The soil of a place owes its characteristics to such things as the parent rock material, climate, topography, and decaying plants and animals. Hundreds of different types of soil result from the interaction of these elements. Any particular soil is unique because of its mix of properties (such as color and texture) and composition (including organic content and the action of soil colloids).
Colloids are small soil particles. Their properties and influences on soil are complex and often important. Soil acidity (or alkalinity), for example, is a result of the alteration and integration of soil colloids. Acid soils are characteristic of cold, moist climates; alkaline soils typically are found in dry areas. Most soils of the major agricultural zones of the eastern United States are moderately to strongly acidic. Lime must be added periodically to neutralize that acidity before these soils can be used to produce most row crops.
Color is perhaps the most obvious soil property. A dark color usually indicates an abundance of organic materials, and red, the presence of iron compounds. Generally, however, color is a result of the soil-forming processes. For example, the pale-gray soil of the northern needleleaf forest results from the leaching of organic matter and minerals from the soil's surface layer.
Soil texture, which determines a soil's ability to retain and transmit water, refers to the proportion of particles of different size in the soil. Sand is the coarsest measure of soil texture, silt is intermediate, and clay is the finest. Soils called "loams" contain substantial proportions of each of the three particle grades and are considered best. They are fine enough to hold moisture yet are not so fine that they cannot easily take up water.
The U.S. Department of Agriculture has developed a soil classification system that indicates the most important soil types for an area of the country. Aridisols, found mostly in the Southwest, gain their name from arid. These soils of dry climates are low in organic content and have little agricultural value. Spodosols generally develop in cool, moist climates, although they are found in northern Florida. They are quite acidic and low in nutrients, and are of agricultural value only for acid-loving crops. Tundra soils, which also have little agricultural value, are associated with a cold, moist climate such as Alaska. The soil is shallow, frequently water saturated, and with a subsurface of perennially frozen ground. Highland soils, found in West Virginia, Utah, and Alaska, are little developed and agriculturally worthless.
Mollisols are grassland soils of the semiarid and subhumid climates of the Central, North Central, and Pacific Northwest sections of the country. They are thick dark brown to black and have a loose texture and high-nutrient content. They are among the most naturally fertile soils in the world and produce most of America's cereals.
Alfisols are second only to mollisols in agricultural value. They are soils of the mid-latitude forest and the forest-grassland boundaries. They are very much "middle" soils in a climatic sense. They are located in areas moist enough to allow for the accumulation of clay particles but not so moist as to create a heavily leached or weathered soil.
Alfisols are divided into three categories, each with its own characteristic climatic association. Udalfs are soils of the deciduous forests of the Middle West. Somewhat acidic, they are nevertheless highly productive when lime is used to reduce the acidity. Ustalfs, found in warmer areas with a strong seasonal variation in precipitation, are most common in Texas and Oklahoma. They are highly productive if irrigated. Xeralfs are soils of cool, moist winters and hot, dry summers. Found in central and southern California, they too are highly productive.
Ultisols represent the ultimate stage of weathering and soil formation in the United States. They develop in areas with abundant precipitation and a long frost-free period, such as the South. Particle size is small, and much of the soluble material and clay has been carried downward. These soils can be productive, but high acidity, leaching, and erosion are often problems.
Entisols are recent soils, too young to show the modifying effects of their surroundings. They are widely scattered and of many types, from the Sand Hills of Nebraska to the alluvial floodplains of the Mississippi River Valley. The agricultural potential of entisols varies, but the alluvial floodplain soils, drawn from the rich upper layers of upstream soils, are among America's most productive.
Source: U.S. Department of State