Politics means the world of government in general:
The vice-president has a long career in politics.
I hate discussing politics with my family because it always turns into an argument.
Sometimes people talk about "office politics" outside the context of government -this means the connections and actions that people inside a company take in order to gain power, forming relationships and manipulating situations to their advantage.
A policy is like a rule. Countries have policies (foreign policy, immigration policy, gun control policy) but schools and companies do, too:
Our policy is that customers can return an item and get a refund within 30 days of their purchase.
My school had strict policies regarding lateness - if you were late more than five times, you got suspended.
The use of compost for planting, adds water holding organic matter to the soil. it feeds the soil micro organisms that help the soil and the plants stay healthy. It also reduces your yard and kitchen scraps that you would normally throw away.
I made a list of the Instructables that are how to build a compost pile