I have two favorite afterschool snacks: carrots dipped in ranch dressing and apple slices dipped in peanut butter. Both are delicious and healthy and both grow from plants in the ground. Carrot plants and apple trees both need to be rooted in healthy soil in order to grow healthy food.
Unfortunately, some of the Earth's soil is not healthy. It is contaminated. Some soil has been polluted by waste or acid rain. Some has also been adversely affected by landfill spillovers. Chemicals such as fertilizer and pesticides can contaminate it as well. All of these things lessen the quality of the soil.