There are so many things you can add to your soil. If you want to correct your soil, you first need to find out what kind of soil you have. Find out what state your soil is in by determining the pH level of your soil before deciding what needs to be added to it.
There are test kits at your local garden centers and it takes just a few minutes to do. Just take a sample of soil, add water to the chemical in the tube and it will turn a certain color which will tell you whether you have acidic soil or alkaline soil. In addition to everyones comments, lime is also important. It supplies calcium, an essential element for plant growth, unlocks soil fertility (making other nutrients available in acid soils), helps to decompose organic matter, release nitrogen, and stimulate the work of microbes and root bacteria.
For more on soil pH, click here
Here's a listing of options for things you can add to your soil. Keep in mind that different plants prefer different mixtures.
Slag, an industrial by-pruduct good for legumes.
Bloodmeal, from slaughterhouses for hastening plant breakdown in compost
Bone Meal for phosphorous and nitrogen
Compost a gardener's gold. Basically a mixture of any of the above and or plant material allowed to heat up in the sun to decompose with aeration and water.
Grass Clippings, make sure no pesticides were used
Greensand and Granite Dust, source of potash
Hulls and Shells of cocoa beans, buckwheat, oats, rice and cottonseed for fertilizer and mulch.
Leaf Mold, dampened shredded leaves with lime.
Leaves, use as compost or mulch.
Manure - make a manure tea by putting say 4 cups of manure in an old cotton tee shirt. Tie it up. Fill up large plastic bucket with water and immerse the manure into it. Let it sit a day or overnight and you will have a rich tea to feed your plants.
Peat Moss, great as a mulch
Phosphate, lots of minerals
Sawdust, a great mulch
Seaweed and Kelp, high in potash
Sludge, similar fertilizer as barnyard manure
At home you can save your eggshells, coffee grounds, tea grounds, any vegetable waste, & ashes from hardwood coals (not charcoal) to add to your grass clippings and leaves for compost. Try to avoid any fruits as these attract fruit flies and other inscets you don't want in your compost pile.
If you have a lot of clippings, I would recommend a compost pile with plastic on tip. This will help heat the pile and you won't have to turn it. It will yield rich, black compost much quicker. This instead of spot composting which I'm not sure is as effective. Let the clippings deteriorate outside of the soil before adding. It will add much more in the long run. ;)