Soil is a general classification of various amounts of primarily Clay, Sand, and Silt. These three materials are essentially the same thing broken down to varying degrees, which are various mineral crystals that vary by region. Certain minerals break down easier and so are more likely to be in clay (lending clay its unique properties) while others (such as quartz) don't break down as easily and are more likely to compose sand. Thus native minerals will determine your soil type, its ratio of sand clay and silt.

Soil can contain suspended amounts of decayed organic matter such as peat. But peat itself or other organic materials are not a true soil. Organic materials suspended in soil are beneficial to farming, and it is the suspended particles that contain the nutrients needed to grow plants. The soil itself primarily affects drainage characteristics.

Loam is a mixture in varying degrees of Clay, sand, and silt. A medium loam has nearly even mixtures of the three. Loam therefore contains aggregate and binder premixed for you, so it is tremendously useful.

in Indiana a we have a topsoil that is a medium loam of some sort, with an subsoil that is clay, the deeper you go the purer the clay. The topsoil can vary over a very small area from a high clay loam (especially on 'clay knobs") to a silty loam, to a sandy loam in lower areas.

If you are to mix in straw, than you should search for a high clay loam. You can use straight clay, but you might want to mix in some sand as an aggregate. Even fired clay bricks must contain aggregate for stability. Modern bricks are 50% clay and 50% ground shale. Older bricks were made of soft clay with at least 25% sand.

-This knowledge of soils comes from growing up in a life where the soil is very important, and so knowing the soil is important.

For my bricks, as an example, a very high concentration of clay was desirable. That is because I have an aggregate already present, charcoal. So I need primarily a binder. A small amount of sand is good but not vital. I do not want a good loam because it would not be 'sticky' enough or strong enough.

By introducing lime, you are increasing the calcium content. we do this all the time on our fields because we have acidic soil (because our land was mostly peat bog 100 years ago) Calcium hardens the soil, which is why we use it for walls, and if you have a loam of a neutral pH then it will kill microorganisms. In acidic loam it will actually encourage microorganism growth which is one of the reasons we use it for farming. microorganisms are a vital part of the life cycle of a plant, but not as vital in your wall.

here is a handy diagram from Wikipedia showing the soil types and how the relate.