This is a summary of the uses of substrates and the benefits of soil in a planted aquarium. I have also included a chart that describes different types of substrates that may prove useful.  This is material that I have been going over in a few books I have. Hope this helps someone. Feel free to leave comments and opinions.

The Importance of Substrates

Your substrate is one of the most important parts of your aquarium. The substrate acts as a “nutrient sink” trapping nutrients where plant roots can easily attain them. These nutrients are moved around by the aquariums currents. The size and shape, depth, and organic and inorganic (mineral) contents are the keys to keeping and maintaining healthy plants. Plants use the substrate not only as a place to root, but also as a source of nutrients, and in some cases, a medium through which to reproduce. This is why it is important to have a good nutrient rich substrate that will promote root growth and plant stability. For example if you were to use a clean pea gravel it would create a fairly biologically inactive substrate because, the water flow would easily wash away nutrients and create an oxygen rich area that would be undesirable and choke the development of a healthy and robust root structure. (This does not hold true to all plants, as some plants do not need a specialized substrate or any substrate at all.)

Size and Shape

The size and shape of your substrate is as important as its contents. A good rule of thumb is to use a substrate that is neither to large or too small. If the substrate is too large it will have too much water flow and will not promote a good bio-layer. If the substrate is too small it may compact and stop the movement of important nutrients. An ideal substrate is a rounded (not jagged) medium that is 1-3mm in diameter.

Nutrient Content

The inorganic and organic content is also very important. All plants need minerals to grow but these minerals are used in small amounts over time. Many commercial substrates will provide many of the nutrients needed for plant growth. Even regular tap water will provide many of the needed minerals for healthy plant growth if the water is not too soft. If your water is soft there are many fertilizers that can be added to increase mineral contents. An important note when using natural substrates is that it should not contain coral or limestone. They are high in calcium and will raise the alkalinity and PH of the water making it harder for your plants to absorb CO2. Another thing to remember is that a substrate without organic matter is only an anchor. The organic content of a substrate contains organic nutrients combined with fish waste that will promote plant growth. Many times aquarists will mix or layer substrates in order to reach a desired mixture of organic and inorganic nutrients.

Using Dirt

Soil and peat have a very high organic content and must be used with care to avoid overloading the aquarium with organic nutrients. Many aquarists who use soil in their aquariums find that it is indeed one of the most effective long term planting substrates. It contains large amounts of carbon, iron, and many other useful nutrients that can be readily used by plants. It is highly recommended that sterilized organic potting mixtures are used to prevent contamination of unwanted substances (i.e. chemical fertilizers and unwanted bio contaminants). One of the great benefits of soil is that as it breaks down it will release small amounts of CO2.   Laterite based substrate are ideal as a long term source of nutrients. Laterite can be mixed in with the main substrate or used toward the lower half of the substrate, where plant roots are most effective at obtaining nutrients.  Laterites are clay like soil types rich in iron and aluminum, formed in hot and wet tropical areas. Nearly all laterites are rusty-red because of iron oxides.  

When using soil it is important to layer your substrates using 2-4 cm of soil topped with 3-4cm of fine gravel as an anchor.

Important

During the first few weeks of soil use the aquarium may experience a high release of nutrients and organic matter. Most plants cope with these changes and may not be affected although these conditions may become toxic to fish and crustaceans.