In general clay is compact and retains too much moisture, while sandy soil has trouble holding water and is poor in nutrients needed for plant growth. Loam and its varieties will provide you with the best soils for your garden.
Loam is simply a combination or mixture of clay and sandy soil that seeks to combine the strengths of both, while eliminating the weaknesses. A good loam with retain water, but still be open enough that flooding does not occur and plants can grow deep, strong roots.
Depending on your location a different loam will give you ideal growing conditions. More sandy loams are best for plants that require less water or for areas that receive above average rainfall. Opt for a more clay based loam if your area experiences extended dry seasons or if the plants you are growing require large amounts of water.
In addition to the mixture of these parts all soil needs to contain organic material and nutrients that will allow plants to grow. The best soils will always be those that have nutrients added to them through composting, manure, or fertilization. Another important consideration for your soil is the insect life. Insects help keep the soil fresh and well mixed as the move around in it. With the proper loam and good nutrients and insect life your soil will be ready to nourish your garden and keep your plants healthy.