Biogeochemical Cycles

The natural reservoir of phosphorus is Rocks.

(A) Rocks - Correct: Phosphorus exists primarily in phosphate minerals (apatite) in Earth's crust.

(B) Soil solution - Incorrect: While present here, it's not the primary reservoir.

(C) Detritus - Incorrect: Organic matter contains phosphorus but isn't the original source.

(D) Atmosphere - Incorrect: Unlike nitrogen, phosphorus has no significant atmospheric pool.

The correct answer is (A) Rocks, as phosphorus cycles slowly through weathering of phosphate rocks.