OVERVIEW

This chapter examines the characteristics, the processes of formation and the global pattern of distribution of soil that develops at the interface between the lithosphere, the biosphere and the atmosphere.

• Soil is a complex mixture of solids, liquids and gases. It contains mineral material derived from the parent material and organic matter derived from living plants and other organisms in the soil.
• The major soil-forming factors are parent material, climate, vegetation, and time.
• Soil texture refers to the proportion of soil particles that fall into each of three size grades; sand, silt and clay.
• Soil colloids are the smallest particles in soils. They are important because they hold plant nutrients in the soil in the form of ions.
• Soil pH can range from acid to alkaline.
• Soil structure refers to the way in which the soil grains are bound together by colloids into peds.
• Chemical and organic processes in soils change primary minerals into secondary minerals such as oxides and clay minerals.
• The nature of the clay minerals determines a soil's base status which, in turn, affects its fertility or ability to retain nutrients.
• The storage capacity of a soil is the maximum amount of water that it can retain by capillary tension when it is allowed to drain.
• Water stored within a soil is depleted by evaporation and transpiration until it is recharged by precipitation.
• The soil water balance describes the inter-relationship between water need (potential evapotranspiration), water use (actual evapotranspiration), precipitation and storage in the soil water zone.
• A soil water budget is a numerical accounting, usually done on a monthly basis, of the soil water balance components.
• Most soils have distinct horizontal layers, known as soil horizons, that develop by the processes of enrichment, removal, translocation and transformation.
• Enrichment is the addition of organic and mineral material to the soil by sedimentation and biological activity.
• Removal takes place by leaching, as dissolved material is carried away in percolating soil water, and by erosion.
• Translocation occurs by the removal of material from upper horizons (eluviation) and its accumulation in lower horizons (illuviation).
• Humification is an important transformation process in which organic material is decomposed to humus.
• Soil classification systems are used to study the distribution of soils and their relationship to a variety of environmental factors.
• The U.S. Comprehensive Soil Classification System groups the soils of the world into 11 soil orders that are distinguished primarily by the presence of diagnostic horizons.
• Seven soil orders (Oxisols, Ultisols, Vertisols, Alfisols, Spodosols, Mollisols, and Aridisols) have well-developed horizons and can often be associated with particular climatic regimes.
• One soil order, Histosols, includes soils with a large proportion of organic matter.
• Three soil orders (Entisols, Inceptisols and Andisols) have poorly developed horizons or are capable of further mineral alteration.

KEY TERMS

soil               soil horizons      Alfisols           
parent material    soil profile       Spodosols          
soil texture       eluviation         Histosols          
soil colloids      illuviation        Entisols           
bases              soil orders        Inceptisols        
secondary          Oxisols            Andisols           
minerals           Ultisols           Mollisols          
water use          Vertisols          Aridisols          
water need                                               

STUDY QUESTIONS

1. What is soil and how is it distinguished from regolith?
2. What are the major factors that affect the properties and formation of soils?
3. What is soil texture and why is it considered an important soil property?
4. Colloids play an important role in determining soil base status and fertility. Why is this?
5. Identify two important classes of secondary minerals produced by mineral alteration and give an example of each.
6. How does the ability of a soil to retain moisture vary with soil texture? Why?
7. Sketch a diagram to illustrate the concept of the soil water balance. Your diagram should show the important inputs, outputs and storage components of the soil column.
8. Define water use and water need and discuss how they affect the soil water balance.
9. What are soil horizons and how do they form?
10. What are the four classes of soil forming processes? Describe each of them.
11. Define calcification and decalcification. Under what conditions does each prevail.
12. List and describe the 11 soil orders of the U.S. Comprehensive Soil Classification System.
13. Which soil order dominates the landscape of the area in which you live? Why?
14. Sketch a schematic diagram to show how soil order and soil profile characteristics would change along a transect across North America from a cool, dry desert in the west to a cool, moist climate in the east.