Vinegar helps neutralize and balance higher Ph. levels it also slowly releases carbon already in mixes/soils. One cap full apx. 3/4 of one ounce of vinegar to every gallon of water let stand over night or even longer if you want to have a more neutral water on hand ahead of time.

Can you collect rain water?

Are you talking about ground soil, or container soil? What makes you think it needs acidification?

BTW nomen, vinegar's acidifying effect is only temporary. It ionizes into H+ (the acid part), and acetate. But bacteria will eventually metabolize the acetate into hydroxide (OH-), which neutralizes the H+ previously added. I don't know how long it might take bacteria to metabolize the acetate, but the longer you wait, the sooner pH will rise again, so I would use water + vinegar immediately.

-Chris

Use rainwater or add some hydrogen peroxide to your water every few waterings. Low concentration peroxide is very good for root systems and helps combat anoxic conditions, pests, salt buildup, and fungal attack. However, plants such as Ariocarpus require nearly neutral or alkaline water instead of acidic, so you need to research your plants to determine which ones appreciate higher or lower pHs.

You can't simply use someone else's dilution rate. You have to measure the pH of your tap water first. Get a pH test kit and then start adding vinegar to a fill watering can until you get the pH into the 5-6 range.