what is required of soil to be considered good agricultural soil?

Answer 1

Water, oxygen (air), nitrates, and different minerals depending on what you want to grow. Density is important so that water does not build up or drain off too quickly.

Also, living organisms in the soil help break down organic compounds, making them usable by plants.

Too much wind, rain (leeching) or erosion will negatively affect soil growing quality.

Answer 2

Decomposed organic matter (compost) which then allows beneficial bacterias and fungi to grow within it. Google the soil association website.