Soils - Part 2: Physical Properties of Soil and Soil Water

Overview

This lesson will help you understand the major components of the physical properties of soil. You will learn such terms as texture, aggregation, soil structure, bulk density, and porosity as it relates to soils. You will learn how soil holds and transmits water and cultural practices that enhance or degrade physical properties of the soil.

[This lesson, as well as the other nine lessons in the Soils series, is taken from the "Soils Home Study Course," published in 1999 by the University of Nebraska Cooperative Extension.]

 

Introduction

As complex as it is, soil can be described simply. It consists of four major components: air, water, organic matter, and mineral matter (Fig. 2.1). In an ideal soil, air and water fill the pore space and compose about 50 percent of the volume; organic matter accounts for about 1-5 percent of the soil volume; and mineral matter accounts for the remaining 45-49 percent. The partitioning of these four components vary considerably. For example, an organic soil in Michigan may be 45 percent organic, while a desert soil from Arizona may be 60 percent mineral.

The mineral and organic matter fractions of the soil are the solids and serve as the storehouse and exchange sites for plant nutrients and other chemicals.  They are important from a fertility and environmental standpoint. It is these fractions, along with cultural practices, that influence other physical properties and processes.

 
 Figure 2.1.  Major components of soils.

Soil Texture

A close look at soil will clearly indicate that the makeup of the mineral portion is quite variable. The soil is composed of small particles. These small particles are the result of massive rocks of different mineralogy that have weathered to produce smaller rock fragments and finally soil particles. Soil particles vary in size, shape and chemical composition. Some are so small they can be seen only with a microscope.

Three categories for soil particles have been established — sand, silt and clay. These three groups are called soil separates. The three groups are divided by their particle size. Clay particles are the smallest, while sand particles are the largest. The size ranges for the soil separates and the relative size of the particles are shown in Figure 2.2.

 

Figure 2.2.  Relative size of soil separates. 

Sand particles are clearly visible, but a microscope must be used to see silt particles. An electron microscope is needed to see clay particles. In comparison to spheres we know and understand, a sand particle may be equivalent to a basketball; a silt particle to a golf ball; and a clay particle to the head of a pin.

The proportion of the different soil separates in a soil defines its soil texture. There are 12 classes of soil texture. For example, if most particles are large and coarse the soil is called a sand. It looks and feels sandy. A silt soil is dominated by medium-sized particles and feels like flour. Small-sized soil particles primarily make up a clay soil which feels slippery or greasy when wet.

The laboratory procedure used to identify soil separates is known as mechanical analysis. This process records the time it takes a specific weight of soil particles to fall to the bottom of a tall cylinder filled with water. A textural triangle can be used to determine soil textural class from the results of a mechanical analysis (Figure 2.3). Often 100 units of soil are used in the analysis, so that the sum of the weights of the three soil separates total 100 and can be easily converted to percentages. The textural triangle represents all possible combinations of soil separates.

 

 

Figure 2.3.  Soil textural triangle and textural classes.

The three sides of the textural triangle represent increasing or decreasing percentages of sand, silt and clay particles. The textural triangle is easy to use once it is understood. Assume that you have a soil that is 60 percent clay, 20 percent silt and 20 percent sand. The percent of clay is identified on the left side of the triangle. From the lower left corner to the top of the triangle, the percent clay increases from 0 to 100 percent. Move along the left side of the triangle until you reach 60 percent clay. Then draw a line at 60 percent clay that is parallel to the bottom of the triangle. The percent silt is identified along the right side of the triangle. From the top of the triangle to the lower right, the percent silt increases from 0 percent to 100 percent. Move along the right side of the triangle until you reach 20 percent silt. Now draw a line at 20 percent silt that is parallel to the left side of the triangle. The bottom of the triangle identifies the percent sand. From the lower right corner to the lower left corner, the percent sand increases from 0 percent to 100 percent. Move along the bottom of the triangle until you reach 20 percent sand. Draw a line at 20 percent sand that is parallel to the right side of the triangle. The point at which these three lines intersect will define the soil’s texture.

Determine soil texture for the soils in Table 2.1. The soil textural class you determine from the triangle should match the texture listed.

 Table 2.1.  Soil separates and textural class
 

 

By using samples of known texture, and with a lot of practice, it is possible to determine soil texture by hand texturing. With this procedure, moistened soil is worked between the thumb and fingers to form a ribbon. Sand and clay percentages are estimated. A guide for estimating soil texture by hand is given in Table 2.2.

 Table 2.2. Soil texture as defined by soil textural class and estimated by hand
 

 

Some small rock fragments may be present in soil as stones or gravel. While these rock fragments play a role in the physical properties and processes of soil, they are not considered in the determination of soil texture.

The soil texture or textural class, as described here, is the same as the soil texture mentioned in Soils - Part 1: The Origin and Development of Soil. A Holdrege silt loam, for example, describes the texture of the surface horizon. It would contain from 0 percent to 27 percent clay, 50 percent to 80 percent silt, and 0 percent to 50 percent sand.

Soils of different textural classes often have a similar amount of a soil separate and behave alike. As a result, we often speak of fine- and coarse-textured soils. Fine-textured soils have a dominance of clay, while coarse soils have a dominance of sand. Medium-textured soils have a dominance of silt. Using this concept, the 12 soil textural classes have been combined into three groups.

Surface Area of Soil Separates

Clay is the smallest mineral particle in soil. Clay particles are the active portion of a soil, because chemical reactions occur at their surface. The chemical reactions control the adsorption and release of plant nutrients and many other chemicals in the environment. Sand and silt particles are much larger than clay and are quite inactive chemically because of their mineral composition and limited surface area.

Clay particles have about 1,000 times as much external surface area as the particles in an equal weight of sand. The effect of decreasing particle size on surface area can be illustrated with a deck of cards. Stacked together, the deck has only 25 square inches of surface area. When separated as individual cards, the deck has a surface area of nearly 1,000 square inches (Figure 2.4). As another example, consider a room in your home. Pack in as many basketballs or golf balls or pinheads as possible. Then multiply the number of balls or pinheads by the surface area of each. You will find that the total surface area of pinheads is substantially greater than for basketballs and golf balls.

 
Figure 2.4.  Subdivision of playing cards, like clay particles, increases surface size.

The large surface area of clay and its mineral composition make it the storehouse of plant nutrients. It is not surprising, then, that soils with more clay have more nutrients than sandy soils. Likewise, deep clayey soils have more nutrients than a clayey surface horizon above sandy subsoil horizons. Eroded soil that once had a clayey surface horizon and a silty or sandy subsoil will also have less nutrient-supplying capacity because of the loss of clay — fine, medium, and coarse.

Soil Structure

Soil structure refers to the arrangement of soil separates into units called soil aggregates. An aggregate possesses solids and pore space. Aggregates are separated by planes of weakness and are dominated by clay particles. Silt and fine sand particles may also be part of an aggregate. The aggregate acts like a larger silt or sand particle depending upon its size.

The arrangement of soil aggregates into different forms gives a soil its structure. The natural processes that aid in forming aggregates are:

1) wetting and drying,
2) freezing and thawing,
3) microbial activity that aids in the decay of organic matter,
4) activity of roots and soil animals, and
5) adsorbed cations.

The wetting/drying and freezing/thawing action as well as root or animal activity push particles back and forth to form aggregates. Decaying plant residues and microbial byproducts coat soil particles and bind particles into aggregates. Adsorbed cations help form aggregates whenever a cation is bonded to two or more particles.

Aggregates are described by their shape, size and stability. Aggregate types are used most frequently when discussing structure (Table 2.3, Figure 2.5).

 
 Figure 2.5.  Soil structural types.

 Table 2.3. Structure type and description
 

 

Structure is one of the defining characteristics of a soil horizon. A soil exhibits only one structure per soil horizon, but different horizons within a soil may exhibit different structures. All of the soil-forming factors, especially climate, influence the type of structure that develops at each depth. Granular and crumb structure are usually located at the soil surface in the A horizon. The subsoil, predominantly the B horizon, has subangular blocky, blocky, columnar or prismatic structure. Platy structure can be found in the surface or subsoil while single grain and structureless structure are most often associated with the C horizon. Turn to Soils - Part 1 to identify the structure for different horizons of the Holdrege, Nora, Sharpsburg and Valentine soils.

Aggregates are important in a soil because they influence bulk density, porosity and pore size. Pores within an aggregate are quite small as compared to the pores between aggregates and between single soil particles. This balance of large and small pores provides for good soil aeration, permeability and water-holding capacity.

Tillage, falling raindrops and compaction are primarily responsible for destroying aggregates. As the cutting edge of a tillage implement is pulled through the soil, the shearing action at the point of contact breaks apart aggregates. If tillage is conducted at the same depth for several years, a tillage pan may develop. This is one form of compaction. Particles that were once part of the aggregates may reorient themselves and form platy structures. The amount of aggregate destruction that results from tillage depends on the amount of energy the tillage implement places in the soil. The field cultivator has little down pressure and destroys few aggregates. The disk, however, has both cutting action because of the rotation of the disk and shearing action. Together there is substantial down pressure and destruction.

Aggregates on the soil surface can be broken down by the beating action of raindrops. The single particles that were once part of the aggregate can easily form a crust when the soil dries. The crust looks very similar to the crust formed on a puddle after it rains. It is very difficult for water to infiltrate a crust and for seedlings to push up through a crust. Thus, field operations that lead to aggregate destruction at the soil surface have detrimental secondary effects. The particles also can be eroded if they become detached by rainfall.

Compaction can lead to the breakdown of aggregates in the surface soil and subsoil if the applied force from wheel traffic, animal traffic or human traffic is greater than the force holding an aggregate together. Field observations have shown that compaction can cause granular structure on the soil surface to break down and reform as blocky structure and blocky or subangular blocky structure in the subsoil to become structureless.

Aggregation is promoted by root growth and the addition of organic material. Roots excrete compounds that are used as food by microorganisms. Also, as roots absorb water and dry the soil, cracks form along planes of weakness. Lastly, when roots decay, root channels serve as conduits for water that facilitate wetting/drying and freezing/thawing.

Organic material may be added in the form of crop residue, animal manure, sludge, and green manure. These additions are usually made to the surface soil and are critical to the development of granular and crumb structure. As organic material is incorporated by tillage, soil animals and microorganisms, it aids in subsoil structure development.

 

 

 

 

 

 

 

 

 

 

Particle Density

A soil particle has no pore space, and is nothing more than a very small piece of rock. The weight of an individual soil particle per unit volume is called particle density.  Usually, particle density is expressed in units of grams per cubic centimeter (g/cm3). An average value for particle density is 2.66 g/cm3. This means that a soil particle that is 1 cubic centimeter in volume weighs 2.66 g. In comparison, water has a density of 1 g/cm3, and organic matter has a density of 0.8 g/cm3. In English units, water has a value of 62.4 lbs/ft3. Traditionally, most measurements of soil physical properties are expressed in the metric system. For your reference, 2.54 cm = 1 inch; 454 g = 1 pound.

Particle density plays an important role in our understanding and determination of other physical properties including bulk density and porosity.

Bulk Density

Soil weight is most often expressed on a soil volume basis rather than on a particle basis. Bulk density is defined as the dry weight of soil per unit volume of soil. Bulk density considers both the solids and the pore space; whereas, particle density considers only the mineral solids. Figure 2.6 illustrates the difference between bulk density and particle density.

 

 Figure 2.6.  Illustration of bulk density and particle density.  

For our ideal soil, one-half of it is solids, and one-half is pore space. Using our example of a 1 cm3 volume, the ideal soil would have 0.5 cm3 of pore space and 0.5 cm3 of solids. Pore space filled with air weighs 0 g. Organic matter is a very small portion of the solids, so it is usually ignored in this calculation. The mineral solids would weigh 1.33 g when dry, and is determined by multiplying particle density by the volume of solids:

2.66 g/cm3 x 0.5 cm3 = 1.33 g

The bulk density, then, is the dry weight of soil divided by the volume of soil:

1.33 g / 1 cm3 = 1.33 g/cm3

For practice, consider a box of undisturbed soil from the field. The box has dimensions of 2.5 cm by 10 cm by 10 cm. The volume of the box can be determined by multiplying the height of the box times its width and its depth. The wet soil in the box weighed 450 g. The dry soil weighed 375 g. Now calculate the bulk density. Your answer should be 1.5 g/cm3. In this calculation, you did not have to use the particle density because the weight of soil in the box was already known.

Bulk density of the surface soil is lowest in the spring immediately after soils have thawed and before field operations have begun. Each field operation compacts soil beneath the tires. If soils are wetter than field capacity, bulk density may increase. However, if soils are dry, bulk density is not affected much. Root growth, in general, starts to be restricted when the bulk density reaches 1.55 to 1.6 g/cm3 and is prohibited at about 1.8 g/cm3. Tillage can increase bulk density if it breaks down aggregates and allows soil separates to pack more tightly. Adding organic material decreases bulk density because organic material has a lower bulk density. However, additions are typically so small in proportion to the weight of soil that they do not markedly influence bulk density except at the soil-atmosphere interface. Bulk density is also important because it tells us about the porosity of a soil.

 

 

 

 

 

 

 

 

 

 

 

 

Porosity

Porosity or pore space refers to the volume of soil voids that can be filled by water and/or air. It is inversely related to bulk density. Porosity is calculated as a percentage of the soil volume:

                                                              Bulk density            x 100 = % solid space
                                                            Particle density

                                                         100%  –  % Solid Space  =  Percent Pore Space

Loose, porous soils have lower bulk densities and greater porosities than tightly packed soils. Porosity varies depending on particle size and aggregation. It is greater in clayey and organic soils than in sandy soils. A large number of small particles in a volume of soil produces a large number of soil pores. Fewer large particles can occupy the same volume of soil so there are fewer pores and less porosity.

Compaction decreases porosity as bulk density increases. If compaction increases bulk density from 1.3 to 1.5 g/cm3, porosity decreases from 50 percent to 43 percent. Aggregation also decreases porosity because more large pores are present as compared to single clay and silt particles that are associated with smaller pores.

Pores of all sizes and shapes combine to make up the total porosity of a soil. Porosity, however, does not tell us anything about the size of pores.

Pore Size

Next to soil texture, pore size is probably one of the most important physical features of a soil. It controls water and air movement and storage. Pores come in all sizes, although clays have predominantly small pores, and sands have large pores. Most soils are a mixture of sand, silt and clay particles, so there is a mixture of different sized soil pores (Figure 2.7).

 

Figure 2.7.  Pore size arrange- ment in clay and sand.


An ideal soil condition is one with an equal number of large and small pores. Large pores allow for soil aeration. Aeration is needed for the exchange of oxygen from the atmosphere and carbon dioxide given off by plant roots and microorganisms. About 10 percent of the pores must be large enough for aeration so that root growth is not restricted.

Small pores are connected to large pores that are connected to medium-sized pores. This complex connection of pores can be compared to a maze with wide, medium and narrow passageways. In addition, some passageways, and soil pores, may be dead ends.

Within an aggregate, the pores are small. Between aggregates, pores are large. Small pores are usually called micropores, and large pores are called macropores. As organic matter is added, the number of macropores increases. These increases result from the increase in aggregation, decay of root channels and creation of earthworm channels. Macropores are crushed when a soil is compacted. Tillage tends to increase macropores in the short-term, but reduces the number of macropores in the long-term because of the loss of aggregation and severing of earthworm channels.

Soil Color

Organic matter (humus), manganese and iron are the primary coloring agents in soil. The dark color of many productive soils in Nebraska and throughout the Midwest is due to organic matter. The dark soil color from organic matter at the soil surface aids in the absorption of heat from sunlight to warm the soil.

Soil shades of red, yellow and gray are due to the amount and chemical form of iron and manganese present. Red soils contain oxidized iron. Oxidized iron is also observed on metal objects that have been exposed to the atmosphere. We call it rust. Yellow soils contain hydrated iron. Gray soils indicate chemical reduction of iron and/or manganese due to wetness and lack of oxygen. Yellow and gray coloration can be found in the subsoil of some Nebraska soils which remain wet for some portion of the year. These subsoil colors serve as an important indicator of natural drainage conditions. In a soil series description, these colors are designated by the term “mottles” as for the Aksarben series.

Sloping land that has been eroded excessively may expose subsoil horizons that are lighter in color and possess little organic matter.

Soil Water

Soil acts as a sponge to take up and retain water. Movement of water into soil is called infiltration, and the downward movement of water within the soil is called percolation, permeability or hydraulic conductivity. Pore space in soil is the conduit that allows water to infiltrate and percolate. It also serves as the storage compartment for water.

Infiltration rates can be near zero for very clayey and compacted soils, or more than 10 inches per hour for sandy and well aggregated soils. Low infiltration rates lead to ponding on nearly level ground and runoff on sloping ground. Organic matter, especially crop residue and decaying roots, promotes aggregation so that larger soil pores develop, allowing water to infiltrate more readily.

Permeability also varies with soil texture and structure. Permeability is generally rated from very rapid to very slow (Table 2.4). This is the mechanism by which water reaches the subsoil and rooting zone of plants. It also refers to the movement of water below the root zone. Water that percolates deep in the soil may reach a perched water table or groundwater aquifer. If the percolating water carries chemicals such as nitrates or pesticides, these water reservoirs may become contaminated.

 

 Table 2.4. Permeability classification system
 

 

Infiltration and permeability describe the manner by which water moves into and through soil. Water held in a soil is described by the term water content. Water content can be quantified on both a gravimetric (g water/g soil) and volumetric (ml water/ml soil) basis. The volumetric expression of water content is used most often. Since 1 gram of water is equal to 1 milliliter of water, we can easily determine the weight of water and immediately know its volume. The following discussion will consider water content on a volumetric basis.

Saturation is the soil water content when all pores are filled with water. The water content in the soil at saturation is equal to the percent porosity. Field capacity is the soil water content after the soil has been saturated and allowed to drain freely for about 24 to 48 hours. Free drainage occurs because of the force of gravity pulling on the water. When water stops draining, we know that the remaining water is held in the soil with a force greater than that of gravity. Permanent wilting point is the soil water content when plants have extracted all the water they can. At the permanent wilting point, a plant will wilt and not recover. Unavailable water is the soil water content that is strongly attached to soil particles and aggregates, and cannot be extracted by plants. This water is held as films coating soil particles. These terms illustrate soil from its wettest condition to its driest condition.

Several terms are used to describe the water held between these different water contents. Gravitational water refers to the amount of water held by the soil between saturation and field capacity. Water holding capacity refers to the amount of water held between field capacity and wilting point. Plant available water is that portion of the water holding capacity that can be absorbed by a plant. As a general rule, plant available water is considered to be 50 percent of the water holding capacity.

The volumetric water content measured is the total amount of water held in a given soil volume at a given time. It includes all water that may be present including gravitational, available and unavailable water.

The relationship between these different physical states of water in soil can be easily illustrated using a sponge. A sponge is just like the soil because it has solid and pore space. Obtain a sponge about 6 x 3 x 1/2 inch in size. Place it under water in a dishpan, and allow it to soak up as much water as possible. At this point, the sponge is at saturation. Now, carefully support the sponge with both hands and lift it out of the water. When the sponge stops draining, it is at field capacity, and the water that has freely drained out is gravitational water. Now, squeeze the sponge until no more water comes out. The sponge is now at permanent wilting point, and the water that was squeezed out of the sponge is the water holding capacity. About half of this water can be considered as plant available water. You may notice that you can still feel water in the sponge. This is the unavailable water.

Water in the form of precipitation or irrigation infiltrates the soil surface. All pores at the soil surface are filled with water before water can begin to move downward. During infiltration, water moves downward from the saturated zone to the unsaturated zone. The interface between these two zones is called the wetting front. When precipitation or irrigation cease, gravitational water will continue to percolate until field capacity is reached. Water first percolates through the large pores between soil particles and aggregates and then into the smaller pores.

Available water is held in soil pores by forces that depend on the size of the pore and the surface tension of water. The closer together soil particles or aggregates are, the smaller the pores and the stronger the force holding water in the soil. Because the water in large pores is held with little force, it drains most readily. Likewise, plants absorb soil water from the larger pores first because it takes less energy to pull water from large pores than from small pores.

Use of soil water estimates on a percentage volume basis does not allow for any practical interpretation. Therefore, water is usually converted from a percentage volume basis to a depth basis of inches of water/foot of soil (Table 2.5).

 

 Table 2.5. Estimated soil water for three soil textures
 

 

The table values are derived from laboratory analysis of soil samples. Some of this information is also published in the Soil Survey. Other techniques have been developed to estimate soil water if laboratory data is not available. Generally, field capacity is considered to be 50 percent of saturation and permanent wilting point is 50 percent of field capacity.

Water holding capacity designates the ability of a soil to hold water. It is useful information for irrigation scheduling, crop selection, groundwater contamination considerations, estimating runoff and determining when plants will become stressed. Water holding capacity varies by soil texture (Table 2.6).

 

 Table 2.6. Range of water holding capacity for different soil textures
 

 

 

Medium textured soils (fine sandy loam, silt loam and silty clay loam) have the highest water holding capacity, while coarse soils (sand, loamy sand and sandy loam) have the lowest water holding capacity. Medium textured soils with a blend of silt, clay and sand particles and good aggregation provide a large number of pores that hold water against gravity. Coarse soils are dominated by sand and have very little silt and clay. Because of this, there is little aggregation and few small pores that will hold water against gravity. Fine textured clayey soils have a lot of small pores that hold much water against gravity. Water is held very tightly in the small pores making it difficult for plants to adsorb it.

Since soil texture varies by depth, so does water holding capacity. A soil may have a clayey surface with a silty B horizon and a sandy C horizon. To determine water holding capacity for the soil profile, the depth of each horizon is multiplied by the available water for that soil texture, and then the values for the different horizons are added together. These determinations are shown for two soils in Table 2.7.

 

 Table 2.7. Calculation of water holding capacity for a soil profile
 

 

Water relations are greatly affected by cultural practices, but the effect is largely indirect. For instance, tillage breaks down aggregates, decreasing the number of large pores. This would cause a decrease in infiltration rate and percolation, the water content at field capacity would increase, and gravitational water would decrease. If compaction causes an increase in the number of very small pores, unavailable water may increase, and water holding capacity may decrease. As a result, the amount of plant available water would also decrease.

On your own, consider the effect of different crops, crusting and organic matter on water relations and their relationship to other physical properties and processes.