Sand or dirt does not have the great variety of soil microbes that help plants grow. Soil is full of many life forms including the more familiar earthworms which help aerate soil with their tunneling abilities. Soil microbes make nitrogen more available for plant roots and decompose old vegetation into reusable nutrients.
Agricultural practices can enrich soil life or make it more deadly for soil microbes and earthworms. Erosion and loss of soil in wind storms or flooding situations also can risk soil biodiversity.
For images of some unfamiliar soil microbes and to read more about soil health see: "Take a Look at Some of the Billions of Organisms that Keep Our Soil Healthy," by Rachel Cernansky, Ensia.com.
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