The answer to whether you can water plants with alkaline water depends on the specific plant and the existing soil chemistry. The pH scale measures acidity and alkalinity, with 7 being neutral. Water with a pH above 7 is considered alkaline, often due to dissolved calcium and magnesium carbonates. While the alkalinity itself is rarely harmful, the long-term impact on the soil environment is the main concern. The cumulative effect of high-pH water gradually shifts the soil balance, potentially leading to plant health issues over time.
How Water pH Influences Nutrient Uptake
The primary concern with using alkaline water is the indirect effect it has on nutrient availability in the soil. Plants absorb dissolved nutrients through their roots, and nutrient solubility is highly dependent on the soil solution’s pH. When soil pH rises above 7.0, essential micronutrients begin to change into forms that plants cannot easily absorb.
High alkalinity causes elements like iron, zinc, and manganese to bind with other compounds in the soil, precipitating out of the root-accessible solution. This process is known as “nutrient lockout,” where the nutrients are physically present but chemically unavailable to the plant.
This lack of solubility is particularly noticeable with iron, which is necessary for chlorophyll production. Even if the soil contains adequate iron, high pH prevents the roots from taking it up in its usable form. The effect is compounded over time, as repeated watering with alkaline water contributes more compounds that raise the soil’s pH further.
Recognizing Signs of High pH Stress in Plants
The nutrient unavailability caused by high pH translates into distinct, visible symptoms on the plant’s foliage. The most common sign is chlorosis, which indicates a deficiency in chlorophyll. Iron deficiency, often induced by high alkalinity, typically manifests as interveinal chlorosis. In this condition, the tissue between the leaf veins turns yellow while the veins themselves remain green.
These symptoms usually appear first on the newly emerging growth because iron is not a mobile element within the plant. If the high-pH stress is not corrected, the yellowing may worsen, potentially affecting the entire leaf structure. Other indicators of high-pH stress include stunted growth, poor vigor, and reduced flower or fruit production.
Plants That Tolerate or Prefer Higher pH
Many common garden plants thrive in a slightly acidic to neutral pH range (6.0 to 7.0), but some species are well-adapted to, or even prefer, alkaline conditions. Understanding these preferences helps gardeners work with their existing water and soil chemistry. Plants commonly referred to as “acid-loving,” such as blueberries, azaleas, and camellias, are highly susceptible to high-pH induced deficiencies and should be avoided in alkaline environments.
Conversely, a wide variety of ornamental and edible plants can tolerate or flourish in alkaline soils (pH above 7.0). These include Mediterranean herbs and certain vegetables.
Tolerant species include:
- Rosemary, thyme, and lavender.
- Cruciferous vegetables like cabbage, broccoli, and kale.
- Lilac shrubs and forsythia.
- Perennials such as bearded iris and yarrow.
Adjusting Soil and Water pH
Gardeners dealing with alkaline water have several options for maintaining a healthy soil environment, involving adjusting the pH of either the water or the soil itself.
Adjusting Water pH
For immediate, small-scale adjustments, a weak acid like common white vinegar can be used to lower the pH before watering. However, the exact amount of acid needed varies significantly based on the water’s initial alkalinity and hardness. Careful measurement and testing are required to avoid damaging the plants.
Adjusting Soil pH
For long-term, large-scale changes, amending the soil is generally a more effective strategy than treating every gallon of water. The most common chemical amendment used to reduce soil pH is elemental sulfur, which is slowly converted by soil microbes into sulfuric acid. This process requires soil temperatures above 55 degrees Fahrenheit to be effective and is slow-acting, often requiring months to fully adjust the pH level.
Organic materials can also help to acidify the soil over time, though their effect is less dramatic than elemental sulfur. Canadian sphagnum peat moss can be thoroughly mixed into the soil before planting to provide a lasting acidic environment. Additionally, using acidifying fertilizers, such as those that are ammonium-based, can help maintain a lower pH as the nitrogen compound breaks down.