Final answer:
The function not served by humus and organic matter in making soil healthy is C)excessively retaining water; humus actually helps to properly retain and moderate water in the soil.
Step-by-step explanation:
The function on the list that is not a function that humus and other organic matter serve in making a soil healthy is c) Retaining water excessively. While humus does help to retain water in the soil, it does not retain water excessively. Instead, humus and organic matter help to improve soil structure, which can moderate water retention in a way that benefits plant growth. More specifically, humus and organic matter provide nutrients to plants, improve soil structure, support microbial life, which is crucial for nutrient cycling, and aid in rendering the soil more porous, thereby improving its water retention capabilities appropriately rather than excessively.
Therefore, the correct answer to the question is option c) Retaining water excessively.