ADRIAN THOMAS / SCIENCE PHOTO LIBRARY ADRIAN THOMAS / SCIENCE PHOTO LIBRARY
Effect of soil acidity. The flower of a hydrangea (Hydrangea macrophylla) that has been grown in alkaline soil (pH more than 7). This is shown by the pink colour of the flower. In acidic soils the flowers are blue. The difference is due to the way aluminium ions in the soil react with the plant's pigments.
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