Water has a pH of 7. Rainwater is slightly acidic and has a pH of about 5.5. This is because as the water vapor tumbles through the air, as clouds, it reacts with carbon dioxide to form carbonic acid. When the rain falls to earth and seeps into the ground, it can create caves like the one seen in the picture above. The slightly acidic rainwater slowly dissolves the limestone (calcium carbonate). This adds calcium to the water as well as carbonate and bicarbonate ions. This stabilizes and buffers the pH of the water. This buffering system is seen throughout nature. It helps stabilize pH in the soil, in the oceans, and even in the human body.
Limestone (calcium carbonate) can be difficult to dissolve in water. For this reason, we will use calcium hydroxide. I usually use Mrs. Wages Pickling Lime. Calcium hydroxide is just calcium carbonate that has been heated treated. When calcium carbonate (CaCO3) is heated, it released carbon dioxide (CO2) and becomes Calcium Oxide (CaO). Finally, a small amount of water (H20) is added to the calcium oxide creating calcium hydroxide (Ca(OH)2).
When calcium hydroxide is added to circulating water, it will react with the CO2 in the water and create carbonate and bicarbonate ions. Thats right, we have just come full circle. If the water is evaporated, we will be back to calcium carbonate (CaCo3)!!
Anybody with a saltwater aquarium is probably well acquainted with calcium hydroxide also known as kalkwasser. I like to use it in the fresh water planted aquarium which is kept at 3-4 dkh. .5 grams calcium hydroxide per 10 gallons is about 1 dkh. Add it slowly so you don’t cause a spike in the pH. This will add calcium and buffer the pH. This, in conjunction with a slightly acidic (~6.5) planting media will help create a balanced aquarium environment.
For container plants with an acidic media like peat or pine bark, you can also use pH adjusted rainwater. I like to add 20g calcium hydroxide per 100 gallons. This is going to cause your rainwater pH to shoot up very high. Next add a circulating pump with a shower nozzle and run until the pH returns to somewhere in the low 7’s. The more you pump and the more reaction the water has with the air, the faster this will happen. Once in the low 7’s it’s safe to water your plants with. This should reduce your need to lime the media. If the pH of the grow media starts to creep up too high, switch back to regular rainwater.
