Chapter 4: Soil Physical Properties

Main Objectives • Capable of explaining soil physical properties, their

importance, and factors influencing soil physical properties.

• Understand the intricate relationship between soil physical properties, soil tilth, and the relative distribution of air, water and solids.

Key terms and Concepts

• Texture • Particle size distribution • Specific surface area • Soil structure • Aggregate • Particle density • Bulk density • Soil pore space • soil tilth

1. Soil Color

2. Texture (size distribution of primary soil particles) ● Soil texture is a basic property of a soil, and is not

readily subject to change in hundreds of years. ● Gravels (2-75 mm), cobbles (75-250 mm), boulders

(>250 mm), and other coarse fragments are not normally considered as part of soil texture.

Germany Spotosol

Minnesota Spotosol

Australia Oxisol

Minnesota Mollisol

A. Soil separates: Sand: <2 mm & >0.05 mm Silt: <0.05 mm & >0.002 mm Clay: <0.002 mm

Figure 4.2 General relationship between particle size and kinds of minerals present. Quartz dominates the sand and coarse silt fractions. Primary silicates such as the feldspars, hornblende, and micas are present in the sands and, in decreasing amounts, in the silt fraction. Secondary silicates dominate the fine clay. Other secondary minerals, such as the oxides of iron and aluminum, are prominent in the fine silt and coarse clay fractions.

B. Texture and surface area SPECIFIC SURFACE AREA: the surface area for a given mass (or volume) of particles. When particle size decreases, specific area increases geometrically. Why is soil surface area important? 1). Maintain water films 2). Chemical attachment and adsorption 3). Weathering at the surfaces 4). Electromagnetic charges as forces of soil aggregation 5). Microbes tend to grow on particle surfaces.

Figure 4.3

The finer the texture of a soil, the greater is the effective surface exposed by its particles. Note that adsorption, swelling, and the other physical properties cited follow the same general trend and that their intensities go up rapidly as the colloidal size is approached.

3. Texture classes A. Loams. Any soil not having the extreme proportion of clay, silt

and sand fractions is a kind of loam (see the triangle). B. The soil texture triangle

C. Particle size analysis: The bigger they are, the faster they fall. V = k d 2

V: settling velocity; d: particle diameter; k: a constant (gravity, density and viscosity of water) So, by measuring the amount of soil particles still in suspension after various settling time, the percentage of each size fraction can be determined (either by the pipet method or by the hydrometer method). Determining soil texture by feel (read Box 4.1, see figure 4.6, and try it)

Figure 4.8

4. Structure ● Structure relates to the arrangement of primary soil particles into secondary aggregates of peds. Why is soil structure important? A. Structural peds (see figure 4.8)

B. Aggregates (>0.01 mm & <5 mm) (a) Hierarchical organization (see Figure 4.9)

B. Aggregates (>0.01 mm & <5 mm) (b) Processes influencing formation and stability of aggregates Biological processes: (1) microorganisms; (2) roots; (3) fauna; e.g., organic glues, hyphae, glomalin, all can be effective cementing agents. In general SOM is the most important type of agent for aggregate formation and stabilization. Physical-chemical processes (clay flocculation, and Na+ dispersion, wet-dry cycles, and freeze-thaw cycles)

Figure 4.11 The aggregates of soils high in organic matter are much more stable than are those low in this constituent. The low-organic-matter soil aggregates fall apart when they are wetted; those high in organic matter maintain their stability.

Figure 4.12 Puddled soil (left) and well-granulated soil (right). Plant roots and especially humus play the major role in soil granulation. Thus a sod tends to encourage development of a granular structure in the surface horizon of cultivated land. (Courtesy USDA Natural Resources Conservation Service)

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Soil aggregates in a Mollisol in Iowa are larger and more stable under native prairie vegetation than where cultivated crops had been grown for some 90 years. In this study, soil samples were taken from a prairie area and from two nearby fields, where either corn or soybeans had been grown the previous year. Differences in past management may in part account for differences between the corn and soybean fields, but the soil in both of these fields shows distinct aggregate breakdown compared to the native grassland area. [Drawn from data in Martens (2000)]

5. Bulk density A. Particle density (Dp): is defined as the mass per unit of

volume of soil solids without any pore space. For most soils Dp may range from 2.6 to 2.75 Mg/M3.

B. Bulk density (Db): is defined as the mass of a unit volume of

dry soil with preserved pore space. ● Bulk density indirectly tells us the total pore space in a soil. Because: % pore space = 100 - (Db / Dp X 100) The equation above gives the mathematical relationship between particle density, bulk density, and total pore space in a particular volume of a soil.

Figure 4.17 Bulk density Db and particle density Dp of soil. Bulk density is the weight of the solid particles in a standard volume of field soil (solids plus pore space occupied by air and water). Particle density is the weight of solid particles in a standard volume of those solid particles. Follow the calculations through carefully and the terminology should be clear. In this particular case, the bulk density is one-half the particle density, and the percent pore space is 50.

C. Factors affecting bulk density: Texture Clay types Structure SOM content Depth in a profile Compaction Tillage

Figure 4.22 Tractors and other heavy equipment compact the soil to considerable depths, increasing bulk density and reducing plant growth and crop yields. The effects are especially damaging if the soil is wet when trafficked. (a) The tires of a heavy vehicle compact a sandy loam soil to about 30 cm, creating a traffic pan. Plowing temporarily loosens the compacted surface soil (plow layer), but increases compaction just below the plowed layer, creating a combined traffic pan and plow pan. Bulk densities in excess of 1.8 Mg/m3 prevented the penetration of cotton roots in this case. (b) The yield of potatoes was reduced in two out of three years in this test on a clay loam in Minnesota. Yield reductions are often most pronounced in relatively dry years when plants have the greatest need for subsoil moisture. [Based on data from Camp and Lund (1964) and Voorhees (1984)]

Root distribution of a cotton plant. On the right, interrow tractor traffic and plowing have caused a plowpan that restricts root growth. Roots are more prolific on the left where there had been no recent tractor traffic. The roots are seen to enter the subsoil through a loosened zone created by a subsoiling chisel-type implement. (Courtesy USDA National Tillage Machinery Laboratory)

6. Pore space:

A. Factors influencing pore space All the factors influencing bulk density also affect pore space.

Figure 4.19

B. Pore size (micro <0.03 mm; meso>0.03 and <0.08; macro >0.08 )

7. Tillage and soil physical properties A. The purposes of tillage: (a) Accelerating SOM decomposition/soil mining (b) Preparing seedbed (c) Control weeds (d) Regulate moisture (e) Plant residue incorporation (f) Reduce soil pests No-till is a way of agricultural practice by growing crops without plowing soils. Conservation/reduced tillage is a agricultural management regime which reduces soil plowing to a minimum level.

B. Soil tilth Soil tilth refers to the physical condition of the soil in relation to plant growth, and is an integration of the following properties: Texture, Aggregate formation and stability, Bulk density, Moisture, Aeration, Water infiltration rate, Drainage, Capillary water capacity. (see the 7 guidelines in the textbook) C. The pros and cons of soil tillage Soil tillage might have served the above-mentioned purposes, but it has also produced unwanted consequences. Have you heard about the dust bowls in North American plains in the past?

Both water content and bulk density affect soil strength as measured by penetrometer resistance. The data are for the clay textured Bt horizon of a Tatum soil in Virginia (Hapludults), which was either severely compacted (bulk density 1.7 Mg/m3) or not compacted (bulk density 1.3 Mg/m3). Note that soil strength decreases as water content increases and is very low regardless of bulk density when the soil is nearly saturated with water. [Unpublished data of R. Gilker, R. Weil, and D. Krizek, University of Maryland and USDA/ARS]

Discussion questions:

1. Please give an example for using the knowledge in this chapter in a real world situation?

2. Exercise to use the texture triangle to figure out the textural class of a soil given the percent clay, silt and sand.

3. Can you describe the mathematical relationship between particle density, bulk density and total pore space in a particular volume of a soil?

4. Can you discuss the possible effects of organic farming on soil aggregate formation and stability in comparison with a nearby conventional farm?

5. Can you think of the potential effects of no-tillage practices on soil bulk density at different soil horizons and the implications for plant growth and environmental protection?

5. Bulk density A. Particle density (Dp): is defined as the mass per unit of

volume of soil solids without any pore space. For most soils Dp may range from 2.6 to 2.75 Mg/M3.

B. Bulk density (Db): is defined as the mass of a unit volume of

dry soil with preserved pore space. ● Bulk density indirectly tells us the total pore space in a soil. Because: % pore space = 100 - (Db / Dp X 100) The equation above gives the mathematical relationship between particle density, bulk density, and total pore space in a particular volume of a soil.

Soil aggregates in a Mollisol in Iowa are larger and more stable under native prairie vegetation than where cultivated crops had been grown for some 90 years. In this study, soil samples were taken from a prairie area and from two nearby fields, where either corn or soybeans had been grown the previous year. Differences in past management may in part account for differences between the corn and soybean fields, but the soil in both of these fields shows distinct aggregate breakdown compared to the native grassland area. [Drawn from data in Martens (2000)]