Relative Abundance and Properties of Nitrates and Phosphates

To determine the correct statement, let us analyze each of the given options:

1. Option (A): "Phosphates have no biological significance in humans"
   Phosphates play an essential and fundamental role in human biology:

   • They form the structural backbone of nucleic acids ($\text{DNA}$ and $\text{RNA}$).
   • They constitute the primary energy currency of cells in the form of adenosine triphosphate ($\text{ATP}$).
   • They are a major component of bones and teeth in the form of calcium phosphate (hydroxyapatite, $\text{Ca}_5(\text{PO}_4)_3\text{OH}$).
     Therefore, statement (A) is incorrect.

2. Option (D): "Oxidation of nitrates is possible in soil"
   In the nitrate ion ($\text{NO}_3^-$), the oxidation state of nitrogen is $+5$, which corresponds to its maximum possible oxidation state (since nitrogen has five valence electrons). Therefore, nitrogen in nitrate cannot be further oxidized. In soil, nitrates undergo reduction (denitrification) by microbial action.
   Therefore, statement (D) is incorrect.

3. Options (B) and (C): Relative abundance of nitrates and phosphates in the Earth's crust

   • As stated in the passage, nitrates are highly soluble in water. Due to this high solubility, nitrate deposits are easily leached and washed away by rainwater and surface runoff into the oceans, resulting in very sparse solid deposits in the Earth's crust (with rare exceptions like the Atacama Desert in Chile).
   • In contrast, phosphates primarily occur in the form of insoluble or sparingly soluble minerals of the apatite family (e.g., fluorapatite, $\text{Ca}_5(\text{PO}_4)_3\text{F}$) and are not readily washed away.
   • Consequently, nitrates are significantly less abundant than phosphates in the Earth's crust.

Thus, statement (C) is correct and statement (B) is incorrect.

Correct Option: C