Adding a soil amendment, also called a soil conditioner, helps improve plant growth and health. The type of amendment or amendments added depends on the current soil composition, the climate, and the type of plant. Some of the various amendments include, Lime (makes soil less acidic), Fertilizers for plant nutrients (i.e. manure, peat, or compost), Materials for water retention (i.e. clay, shredded bark, or vermiculite), Gypsum (releases nutrients and improves structure), Clay (allows water to reach the plant root).
If you are looking for soil amendments that have a fertilizer component to them like bat and seabird guanos, check out the Dry Nutrients category on the site.