Testing on Tobacco Greenhouse Water Quality

The quality of source water for irrigation is crucial for any greenhouse operation.
This is especially true for floatbed greenhouses, where Styrofoam trays filled with tobacco transplants float in bays of water. Well water is typically used to fill the bays, and its pH and mineral levels can vary somewhat from year to year, with the potential to significantly impact plant growth.

Vigilant growers check their well water annually by sending samples to the Agronomic Services Division of the North Carolina Department of Agriculture and Consumer Services, which assesses the suitability of water for plant production through a laboratory test known as solution analysis. This test includes measurements of pH; electrical conductivity; total alkalinity (a measure of carbonate and bicarbonate ions); hardness; sodium adsorption ratio; and levels of nutrients, sodium and chloride.

The solution analysis report often points out potential problems and provides management recommendations. Regional agronomists provide further assistance by making site visits, explaining report findings and offering advice.

Solution analysis is a useful monitoring tool because water quality may change in response to environmental conditions such as drought or heavy rain.

Wells close to each other can also have vastly different characteristics depending on their depth. For these reasons, NCDA&CS recommends that growers have their well water tested at least once a year.

Tommy Castelow of Castelow Farms in Hertford County routinely uses solution analysis and other agronomic tests to help prevent production problems.

Castelow is a veteran tobacco grower who is in the process of expanding his floatbed operation. To supply sufficient water for that effort, he recently invested in drilling a new, deeper well to supplement several shallow wells.

“I had hoped the new well would provide better output, but the volume was about the same,” Castelow said. “Unfortunately, the quality was not. When I had the water tested, my report indicated high sodium levels and suggested that I contact regional agronomist Wayne Nixon for management advice. The transplant seedlings were not growing a good stem. They were real wiry.”
Nixon made a site visit to assess the situation.

“Tommy is one of my best tobacco growers,” Nixon said. “He was concerned about high levels of sodium and bicarbonates in the water and how he could overcome that. The deeper the well went, the worse the water quality was. I had never seen sodium levels so high. Excess sodium leads to spindly, yellow transplants.”

Excess sodium is a water problem that tends to occur in eastern counties of North Carolina. Two values on a solution analysis report relate to sodium: one is Na, which is a measure of sodium in parts per million; the other is SAR, which stands for sodium adsorption ratio.

Na values of 70 or greater are higher than recommended.

SAR values greater than 4.0 indicate that sodium uptake may be interfering with uptake of calcium and magnesium. These two nutrients are required for optimal plant growth; sodium is not.

Management usually involves application of soluble materials high in calcium because plants will preferentially take up calcium over sodium. Gypsum, also know as calcium sulfate, is usually used in these cases. It is not very water soluble, however, which poses a challenge in floatbed situations.

In working to solve Castelow’s problem, Nixon decided to try using plaster from a home improvement store instead of gypsum. It is essentially the same compound, but goes into solution more readily because particles are much finer.

“Adding plaster to the floatbed water was a phenomenal success,” Nixon said. “We saw right off with the first solution sample that it had made a big difference. We achieved at least a 50 percent reduction in SAR values. We also found it is much easier to get plaster into solution if it is spread out in the bays before water is added.”

The other water problem Castelow wanted to address was high total alkalinity, indicated by the value of 385 ppm on his solution report. The higher the total alkalinity value, the more difficult it is to lower pH.

If pH remains high, then micronutrients become less available to plants. The remedy is to add battery acid (35 percent sulfuric acid) to the floatbed water as recommended on the report.

“Many growers think they are adding battery acid to lower pH. That is true, but only in an indirect way,” Nixon said. “Adding battery acid lowers bicarbonate levels, which then lowers pH. That is what is going on.”

“Adding plaster and battery acid has helped make my water more suitable for plant production,” Castelow said. “Nixon’s advice has been a real asset. I called him in mid-stream to work on this problem last year, but this year he’s been with me from the beginning. We’ve seen improvement in the water and in the plants, too.”

Nixon and Castelow are continuing to look for better solutions to this well water problem. Adding plaster can be messy, and battery acid tends to be expensive, driving up costs. The two are intrigued by endorsements they have heard regarding reverse osmosis filtration systems and plan to investigate this option.

“Regardless of how we decide to fix the problem, we will continue to collect solution samples regularly so we know where we stand with water quality,” Nixon said.

“In dealing with this problem, Tommy has realized the need to test more often, and in doing so, he has been able to significantly fine-tune and improve his fertilizer applications. Overall, it has been a beneficial learning experience.”