On a very basic level (no pun intended), pH is a measure of the acidity or alkalinity of a substance. pH is a term used in chemistry to measure the activity of hydrogen (hydroxyl and hydroxide) ions in an aqueous solution.

The concept of pH was primarily introduced to science in 1909 and shortly thereafter revised and developed into the modern definition and measurements that are used today. While it is agreed that the “H” in pH represents “hydrogen,” it is debated whether the “p” stands for “power” or “potential.” Either way, “the power of hydrogen” or “potential hydrogen” both help to clarify the abstract definitions often associated with the mysterious symbol.

The pH scale

The pH scale is a tool used to determine the pH level of a given substance. The pH scale ranges from 0 to 14 with 0 being the most acidic, 14 the most alkaline (or, basic) and 7 the neutral midpoint. The pH scale is logarithmic, meaning that each value above or below neutral represents a tenfold increase in acidity or alkalinity from the previous unit. For example, a pH level of 4.0 is ten times more acidic than a pH level of 5.0, and a hundred times more acidic than 6.0. On the other end of the spectrum, a pH level of 9.0 is ten times more alkaline—or basic—than 8.0. This makes it crucial to take accurate readings when testing pH levels of a nutrient solution.

Why is pH important for plants?

pH affects just about everything, and plants are no exception. pH levels affect many factors in a plant’s life cycle. That’s why it is important to consider the pH level of the soil or medium plants are being grown in, as well as the solution they are being watered with…

Certain plants grow better in acidic or alkaline environments

Just like animals, different plants have different food preferences. Plants such as citrus fruit, blueberries, blackberries, strawberries, potatoes and orchids are prime examples of acid loving plants. Plums, poppies, sage, cherries and sunflowers are plants that prefer a more alkaline environment. Many plants are tolerant of a wider range of acidity and alkalinity, including beans, broccoli, garlic, peas, melons, onions and corn, to name a few.

Availability of nutrients

Plants rely on a specific group of macro- and micronutrients to provide the bulk of their sustenance. Without proper nutrition, plants experience deficiencies and perform under par. Mineral nutrients are only soluble within a solution when the pH level is balanced within a prime range (usually around 5.5 to 6.2). Macronutrients tend to be less available in soils with low pH, whereas micronutrients are less available in high pH soil. If pH levels stray from the ideal range, vital nutrients will be rendered unavailable to the plant, and the plant will be unable to eat the food it needs.

Disease cure and prevention

Certain diseases thrive in environments that are too acidic or too alkaline. For example, powdery mildew (a common garden nightmare) thrives on the surfaces of leaves with a slightly acidic pH. By treating the plants with a foliar application with a high pH (around 8.0), it will kill the mildew spores and help prevent reoccurrence of the disease.

What is the optimal pH range for plants grown in soil?

While different plants have varying ranges of pH to achieve optimal growth, most plants cultivated in soil will perform best when grown in a pH range of about 6.0 to 6.5. When growing in soil, ideally the grower will test the soil and the solution used to water the plants. If the soil is homemade, there are test kits available to check the pH of the soil.

Most commercial-grade specialty soils that come pre-bagged have already been tested and pH balanced The water is what differs from region to region and must be carefully monitored on a regular basis. The pH of the nutrient feed solution should be checked after adding fertilizer and adjusted to reach the optimal range of 6.0 to 6.5. pH adjusters designed for horticultural applications are available at most hydroponic stores or specialty nurseries.

What is the optimal pH range for hydroponically grown plants?

As with soil, plants grown hydroponically have pH preferences depending on plant variety. In general, plants in hydro tend to do better in a slightly more acidic environment than plants in soil. A pH range of about 5.5 to 6.0 is ideal for most hydroponic growth.

It is particularly important to consider pH in hydro, as all of the plant nutrition is coming from the fertilizer solution. Too high or low of a pH level will prevent either macro- or micronutrients from being absorbed by the plants. When growing with hydro, the medium used—whether it is inert or not—will have a slight effect on pH levels.

Why does the pH of a nutrient solution fluctuate?

In a nutshell, the explanation of this question is that plants use different elements, such as nitrogen, at different rates. As plants eat, they deplete certain substances and tend to raise the pH as they digest the fertilizer in the solution.

How is pH tested for horticultural purposes?

  • Paper test strips: These are among the most inexpensive and simple methods of testing pH. Paper strips infused with pH sensitive dye are dipped into the nutrient solution being tested. The color that appears on the strip corresponds with a color chart that determines the pH level.
  • Liquid indicator solutions: Similar to the paper test strips in that they use color, the liquid indicator solution is a popular and reliable method for novice or budget growers. A small sample of nutrient solution is taken from the reservoir in a container provided in the test kit. A few drops of pH sensitive liquid are added to the solution and change to specific colors depending on the pH level. The colors correspond with a chart that determines the pH.
  • Digital meters: High tech (compared to the previous methods), accurate, and easy to use, digital meters are the most logical choice for professional growers. They use a glass bulb electrode to precisely determine the pH of the solution being tested. Digital meters must be stored properly and calibrated often. They are notorious for breaking down and can be a source of frustration for growers who rely upon their precision. When using digital meters, it is important to keep them clean and wet while storing them. It’s also a good idea to have a liquid test kit on hand for backup.

How is pH adjusted for horticultural purposes?

Many common household items will have an effect on the pH level of pure water. Some of these include lemon juice to lower the pH, or baking soda to raise it. However, for the serious grower, it is recommended to use pH adjusters that are specifically designed to work well within a hydroponic nutrient solution. pH down (acid) lowers pH levels, while pH up (base) raises them.

There are several brands of pH up and down available in the hydro industry. Choose brands that do not add dyes to their pH adjusters, as artificial dyes have no added benefit to the health of plants. Look for adjusters that contain at least two to three acids in the pH down and two to three bases in the pH up. This will provide for a more stable pH level with less drastic fluctuation.

There are many important factors to consider when growing beautiful and bountiful plants, and pH ranks high in importance among them. Understanding why pH is important and how to manage pH balance helps to ensure happy plants. Attention to detail in the garden provides for a much deserved and rewarding end result; treat your ladies right and enjoy the bounty of healthy harvests!