Whether using municipal or well water, monitoring and testing are important to ensure successful production of greenhouse hydroponic lettuce and leafy greens.
An increasing number of ornamental plant growers are looking to take advantage of the growing interest in local food sales by expanding their production with seasonal crops of lettuce, leafy greens and herbs. Unlike ornamental plants, growers of edible crops have the added concerns of food safety.
“If growers are using municipal water for growing ornamental crops and then add lettuces and leafy greens, there should be no concerns related to water quality from the standpoint of human pathogens that might be associated with surface water,” said horticulture professor Sarah White at Clemson University. “There are pH issues associated with municipal water because most municipal water is neutral or alkaline to prevent the corrosion of pipes. Ornamental growers would likely need to acidify their water if the pH is above 7.5, especially for leafy greens and lettuces. These growers may already be acidifying their water for the ornamental plants they are producing.
“For new growers who are planning on using municipal water, they need to know what the water pH is. Because the pH is likely to shift during the year, growers need to be cognizant of the shifting pH and how injecting acid needs to be responsive to these changes. Some bedding plant crops may require more or less acid injection than lettuces and leafy greens.”
White said most municipal water sources are drawn from surface water reservoirs, which can cause some seasonal variation in water pH.
“Usually during the winter the water source quality is consistent,” she said. “If growers are producing during the winter and carrying production into spring there might be some changes in the water source that can affect the pH.
“Regardless of whether growers are producing lettuce and leafy greens in nutrient film technique (NFT) or deep water raft systems, they need to actively monitor pH year round. There are Bluetooth pH meters that can be stuck into a water source that will log pH. It’s easy to do. Growers should monitor and track their pH and know what they have to do to adjust it.”
White said for ornamental growers looking to add lettuces and leafy greens, it isn’t going to matter what type of acid is used to lower the water pH.
“Growers should be able to use the same acid for both ornamental and edible crops,” she said. “Usually they pick an acid based on the cost. If they are going to adjust the water pH they should inject fertilizers after the water pH has been adjusted.”
White said municipal water usually has a pH of 7.5 to 8. Most plants grow best at a pH of 6 to 6.5.
“Nutrient availability changes with different pH,” she said. “That is why the pH needs to be adjusted in order for the nutrients in the water to be available to the plants.”
White recommends if growers have never produced lettuces and leafy greens that they monitor the water pH more often.
“If growers don’t know how sensitive these new crops are to pH, they might try doing some trials with lettuces, leafy greens or herbs,” she said. “This will enable growers to determine the best pH for producing these new crops before they invest in filling a whole greenhouse.”
Adjusting water alkalinity
White said depending on where growing operations are located in the country, municipal water sources can have different alkalinities.
“In some western states and coastal regions of the United States, alkalinity issues are more likely,” she said. “In locations with higher alkalinity, more acid is required to get the water pH to the desired range for crop production. A lot of plants don’t do well with high alkalinity vs. low alkalinity. If the pH is being adjusted by injecting acid this coincidentally manages the alkalinity level as well. It typically requires more acid to accomplish the same pH change in water with higher alkalinity. Water that has high alkalinity will also have a high pH.
“If growers have a water source that is highly alkaline in a certain region of the country, chances are it won’t matter what source growers pull from because there are going to be alkalinity issues. The only thing they could do differently is if they capture rain water, filter it, and then blend it with their other water source.”
White said well water is the most common water source used by growers.
“We have done two surveys in the last 10 years and about 65 percent of all growers indicated they use well water,” she said. “The reason is because it is a clean water source and there are not usually any issues with plant diseases. The contaminants that most growers might encounter are salinity and iron. If growers have a lot of salts in their water, how it is managed becomes very critical. Many Southwestern growers deal with this issue.”
White said water with a high salt level can be caused by a mix of elements and it is regionally specific.
“Sodium, chloride, calcium and magnesium are the biggest contributors to high salinity water sources,” she said. “If growers are having high salt issues, it’s probably caused by chlorine or sodium. Growers can manage fertilizers to help balance the high salts.”
White said the other contaminant growers might find in well water is iron depending on the region of the country where they are located.
“There are typically problems with iron and iron-oxidizing bacteria associated with well water use. If there is iron in the water, growers should aerate it before they use it. Aerating the water oxidizes the iron so that it precipitates out. The aeration should be done before the water goes into the fertilizer tank and before growers start adding salts. Once fertilizers begin to be added it might be more difficult to remove the iron.”
White said the pH for well water is usually in a good range for growing plants. She said growers should still test the pH of their water.
“If growers are drawing from a salty water source, chances are they are going to have alkalinity and pH issues. If growers are using salty water sources on ornamental plants and decide to try growing lettuces, leafy greens and herbs, whether they can use that water and how it is being treated will depend on the type of ornamental plants being grown. Some ornamental plants tolerate salts more than others. Growers may not have to do much to bring the salts to an acceptable level for lettuces, leafy greens and herbs.”
White said most growers won’t put in a reverse osmosis system to remove high salts because of the high cost associated with the equipment and having to manage it along with the waste water it produces.
“Growers are more likely to manage high salts by blending water sources, by heavily irrigating the crops or by their choice of which plants to grow,” she said. “Growers may want to use a municipal water source to blend with well water so that salts are at a manageable level for the plants. I highly recommend that growers get water quality analyses done periodically. They should also have an inline monitoring system if they chemically treat the water so that they know the real-time pH and salinity (electric conductivity is a proxy) levels of their water.”
White said there really isn’t a need to filter municipal or well water unless growers are recirculating the water.
“A rapid sand filter, which is cheap and fast, will remove organic matter and other debris that might get into the water,” she said. “This filter might remove some disease organisms, but it’s not 100 percent. If growers are concerned with plant diseases, they are going to need to add a sanitizer like chlorine, ultraviolet light or ozone. A rapid sand filter is easy to pair with a chlorination system like Accu-Tab.
“Growers could also use a slow sand filter. This is a biologically-based system, but it just takes longer to filter the water. The slow sand filter removes both particulate and plant disease propagules. Depending on what a grower’s goal is, a slow sand filter would accomplish the same thing as a sanitizer.”
White said a lot of ornamental plant growers who use well water route it into an open containment pond.
“Most growers have a pond that they pump the well water into before irrigating their crops,” she said. “These growers might have an issue with using that water to irrigate edible crops. They would need to use a sanitizer, which would take care of plant pathogens as well as potential human pathogens such as Salmonella and E. coli. Those are the main pathogens growers would have to be worried about.”
For more: Sarah White, Clemson University, Plant and Environmental Sciences Department, (864) 656-7433; email@example.com; http://www.clemson.edu/cafls/faculty_staff/profiles/swhite4; http://cleanwater3.org.
David Kuack is a freelance technical writer in Fort Worth, Texas; firstname.lastname@example.org.