Deficiency of Calcium directly affects the structural integrity of hard tissues like bones and teeth because calcium is the primary mineral responsible for their hardness and strength. Calcium combines with phosphate to form hydroxyapatite crystals, which provide rigidity and resistance to mechanical stress. A lack of calcium leads to weakened bone matrix, increasing the risk of fractures and dental problems such as tooth decay and periodontal disease. Other minerals like iron, potassium, and magnesium play different physiological roles but are not the main components of bone and tooth hardness. Thus, calcium deficiency is most critical for weakening hard tissues.

Reference: Gray's Anatomy, Henry Gray, 42nd Edition.