Difference Between Compost and Top Soil

This article first appeared on Pat's Garden Forum Question from Karen:

What is the difference between compost and top soil and do you need both when amending the soil in your yard?

Answer from Pat:

When you purchase something called “top soil” this should be the mainly composed of the stuff that is the top layer of earth that is made up of pulverized rock, microbes, water, air and humus and is found on top of the ground all over the world and is the stuff plants grow in. In your garden it is the soil that is in your flower beds and all over the ground under your lawns, trees, shrubs or whatever you are growing and is what they put their roots into. Soil goes down to bedrock, top soil is the top layer of it. When companies sell top soil they used to sell river bottom silt that they have mixed with composted amendments (such as manure or aged woody products sometimes mixed with some nitrogen to make it rot and perhaps other nutrients as well.) What I have just described is what top soil that you purchase SHOULD be. However, I have just purchased 2 cubic yards of “top soil” with which to fill a raised bed in my garden and I am distressed to find that it has very little actual soil in it and it is fluffy stuff that is mostly woody product that I hope is fully rotted but may not be. I think what is happening is that companies are running out of river bottom silt or other forms of top soil to mix with amendments. I have heard of top soil being sold that is mostly nothing but sand...