Final answer:
The correct statement about soils is that the parent rock from which the soil is derived is an important source of nutrients for the soil. Other statements are inaccurate with regards to the organic content at different soil depths, the development of soil on slopes, and the impact of plants and animals on soil fertility.
Step-by-step explanation:
The question is asking about various attributes of soils. Only one of the given statements is true. To clarify the true statement, we need to look at the characteristics of different soil horizons as well as factors influencing soil quality and formation.
Analysis of the Statements
- The organic portion of soil is not referred to as regolith; regolith consists of loose, inorganic material on the Earth's surface.
- The highest soil horizons, such as the O horizon and A horizon, typically contain more organic matter due to plant and animal activity, contrary to the claim about lower horizons having greater contents of organic material.
- Steep slopes generally lead to soil erosion, resulting in thinner soils, not the thickest as the statement suggests.
- Contrary to belief, the presence of plants and animals enhances soil fertility by contributing organic material, facilitating nutrient cycling, and improving soil structure.
- The parent rock, or the C horizon, is indeed an important source of nutrients for the soil, making this statement true.
To summarize, among the given statements, the correct assertion is that the parent rock from which the soil is derived is an important source of nutrients for the soil.