Soil Structure and pH
What is soil structure?
Soil structure refers to the way soil particles are bound together into aggregates of different sizes and shapes. It determines the amount of air and water that may penetrate into the soil. This is important because a plant’s ability to take root and grow to maturity is directly affected by the porosity of the soil.
For instance, heavy or dense soils will reduce the flow of air, water and nutrients into the soil and result in a reduced ability of the plant to flourish. Aggregates of soil that occur naturally are called peds. Aggregates that occur as a result of artificial actions are called clods. Soil structure is described by the geometric shape of the peds or clods. Generally, structural types of soil are described as columnar, blocky, granular, plate-like, prismatic and structureless. Each type affects the movement of water, air and nutrients in a different way. Some soils have simple structures made up of one structural type. Other soils are more complex, having compound structures with more than one structural type.
What is Columnar Soil Structure?
Columnar soil structure is characterized by rounded vertical shapes similar to columns. Cracks run longer in the vertical direction than the horizontal. Columnar soil structure is common in subsoils containing sodium. Columnar structure is dense and, therefore, difficult for plant roots to penetrate, but water penetration, aeration, and drainage are all good.
What is Blocky Soil Structure?
The blocky structure contains large blocks of soil with the same number of cracks running vertically and horizontally. They appear as flat or slightly rounded surfaces and are usually the same dimension. Other forms of blocky structures are named subangular and angular, depending on their appearance when they intersect with other peds. Blocky structures occur mainly in subsoil but also occur in surface soils with a high clay content such as the surface of a dried pond. Blocky soil structure provides good water penetration into the soil but only moderate aeration and drainage.
Granular blocks of soil are small and round with the same number of cracks running in the vertical and the horizontal. They appear as cookie crumbs and not similar to adjoining peds. Granular structure provides good water penetration. It is the most porous type of soil structure and results in the best aeration and drainage of all the soil structures. The granular structure occurs in surface soils and garden soils where there is an ample mixture of mineral and organic matter.
What is Plate-Like Soil Structure?
In a plate-like soil structure, peds are flat or plate-like. The blocks of soil are generally longer in the horizontal than the vertical. A particular form of platy soil structure is called lenticular where the ped is thick in the middle and thin running down to the edges. The platy structure is usually found in subsurface soils that have been compacted by animals or machinery. Platy soil structure has only moderate water penetration, aeration and drainage as well as a reduction in the ability for roots to penetrate the soil.
What is Prismatic Soil Structure?
Prismatic structures are similar to columnar structures except that the tops of these structures are flat. Structureless soil occurs where there is no discernible shape to the peds. All the above soil structures can be determined by breaking apart each layer and matching it to the appropriate structural form.
What are Soil Aggregates?
Soil aggregates occurring naturally are formed as a result of various physical forces such as wetting and drying conditions, freezing and thawing cycles and organic matter activity. Soil structures are formed and affected by artificial means such as tilling, cultivation and irrigation. Since a well-structured soil most effectively encourages good plant growth and maturity, there is a great deal of activity in the area of soil structure improvement. This involves the proper management of insects residing in the soil as well as control of the amount and type of organic matter introduced into the soil.
Resources about Soil Structure and Types
Illinois.edu Soil Types People describe soil types in all kinds of ways such as heavy, light, sandy, clay, loam, poor or good. Soil scientists describe soil types by how much sand, silt and clay are present.
The Twelve Soil Orders Soil Taxonomy: In 1975, Soil Taxonomy was published by the United States Department of Agriculture’s Soil Survey Staff.
The Secrets of soil
Agricultural Service Laboratory at Clemson University, Soil Analysis Procedures
What is Soil pH?
Soil pH is a measurement of the acidity or alkalinity of soil. The measurement represents the negative logarithm of the amount of hydrogen ion in the soil. pH is measured on a scale from 1 to 14. A pH of 7 is considered neutral. For instance, pure water has a pH of 7. As the amount of hydrogen ions in the soil increases, the soil pH decreases and becomes more acidic so that a pH reading of 1 to 7 is considered acidic. As the amount of hydrogen ions decreases, the soil pH increases and becomes more alkaline or basic so that a pH reading of 7 to 14 is considered basic.
How Soil pH Affects Plants?
Plants need nutrients and minerals for growth. In order for a plant to absorb a nutrient or a mineral, it must be dissolved in the soil solution. Soil pH has a strong effect on the solubility of minerals and nutrients. If the pH is too low, the nutrients, although more easily soluble, may be easily leached or swept away and removed from the soil. At a high pH, the nutrients may become insoluble, and the plants will not be able to absorb them. Acidic soils having a pH in the range of 6.0 to 7.2 are most effective for dissolving minerals and nutrients.
Since hydrogen ions present in the soil are subject to water movement and movement of roots in the soil, as well as the chemical effects of weather reactions to the soil, soil structure that affects much of these activities, likewise, will have an effect on soil pH. Soils said to have good structure are granular in nature. Because of its crumbly nature, soil is porous and, therefore, provides very good water and air movement through the soil, but at the same time, it is resistant to erosion and leaching. The granular nature of soil is also characterized by a variety of pore sizes that can accommodate many different types of plant roots and soil organisms that use different pore sizes to circulate in the soil. Good soil structures encourage the free flow of water and air, which enhances the function of microorganisms in the breakdown of animal and plant residues. This assists in plant growth and development. However, even such soils can become acidic as a result of rainwater leaching away basic ions affecting pH levels as well as the acidic effects of the formation of organic and inorganic acids during decomposition in the soil. In these cases, artificial means, such as composting or liming, are used to maintain the proper pH levels. The introduction of organic matter enhancing soil biological activity can improve soil structure and provide the environment that results in a stable pH level that will encourage proper growth and development.
How Does Soil Structure Influence Soil pH?
The effect of soil structure on soil pH is mainly indirect. The main example is water movement through the soil known as leaching. Leaching removes valuable minerals from the soil when water seeps through or flows through the structure, but it can also wash away various toxins, such as fertilizers, that might have built up. Drainage and infiltration of water depend on soil structure. Pores larger than 0.05 millimeters allow excess water to drain away. Permeability is the measure of the soil’s ability to absorb and allow the flowing of water, which is controlled partly by the soil structure. Infiltration is concerned with the speed at which water can enter the soil. This is also partly affected by the surface soil structure, specifically, the type of cracking that manifests itself in the soil. All these factors influence the proper health of plant life in the soil and provide the environment for stable pH levels in the soil.
How are Dioxins Removed From Soil?
Dioxins refer to the family of dioxins and furans while the specific term “dioxin” specifically refers to the 2, 3, 7, 8 -tetrachlorodibenzo-p-diocin compounds. These are usually abbreviated to 2, 3, 7, 8-TCDD or even TCDD. In simpler terms it is referring to the family of chemicals which consists of 135 related compounds known as furans and 75 dioxins. The less chlorine atoms on the molecule, the higher the toxicity and the more chlorine atoms on the molecule, the lower the toxicity. Dioxins are formed by the presence of oxygen, carbon, hydrogen, heat and chlorine. They are highly lipophilic but do not dissolve easily in water. Due to their high lipophilic chemical build they are soluble in organic matters with fat like properties and fatty substances. They are odorless and colorless in their pure form with high boiling and melting points. Evaporation is also very slow due to their low vapor pressure. These properties explain why dioxins are found and dissolved in fat tissues. Dioxins have been in the environment and soil from early back as the 1800s. The amount was heightened during the 1930s and 1960s and peaked between the 1960s and 70s. The levels of dioxins started to decline after 1970. There has been a large focus from regulatory authorities around the world to remove dioxins from the environment, including the removal of dioxins from soil.
The contamination levels will suggest how high the concentration of dioxins are in most soils. Typically speaking they are usually quite low. If they do infiltrate the soil, they attach strongly to particles which are high in organic matter and bind tightly. Since the connection is so strong, it is difficult to remove these dioxins. Typically, water alone is not strong enough to move dioxins nor are they able to move freely within the soil. Dioxins which are able to get free are moved through a natural process of soil erosion, which is a combination of wind and water. Soil erosion only works in moving dioxins if the dioxins have a low organic content.
Polychlorinated biphenyls (PCBs) are a related chemical and once burnt they become dioxins. PCBs are a group of organic compounds comprising of one to ten chlorine atoms attached to biphenyl. Biphenyl is a molecule which is composed of 2 benzene rings. They are created by electrophlic chlorination of biphenyl with chlorine gas. Their properties and characteristics are similar to what was explained above for dioxins, for example they are tasteless and odorless. They do not degrade easily and are very stable compounds. They have high thermal conductivity and high dielectric constants. PCBs live in the organic fraction of the soil. This is due to their low vapor pressure.
A recent test has analyzed the effect of removing PCBs from soil. This was achieved by first mixing PCB- contaminated soil with a hot (150C) KPEG reagent in a rotating industrial mixer. The regent was made up of a combination of poluethlyene glycol (PEG), potassium hydroxide (KOH), polyethylene glycol monemethylether (PEGM) and dimethylsulfoxide (DMSO). Soil moisture was then volatilized and recovered to use later in the process. These solutions were mixed for thirty minutes to two hours. Once mixed, the bulk of the solution was decanted from the treated soil. Then the dechlorinated by-products and residual reagent were removed from the soil. This was done by mixing the soil with an equal volume of water and then decanting the water. This process was repeated two to three times which produced a 99% recovering of the regents. The dehalogenated products are removed in the last wash through a bed of activated carbon. The contaminated carbon is then burned in a PCB-approved incinerator.
Soil PH and Sciences Resources
Information for kids about soil chemistry and facts
about soil pH. The importance of soil pH
How it affects plants
esf.eduSoil pH: What it Means From the SUNY College of Environmental Science and Forestry
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