Gardening Information: Soil - - Collection of articles from Oregon State University on various aspects of soil: use of amendments and cover crops, temperature, acidity, moisture, and testing.
Improving Soil - - Oregon horticulturalist offers information on soil components, pH, testing methods, and nutrients, and means of improving it by using amendments and improving drainage and aeration.
The Schundler Company - - Collection of articles and resources about the horticultural uses of perlite and vermiculite.
Vermiculite.com - - One-page site giving suggestions on where to purchase vermiculite in the United States, along with some uses and properties of vermiculite.