Soil fertility can best be conserved by the activities of

Soil fertility can best be conserved by the activities of microbes. Microbes are tiny living organisms, including bacteria and fungi, that play an essential role in maintaining soil fertility. They break down organic matter in the soil, releasing nutrients that plants need to grow. Additionally, they help to create a healthy soil structure by producing substances that bind soil particles together, making the soil more resistant to erosion. Earthworms and rodents can also contribute to soil fertility by aerating the soil and mixing organic matter throughout it, but their impact is typically smaller compared to that of microbes. While human activities such as composting and crop rotation can help improve soil fertility, they are not considered as "activities of" in the context of the given options.