Certified Crop Advisor Practice Exam

What is a primary component of soil organic matter?

1. Sand

2. Clay

3. Humic Substances

4. Mineral Aggregate

The correct answer is: Humic Substances

Humic substances are a primary component of soil organic matter, playing a critical role in the structure and function of soil ecosystems. These compounds are formed from the decomposition of plant and animal materials and contribute to several key soil properties. One of the most significant roles of humic substances is their ability to improve soil structure. They help to bind soil particles together, which enhances aeration and water infiltration, leading to better conditions for root growth and microbial activity. They also act as a reservoir of nutrients, as they can hold onto essential elements like nitrogen, phosphorus, and sulfur, making them more available to plants over time. Additionally, humic substances have a high cation exchange capacity, meaning they can hold positively charged ions (cations) that are essential for plant nutrition. This characteristic enables the soil to retain nutrients that might otherwise be leached away. In contrast, sand and clay are mineral components of the soil and do not contribute to organic matter in the same way; rather, they affect the physical and chemical properties of the soil. Mineral aggregates refer to the clumping of soil particles which can be influenced by organic matter but are not themselves a component of it. Therefore, humic substances stand out as the primary component of soil organic matter due to