If the issue is dissolved salts coming from the irrigation water source (i.e., not primarily from the fertilizer) there are not a lot of options in flood floor systems because no leaching occurs with subirrigation. If the irrigation water electrical conductivity (EC) without fertilizer added is much above 0.7 mS/cm, using flood irrigation alone is difficult on long-term crops because ions accumulate in the root substrate.
The first priority with irrigation water that has a high EC is to try to purify the water (for example, reverse osmosis or blend with rain water). A periodic top watering with booms or hose can help to leach out salts and rebalance the nutrients in the root substrate. However, do not recycle the resulting runoff to the same crop because it is likely to have high levels of ions, such as sodium or chloride.
If the high EC is coming from over-application of fertilizer, and the EC of the irrigation water itself is low (0.7 mS/cm or below), there are more options. There is plenty of good research and grower experience that plants grown with flood irrigation can thrive with lower fertilizer rates compared with top watering because there is no leaching with subirrigation. For example, http://journal.ashspublications.org/content/118/6/771.full.pdf.
Ethephon is a plant growth regulator used widely to improve branching and control flowering in horticultural crops. The pH of the spray tank is definitely important for ethephon effectiveness, as well as other pesticides. See http://edis.ifas.ufl.edu/pi193 and https://ag.umass.edu/fact-sheets/effects-of-ph-on-pesticides-growth-regulators.
In the US there is a number of commercially available adjuvants that are marketed as buffering agents, and can be used to adjust spray tank pH. Many of these products include a pH indicator dye.
Some growers adjust pH using a mineral acid such as 35% sulfuric acid (battery acid). However, these acids are very caustic and require appropriate gloves, safety glasses, and other protective clothing. Always add acid to the pesticide mix, not vice versa. Depending on the alkalinity of your spray tank, you may only need a very small amount of acid to drop tank pH. Add a little acid, mix, and then measure pH. You can measure pH with a pH meter (ideally around pH 4.0 to 4.5 for ethephon, but this varies for different pesticides).
Most plants grow best at a pH of 5.4 to 6.4 in container substrates. The irrigation water quality (especially pH and alkalinity) is one of the factors that drive substrate-pH up or down. You can find out more at the University of Guelph website.
Nutrient availability (micro- and macro-nutrients) to plants is best when the pH of your water source is between 5.5 and 6.5. Though higher pH values generally don’t harm plant growth (exception – acid-loving crops), availability of micronutrients may decline, along with increased potential for problems related to salinity.
First have your water tested – irrigation water with over 150 ppm CaCO3 alkalinity often requires injection of an acid to avoid high substrate-pH problems in your crop. Select the acid used (for example, sulfuric, nitric, phosphoric, or citric acid), being aware the acids differ in the nutrients added and safety issues in handling concentrated acid. To select the appropriate acid concentration for your water quality use the AlkCalc online tool from the University of New Hampshire.
Don't add acid into the pond. Instead, add acid into the irrigation water with an injector at the time when the solution is pumped out of the pond onto crops. This approach is more likely to control pH than adding acid into the pond when pH will naturally fluctuate.
Don't add chlorine into the pond. Chlorinating the pond is also unlikely to reduce algae in the presence of high nutrient levels, sunlight, and high pH. Injecting chlorine after the water is pumped from the pond and irrigation water is also acidified is most likely to be effective.
Don't be too concerned about the high pH. The pond's pH will increase during the day because algae remove carbon dioxide from the water during photosynthesis in the presence of sunlight. Carbon dioxide (CO2) is acidic in water. Algae and microbes tend to drop pH in ponds during the night as they respire and add CO2 back into the water.
Send a pond water sample to a lab for nutrient testing. High pH in the pond can cause micronutrients to precipitate out (become insoluble). As algae and microbes grow in the nutrient solution, the nutrient balance may change - so it will be useful to know what nutrients are actually dissolved in the water. That can help you decide if any extra nutrients should be injected back into the solution when reusing this water.
A diagnostic test of the pond water can also measure alkalinity (think of this as dissolved limestone). High water alkalinity causes more issues than high water pH if the pond water is used for irrigation. Highly alkaline water has the greatest effect on increasing substrate-pH of irrigated crops over time.
If practical, you could cover the pond to reduce algae growth and pH fluctuations.
The pH of a pond containing algae will increase during the day because algae remove carbon dioxide from the water during photosynthesis in the presence of sunlight. Carbon dioxide (CO2) is acidic in water. Algae and microbes tend to drop pH in the pond during the night as they algae respire and add CO2 back into the water.
It depends on a variety of factors: how you irrigate your crops, the crop mix grown (as some crops are more sensitive than others), and the relative ratios of positively charged minerals (cations) like calcium, magnesium,and sodium in solution.
Reclaimed wastewater is treated wastewater from a wastewater treatment plant - it's safe to release into the environment, but not safe to drink.
It depends. Sometimes reclaimed water can contain relatively high levels of salts. If irrigation is managed properly, for example with adequate leaching during irrigation, moderate levels of salts (salinity) will not reduce crop growth. If you are considering use of reclaimed water - contact your extension specialist or local agent for help.