Continuous application of ammonium-based nitrogen fertilizers increases soil acidity, whic… — JAMB UTME

Continuous application of ammonium-based nitrogen fertilizers increases soil acidity, which mobilizes toxic heavy metal cations by displacing them from soil colloid exchange sites into the soil solution.

Answer: Answer

Answer

The statement is true. Biological nitrification of ammonium fertilizers generates hydrogen ions (

H^+

), lowering soil pH. The increased acidity causes

H^+

ions to displace bound heavy metal cations (

Cd^{2+}

Pb^{2+}

Al^{3+}

) from soil colloids into the soil solution, enhancing their mobility and bioavailability.

The statement accurately describes environmental soil chemistry. Soil nitrification of ammonium produces

H^+

ions, decreasing pH. The elevated

H^+

concentration displaces toxic heavy metal cations from negatively charged clay-humus exchange complexes, bringing them into the soil solution.

Step-by-Step Solution

Analyze the chemical effect of ammonium-based fertilizers on soil pH

Nitrifying bacteria oxidize

NH_4^+

NO_3^-

, releasing

H^+

ions according to the reaction:

NH_4^+ + 2O_2 \rightarrow NO_3^- + H_2O + 2H^+

. This increases soil acidity.

Establishing the source of increased soil acidity is essential for evaluating soil chemical dynamics.

Examine the interaction between hydrogen ions and heavy metal cations on soil colloids

Soil colloids carry negative surface charges that adsorb metal cations (

Ca^{2+}

Mg^{2+}

Cd^{2+}

Pb^{2+}

Al^{3+}

). High concentrations of

H^+

ions displace these bound cations into the soil solution through cation exchange.

Understanding ion-exchange equilibria on soil particles explains how acidity affects metal solubility.

Determine the environmental mobility and toxicity outcome

Displaced heavy metal cations enter the soil water solution, significantly increasing their mobility, leaching potential, and root uptake toxicity.

Confirms that increased soil acidity leads to heavy metal mobilization.

Key Concept

Soil Acidification and Heavy Metal Mobilization

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