Soil horizons can be differentiated on the basis of structural type, class, or grade. What causes aggregates to form and what holds them together? Clay particles cohere to each other and adhere to larger particles under the conditions that prevail in most soils. Wetting and drying, freezing and thawing, root and animal activity, and mechanical agitation are involved in the rearranging of particles in soils--including destruction of some aggregates and the bringing together of particles into new aggregate groupings.
Organic materials, especially microbial cells and waste products, act to cement aggregates and thus to increase their strength. On the other hand, aggregates may be destroyed by poor tillage practices, compaction, and depletion of soil organic matter.
The structure of a soil, therefore, is not stable in the sense that the texture of a soil is stable. Good structure, particularly in fine textured soils, increases total porosity because large pores occur between aggregates, allowing penetration of roots and movement of water and air. Consistence is a description of a soil's physical condition at various moisture contents as evidenced by the behavior of the soil to mechanical stress or manipulation.
Descriptive adjectives such as hard, loose, friable, firm, plastic, and sticky are used for consistence. Soil consistence is of fundamental importance to the engineer who must move the material or compact it efficiently. The structure and stability of soil changes as it gets wet, dry and wet again — something architects and engineers know only too well.
New US research combining geophysics and fluid mechanics provides some clues to how and why this happens and, more generally, to how particles stick together and then pull apart. The work by a team from the University of Pennsylvania — described in a paper in the journal PNAS — grew out of earlier research to determine how the needle-like fibres in asbestos stick to each other, and to other materials, to form aggregates.
Ali Seiphoori and colleagues began thinking more generally about what determines the strength and stability of an aggregate — so they created a simple model. They suspended very small glass spheres of two sizes — three and 20 microns — in a droplet of water.
As the water evaporated, the edges of the droplet retreated, dragging the particles inward. These new insights about the contribution of particle size to aggregate stability open up new possibilities for considering how to enhance stability of materials like soil or cement when desired. In addition, the production of a variety of materials, from medical devices to LED screen coatings, relies on thin film deposition, which the researchers say might benefit from the controlled production of aggregates that they observed in their experiments.
Materials provided by University of Pennsylvania. Note: Content may be edited for style and length. Science News. Story Source: Materials provided by University of Pennsylvania. Arratia, Douglas J. Formation of stable aggregates by fluid-assembled solid bridges. ScienceDaily, 4 February University of Pennsylvania.
Tiny 'bridges' help particles stick together. Retrieved November 12, from www. Some of the smallest clay particles may remain permanently in suspension and will not settle out. By measuring the depth of each layer of soil particles, you can figure the percentage of sand, silt, and clay in your soil.
Dig holes 1 foot deep by 2 feet wide in various places in your garden or landscape. Cover the holes with plastic to let the soil dry out. Once it's dry, fill the hole to the top with water and time how long it takes for the water to completely drain. The ideal time should be between 10 and 30 minutes. If the water drains in less than 10 minutes, then your soil will tend to dry out quickly in summer.
If it takes 30 minutes to 4 hours to drain, you can still grow most plants but will have to water slowly to avoid runoff and to allow the water to soak deeply. If your soil takes longer than four hours to drain, you may have a drainage problem. One caveat: Extremely dry soils, especially those with large amounts of clay, tend to crack.
The water in the drainage test will leave quickly because of these cracks, not because of good structure. The simplest way to see if your soil has a hardpan or compaction layer below the surface is to take a metal rod and walk around your property sticking it into the ground.
If you can't easily push the rod into the soil at least 6 to 8 inches deep, then you need to improve the aeration of your soil. If you push it down and consistently meet resistance at a certain depth, then there may be a hardpan layer. To further check for a compaction layer, dig a 1- to 2-foot-deep hole in the garden and look at the profile. A hardpan appears as a horizontal layer of soil that looks denser with less crumbly soil particles than the topsoil, has few roots or earthworms, and may even be a darker color than the rest of the soil.
Another way to tell if you have a hardpan layer is to dig up a plant and examine the roots. If they're white, vigorous, and well branched and extend at least 6 to 8 inches deep, then your soil has good structure.
If the roots are 1 to 2 inches deep, mushy, and gray colored, they are infected with a bacterial rot. If they are shallow, brittle, and black, they're infected with a fungal rot. Both diseases are enhanced by poor drainage either from a high water table or a compaction layer. To check if your soil is severely alkaline, take 1 tablespoon of dried garden soil and add a few drops of vinegar.
If the soil fizzes, then the pH is above 7. The free carbonates in the soil react with the acid at a pH of 7. To check for acidity in the soil, take 1 tablespoon of wet soil and add a pinch of baking soda. If the soil fizzes, then the soil is probably very acidic pH less than 5. The ideal pH for most plants is 5. A few plants prefer more extreme conditions. Try this remedy for acidic or alkaline soil: If your pH is on the extreme end of either range, take a soil test to determine the exact pH.
Add the appropriate amounts of limestone for acidic soils and sulfur for alkaline soils , according to the soil test.
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