Soil Texture and Structure

Soil texture determination requires about three hours of lab time to complete - you will work in teams of three individuals. To make the most efficient use of your time prepare the soil sample for analysis and make the initial hydrometer reading. During the two-hour interval you should continue with other experiments. Remember to take the second hydrometer reading.

This is the equipment necessary to complete the texture determination (mixer, mixer cup, cylinder, the hydrometer, wash bottle, and stirring rod). Notice the precaution in step 2 (DO NOT fill the cup within 2 inches of the top, otherwise it will splash over while being mixed].

When attaching the cup to the stirrer, the top of the cup should be raised straight upwards and hooked securely under the switch finger, with the base hooked over the lower finger support. When the cup is secure on the mixer, plug in the electrical cord and mix the sample for five minutes. Unplug the mixer before removing the cup. This prevents damage to the cup e.g. in Slide 3.

Transfer the soil sample to the cylinder using the wash bottle, as illustrated. Make certain that all of the soil is transferred from the cup to the cylinder. In Step 5, the hydrometer is carefully placed in the cylinder when filling to the 1000 ml mark illustrated in Slide 4.

Here you see how to hold a hydrometer,which is extremely fragile ($85 replacement cost). The long thin stem is especially vulnerable to breakage. When placing the hydrometer into the cylinder slowly lower it, holding it vertically with the stem between the thumb and forefinger. Do not permit the hydrometer to bob to the bottom of the cylinder as the tip will break on impact. When you are not using the hydrometer, leave it resting in the box so it does not roll onto the floor.

Stir the suspension using the stirring rod, making certain that you do not lift the stirring rod so rapidly or push down so strongly that the suspension sample is splashed out of the cylinder. Note the timing sequence that is described in Steps 7, 8, and 9 of the lab manual.

The hydrometer graduations are shown here. The scale is in units of grams / liter. When reading the hydrometer in suspension, read the bottom of the meniscus.

The relative size of the primary particles is shown here. The weather balloon represents a sand particle and the basketball represents silt particles. The glass beads represent clay particles. In reality, clay particles are plate-shaped.

Texture is a basic physical property of soil that differentiates soils. A soil with most of the particles within the range between 2 mm and 0.05 mm would be considered a sandy soil (illustrated here).

A photomicrograph of a sandy soil is shown in Slide 12. Sand grains are angular in shape and tend to fit together rather closely in the natural state. But, they are relatively large in size and there are no forces of attraction between the individual grains; hence, the looseness. Large-sized pores exist between the grains, thus, the low water holding capacity and excessive drainage characteristics. Because sand grains do not exhibit surface activity (i.e., cation exchange and adsorption of water and gases) they do not contribute to the fertility status of a soil as do clay and organic matter.

As clay and organic matter contents increase, the physical characteristics of the soil change. Silt particles, because of their smaller diameter, impart greater water holding capacity and slower drainage, but do not contribute significantly to the fertility status of the soil other than through secondary effects. Silt particles are angular in shape and lack surface activity. Clay particles are very small in size, plate-shaped, and have surface activity. All of these factors combine provide clay with high water holding capacity, slow drainage, and enhanced fertility status. Clay particles may adsorb large amounts of water, and the soil becomes very cohesive and plastic. Soils high in clay content are also sticky when wet. Because of the high water holding capacity and slow drainage, gaseous diffusion is restricted, tending to inhibit root extension. Sandy soils, in contrast, are generally well-aerated, and root extension is not inhibited.

The extremes of soil texture are not particularly desirable for most uses. Generally, soils of medium texture (loams) would be most desirable, especially for plant growth.

The field determination of texture is described for in the text. This slide illustrates a cast of moist sand. This material is extremely gritty. As the texture becomes finer and more coherent, the soil material becomes more workable in the hands. The ribboning effect of clay and clay loam soils is illustrated in here. When clay loams dry and harden, they are broken only with difficulty as shown (recall walking on the Tully mudslide). Sometimes a hammer is needed. The increasing coherence and plasticity with increase in clay content of the soil material is due primarily to the great forces of attraction between clay particles, either as clay aggregates or as coatings on sand and silt particles, and the increase in adsorption of water.

Recall that clay transport from the upper horizons to the B horizon is one of the soil-forming processes. When texture in soil profiles changes, it is described as 'textural discontinuity'. For example, a soil with a loam texture A horizon overlying a B horizon consisting of coarse sands and gravels exhibits textural discontinuity.

Texture is an inherited property of soil and is not changed by agricultural cultivation except when the topsoil might be mixed in with the subsoil during deep plowing or when some of the topsoil has been eroded away. However, cultivation or other disturbance to the soil does have a profound influence on soil structure. Structure is the aggregation of primary particles into peds, and is dependent upon many factors that can be readily influenced by soil manipulation.

This slide illustrates the round shaped peds of granular structure, typical of a surface soil under grassland or pasture cropping, and in some cases, under a forest soil surface horizon. Granular structure results from high organic matter content and soil organism activity coupled with the binding effects of fine roots. With few exceptions, granular structure is confined to surface horizons. Ggranular structure is porous, has a high water infiltration capacity, permits root penetration and gaseous diffusion. Consequently, granular structure promotes optimum root growth and development.

Several peds of subangular blocky structure, characteristic for many subsoils, are shown here. Although many very fine pores may exist within each ped, the major portion of the pore space exists BETWEEN peds as channels and planes where major root extension and water movement normally occur. Subangular blocky structure is not as porous as granular structure, but its porosity or lack thereof, is detrimental only under compacted or dense subsoil conditions.

Platy structure, illustrated in this slide , has longer axes in the horizontal plane. This type occurs almost exclusively in the subsoil and forms under conditions of compaction, high bulk density, or is inherited from the parent material (i.e. horizontally bedded shales or siltstone). Root extension and water movement are restricted.

To illustrate structural differences within the soil profile, this slide shows granular structure in the Ap, and strongly developed, nearly massive to subangular blocky structure in the subsoil (between 8 and 17 inches. Notice where most of the roots occur in this B horizon.

Little structure differences occur within this ( profile, with the A horizon having very weak granular structure and the remaining horizons having single- grained structure.

Differences in color due to differences between the inside and outside of the peds (structural units) are illustrated in here from left to right. The yellowish-brown color on the left is due to the relatively dry conditions that exist within the tight peds, whereas the gray-brown color on the right is due to the more moist conditions outside the ped. The moist condition persists for an extended period of time in this soil. The peds are very dense, and both root penetration and water movement are confined to the transpedal pores. Reduction conditions exist in the spaces between the peds leading to the reduced iron color. In many cases, well developed structure offsets the detrimental effects of texture extremes by enhancing total porosity and by creating a more favorable distribution of pore space, and a wider range of pore sizes.

Bulk Density and Total Pore Space

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. We are primarily interested in bulk density and pore space as they affect water, aeration status, root penetration and development.

Clay particles are flat, or plate-shaped and 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. Particle size and shape have a significant effect on the soil bulk density.

Important Generalization: Total pore space increases as soil texture becomes more fine.
Sand grains are relatively large and angular in shape. Large-sized pores predominate because smaller silt and clay particles do not fill the spaces between sand grains. The single grain structure 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 pore sizes and shapes. This in turn influences pore size distribution.

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. This does occur under some natural conditions. Fragipans are formed in silt loams. The large proportion of silt, which does not promote aggregation, acts to plug the pores between the larger sand sized grains, producing high bulk density. Increasing clay and organic matter content increases the volume of small pores and promotes aggregation (formation of structure) resulting in reduced bulk density. 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. Macropores between the peds (interpedal). Micropores within peds are referred to as 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.

In (e) granular structure, roots permeate 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, furtherenhancing the degree of aggregation. Single-grain structure has somewhat the same appearance, but on the smaller scale of individual sand grains. Granular structure has a predominance of macropores, and bulk density is low. Blocky structure is illustrated in (f) of the lab manual 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. Conditions are more restrictive for roots. 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.

Water is retained in small pores and easily drained from large pores (i.e. fine-textured soils or those with platy structure). 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 have intermediate conditions of drainage and water retention.

Here you see the equipment displayed for obtaining an undisturbed soil sample for bulk density and pore space determination (we used this equipment to collect samples at 3-Rivers and Heiberg field sites. 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.

When a soil is compacted, as under vehicle traffic, or artificially during construction, 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.

Revised October 6, 2006