Subsoils are either porous or non-porous. Nonporous subsoils do not drain well. The worst condition is a hardpan subsoil. This type has an impervious layer that will not drain. This layer can be relatively shallow or quite deep. The lack of drainage causes water to collect in the soil above, which forces out the air in the soil particles and drowns plant roots that reach it. If you have this type of subsoil, you will need to break up the hardpan, if possible, or install a drainage system to carry off the excess water.
Topsoil can vary from only a few inches deep on rocky hillsides to several feet deep in the grasslands of the US Midwest. It is the upper layer of soil below living plant material and often consists of, or at least contains, organic material.
Organic material is the key word here. If your soil is subsoil, you can actually turn it into 'topsoil' by adding sufficient organic material. This is what Nature does over a long period of time. The plants that grow, die; leaves fall and decay; creatures die and decay and all of this 'organic' material accumulates to form topsoil. You can imitate Nature and simply pile a deep (12" or more) layer of wood chips, sawdust, leaves or other garden and kitchen waste and let it rot for a few years.
You can speed up the process if you're willing and able to make the effort. I have several beds that started out as solid orange clay subsoil, interspersed with various pieces of concrete rubble, soda bottles and other construction debris. Now, the soil is dark, easy to work, and full of earthworms.
To accomplish this, I double-dug the beds and incorporated two wheelbarrows of organic material - sifted compost, rotted woodchips or whatever I had handy - in each trench, along with a few shovels full of coarse sand. For a simple illustration of this process, digging only one spit deep, check out my resource list.
This isn't quite the standard double digging, which assumes that your first layer or 'spit' will be topsoil and is more concerned with loosening soil than changing its characteristics. Since the object is to create 'topsoil', you must incorporate more organic material than is usually recommended into both the top and bottom layers of each spit. While you're at it, if you have clay soil, it doesn't hurt to fork over the bottom of the lower trench to improve drainage.
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