What is the function of humus in soil?

• A. To replace sulfur
• B. To aid in compaction
• C. To decrease water retention
• D. To increase water retention

Answer: (D)

Humus, the decomposed organic matter in soil, improves the soil's ability to store water. Its structure forms stable aggregates and creates a high surface area that holds onto water and releases it slowly as plants need it. This increases the soil’s water-holding capacity, helping roots access moisture between irrigation events and during dry spells. The organic matter also enhances soil porosity and resilience against erosion, contributing to a more moisture-retentive environment overall. Replacing sulfur isn’t a function of humus, and humus actually helps prevent compaction by improving soil structure rather than aiding it; it does not decrease water retention—the opposite is true, as it increases retention.