The case of the curious pH drop

I teach one of my greenhouse crop production courses in the fall, and in that course, we focus on flowering potted plants, cut flowers and foliage plants. As part of the lab, we grow numerous potted plants, including poinsettias. In lab, we practice using Total Crop Management — a holistic approach to containerized crop production based on integrating data and data collection with critical evaluation (Fig. 1). It begins with regularly monitoring greenhouse environment factors such as the daily light integral, average daily temperature and difference between the day and night temperature; plant growth or when graphical tracking curves exist, plant height; substrate and irrigation water properties such as pH and electrical conductivity (EC); and pest populations. When using data on the greenhouse environment, plant growth, substrate, water and pests, decision-making can be much easier. You do not have to guess what the conditions are like in your greenhouse or in containers; rather, you know and make better-informed decisions.

At the beginning of the semester, the students were provided targets for final height, greenhouse conditions, water quality and pour-through substrate test ranges. For poinsettia, target pour-though values of a pH of 5.8 to 6.2 and an EC of 2.5 to 4.5 mS/cm are ideal. Poinsettias are considered a “general” crop with respect to pH requirements (not too low, not too high), but are considered a “heavy feeder” as reflected by the higher EC values.

Two weeks after planting our poinsettias, we performed our first pour-though substrate tests, and this is where the mystery began. A student returned from the greenhouse and seemed a little puzzled. The pH of their poinsettias was measuring between 4.8 and 4.9. Initially, I suggested that the hand-held pH and EC meter may not have been properly calibrated. So, they went out, recalibrated the meter, and got similar results. Other students were filtering back into lab and they too had low pH values. I thought that perhaps the calibration solution was not good, so we opened a new bottle of solution and recalibrated. Again, we got similar results. As a result, we tried a different hand-held meter, and then a different brand of calibration solution. The pH of the substrate was definitely low.

What was the cause of the low pH? We next investigated our diluted fertilizers, clear water, fertilizer stock solution and injector. The pH and EC of the diluted fertilizer solution we were applying seemed normal and the results did not indicate a problem. Working backwards from the hose-end, we tested our clear municipal water. Again, the values seemed within range. We don’t acidify our water, since the municipal water we use has an alkalinity of ~60 ppm — “plug quality” water. We next looked at our fertilizer stock solution and our fertilizer injector. We use a blend of 21-5-20, which reduces pH, and 15-5-15 fertilizers, which raises pH, to make a fertilizer solution that complements our water in managing substrate pH. We mixed up a brand-new stock solution and, sure enough, the injectors were indeed calibrated and injecting properly.

So, what in the world was causing the pH drop? I couldn’t think of anything in our facility that would be causing the problem. Our problems had to be caused by something else! I decided that the one thing that we had not measured yet: alkalinity. So, I got out my alkalinity test kit and tested the clear municipal water. Lo and behold, the alkalinity was not the 60s as it usually is; rather, it was in the low teens, about 75 percent lower than usual. Our greenhouse manager called the city to inquire about the low alkalinity. The city had recently changed their practices and, sure enough, they were dropping the alkalinity concentration lower than previous standards.

We finally knew what the culprit was: low alkalinity in our irrigation water. The 21-5-20 was likely causing too much of an acidic reaction with the new lower-alkalinity municipal water. There were a few steps we took to get our substrate pH back on track. First, to bring up the substrate pH quickly, we made flowable limestone applications. For longer-term pH management, we also changed our fertilizer to 100 percent 15-5-15 to take advantage of the pH-raising effect, completely omitting the acidic 21-5-20.

Why am I talking about poinsettias as the spring season is swinging into full-fledge production? The moral of this story has nothing to do with poinsettias. Rather, it underscores the value of regular monitoring and testing. Lord Kelvin, the mathematical physicist and engineer, is attributed with this quote that sums up the value in the regular monitoring: “To measure is to know.” Without performing any pour-through tests after planting, this problem would likely have gone undiagnosed for a while longer. When we discovered the low substrate pH, the shoots still looked fine and had no visual symptoms. However, had we not performed any pour-throughs, our first indication of a problem would likely be symptoms of micronutrient toxicity on the foliage; if the symptoms of a problem are visible, some damage would have already been done. This story also demonstrates the value of a systematic approach to problem solving (Fig. 2). The city changing its water treatment processes was not on our minds when we were first problem solving. However, after thoroughly investigating those in-house factors that we had control over, we felt it had to be something external or beyond our control and expanded the scope of our investigation.

Christopher is an assistant professor of horticulture in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu