Irrigation Chapter 2 - Physical Properties of Soils

Dryland and irrigated agriculture depend on the management of two basic natural resources, soil and water. Soil is the supporting structure of plant life and water is essential to sustain plant life. The wise use of these resources requires a basic understanding of soil and water, as well as the crop. This chapter and Chapter 3 discuss soil and the soil-water system.

Authors:  Dean Yonts, University of Nebraska Lincoln Biological Systems Extension Engineer, Panhandle Research  Extension Center, Scottsbluff, NE Brian Benham, Former University of Nebraska Irrigation Engineer, Extension Specialist and Associate Professor, Director, Center for Watershed Studies, Virginia Tech

The term “soil” refers to the weathered and fragmented outer layer of the earth’s terrestrial surface. Fragmenting and weathering, which break down parent material to form soil, are the result of both physical and chemical processes. Erosion (both wind and water) is the most visible soil “creating” process.

A given volume of soil consists of four parts: mineral matter, organic matter, water, and air (Figure 2.1). The mineral and organic matter of a soil store nutrients required by crops. Changes in environment, erosion, and cultural practices can change soil makeup. The relative amounts of mineral and organic matter determine the physical properties of soil. The remaining volume of soil, composed of spaces between the mineral and organic matter, is the pore space. The pore space is filled with varying amounts of water and air.

Coarse soils, such as sands and gravels, have relatively large pores; however, the number of pores is small when compared to a finer soil. Finer soils, like clays or clay loams, have relatively small pores, but many, many of them. The small but abundant pores allow finer soils to hold more water.

Soil texture is determined by the relative amounts of three groups of soil particles or soil separates. The three soil separates are sand, silt, and clay. Texture provides a means to physically describe soil by feel or by measuring the proportion (percentage) of the three soil particle size ranges. A coarse soil has a relatively large amount of sand and feels “gritty.” A silt soil has the texture and feel of flour. A clayey soil may feel “slick” or “sticky” depending on its water content. A loam soil has nearly equal amounts of sand, silt, and clay. The relative sizes of the three soil separates are compared in Figure 2.2. Sand particles can be seen by the naked eye. A microscope must be used to see silt particles. An electron microscope is needed to see clay particles.

A textural triangle (Figure 2.3) is used to describe soil texture. The three sides of the triangle represent the percentages of sand, silt, or clay. The intersection points of three lines from each side of the triangle determine how the soil texture will be classified. For example, if a soil has 20 percent clay, 40 percent sand, and 40 percent silt, it is a loam (see the triangle in the area labeled loam).

Soil structure refers to the arrangement and organization of soil separates or individual soil particles into units called soil aggregates. The arrangement of soil aggregates gives soil its structure. There are three broad categories of soil structure — single grained, massive, and aggregated. Generally the most desirable structure for plant growth is aggregated, especially in the critical early stages of germination and seedling establishment. The principal types of soil aggregates are platy, prismatic, columnar, blocky, and granular (Figure 2.4).

The processes that form aggregates are:

1) wetting and drying,

2) freezing and thawing,

3) decaying organic matter,

4) activity of roots, small animals, and bacteria, and

5) soil tillage.

The wetting/drying and freezing/thawing actions, as well as root and animal activity, push particles back and forth to form granules. Decaying plant residues and bacterial slimes coat these granules and bind them together to form aggregates. Tillage can expose soil near the surface to the destructive forces of erosion. Repeated traffic, especially when soil water content is high, destroys near surface aggregates and compacts the soil.

A soil’s physical properties are expressed numerically by the following characteristics: particle density, bulk density, and pore space or porosity.

Particle density is the weight of a given soil particle per unit volume. In other words; it is the weight of a soil particle or separate, divided by the volume of that soil particle or separate. In most mineral soils, like those found in Nebraska, the average particle density is between 2.6 and 2.7 g/cm³.  By contrast, organic matter typically has a particle density of about 0.8 g/cm³.  Water, by definition, has a density of 1 g/cm³.

Bulk density of a soil is defined as the weight per unit volume of soil. A unit volume of soil includes both the solids and the pore space (see Figure 2.5). Bulk density is important because it reflects the porosity of a soil. Loose, porous soils have lesser bulk densities than tight, compacted soil. The bulk density of a soil increases with compaction. Bulk density indicates how easily a soil will till, how easily water will infiltrate, how it will hold water, and its suitability for growing plants.

Using the numbers shown in Figure 2.5, the bulk density for this example is determined as:

Equation 2.1

Bulk density B_V =  __Mass of soil            =   1.3 g       = 1.3 g/cm³                                 Volume of soil unit         1.0 cm³

In other words, the soil in this example is 1.3 times heavier than the same volume of water.

The particle density for this example is:

Equation 2.2

Particle density =       Mass of soil         =      1.3 g       = 2.6 g/cm³                                    Volume of solids          0.5 cm³

Typical soil bulk densities for fine sands, silt loams, and silty clay loams are 1.5, 1.35, and 1.25 g/cm³, respectively.

Stable soil aggregates are important in a soil because they help maintain good soil structure. Good soil structure translates into low bulk densities (1.3 - 1.5). If an aggregate is crushed, its bulk density increases and pore space decreases. High bulk densities  (>1.6 g/cm³) can result from compaction. Compaction can result from tilling a soil when it is wet. Compaction caused by wheel traffic can increase the bulk density to a depth of at least 1 foot. “Smearing” (the destruction of soil structure caused by shearing) can create a “tillage pan.”Figure 2.6 illustrates the concept of a tillage and/or traffic compaction. As a rule of thumb, bulk densities greater than 1.7 to 1.8 g/cm³impede root penetration.

The space between soil particles is the pore space. This pore space contains varying amounts of water and air. Soil porosity depends on soil texture and structure. Soils with lesser bulk densities have greater porosities. Good porosity is essential to adequate soil aeration, water drainage and root penetration. Silty and clayey soils have smaller pores but many more pores than a sandy soil. Water can be held tighter in small pores than in large pores. For this reason a clay loam with its many small pores can hold more water than a sand. Even though the individual soil particles and pores are larger in sands, the porosity or total pore volume is less in sands than in silty or clayey soils. This characteristic causes the bulk density to be greater for sands.

Soil is the weathered remains of parent material. All soils have distinctive characteristics reflecting the parent material, and the forces which formed it. Most, but not all, soils have three distinct horizons which form the soil profile, Figure 2.7. The A horizon, or topsoil, is the surface layer and usually has the greatest organic matter content. Immediately below the A horizon is the B horizon, or subsoil. The C horizon, or parent material, lies beneath the B horizon. Together, the A, B, and C horizons form the soil profile. Soil horizons usually differ in color, texture, structure, and organic matter content.