Soil

From:http://www.gardenplot.com/text.htm

Soil is made of many different ingredients. Some ingredients are organic, i.e., bits of once living material such as leaves, grass clippings etc., that are now decomposing. The other ingredients are inorganic, i.e., bits of stone that have been ground down into  small particles by water and wind. Some water problems in the garden involve the organic materials. Organic material holds water in the soil. Moist organic material is used up as it decomposes (bio-degrades) and becomes food for the plants grown in the soil. When organic material becomes very dry (as in a drought), the surface tension increases, preventing absorption when water becomes available again. As long as it stays dry, it can and will be carried off the surface by wind.The inorganic materials sand, silt and clay remain behind. Soil particles vary in their ability to allow water into the soil, to a great extent due to particle size. Clay is by far the worst due to its microscopic size--approximately 1/12,500 of an inch. Over long periods of time, years and years, these inorganic materials fractionate, i.e., separate into layers. The large particles, such as sand drop, the smaller ones, clay and organic material float to the top. As the quantity of organic material is reduced in the surface layer, the clay packs tighter and tighter together eventually forming a crust that water can't penetrate. When rain, dew, or irrigation water can't penetrate the crust, it runs off and is unavailable to the plants living in the soil.  The sum total of all this is that water is available to our gardens which doesn't get to the root zone of our plants.