Phosphorus is a critical nutrient in ecosystems but lacks a gaseous phase.
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.
The natural reservoir of phosphorus is primarily found in rocks and minerals. Phosphorus exists in nature as phosphate salts, often found in apatite rocks. Soil solution, detritus, and atmosphere are not the main reservoirs of phosphorus; phosphorus is not readily available in the atmosphere in its usable form.
Option (A) is correct because phosphorus is primarily stored in rocks.