Garden soil is usually spoken of on a continuum from sandy to clay soil. On the sandy end, the soil is primarily comprised of larger particles with poor bonding. The soil is loose and very porous. Clay soil is dominated by extremely small particles that bond together tightly. The soil is dense and compact. The best garden soil is somewhere between the two extremes. Low soil moisture content affects the types of soil differently.
Testing for Moisture
The true test of the moisture level of soil is only found in a lab. A visual on-the-spot examination by an experienced gardener can give estimates which may come close. A lab test consists of measuring the weight of the new sample and then driving the moisture out by drying it. The difference in the weight tells how much water was in the soil.
The moisture level varies widely in areas with a multi-season climate. The type soil and the amount of organic material present also help determine the amount of moisture.
Sandy soil becomes less and less bonded when water is unavailable. The ultimate expression of this is in a desert, where sand blows with the wind and forms large dunes. Sandy soils drain water very rapidly so it is hard for plants to gain a foothold when the soil is dry. The microorganisms that the plants depend upon for food are unable to survive when their source of water disappears.
Clay soil bonds together even tighter in the absence of water. Together with a hot sun it may even bake the soil into a hard, pottery-like condition. It becomes very hard for plant roots to penetrate the soil to gain the nutrients they need.
The Dust Bowl
The exposed surface of clay soil can crumble under these conditions and form into a very fine dust that is easily blown about with the wind. The worst example the United States experienced was in the 1930s when the drought came and what was to become known as the Dust Bowl began in the Southwest. The farmers and ranchers had removed the native grasses that had protected the soil and there was nothing left to keep it from blowing away.
Organic Material and Soil
Soil with a good mixture of clay, sand, and decaying organic matter can much easier deal with a period of little to no moisture. The organic humus can hold many times its weight in water. By providing any type of soil with a good amount of compost and other organic material it is like putting money into a bank for when economic times are lean. The hard work today will pay off when the moisture is missing