When something soils, it gets dirty or stained. Soils are also the stuff that plants grow in.
If something soils, it is usually not good because it means it gets dirty. Soils that plants grow in are good because they help plants grow.
Examples:
She soils her dress by playing in the mud.
The rain soils the clean sidewalk.
The dog soils the carpet when it has an accident.
He soils his shirt with spaghetti sauce.
The soil in the garden is good for growing plants.
The soil in the pot helps the flower grow.