CHAMBERSBURG, Pa. — This time of year, many greenhouse production areas are in full swing, producing numerous bedding plant and other ornamental varieties for sale later this spring. As plants begin to develop and approach flowering, the last thing a grower wants is to see damage to their plants. In addition to damage from insect and mite pests or pathogen infection, a cause of damage that is often overlooked in greenhouse crops is damage due to water quality.
Water quality issues from pH, alkalinity, or high salt levels can first appear as mild chlorosis or yellowing of leaf tissue along the veins or between them in some cases. A first instinct may be to increase the rate of fertilizer or add in micronutrients that could be missing, such as iron or magnesium. However, this won’t do the trick if water quality is actually the problem. As symptoms worsen, tips can begin to dieback or leaf edges will begin browning and curling. Without first adjusting the irrigation water, any added nutrients won’t be available to the plant that needs them.
Understanding the pH and alkalinity of water is an important part of fertilization because it influences how available nutrients are for plants to take up. A pH of 7.0 on a 0-14.0 scale is considered to be neutral. Anything below 7 is considered acidic while anything above 7 is considered basic. Total alkalinity of water represents water’s buffering capacity, or how resilient it is to changes in pH. If alkalinity is low, fluctuations in pH can occur fairly easily. This can be risky for crop production, so having some amount of alkalinity is desirable. However, alkalinity that is too high can make it very challenging to adjust pH to a suitable level. Because of the karst topography throughout much of this area, there has often been a long history of dissolved limestone ending up in groundwater and well water, leading to higher alkalinity water and basic pH.
This increased alkalinity and elevated pH can wreak havoc for many bedding plants, most of which prefer to be kept acidic, between 5.8-6.2 pH. A common practice for greenhouse producers in our area is to add acid, such as sulfuric acid, to the stock tank their fertilizer injector pulls from. However, without accurate assessments of total alkalinity, it can be hard to know how much acid to add. Adding based solely on pH readings will be insufficient if the high alkalinity is buffering the water’s pH. Additionally, high soluble salts in irrigation water can cause further damage to ornamental plants, many of which are quite sensitive to elevated sodium or chloride levels. Thus, having a more comprehensive assessment of water quality can aid in determining the appropriate course of action to care for greenhouse irrigation water and ensure quality plants.
The following list provides steps to follow in assessing and correcting water quality issues for greenhouse irrigation water:
- Test. A crucial first step is to have a water sample tested at a laboratory that is equipped to test water for irrigation purposes. You can have this test done through Penn State or a variety of commercial labs. To use Penn State’s service, visit your local extension office for an irrigation water test kit that will include instructions on how to submit the sample for testing. A proper water test will include pH, alkalinity, soluble salts (including Ca+ and Mg+, the ions that make water “hard”). You may want to consider adding supplemental tests that for heavy metals and nutrients.
- Filter. Cloudy water is caused by solids suspended in the water. These can clog pipes, valves, and nozzles. Thus, it is helpful to filter suspended solids out of your greenhouse watering source. Simple filters are easy to find and install and should include pressure gauges installed on both sides of the filter can indicate when it is starting to clog. Be sure to clean and maintain them regularly.
- Adjust pH and alkalinity. Water for irrigation should fall in the range of 5 to 7 on the pH scale. After determining the needs of the crops you will grow and the alkalinity of your water source, acid is used to adjust the pH of irrigation water to this range. An alkalinity test measures the level of bicarbonates, carbonates, and hydroxides in water, which neutralize acids. Irrigation water tests should always include both pH and alkalinity to determine how much acid to add.
- Check soluble salt levels. In addition to pH issues, soluble salts can limit nutrient uptake in plants and cause harm to ornamental plants. If the water test indicates high levels of soluble salts, as measured by electrical conductivity (EC), it may need to be purified or diluted with lower EC water. Keep an eye out for excess sodium (Na+) and chloride (Cl-), as these can be particularly detrimental to plants.
- Use the correct injector, and make sure it is working properly. If you are using an injector to add acid to your water, make sure it is the right model and is working properly. Parts can wear and need replacement over time. Regular monitoring and maintenance of injectors is critical to the health of your greenhouse plants.
If you’re a commercial grower needing assistance with assessing greenhouse water quality or troubleshooting fertility issues, feel free to reach out to your local horticulture extension educator.
–Karly Regan, Penn State Extension
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