The pH scale measures a liquid’s acidity or alkalinity, ranging from 0 (most acidic) to 14 (most alkaline), with 7.0 being neutral. The pH of irrigation water significantly affects plant health by influencing how a plant absorbs nutrients from its environment. Understanding your water source is crucial for successful gardening, as plants have distinct preferences for the pH level of their growing medium.
How Water pH Affects Nutrient Availability
Water pH directly impacts the solubility of mineral nutrients within the soil or growing substrate. Most plant species thrive in a slightly acidic to neutral range (pH 6.0 to 7.0) because this environment keeps essential compounds in their most soluble form. When the water or soil becomes too alkaline (above pH 7.0), a chemical process known as “nutrient lockout” can occur.
This phenomenon causes certain micronutrients to precipitate, meaning they change structure and become solid rather than dissolved in the water solution. For instance, calcium and bicarbonates in high-pH water react with metallic ions like iron, manganese, and zinc. These reactions form insoluble compounds, such as carbonates and hydroxides, which the plant’s roots cannot absorb, even if the elements are present in the soil.
The plant then suffers from a functional deficiency because the nutrients are chemically unavailable for uptake. Iron deficiency, common in plants watered with alkaline water, manifests as chlorosis, or the yellowing of the newest leaves. A lack of soluble manganese and zinc can also impair photosynthesis and enzyme functions, leading to stunted growth.
Plants That Flourish in High pH Environments
While many garden plants prefer acidity, calciphiles (lime-lovers) thrive where the pH is neutral or alkaline (7.0 and above). These plants have evolved mechanisms to efficiently absorb micronutrients even in less soluble forms. Matching the plant to the water and soil pH is generally better than constantly trying to alter the growing environment.
Vegetables and Herbs
Several popular vegetables and herbs tolerate alkaline conditions. Garden asparagus performs well in soils up to pH 8.0, often benefiting from the calcium present. Members of the brassica family, including cabbage, also tolerate slightly alkaline conditions. Aromatic herbs flourish in the free-draining, high-pH soils common in chalky regions.
- Asparagus
- Cabbage (and other Brassicas)
- Lavender
- Oregano
- Thyme
Ornamentals and Shrubs
Many resilient flowers and shrubs are well-suited to alkaline water sources. The common lilac (Syringa) produces its best blooms in neutral to alkaline soil up to pH 7.5. Other reliable choices include hardy geraniums (Cranesbill), climbing clematis, and the succulent sedum, which prefers a pH range of 6.0 to 7.5.
Trees and Landscape Plants
Certain trees and large shrubs are excellent choices for water with a higher pH.
- Boxwood (Buxus)
- Juniper (Juniperus) varieties
- Silver Maple varieties
Selecting these robust, alkaline-loving species ensures a healthy landscape without the constant need for soil or water amendments.
Practical Steps for Testing and Adjusting Water pH
Determining the pH of irrigation water is the first step for ensuring plant health. Simple, affordable liquid test kits or pH strips provide a quick reading. For more precise measurement, especially in container gardening or hydroponics, an electronic pH meter offers greater accuracy. If soil analysis is needed, local agricultural extension services can provide comprehensive testing, including pH and nutrient composition.
Temporary Adjustments
If water is highly alkaline (above pH 7.5) and you need to grow acid-loving plants, temporary adjustments can be made for small-scale watering. Adding a small amount of white vinegar (acetic acid) to the water effectively reduces the pH before application. A common starting ratio is about one tablespoon of vinegar per gallon of water, but the mixture must be tested to confirm the final pH level.
Long-Term Soil Adjustments
For a longer-term, passive adjustment in container plants, incorporate organic matter that slowly acidifies the medium. Sphagnum peat moss, for example, can be mixed into the potting soil to lower the pH as it decomposes. This method is useful for acid-loving container plants, but it requires regular monitoring to prevent the pH from dropping too low.