Bulk Density and Pore Space

This week I want to guide you through an exposition of bulk density and pore space, and how texture and structure may affect these soil properties.

Bulk Density is defined as the weight of a unit volume of soil including its pore space. Since soil is a porous medium, with water and air contained in the pore space between the solid inorganic and organic particles, the concept of soil bulk density must include the voids. This concept is quite unlike others concerning density in Physics class where you considered only the volume of solid material. Water and air are important components of soil and we must frame our soil concepts so that factors affecting water and air dynamics are included. Thus, we are primarily interested in bulk density and pore space as they affect water and aeration status, and root penetration and development.

Carefully read pages 1 and 2 of the laboratory exercise before proceeding further.

Look at Figure 4.1 on the first page. In (a) and (b) you can see how porous material becomes more dense as the particles decrease in diameter. The smaller particles occupy more of the unit volume than the larger particles in the same volume. It is the same relationship between sand particles versus silt particles. Soils that predominate in sands have more total pore space than soils that have predominately silt-sized particles; hence, silts are more dense than sands. Something else also occurs as density increases with decreasing particle size-- the size of the pores between the particles decreases. This increases the impedance to water movement and root penetration. A different situation arises when we consider density and a soil predominated by clay particles. Consider Slide 1:
You see here that clay particles are flat, or plate-shaped. And, that they pack as random stacks of particles. Because of this random packing, the solids are not as effective as spheres are in occupying a unit volume. If they were perfectly stacked like bricks in a wall, then they would very easily fill all the space. The randomly packed material is less dense than silt or sand. Further, the pore sizes are extremely small-- but, there are many more of them. Briefly then, not only particle size but also particle shape has significant effect on the density of soil.

To carry the discussion into the consideration of field soils, then we must make some generalizations, some of which will first appear to refute what I have just told you. Consider this carefully: Sand grains are angular in shape and fit well into the space between themselves as compares to spheres of the same size tending to make the soil dense. Large-sized pores predominate because there is not much of the smaller silt and clay particles or organic matter to fill the spaces, and there are not many of them since the predominant particle size is large and close fit together. The structure is usually single-grain for sandy soils and does not contribute to the enhancement of the bulk volume as do other types of structure. As the texture becomes finer, or as silt plus clay increases, total pore space increases together with the range and diversity of pre sizes and shapes. This trend is caused by several factors. The increase in silt and clay infers an increase diversity of particle sizes. This in turn influences pore size and the distribution of pores. It would be expected that the greater diversity of particle size would result in a massive or compacted soil since the small particles may fit between the larger particles, and so on. This does occur under some natural conditions, but the usual result is that with the increase in clay, fertility is enhanced together with waterholding capacity. These factors favor the production of greater amounts of organic matter than in coarse-textured soils. There is also an increase in organism activity. All of these factors together form structure. You learned in the third laboratory exercise that the formation of structure increases the soil bulk volume, meaning that total pore space has increased as the result of the solid particles being arranged in macropores within peds. The clay and organic matter, plus root exudates and excretions of soil organisms act as cementing agents for the structural aggregates. It can be said that a soil occupies greater bulk volume after the formation of structure.

Here you see an illustration of pore size diversity as the result of structure formation. Here can be seen the macropores between the peds, or referred to as being interpedal, and the micropores within the peds, being intrapedal. The large pores with connecting channels act as routes for root extension, water movement and gaseous diffusion, whereas the meso- and micropores are involved in the retention of water against the force of gravity. It is probable that the major portion of chemical activity takes place in the meso- and micropores when occupied by fine roots. Soil organism activity may predominate on the walls of the macro- and mesopores, and to a lesser degree in the micropores. However, little is actually known about the exact location of much of the microorganism activity.

The generalized trend of increased pore space and decreased density continues until a critical point is attained where total pore space decreases and density increases. The abundance of clay, much of which becomes mobile, fills or clogs the smaller pores. The reduced waterholding capacity causes the soil to be droughty, in turn reducing the organic matter content. Aeration becomes a problem by the reduction of oxygen concentration and the attendant increase in carbon dioxide. Aggregation of the soil is reduced, and structure is not well-developed. There is a natural tendency towards compaction.

Let's now examine the effect of structure on pore size, pore distribution, and density as illustrated in (e), (f), and (g) in Figure 4.1. In (e), granular structure is shown. Roots can be found permeating the total pore volume, with some fine roots penetrating the granular peds. The root network is generally so profuse that the granules are bound into compound peds, enhancing still further the degree of aggregation. Single-grain structure has somewhat the same appearance, but on the smaller scale of individual sand grains. The granular structure has a predominance of macropores, many very large in size. Bulk density is low. Blocky structure is illustrated in (f) of the figure. Most roots occur in the large pores at the corners of blocks and within the planar channels between the smooth faces of the peds. Some fine roots may penetrate the peds by following fine pores. As I discussed for medium-textured soils, greater resistance to water movement and root penetration occurs here as compared to granular structure, but the conditions are still favorable for good plant growth. In the last illustration labeled (g), platy structure is shown. By a little thought about what has been said, what do you think is the pore space distribution and water relations here? If you said that conditions are more restrictive here, you are correct! Short-channeled, medium-sized pores exist as vertical pores, with horizontal pores as long, thin planar channels. Obviously, root penetration direction, water and gaseous movement tends to be at right angles to the axis of the long horizontal pores. Impedance of all sorts is easily understood.

Before we leave the present discussion, I wish to mention that water is retained in small pores and easily drained from large pores. So, in fine-textured soils or those with platy structure, water is retained in many of the pores. This impedes gaseous diffusion and thus has a detrimental effect on root growth. On the other hand, most of the moisture drains from a sandy soil in response to gravity. Subangular blocky and blocky structure in a medium to medium-fine textured soil has intermediate conditions of drainage and water retention.

This completes the first portion of the presentation, and some of the things I have talked about probably sound familiar to you as material given in the lectures or in the textbook. I feel that it would be most helpful to for me to repeat coverage of some of this material in slightly different terms, as these relationships are difficult for many students to grasp. Don't hesitate to ask for assistance.
Here you see the equipment displayed for obtaining an undisturbed soil sample for bulk density and pore space determination. The sampling head on the block of wood holds the aluminum ring used to retain the soil sample. The head is then screwed on to the T-handle. The weight is used to pound the sampler into the soil. The cans are used to transport the undisturbed sample to the laboratory.
The sampling tool is completely assembled here.

Here, the sampler is being gently pounded into the soil.

The ring with soil core has been removed from the sampling head and is being trimmed to ring volume with a knife. This part of the operation must be done with care to prevent disturbance to the natural structure of the soil sample.

The trimmed core is now ready to be placed in the can. The soil data that you will work with in the laboratory has been obtained by this method.

Soil clods dug from the wall of the soil pit have been placed in hair nets and dipped in liquid Saran to hold them together and to provide a waterproof coating on the clod. They are then hung up to dry. This method is a particularly good one and used by many soil physicists.

Two final points need to be discussed before I close today. First, when a soil is compacted, as under vehicle traffic, or artificially during construction, the soil structure is destroyed, partially or totally, depending upon the degree of compaction. Reduction in bulk volume occurs, and usually at the expense of all the macropores and many of the mesopores. Bulk density is increased and water retention and movement are significantly reduced. It was mentioned earlier that sandy soils do not compact to a large degree under most conditions. But many medium- and fine-textured soils are particularly susceptible to compaction. Moisture content at the time of compaction also is a major influence.