CBSE Class 7 Answered

Roots generally make the soil near them more acid, thereby rendering nutrients available. The theory of contact exchange proposed that a root absorbs cations from soil colloids by exchanging them at its surface for H⁺ provided by the root. Roots release of H⁺ to maintain cation-anion balance within the plant. Thus, when a blue litmus paper is used to test the environment around the roots, it will turn red or pink when it comes into contact with an acidic solution below pH 4.5.