The prime ingredient in successful gardens is an ideal soil. To understand what your soil is like, you have to spend a little time working in it. Digging is the best way to find out about your soil. Shoveling the dirt exposes the soil's texture, and how quickly it dries.
Soil consists of solid mineral particles, decaying organisms, living organisms, air, and water. Ideally a soil should be 45% mineral, 5% organic matter, 20% air, and 30% water. Most of the organic, air and water material is in the top strata of the soil. This is call the "topsoil". This is the coarsest, most fertile layer where most roots grow.
Soil texture is the relative volume of sand, silt, and clay particles in a soil. Soil texture affects the water-holding capacity of soil, movement of water through the soil, and ease of cultivation.
Mechanical modification is just improving the soil's structure by digging in one form or another. The is best done when the soil is neither too wet or too dry. Excess digging / tilling during extreme moisture conditions leads to compaction (reduced pore space).
Rototilling should only be done on a limited basis. Excessively tilled soil reduces the texture of the soil, earthworms leave tilled areas.
Amendments is another way to improve the quality of the soil. Amendments include fertilizers, peat moss, compost, manures, worms, cover crops (a crop that improves the soil in which it is grown), and sand (a coarse sand used in building).
Amendments can be used to break up clay by forming aggregates in the soil.
Good soil is not just dirt. It is a complex relationship of various components that need to be in balance with each other. In other words, dirt is not just dirt, it's an entire environment that exists just below the surface. When all is good, things grow and bloom and thrive. When things aren't right, problems arise. Diseases take hold, insects cause more damage than the plant can handle.
Your soil is the base upon which everything grows. Take care of it.