Webinar #13 - Water pH management for improved horticultural productivity and profit margin (October 2014)

By: Chuan Hong

Webinar Recording

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►Hello, everyone, welcome to the webinar! I'm Chuan Hong, a professor of plant pathology at Virginia Tech. I will be leading today’s webinar on Water pH Management for Improved Horticultural Productivity and Profit Margin.

►This is the thirteenth webinar in a series, organized by the multi-institutional research team on the Specialty Crop Research Initiative Project: Integrated Management of Zoosporic Pathogens and Irrigation Water Quality for a Sustainable Green Industry. This project is sponsored by the USDA National Institute of Food and Agriculture. We greatly appreciate the AmericanHort, the Society of American Florists, the advisory panel and many farmers for their continuing support.

►Here is an outline of what we will cover today

►The first question is why water pH is so important?

►During our last webinar, we discussed a number of water quality parameters and their potential impacts. Today, we will focus on water pH, a major concern for many horticultural farmers. Specifically, we will discuss:

►Here is the first example of pH impact on pathogen survival in an aquatic environment with Phytophthora megasperma.  In this study, Dr. Kong evaluated a range of pH from 3 to 11 for a few seconds (or zero hour) to 7 days. Zoospores of P. megasperma persisted in most pH levels at high rates.  Among a dozen of Phytophthora specie we have evaluated, P. megasperma is most adapted to aquatic environments.  These data help us understand the abundance of P. megasperma in runoff containment ponds across locations over time. P. megasperma attacks some woody ornamental plants like Junipers.

►The second example I like to show you is Phytophthora nicotianaeP. nicotianae is a destructive pathogen of numerous herbaceous annuals and perennials as well as some shrubs and tree species such as boxwood.  It also is a major pathogen of many other horticultural crops including vegetables, citrus, tree fruit, and also tobacco.
As shown in this chart,

This discovery means that this pathogen is not as adapted to aquatic environment as P. megasperma and this is great news for Farmers.

►The third example is Phytophthora ramorum, the sudden oak death pathogen that has generated numerous headline news over the past 10 years. According to the latest nationwide survey data, this pathogen has been detected in waterways outside of nurseries that have received infected plant materials such as camellia, etc. in six southern states (AL, FL, GA, MS, NC, TX) as well as New York.

As shown in this graph, this pathogen is much more adapted to aquatic environment than P. nicotianae, but not quite like P. megasperma.  It persisted in a wide range of pH levels from 5 to 9 at high rates.  In this study also by Dr. Kong, the experiment was extended to 14 days, and as you can see, there is clear tendency of pathogen recovery increase from day 7 to 14. This means the pathogen formed the second generation.  

This pathogen poses a great threat not only to the nursery crops but also to surrounding forests.

► Now I am going to switch gears and look at the water pH impact on chemical performance. 

Chlorination is by far the most widely used water decontamination technology in the green industry.  So let’s look at how water pH may affect the chlorine performance or water treatment efficacy.

There are three species of chlorine: all chlorine molecules in water are called total chlorine. The amount of chlorine that have reacted with different organic and inorganics thus tied up is called combined chlorine. The leftover (or those NOT tied up) is called residual chlorine or free chlorine. It is Free chlorine that is capable of killing pathogens. So, free chlorine is of the most interest and importance to farmers.

It is very important to note that FREE chlorine could be in three MAJOR forms in water

These three forms of free chlorine have very different pathogen-killing power.  It was estimated that hypochlorous acid is approximately 20 times as powerful as hypochlorite ion. What is really tricky here is these three forms of chlorine CO-exist in equilibrium; and their dynamics depends largely upon water pH as illustrated in the chart.

That is why it is very important to make sure water pH between 5.0 and 6.5 to get the most out of chlorine dollars.

►This table presents a few scenarios of chlorine performance at different pH levels, assuming that hypochlorous acid is 20 times as efficacious as hypochlorite ion.

From this table you can see how quickly your chlorine dollars could be watered down with increasing pH. So, it is very important to know water pH when using chlorine to treat irrigation water.

► Another very important group of chemicals that are also sensitive to water pH are pesticides. I reviewed the MSDS of 130 fungicides, 124 insecticides, 67 herbicides, and 19 PGR for optimum pH range, then summarized here by their upper limit. What this means is that if a pesticide has an optimum pH range from 5 to 7.2, it is counted in this green part. As you can see, >50% of pesticides including PGR with upper pH is below 7. For these pesticides, if your water pH is above 7, your dollar is diluted.

►Here is an example with Captan which has an optimum pH range from 1 to 5. At this pH range up to 5, its half-life time is about 600 min or 10 hours. It declines quickly, 3 hours at pH 7 and 1 min at pH 9. This process is called hydrolysis.  So if someone uses water with pH at 9, s/he will make none or very little out of this pesticide.

►Water pH may affect nutrient availability too!

The illustration was downloaded from IGrow website of South Dakota State University.  The solubility of individual nutrient elements depends upon pH. According to the Best Management Practices - Guides for producing container grown plants, the recommended water pH is 6.5 to 7.0.  As water pH increases, iron, manganese, boron, copper and zinc become less and less available, potentially causing of these micronutrient deficiency.

►On the other hand, pH may affect the plant response to salty water as shown in these Yellow Climax marigold plants. The X-axis here is electrical conductivity in a range of from 2000 to 10000 mS/cm.

►These are just a few examples of how water pH may affect crop health and production as well as the performance of agricultural chemicals and the return of your agricultural investment dollars.

►The second question we like to cover today is how may water pH differ among water sources? We will look at found ponds at two nurseries.  Let’s look at their water recycling system setup first.

►This is a Google map of an ornamental nursery in eastern VA, we call it “VA1”. At this production facility, most runoff water is channeled into this sedimentation pond, VA11. When VA11 is full, water overflows to the next pond, VA12. Similarly, when VA12 is full, it overflows to a swamp or to VA13 as needed. There is an adjacent creek (VA10) that does not receive any runoff water from this nursery.  Water is pumped out from VA12 and to lesser amount from VA13 for irrigation.

►Here is another nursery in central Virginia; we call it “VA2”.  Like in VA1, this nursery channels all agricultural runoff into this top pond, VA21. When VA21 is full, water overflows to VA22. Similarly, when VA22 is full, water overflows to VA23. Water is pumped out from VA23 for irrigation.

►This computer screen shot shows water pH readings over a 6-month period from April to October 2011.  In this chart, four different colors represent four different ponds.

As you can see, water pH in the clean water source at VA1, or the creek (VA10) that did not receive any runoff water from the nursery, coded here in Dark Blue, remained between 5.4 and 6 while the dirty pond VA12, coded in Red, fluctuated dramatically.  Similarly, at VA2, water pH fluctuated much more frequently and at greater degree in the dirty pond, VA21, than the cleaner pond, VA23.

This snapshot highlights the potential difference in water pH among water sources and the most dramatic fluctuation occurred in ponds that receive agricultural runoff directly or indirectly.

►The third question is How may water pH change within a runoff containment pond?  We will examine this from three different angles.

►First, let’s look at how water pH may change with location.  Water pH level generally is lower at the runoff entrance than in middle.

►Second, how water pH may change with depth in a water column?  This chart shows the pH difference between surface water and the water in bottom in each of three ponds at VA1 with the X-axis showing the dates when data were taken.  If water pH readings were same for both surface and bottom, then the dots fall on this dash line.  All the dots above this line mean at those dates, surface water pH were higher than bottom water pH.  As you can see here, most dots are above this dash line, thus, pH were higher at surface than bottom water for most times when data were taken in all three ponds at VA1.

►A similar trend was seen in two ponds at two nurseries in Maryland.

►Some of you may have seen this chart before showing the diurnal water pH fluctuation.  We looked at data from all 9 ponds in Virginia, 2 in Maryland and one in Mississippi, and found the same trend like this.  Water pH bottoms in the early morning, increases sharply from 9 to 12, then peaks in late afternoon.  What shown in chart are some of the extremes. Water pH difference within 24 hours could be as great as 3.5 units!

►You might wonder what drives water pH fluctuation and variations in runoff containment pond and that is what we will discuss next.

►Here is a summary of some data from nine ponds in VA and MD in the form of percentage of hourly or quarter hourly water pH readings under each of the three categories: acidic (<7.0), slightly basic (7.0-9.0) and highly basic (>9.0).

As you can see, seven of the nine ponds monitored had at least 61% of the pH readings at 7.0 or above.

In fact, water pH in one pond NEVER went below 7.0.

►There are three major processes that drive water pH change diurnally and seasonally.  As shown in the background photo here, there are A LOT of cyanobacteria and green algae as well as numerous bacteria of different kinds including natural cleaners in ponds!  These microbes are the primary drivers of water pH in this system! 

First, cyanobacteria and green algae use carbon dioxide and water to make sugar with sunlight.  This process is termed as photosynthesis.  Carbon dioxide is a weak acid. As the sun rises each day, photosynthesis intensifies, so more and more carbon dioxide is being removed from water, resulting in less and less carbon dioxide in water and driving up water pH in a pond.  Photosynthesis converts sun energy into biochemical energy.

Second, there is a constant process called cellular respiration taking place in the cells of all organisms to convert biochemical energy from nutrients to into adenosine triphosphate (ATP), and then release carbon dioxide that drives down water pH in the system. 

►There is another process that we found out recently was thermal stratification.  In a stratified pond, warm and light water stays on the top while cooler and heavy water sinks to the bottom, preventing water mixing in a water column.  Dr. Zhang is writing this data for a publication.  We believe this process has contributed to the vertical distribution of water pH at different depths in runoff containment ponds.

►The last and most important question of the day is how to manage water pH for improved horticultural productivity and profit margin.  Based on what we have learned this far about water quality in different water sources in particular of runoff containment ponds, we will recommend several long-term, sustainable solutions to help growers manage water pH for max profit margin.

►The first recommendation is to make an informed decision in selecting water source for specific purpose.  Using nursery VA1 as an example, water pH in one pond, VA12, was above 7 for most time of the year while it was consistently in the range of 5.3 to 6.0 in the VA10. All this farmer has to do to achieve best performance his pesticides is to use water from a right source.  If a pesticide performs better at acidic pH, use the water from VA10, otherwise use the water from VA12. 

►Here is data from another nursery VA2. There are three ponds that are arranged in chain with a stepwise water flow All runoff water is captured in VA21. When this pond is full, water overflows to VA22, then VA23. Water is pumped out form this VA23 for irrigation.  As shown in a previous slide, this nursery pump water out of the VA23 for irrigation where water pH is least fluctuating and mostly in the acidic range for most time of the year.  That is the best set up for water recycling system at that nursery.

Think about whether you have the land to do the same at your production facilities, if so, I can assure you that this will be your best investment of your capital!

►Again, this chart shows a typical diurnal water pH fluctuation pattern.  What time of the day does water pH read the lowest and highest?  What is the range of fluctuation from the lowest to the highest?  Think about how much more you would make out of your chlorine and pesticide dollars if irrigation and pesticide applications are programmed to do in the morning than afternoon?

►Can the above three tips take care of all the water quality issues, probably NOT. So it is very important to check your water pH regularly and acidify your water as need before chlorination and pesticide applications.

►Take-home messages:

►Take-home messages (cont’d)

► Before concluding the presentation, I like to thank my associates in particular of Patti, Haibo, and Ping for running most of the water quality monitoring stations, analyzing some of water quality data, investigation water quality impact on pathogen survival. I also like to thank Dr. John Lea-Cox and his team at University of Maryland, Dr. Warren Copes at USDA ARS facility in Mississippi, Dr. Gary Moorman at the Penn State for their contributions to water quality data collection, analysis and interpretation. I can’t thank enough all the collaborating farmers for their continuing support to this project. I also like thank you ALL for your participation and attention!

►Our next and last webinar in this series will be on November 4. Dr. Jim Pease will be the presenter on economics of water conservation practices in horticultural production.  I look forward to seeing you again next month.