Citric acid is an acidic chemical found in many fruits such as lemons, oranges and certain berries. It affects a variety of organisms, including humans, animals and plants. Its naturally corrosive properties make it detrimental to various plants in large doses.
A chemical's pH is the measure of its acidity or alkalinity and runs on a scale from 0 (acidic) to 10 (alkaline). Citric acid is below 4.0, meaning that it is acidic and potentially corrosive. The optimal pH for plant growth is between 5.5 and 7.5, so some plants have adapted to survive in different levels. Therefore, applying enough citric acid to a plant's water or soil will create an environment unsuitable for the plant.
Citric acid is a corrosive agent and will therefore burn through seeds, which may ultimately prevent them from germinating. Adding too much acid to the soil will create acidic soil, which will acidify any water added to the soil. The acid may burn the plant's root system when it is absorbed, leading to inefficient nutrient absorption and eventual death.
Not all effects of citric acid are negative, and it may even be beneficial in low doses. A study by the U.S. Department of Agriculture designed to create a repellent against certain frogs in various Hawaiian plants found that a 16 percent solution of citric acid applied directly to plants did not severely affect them. There were some cases of discoloration, but the plants were otherwise still healthy even after the acid was applied, indicating that citric acid could be used as a repellent.
The Krebs Cycle
The Krebs Cycle, or Citric Acid Cycle, is used by plants to convert various citric acids into phosphates, which serve as a source of energy for the cell. The cycle relies on a delicate supply of citric acid, which it converts to citrate. Too much citric acid in the water of a plant may interrupt this cycle or lead to excess phosphates.