Leaves and pine needles shredded by a mulching mower provide a free and plentiful source of soil amendments.
Breaking It Down
Organic matter consists of the decaying remains of plants and animals. It does a good job of binding clay particles together (better than gypsum). This results in improved drainage and aeration as well as sofer and lighter soil. It also increases the soil's fertility while creating a friendly environment for beneficial soil microbes and earthworms.
How much organic matter should you work into your soil? In most cases, the answer is as much as you can. It comes in many forms. One of my favorites is the leaves and pine needles that fall from trees in autumn. I shred them with a mulching mower and then either till them into the ground or lay them atop the soil as mulch. They do a great job, and you can't beat the price--free.
Other good sources of organic matter are composted cow manure, garden compost, and ground bark (add a little nitrogen fertilizer in with the bark to help it break down). And if you need to both acidify and loosen the soil, nothing works better than baled sphagnum peat.