SOIL PROPERTIES LAB

We did an abridged version of this lab to focus on soil moisture, and the main points were that coarser soils tend to have 1) lower field capacity, and 2) greater infiltration capacity (once wetted). Furthermore, from lecture and readings you should also realize that coarser soils tend to have less total surface area, and thus less potential nutrient availability (because available nutrient ions adhere to particle surfaces) and less chemical reactivity (since chemical reactions occur on particle surfaces).

"Soil textures" obviously are important. Remember, these are proportional mixtures of different particle size classes ("separates", conventionally sand, silt, and clay). Any particular texture is identifiable using the soil trangle below. Become famiiar with how it works, and keep in mind that "loam"s refer to medium mixtures of all particle sizes.

On the soil moisture issue things get very much more complex. Due in part to the fact that smaller particles can pack more tightly together (which can you get into a smaller box; 5 lbs of eggs or 5 lbs of peas?), the amount of accessible (if you're a plant root) moisture is not so straightforward as being a direct function of soil texture. Coarser soils allow faster infiltration, but can't hold as much water; finer soils can hold huge amounts of water, but won't allow it to get in or out very readily when packed down (which in nature it usually is). There are other factors involved, too, but suffice it to say that the texture that usually has the greatest amount of moisture accessible to growing plants is the medium texture loam. Overly fine soil won't allow enough mobility (below wilting point), and very coarse soil allows such rapid drainage that it dries too quickly.

In Portage County the coarser soils occur east of the Wisconsin River, closer to the glacier's outermost edge during the last Ice Age (about 15,000 B.P.). West of the river lakes existed, providing a stillwater setting that allowed finer soils to settle and accumulate. The green letters below show our sampling sites; the glacier edge is blue.

Our timings for 50 ml water pourings into dry soil samples (masses varied somewhat) were comparable to the following measured during Spring 1996 lab sections. Note that the initial infiltration rates, while the samples were still dry, were rapid compared to later measurements in wetted soil. Note also how the Amherst and Hull rates became relatively consistent by the end of the second consecutive Monday lab section (red); apparently these coarser soils had reached their infiltration capacity.

Infiltration TIME in seconds

                        Infitration TIME in seconds                     

SUPPLEMENTAL EXERCISE

SOIL CLASSIFICATIONS AND REGIONS

PURPOSE

To familiarize you with soil classification systems, pedogenic processes and formative environments, and national soil distribution patterns. Because of their involvement with the atmosphere in moisture cycles, with the biosphere in nutrient cycles, and with the lithosphere in weathering dynamics, an understanding of the differences between natural landscapes is fundamental to explaining the distribution of soil types.

MATERIALS NEEDED

Textbook Goode's Atlas 18th Ed. paper towels

Portage County Soil Samples U.S. Soil Samples slide projector

INTRODUCTION

Scientists classify soils in many ways, but one of the most accepted classification systems in the United States is the USDA Seventh Approximation system, which differentiates soils by their chemical nature and physical qualities (implied in Table 3, but see your text for additional details). The nature of a soil ultimately reflects the environment creating it, including especially the climate, parent material, relief, organic contributions, and time for development at a place.

Distinct soil orders ("kinds of soil"; underscored in Table 1) are typical of particular regions. The formative conditions vary widely within an area as large as the United States, so consequently we have a wide variety of soils. Most soil orders are associated with a particular climatic and vegetative environment, but some can occur in several environmental settings.

Table 1. SOIL CODES (in Goode's Atlas 18th Ed. pp. 18-19)

PROPERTIES USED FOR SOIL CLASSIFICATION

Texture

Soil texture is the proportional mixture of different particle size groups of mineral fragments, with size usually expressed as average particle diameter. The intensity and rate of chemical weathering processes is governed by texture because a soil mass composed of fine ("small") particles has more aggregate surface area (where most chemical reactions occur) than the same mass of coarse ("large") particles.

Furthermore, the greater surface area of fine-textured soils also provides a greater potential availability of a special group of chemicals, the base nutrients. Plants require these nutrients for growth, and get them through roots that contact the surface of soil particles.

Finally, soil moisture (water in the soil) and infiltration capacity (the ability of water to seep into the soil) are functions of texture. Particle size controls porosity (the total amount or proportion of void space between the solid particles). Particle size, and the proportional mixture of different particle sizes, determine a soil's permeability (the connectivity between void spaces), which is governed by the tightness of particle packing.

Color

Soil color has distinct components that can reveal different conditions in a soil. Two that are especially useful for identifying and classifying soils are hue and intensity.

Hue refers to the dominant electromagnetic wavelength (blue, red, etc.) that is reflected off soil illuminated by white light; we often refer to this alone as "color", but color is more complex than just hue. Hue can indicate important chemical components in a soil. For example, reds and oranges often indicate high concentrations of oxidized iron; bluish grey can reveal reduced iron; greens may indicate copper; yellow for oxidized aluminum; purple sometimes shows potassium; and white suggests calcium carbonate or sodium nitrate. Soils with high organic content often are black or dark brown.

Related to hue is luster (glistening vs. dull), which may indicate the presence or absence of elemental metals, natural glass, or mica fragments in the soil.

Intensity is the brightness (light or dark) of a soil viewed by reflected light, and usually this indicates something about soil moisture content. Moister soils appear darker because water is highly absorptive of most light at visible and near-infrared wavelengths; dry soil is lighter.

SOIL ARRANGEMENT IN THE ENVIRONMENT

Zonality

As you dig downward in some soils, you progress through various layers called horizons. Typically, the uppermost soil layer contains most of the decomposed organic material and humus, and is termed the A horizon. Beneath this there may occur a B horizon composed mostly of fine mineral fragments. The C horizon at the bottom of the soil is similar, except that it includes larger chunks of undegraded bedrock.

The vertical sequence of layers at a site is called the soil profile, which differs from place to place depending on what climate and vegetation occur. Soils that exhibit typical A-B-C profiles in otherwise unremarkable situations are called zonal soils.

Some soils develop unusual profiles or uncharacteristic horizons because they exist in special environmental circumstances. Such soils are called intrazonal soils. For some examples: hydromorphic soils form where the ground is regularly saturated with water, halomorphic soils have salt concentrations, and calcimorphic soils are lime-rich because they develop out of limestone parent rock material.

Finally, some soils have little or no layering. Such azonal soils usually are so young that they have not had time to develop any distinct horizons. Examples include sand dunes, volcanic ash mantles, and alluvium.

Pedogenic Processes

Distinctive soils develop under the influence of environmental conditions at a place. Several specific pedogenic processes exist that create zonal soils, and several more that create intrazonal soils. Which specific process dominates at a place depends on the balance there of the five formative factors: climate, [topographic] relief, parent [bed]rock material, organic [materials], and [development] time.

Podzolization is the process in which soluble metal oxides leach vertically from the top (elluviation) into the bottom (illuviation) of the B horizon. A podzolic soil thus has a somewhat bleached upper, and a stained lower, B horizon. This process creates zonal soils in climates that are cool, acidic, and humid.

Laterization is the process in which soluble base nutrients and colloidal silica leach completely out of the profile in gravity water. A lateritic soil often is bright red to yellow because of accumulated residual iron or aluminum oxides. This process creates zonal soils where climates are hot, acidic, and at least seasonally humid.

Calcification is the process where soluble base nutrients leach downward with infiltration from the surface, and are drawn upward by capillary action from deeper horizons, to form a concentration layer in the B horizon. Calcareous soils usually contain a whitish layer of lime enrichment at depth. This process causes zonal soils where climates are too dry for forests, but are wet enough for grass cover.

Salinization is the process where precipitation is so sparce that surface infiltration is absent, so that ground moisture is drawn all the way to the surface where it evaporates, leaving behind dissolved salts as a surface crust. Such saline soils often have a hard white duricrust, or a powdery white dusting, of evaporite salts. Salt concentrations frequently are toxic to most plants. Such intrazonal soils are common in arid lands.

Gleization is the process where a seasonally fluctuating water table causes a soil horizon that has pockets of both oxidized (reddish) and reduced (bluish) iron. These gley soils often have a mottled blue-and-red layer beneath an organic-rich peat surface. Intrazonal gley soils are typical of permafrost and coastal lowland bogs where highly acidic waters inhibit decomposition so as to accumulate peat.

FIELD EVALUATION OF SOIL PROPERTIES

You don't always have lab glassware, color charts, or other equipment available for evaluating soil properties, but you don't necessarily need them either. Color judged subjectively, and several simple hand tests for structure and texture, provide reasonably accurate determination of a soil's characteristics. Try these simple tests to determine differences between a few soil samples.

Color. Using the color guidelines discussed above, tentatively interpret the moisture and chemical content for the soil samples.

Texture. For one sample, roll a bit of moistened soil into a ball, then squeeze it between your fingers. If it won't make a ball then the texture is probably sand; if a ball can be formed but readily crumbles the texture is probably loamy sand. A silt loam will form a more durable cast of your finger impressions. Clay loams can be pressed and drawn out into a ribbon that is relatively brittle and somewhat gritty. Purer clay can be molded and rolled into a long "wire".

You can also determine if any particular separate is absent in a sample by a simple touch test. Sand grains are easily felt, even though they may comprise only a small proportion of the sample. If you feel no grit against your fingers when you squeeze the moist sample, then the sand content is negligible. Now, rub a small bit of the moist sample against your teeth. Any grittiness your teeth feel which your fingers don't probably is silt. A pure clay will feel smooth against your teeth.

IMPORTANT TERMS, PHRASES, AND CONCEPTS YOU SHOULD UNDERSTAND

REFERENCES

Butzer, K. Geomorphology From the Earth. New York: Harper and Row, 1977. pp. 54-77. GB401.5.B87

Froth, H. D. and L. M. Turk Fundamentals of Soil Science, 5th ed. New York: John Wiley & Sons, 1972.

Gersmehl, P. J. Soil Taxonomy and Mapping. Annals of the Association of American Geographers 67 (1977): 423-447.

Jenny, H. Factors of Soil Formation; A System of Quantitative Pedology. New York: McGraw-Hill Books, 1941.

PART 1

USA REGIONAL SOIL ORDERS

Examine and compare the various U.S. soil samples collected at the sites numbered in Figure 1 and Table 2. The sample jars have a green number corresponding to their location; do not open any samples in the closed jars (some are potentially harmful). Other samples, heaped on cards labeled with green sample numbers, can be handled if you wish.

1. Record color (hue only) and texture ("very coarse", "coarse", "fine", or "very fine") in Table 2. Four given examples in the table should help anchor textures for you.

2. Use the classification outline (Table 3, on last page) to try judging soil order for each sample. Note the environmental descriptions in Table 2.

3. Compare your interpretation to page 18 in Goode's Atlas to assess how well these samples conform with the general patterns of soil orders.

Figure 1. USA Soil Sample Collection Sites

Table 2. USA Soil Sample Observations

Table 3. SOIL CLASSIFICATION SUMMARY

I. ZONAL SOILS ... horizoned profiles in normal environments

A. Podzolization - minerals leached to lower horizons

1. Spodosol: ash-colored elluvial horizon; cool & humid

2. Alfisol: forest brown soil; temperate humid

B. Calcification - lime (CaCO3) enrichment horizon

3. Mollisol: soft dark grassland soil; dry temperate

4. Aridisol: coarse desert soil; very dry steppe/desert

C. Laterization - silica leached/residual metal oxides

5. Ultisol: old red/yellow clay soils; humid subtropic

6. Oxisol: intensely oxidized laterite; hot & humid

7. Vertisol: black swelling clay; hot & seasonally wet

II. INTRAZONAL SOILS ... horizons from special environmental conditions

A. Hydromorphic - waterlogged/seasonally saturated; GLEIZATION process

1. Histosol: mottled blue/red bog soils; cool & humid

B. Halomorphic - alkaline evaporite surfaces; SALINIZATION process

2. Aridisol: desert/steppe alkali crust; warm & arid

C. Calcimorphic - limey parent material

3. Inceptisol: limestone bedrock; warm & humid

4. Mollisol: limestone bedrock, lime nodule layer; warm & dry

5. Ultisol: savanna/seafloor limestone, reddish crust; hot & dry

III. AZONAL SOILS ..... young deposits with little or no horizonation

A. Dunes - surf/wind deposition

1. Inceptisol: glacial moraine beach sand

2. Entisol: stablized sand sheets

3. Andisol: volcanic tephra

4. Inceptisol: glacial outwash/loess

5. Inceptisol: desert loess

B. Alluvium - surface runoff deposition

6. Entisol: glacial outwash

7. Inceptisol: fluvial (river) flood sediments

8. Inceptisol: lacustrian (lake) sediments

C. Highlands - highly organic and coarse texture

9. Inceptisol: alpine tundra sediments

10. Entisol: freshly weathered sediments

11. Inceptisol: reweathered sedimentary rock

D. Deserts - coarse regolith debris

12. Entisol: disintegrated bedrock

PART 2

SOIL ENVIRONMENTS, DEVELOPMENT, AND CLASSIFICATION

1. What are some sample sites you classified differently than the atlas did? You are not necessarily wrong if your classification is different. Why might sample classifications legitimately differ from the mapped patterns?

LOCAL VARIABILITY IN CLIMATE, PARENT ROCK MATERIAL, RELIEF, ORGANICS, AND TIME

2. For each of the following places, what soil order would you expect, and what sort of vegetation and climate should also occur there?

3. Where, in general, do you expect to find coarser soils? What climatic and vegetation conditions occur there? WESTERN USA; DRY AND/OR COOL (DESERT, TUNDRA, HIGHLANDS)

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