Generally, it is a good idea to add some organic matter, which loosens clay soil and helps sandy soil retain nutrients and water. If you are going to buy something from the store, then composted cow manure is a good option. It does not have high levels of salts that can be deleterious to the health of the plants and it is fairly cheap. Vegetable manure is also very good assuming you can verify that the composting has been done properly to kill off potentially viable weed seeds. While many gardeners insist of working it into the soil, it is much more preferable to just layer it on top and let the animals do the mixing for you. Trampling and digging into the soil on a constant basis affects the populations of decomposers and overworks the soil destroying its structure. It speeds up oxidation of organic matter, which lowers the soil's ability to retain nutrients and water. It increases the rate of denitrification. It also destroys animal tunnels that aid the movement of air and water through the soil. In other words, digging manure into the soil to some extent defeats the purpose for which it is intended.
As wildlife gardeners, we try to encourage the development of ecological communities. One should not exclude the soil-based communities that are essential to the health of the plants. Many organisms that live in the soil have their life-cycles disrupted by digging. Fungal hyphae bring nutrients and water to plants as well as storing nutrients that would otherwise be leached down the soil profile. Bacteria aid in the decomposition process and produce a slime that binds particles together. Fungi, bacteria, nematodes and protozoa are in balance with each other in healthy soils in a way that controls the amount of nutrients released to plants. During digging, fungal hyphae are shredded and underground tunnels, used by insects for reproduction, are interrupted. These include digger wasps and bees; burrowing spiders; mole crickets; short-horned grasshoppers; cicadas; tiger beetles; dung beetles; scorpionflies; antlions; robber flies; velvet ants; cicada killer wasps and many other groundwelling species. All of these animals together with nematodes, bacteria and fungi play a role in the development of soil structure and chemistry. Their importance is all too often ignored by gardeners.
An even cheaper and better way to manure is to use your raked leaves (but not needles). So many gardeners expend time collecting leaves off their lawns and moving them to the roadside. Then, they expend more time and money going to the garden centre to buy compost or chemical fertiliser, which is then lugged back into the garden to replace the manure they gave away for free. The modernisation of agriculture is mainly responsible for the prevalent attitude amongst gardeners that gardening should be an unsustainable practice that involves the removal of essential nutrients and organic material which is then replaced with chemicals and pesticides.
Many gardeners apply a thick layer of wood mulch to their beds. The advantage is that it cuts down on a considerable amount of weeding and watering. However, there are ecological consequences that result from this practice. This mulch is not only a good barrier to weeds, but also to insects that form part of the soil community. In particular, ground nesting bees are affected by this practice.
A covering of raked leaves may not look as sightly as a manicured flowerbed, but it has several advantages. The leaves will feed many invertebrates as well as protect them from predators. On exposed soil, these invertebrates would not be able to survive the levels of predation that would occur. Many butterflies complete their metamorphosis into adults as a chrysalis in leaf litter close to the host plants. With leaves, populations of invertebrates can form that will support animals in a sustainable way further up the food chain. Ground feeding birds such as Juncos will visit your garden looking for tasty morcels. Snails, which are a major source of calcium for birds do well in leaf litter. Many of the pests that we do not like will be kept in balance by the soil-based community and visiting birds. The carbon-nitrogen ratio is high in leaves. However, a layer of leaves will not rob the plants of nitrogen. The addition of leaves as a manure better mimics the natural process of nutrient cycling in woodlands than the addition of compost that is relatively nitrogen-rich. Wildlife gardening is different from vegetable gardening where nitrogen-rich composting is required for intense food production.
Leaves shade the soil underneath keeping it cooler and reducing the the evaporation rate. This results in a considerable reduction in the water needs of your plants. Leaves are raked onto flowerbeds in the fall and this layer will protect more tender plants from cold weather. Leaves also encourage the development of fungal hyphae that form important symbiotic relationships with perennials. Some plants, which include many members of the mint family, do not do well under mulch and must be left exposed to the air during winter.
As mentioned elsewhere, if you have a sandy or a sandy loam soil, you should leave some soil bare for native bees and wasps to nest in. These areas should remain free of any disturbance to give these wonderful creatures a chance to complete their life-cycle.