Amending your soil is simply adding a material to improve the soil's physical properties. The soil amendment can be organic or inorganic. Organic amendments are alive, were once living, or come from a living creature. We typically think of peat, compost, and manure when talking about organic amendments. Inorganic amendments are mined or manufactured. Perlite, vermiculite, sand, and gravel are typical inorganic amendments.
Soil structure is how the individual soil particles fit together. In sandy soil the particles are large and loose with lots of air pockets, or pore space, in between them. That's why water drains so quickly through sand; it travels easily through the pore space. In clay soil the particles are very close together and the pore space is very small. That's why water is absorbed in clay and it stays wet for so long; the water fills the pore space and has nowhere to go.
Soil amendments will change the structure. While inorganic amendments are a definite option, they tend to be more expensive for a home gardener and don't improve the third component of tilth, soil fertility. Soil fertility is what enables roots to absorb the nutrients in the soil. Because organic amendments improve both structure and fertility, they will be my primary focus.
Organic soil amendments increase the organic matter within the soil. That sounds obvious, but it takes decades or centuries for nature to increase soil organic matter naturally. Organic matter is important because it's the fuel for the billions of microorganisms that live in your garden bed. Microorganisms make plant growth possible. They convert the nutrients in the soil into the ions that plant roots can absorb. Increased soil organic matter means increased root growth.
Organic amendments can be small or large. Small amendments include peat, compost, crushed leaves, aged manure, sawdust, and dried grass clippings. Large amendments include bark, wood chunks, and straw. Generally, small amendments are best for sandy soil and large amendments are best for clay soil. In sandy soil the smaller pieces act as little sponges to fill the pore space and soak up water and nutrients. In clay soil the larger pieces act to push aside the small soil particles and make space for water and air.
You can buy organic amendments or make your own. Nurseries, garden centers, and big box stores will all sell bags of compost, manure, and soil blends. You can buy compost and decomposed manures in bulk from many landscape or sand and stone companies. Depending on how much you need, it may be worth filling your truck or paying for the delivery of a large quantity of amendments. Starting your own compost pile, bagging fallen leaves, and saving some grass clippings are free ways to accumulate good soil amendments. If you have or know someone with horses, use the manure.
The amendment will eventually decompose in the soil. That's the result of microorganisms at work. Bigger pieces will decompose slower than small. If you're amending sandy soil with compost you probably won't see any indication of the organic matter in the next year; it's there in small amounts, but hard to see. On the other hand if you're amending clay soil with wood chunks, you may see the wood pieces still evident in the soil for years.
For your initial application, spread the amendment over the top of the soil to a depth of two or three inches. This may seem like a lot, but for poor soil or soil that has never been amended it is necessary. In subsequent years you can use less. In well-amended soil you may only need to add an inch of amendment each spring when you get the beds ready.
The quantity of amendment will be printed on the bag, usually in cubic feet. Determine how big your garden bed is and buy enough to cover it. One cubic foot of compost will cover an area of six square feet with two inches of compost. You can buy in bulk by the yard; a yard of amendment is 27 cubic feet. One yard of compost is enough for about five average four feet by eight feet raised beds. Three yards of compost will cover an area of about 500 square feet with two inches of compost.
After spreading it, dig it in as deep as possible. You want the amendment to get down to root level. I like to make two applications of amendments when I first amend a garden. I dig deep and turn over the soil so that the amendment falls to the bottom of the hole. After digging up the entire bed, I spread more amendment and then use a tiller to incorporate it. Tillers don't usually get as deep as a spade can so I use both tools to get more amendment into the soil.
Also, adding "topsoil" is not the same as amending the soil. Topsoil is rarely regulated by states, while amendments often are. That means when you're buying compost you know you're buying compost, but when you buy topsoil you may not know what you're buying. Topsoil from bag to bag or truckload to truckload can be drastically different and it's possible that the soil you're buying is worse than what you have in your garden. Except for filling large holes outside the garden, stay away from topsoil.
It takes a lot of labor to add organic amendments to your garden soil. The initial application as I described is hard work, but it usually only needs to be done once. Annual amendment application is easier, but still time-consuming. Don't avoid doing it just because it's hard. Your plants will grow better and in the long run you'll save time that would be spent weeding and fertilizing. Make the effort and your garden will be better for it.