilized growing media or when nutrient‑rich irrigation water is applied. This can result in deficiencies and reduced growth despite adequate nutrient supply.

What pH Is and How It Relates to Ornamental Plant Production

From a technical standpoint, pH is defined as “the negative logarithm of the hydrogen ion (H⁺) concentration in a solution.” In practical horticultural terms, pH expresses the acidity or alkalinity of irrigation water or of the nutrient solution present in the growing substrate. pH is expressed on a logarithmic scale ranging from 0 to 14, where 0 represents extreme acidity and 14 extreme alkalinity. A pH value of 7 is considered neutral.

pH Values and their Interpretation

• pH 3.5–5.0: Strongly acidic
• pH 5.1–6.5: Acidic
• pH 6.6–6.9: Slightly acidic
• pH 7.0: Neutral
• pH 7.1–7.5: Slightly alkaline
• pH 7.6–8.5: Alkaline
• pH 8.6–14.0: Strongly alkaline

Ideal pH for Plants

In ornamental plant production, pH is primarily evaluated in terms of its impact on the availability and behavior of macro‑, meso‑, and micronutrients in both irrigation water and the growing substrate. pH levels that are either too low (acidic) or too high (alkaline) can limit nutrient availability or cause certain elements to accumulate to toxic levels.

For most ornamental substrates, a pH range between 4.0—for acid‑loving species—and 6.5 is considered optimal, as the majority of ornamental plants achieve their best growth within this interval.

Understanding pH Variability: pH Dynamics in Closed Cultivation Systems

In container‑grown ornamental crops, pH often changes during cultivation, particularly in crops with longer production cycles. In subirrigation systems, pH variation may occur even within a single container. The nutrient solution in the lower portion of the pot often exhibits lower pH values than the upper layer. This vertical pH gradient is especially pronounced when high proportions of ammonium nitrogen are supplied. The use of water‑soluble fertilizers with a higher share of nitrate nitrogen—such as those in the Peters Excel range—helps limit pH stratification and promotes more uniform pH conditions throughout the root zone.

While this paragraph is about understanding pH variability, you might find useful to learn more about pH stability in new substrates by reading this article.

In professional plant production, both substrate pH and irrigation water pH must be monitored. In the following paragraphs we see why.

Factors Influencing Substrate pH

Substrate pH evolves over time as a result of several interacting factors, including:

• Fertilization strategy and nutrient formulation
• The presence of calcium and other cations in irrigation water or substrate
• Uptake of cations by plants, which increases hydrogen ion (H⁺) concentration in the rhizosphere and lowers pH
• Uptake of anions, which increases hydroxyl ion (OH⁻) concentration and raises pH
• Excess nutrient availability relative to plant uptake capacity, disrupting the cation–anion balance
• Irrigation practices, as both water composition and application volume influence cation distribution within the substrate
• The intrinsic chemical and physical properties of the growing media; each substrate responds differently to pH changes, as explained in our article on pH Measurement on Peat-based Substrates vs Peat Alternatives

Proper pH management is therefore essential to preventing nutritional disorders and improving plant health, quality, and structural strength—key drivers of commercial value in ornamental production.

Why the pH of the Substrate Solution Matters

As previously mentioned, pH directly influences nutrient solubility and uptake. The substrate serves not only as a physical support for the root system, but also as a reservoir of nutrients. An imbalanced pH can therefore lead to deficiencies of macro‑, meso‑, or micronutrients, or to excessive availability of certain elements—such as manganese and iron—which may become toxic under strongly acidic conditions. (If interested, we have dedicated an extensive section of our web site to symptoms and management of 12 nutrient deficiencies in ornamental plant production.)

While the optimal pH of the substrate solution varies among species, it typically falls between 6.0 and 6.5 for most ornamental crops.

Acid-Loving Plants

Acidophilic species prefer lower pH values, sometimes as low as 4.0. Typical examples include azalea, camellia, hydrangea, heather, pieris, kalmia, gardenia, and rhododendron. Among bulb crops, clivia, calla lily, and lily perform best in acidic substrates. Several tree species—such as Norway spruce, Japanese maple, birch, and beech—also thrive under lower‑pH conditions.

Effects of Substrate pH Below 5.0

• Reduced uptake of potassium, magnesium, calcium, and molybdenum
• Increased solubility and potential toxicity of manganese, zinc, boron, and copper
• Increased iron availability
• Higher susceptibility to fungal diseases (e.g., Phytophthora in cyclamen)

Effects of High pH

• Reduced availability of phosphorus and iron, resulting in iron chlorosis
• Increased availability of molybdenum
• Insolubilization and deficiency of manganese, zinc, boron, and copper
• At pH values above 7.5, alkaline hydrolysis of certain plant protection products may occur, particularly when spray solutions are applied several hours after preparation

Water pH: Why It Is Important

Water plays a central role in plant nutrition. Its pH affects fertilizer efficiency and directly influences the pH of the substrate solution. Highly acidic water may disrupt substrate chemistry, while alkaline water can neutralize essential nutrients.

As illustrated in this nutrient availability charts, pH strongly determines the availability of macro‑, meso‑, and micronutrients.

• low alkalinity has limited buffering capacity,
• high bicarbonate levels can increase pH over time.

Why pH Changes During Cultivation: Monitoring Water pH

During crop production, pH variation is primarily driven by two factors: bicarbonate concentration and nitrogen form.

• Bicarbonates: Hard water with high bicarbonate levels can significantly raise substrate pH if fertilization strategies or water treatment are not adapted.
• Nitrogen form:
  • Nitrate nitrogen (NO₃⁻) tends to slightly increase pH
  • Ammonium nitrogen (NH₄⁺) significantly lowers pH

Preventing undesirable pH fluctuations starts with monitoring bicarbonate levels and aligning fertilization strategies with water quality:

• Irrigation water with high bicarbonate concentrations raises substrate pH and typically requires treatment to reduce bicarbonate levels using acids such as nitric, phosphoric, sulfuric, or citric acid.
• In contrast, in soft water with low bicarbonate content, the use of ammonium‑based fertilizers may overly reduce pH in the lower part of containers. Under low‑oxygen conditions, incomplete ammonium conversion can lead to nitrite accumulation, which is toxic to plants.

The  Peters Excel Acidifier range includes formulations designed for hard water that reduce bicarbonates and stabilize pH. For soft water conditions, the Peters Excel CalMag range provides calcium and a nitrogen composition with more than 90% nitrate nitrogen.

Now that we have explored both substrate’s and water’s pH, let’s dig into its measurement.

How to Measure Substrate and Irrigation Water pH

Imbalanced pH—whether in irrigation water or within the substrate— compromises plant metabolism, limits nutrient uptake even when nutrients are present, inhibits root development, and weakens plant resistance to stress and disease.

ICL recommends regular water and substrate analyses through specialized laboratories. For on‑site monitoring, digital pH meters offer high accuracy, while litmus strips represent a simpler alternative. Further details are available in our guide to measurement tools for substrates.

Analytical results are essential for building accurate fertilization strategies. Using the AngelaWeb3.0 digital tool, ICL technical teams evaluate multiple variables simultaneously to develop customized nutrition programs in close collaboration with growers.

Monitoring pH of Irrigation Water and the Substrate Solution

Regular monitoring of both irrigation water and substrate pH is strongly recommended. Substrate pH should generally be checked at least once per season when peat‑based substrates are used, and more frequently with peat‑reduced or peat‑free growing media, as discussed in our article on pH measurement on peat-based substrates vs peat alternatives. Water pH should also be measured periodically—and always when changing the water source. Irrigation water may originate from rainwater, wells, recirculation systems, surface water, reverse osmosis installations, or municipal supply, each with distinct chemical characteristics.

Recommended Fertilizers for pH Regulation in Ornamental Production

When irrigation water is excessively alkaline, pH can be reduced by acid dosing using nitric, phosphoric, sulfuric, or citric acid. However, a more stable and safer approach is the use of water‑soluble fertilizers specifically formulated for hard water, such as Peters Excel Acidifier or Universol HW. At an application rate of 1 g/l, these fertilizers can neutralize up to two moles of bicarbonates.

To increase pH, growers may use specific correctors, basic‑reactive fertilizers, or substrate with adjusted pH. Nevertheless, the level of nutrient safety and consistency achieved through carefully formulated water‑soluble fertilizers—such as Peters Excel CalMag or Universol SW—is difficult to replicate with alternative methods.

Developed for both soft and hard water conditions, the Peters and Universol ranges address most pH‑related challenges directly through their formulation. Fertilizer selection should always be based on accurate knowledge of irrigation water composition, making regular water analysis a key component of effective pH and nutrient management.

Conclusion

Effective management of irrigation water and substrate solution pH is a cornerstone of successful ornamental plant production. Regular monitoring and targeted corrective actions help prevent nutritional imbalances (learn more about this topic in our article on nutritional deficiencies in ornamental plants), improve crop performance, and ensure consistent quality. A well‑controlled pH ultimately results in healthier, more uniform, and more productive ornamental crops.

For personalized guidance, contact your regional technical support team via your country’s website.