CHAPTER ONE: SOIL TEXTURE, STRUCTURE AND COLOR

Study Questions

1. List the particle diameter ranges (U.S.D.A. system) corresponding to the three main size classes of soil texture. How might a person distinguish between the size classes using the feel method?

sand (2 to 0.05 mm) - gritty feel

silt (0.05 to 0.002 mm) - smooth, usually nonsticky

clay (less than 0.002 mm) - smooth, usually sticky, exhibits plasticity (forms ribbons)

2. When manipulating a moist soil sample, you observe noticeable grittiness yet you are able to form a ribbon of 4 inches consistently. What is the probable textural class for this soil?

The sample contains both a high percentage of sand and clay - probably a sandy clay or a sandy clay loam.

3. In general, how does the texture, structure, and color of surface horizons compare with the texture of subsurface horizons?

In general, clay content increases with profile depth. Surface horizons usually exhibit granular/crumb structure and a dark colored. Subsurface horizons usually exhibit blocky, platy, prismatic, or columnar structure, and are lighter in color.

4. What is the significance of structure to plant growth?

Soils with good structure contain many macropores (pores with a large diameter). Macropores facilitate good aeration, good drainage, and rapid water infiltration. In addition, good soil structure is often associated with strong cohesion of soil particles, decreasing the potential for erosion.

5. Name five soil constituents and the soil color you would expect them to be associated with.

ferric iron (Fe³⁺) - red, orange, yellow

ferrous iron (Fe²⁺) - grayish-blue, olive-green

humus - dark brown to black

salts (lime, gypsum) - white, gray

volcanic ash - gray

6. Briefly describe the soil conditions necessary for mottle formation. What is the significance of mottles in making interpretations for soil use and management?

The presence of mottles at a particular depth in the soil profile indicates the soil is saturated at and below this depth for some time each year. Mottles present near the soil surface suggest the soil exhibits poor drainage and poor aeration. These problems may limit crop selection, restrict the performance of septic systems, and result in seepage problems for basements.