Do you have any suggestion on what acid to use to lower pH so that we can use ethephon?

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 and

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).

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