Tannic acid can build up in garden soil over time as a result of the decomposition of leaves. While it can be helpful in getting rid of salts and increasing soil acidity, you can have too much of a good thing. In an organic garden, you can neutralize tannic acid with a natural alkaline soil amendment like dolomitic limestone or ground oyster shells.
If you don't know your soil's currently acidity, check it with a soil pH test kit. A soil pH of 5 to 6.5 is considered acidic, while 7 is neutral, and higher than 7 is alkaline.
Determine how much limestone you will need. See the Resources section for a handy lime calculator.
Dig up the area where you plan to apply limestone. You can either work around existing plants, taking care not to damage their base and roots, or temporarily move them while you work in the limestone.
Spread the limestone evenly throughout the area. Consult the directions on the bag for exactly how much to use; in sandy soils it is usually about 25 pounds per 1,000 square feet, while clay soils can take as much as 100 pounds of limestone per 1,000 square feet.
Mix in the limestone using a cultivator or rake and water the area thoroughly to help it disperse throughout the soil.
Test the soil's pH again in a week to be sure you have achieved the desired pH level. If not, another application of limestone may be in order.