Soils 205- General Soils

Fall 2001

Quiz 1 Study questions

1. What are the five main functions of soils in any ecosystem?

1. medium for plant growth. The soil provides:

-anchorage

-water storage

-temperature modification

-ventilation

-nutrient supply (remember that all but three of the 18 essential nutrients come from soil; and are taken up through the soil solution)

2. regulator of water supplies

-stores water

-filters out impurities

3. Recycler of raw materials

-facilitates the decomposition of organic materials

-recycles nutrients

4. Habitat for soil organisms

-soil contains great diversity of organisms

-handfull of soil may contain a billion organisms from 1000 different species

5. Engineering medium

2. In an ideal agricultural soil in good physical condition, approximately what proportion of the total volume is comprised of pores? 50% solids, 50% pores. Solids consist of mineral (~45%) and organic (~5%). Pores are either filled with air or water.

3. How are the proportions of soil air and water related?

They are inversely related. There is a finite amount of pore space that is either filled with air or water. As the water content increases, air is displaced. Oxygen diffusion is very slow through water-filled pores so that high water contents lead to anaerobic conditions.

4. How are primary and secondary minerals different?

Primary minerals have not been altered since crystallizing from molten magma. Examples include quartz (the most resistant to weathering), mica, and feldspars. Secondary minerals are formed from the weathering products of other minerals. This group of minerals includes Fe/Al oxides and layer silicate clay minerals.

5. Explain the relationship between the following terms:

a. soil profile and pedon

A pedon is the smallest three-dimensional body that encompasses the range of a soil’s characteristics. It is generally about 1m³ in volume. The profile is one vertical face of the pedon that contains horizons.

b. horizon and profile

A horizon is a horizontal layer that is different than the layer above or below it. Master horizons are A, E, B, C, and R. The profile contains horizons.

c. E and B horizons

E horizons have been leached of clay, organic matter, and Fe oxides. As a result they are light in color. The dominant process is eluviation. B horizons are zones of accumulation (illuviation). If a soil has an E horizon, it will be over a B horizon.

d. O and A horizons

O horizons are found above the mineral soil surface. They have >20% organic matter. They are typically found in forested areas and in wet, cool areas where plant production is greater than decomposition.

e. hue, value, and chroma

These three terms are used to describe soil color using the Munsell system. Hue is the dominant spectral color, value is the lightness or darkness, and chroma is the brightness. They are written as: hue value/chroma

6. Give the most complete horizon designation (master symbol and subordinate distinction, if appropriate) for each of the following.

a. sandy horizon with little soil development ___C____

b. horizon with segregated CaCO₃___Bk___

c. eluvial horizon__E____

d. horizon with properties of E and B horizon superimposed upon one another___EB____

e. B horizon with accumulation of layer silicate clays___Bt_______

7. What are the main agents that determine soil color?

Organic matter causes dark brown to black colors

Fe oxides (Fe⁺³) cause yellow to red colors (oxidized conditions)

Primary minerals, salts, and unweathered volcanic ash produce white or light colors

Fe⁺² (reduced iron) causes “gleyed” or “low chroma” colors

8. What are gleyed colors and what type of environmental conditions do they reflect?

Gleyed colors are dull, low chroma colors produced when Fe⁺³ is reduced to Fe⁺² in the absence of O₂. Gleyed colors indicate that the soil is seasonally saturated with water.

9. What valuable information can be gathered by studying soil morphology or horizonation?

The presence of certain horizons indicates processes that are occurring in the soil. Colors can reflect environmental conditions.

10. What are the three soil separates and how are they defined? What properties or behavior does each soil separate contribute to soil?

Sand- 0.05-2mm

-mostly quartz

-low fertility

-large pores lead to rapid drainage and good aeration, but low water holding capacity

-low surface area

Silt 0.002-0.05

-mostly quartz with some weatherable minerals

-higher total porosity but smaller pores than sand

-little cohesion (susceptible to erosion)

Clay <0.002 mm

-high surface area

-high nutrient content

-high adsorption of water and pollutants

-very small pores slow water movement and aeration may be a problem

11. What is the difference between the texture of a soil and its structure?

Soil texture is the proportion of sand, silt, and clay in a soil. It is determined by hand texturing in the field and by using sedimentation in the laboratory. Structure refers to how the soil separates are aggregated into peds.

12. Using a textural triangle, determine the textural class name for three soils with the following particle size distribution:

Soil	% sand	% silt	% clay
1	22	60	18
2	38	17	45
3	59	13	28

Soil 1 = silt loam

Soil 2 = clay

Soil 3 = sandy clay loam

13. Which is more susceptible to modification by soil management: Soil texture or soil structure? Soil structure. Soil texture can only be modified by adding large amounts of a soil separate. This is generally not done. Soil structure is dependent on aggregation and is affected by organic matter. Management such as tillage decreases organic matter and breaks down aggregates thereby destroying soil structure.

14. Describe 6 types of soil structural units or peds and tell where each is most likely to occur in the profile.

1. granular- spherical aggregates found in A horizons; especially in grasslands or where earthworms are present.

2. platy- thin horizontal plates that can be found in A and subsurface horizons where the soil has been compacted.

3. blocky

angularblocky- cube-like peds with sharp faces that are generally found in subsurface horizons.

Subangularblocky- cube-like peds with rounded edges that can be found in surface and subsurface horizons.

4. prismatic- vertical pillars with flat tops. Found in subsurface horizons with swelling type clays.

5. columnar- prism like but with rounded white tops. Common in subsurface horizons in arid and semi-arid regions where salts have accumulated.

6. structureless soil

massive- rock like structure, no planes of weakness

single grained- particles are not interacting, like beach sand

15. What is the difference between particle density and bulk density of soils?

Particle density is the weight of solids divided by the volume of solids. Bulk density is the weight (oven dry) of soil divided by the total volume (solids plus pores). Since the bulk density includes pore space, which is generally about 50%, the bulk density of soils should be approximately ½ of the particle density.

16. What is the relationship between the compaction of a soil and its bulk density?

As a soil is compacted, the bulk density goes up. The more compaction, the greater the bulk density.

17. If a soil has a bulk density of 1.4 Mg/m³ and a particle density of 2.6 Mg/m³, what portion of the soil volume is pores and what portion is solid particles?

% pores = 100% – (Db/Dp *100%) = 46%

18. A cylindrical core sampler (10 cm high x 10 cm diameter) is used for bulk density determination. The collected sample weighs 1420 g. After oven drying it weighs 1300 g. (show your work, and include units).

a. What is the bulk density of this soil? Db = oven dry weight/vol.

volume of a cylinder = πr²h = π * (5)² * 10 = 785.4 cm³

Db = 1300 g/785.4 cm³ = 1.66 g/cm³ or Mg/m³

b. What is the % pore space?

%pore space = 100 – (1.66/2.65 *100) = 37.5%

c. What can you say about the quality of this soil for plant growth based on your

calculations?

If the typical porosity is 50%, this soil appears to have been compacted. Since macropores are generally lost with compaction, aeration and drainage may be slow. Roots may have trouble penetrating the soil.

19. Why are the relative amounts of macropores and micropores important to air and water movement in soils and how can these be influenced by soil management?

Macropores are important in drainage and aeration. They are also large enough to provide niches for important soil organisms. Macropores are highly influence by management and are lost when aggregation and soil structure are destroyed by management such as tillage. Micropores are important in holding water against gravity and are less susceptible to management.

20. What are the components of soil strength in cohesive and noncohesive soils?

Cohesive soils- attraction between clay particles

- attraction between clay particles and water

- frictional resistance between particles

noncohesive soils- frictional resistance between particles

- water content

21. How do we measure soil strength in cohesive and noncohesive soils?

Cohesive- compression test

Noncohesive- angle of repose

22. What are “expansive soils” and why are they a problem?

Expansive soils contain swelling type clays called smectites. They are a problem because as they shrink and swell during dry and rainy periods, they destroy foundations.

23. How does organic matter influence aggregation?

One of the main aggregating agents in soil is organic matter. Roots and fungal hyphae enmesh soil particles. Polysaccharides act as glues that hold particles together.

24. What four guidelines for maintaining soil tilth. Explain the reasoning behind each guideline.

1. Leave a protective mulch on the surface. This will protect the soil from raindrop impaction.

2. Minimize tillage operations. Conventional tillage will break down soil aggregates over time.

3. timing of tillage- Do not till when the soil is wet and aggregates can be easily crushed or smeared.

4. Add conditions such as gypsum (CaSO₄) and PAM if necessary.

Gypsum provides Ca⁺² which leads to flocculation.

PAM is a synthetic organic that acts as natural polysaccharides.