There is nothing you can do that will improve a garden more than adding organic matter, like compost. Black Gold Compost Company makes several high quality composts and sells them in convenient bag form. I only use Black Kow and Black Hen. Black Hen is a little more expensive, so I only use it when I need to- specifically when I have extra poor soil or an expensive and/or rare plant that I'm not willing to risk losing. The reason I do this is because, on top of all the benefits of organic matter, Black Hen has a some nutrients. These nutrients are in a very plant friendly form that won't burn roots. I'm also convinced (no data to back this up, it's just an informed hunch) that the bacteria in Black Hen are different and a little better. Use your regular compost source when you can. When you need a little extra TLC, use Black Hen.