Diagnosing Yellowing Arms on an Argentine Giant Cactus

A large, 5-foot-wide Argentine Giant (Echinopsis candicans) with multiple arms is yellowing. No mushiness or black spots reported. Possible causes: natural aging of older arms (energy directed to new growth), nutrient deficiency (apply balanced 10-10-10 or cactus-specific 1-2-2 fertilizer), depleted soil (repot even large specimens every few years), or overwatering (outdoor plants need water only when soil is bone-dry; rain often suffices).

A photo would help narrow the diagnosis. In the meantime, assess watering practices, check soil condition, and consider fertilizing if it's been years since the last repot. Argentine Giants are tough but respond poorly to soggy conditions.