Bulk soil amendments are similar to mulch, and in some cases, they are made of the same materials. The difference is that amendments are for mixing into the soil and then decomposing. Mulch is for laying on top of soil and decomposing slowly, if at all.
The best kind of soil amendment is compost that you either buy or make yourself. Composted agricultural by-products are available in many areas and make excellent soil amendment or mulch.
If you're making a new lawn, spread a 2-inch layer of compost over the soil and use a rotary tiller to cultivate it into the soil to a depth of about 6 inches. At this rate, you are adding 30 percent (by volume) organic matter to the soil.