The pH factor

Capturing runoff water in containment ponds and using it for irrigation has become a common practice, but few growers know their recycled water quality dynamics and its profound impacts on crop production. The range and variations of pH in captured water has implications on several production practices.

Chlorination is a commonly-used water decontamination technique in nursery production and its performance depends largely upon water pH. The optimal pH for chlorination is between 5.0 and 5.5. Increasing pH dramatically reduces chlorine killing power, by 4 percent at pH 6.0 and 94 percent at pH 10.0.

When water is used to prepare a spray solution, water pH affects the life of pesticides due to alkaline hydrolysis. The more alkaline the water is, the more rapidly many pesticides break down. For example, the half-life of Captan drops from 8 hours to 2 minutes when pH increases from 5.0 to 9.0.

Water pH also affects the solubility of nutrients. Low pH may increase the availability of some nutrients, causing phytotoxicity, while high pH limits the solubility of other elements, leading to nutrient deficiency. According to current BMPs, the recommended pH for most ornamental crops is between 6.0 and 7.0. Some plants grow best at pH between 4.5 and 6.0.

Recycled water pH range

In our tests, water pH in recycling irrigation ponds differed greatly from that in natural streams and wells.

According to our 24/7 monitoring in seven ponds across Virginia from 2010 to 2015, water pH fluctuated from 4.5 to 12.8 with more than half of the time in the alkaline range. This huge range and dramatic fluctuation resulted from intense photosynthesis, respiration and decomposition of algae and aquatic plants in water. They have great impact on the performance of all crop production practices that involved in use of these pond waters.

Seasonal and diurnal water pH patterns

Water pH in recycling irrigation ponds changed seasonally. Surface water pH changed from acidic to neutral condition during winter and spring, then to mostly basic conditions during summer and fall.

Water pH in these ponds also varied diurnally with the lowest in early morning and the highest in late afternoon.

Water pH variations within a pond

Water pH also changed with depth and location within the same ponds. Water pH was always higher at the surface than lower depths. This difference could be as great as 4.2 units as observed in one pond with an average depth of 3.8 m. Location wise, surface water pH was higher in the middle than at the entrance and outlet. Such location differences were mostly within 1.0 pH unit.

Water pH in multi-pond recycling system

Water pH dropped every step in multi-pond water recycling systems with a stepwise water flow. The proportion of alkaline pH readings was 68 percent in the top pond. It declined to 33 percent in the middle pond and dropped to 16 percent in the bottom pond in one of the three-pond recycling systems monitored.

Management implications

Recycled water pH is mostly in alkaline range and fluctuates dramatically, which are not good for the performance of chlorine and some pesticides. It also affects the availability of individual nutrient elements. Lowering water pH is the key to improving crop health. Specifically, we highly recommend the following:

• If land and funds are available, design and implement a multi-pond recycling system with runoff water from all production areas being captured in the top pond and water pumped out from the bottom pond for irrigation.
• Place the pump inlet at a deeper depth of water column, rather than close to the surface.
• Irrigate plants in early morning when water pH at its lowest of the day. Avoid pumping water in late afternoon when water pH is at its highest; otherwise, more acid is required to adjust water pH to the desired levels.

Test water pH before applications, especially during summer and fall when recycled water pH fluctuates more frequently and at a greater range.

Haibo Zhang was a postdoc and Chuan Hong is a professor at the Virginia Tech’s Hampton Roads Agricultural Research and Extension Center in Virginia Beach, VA, chhong2@vt.edu