Soil

Cation and anion exchange

Cation exchange, between colloids and soil water, buffers (moderates) soil pH, alters soil structure, and purifies percolating water by adsorbing cations of all types, both useful and harmful. The negative charges on a colloid particle make it able to hold cations to its surface. The charges result from four sources. Isomorphous substitution occurs in clay when lower-valence cations substitute for higher-valence cations in the crystal structure. Substitutions in the outermost layers are more effective than for the innermost layers, as the charge strength drops off as the square of the distance. The net result is a negative charge. Edge-of-clay oxygen atoms are not in balance ionically as the tetrahedral and octahedral structures are incomplete at the edges of clay. Hydrogens of the clay hydroxyls may be ionised into solution, leaving an oxygen with a negative charge. Hydrogens of humus hydroxyl groups may be ionised into solution, leaving an oxygen with a negative charge. Cations held to the negatively charged colloids resist being washed downward by water and out of reach of plants’ roots, thereby preserving the fertility of soils in areas of moderate rainfall and low temperatures. There is a hierarchy in the process of cation exchange on colloids, as they differ in the strength of adsorption and their ability to replace one another. If present in equal amounts: Al3+ replaces H+ replaces Ca2+ replaces Mg2+ replaces K+ same as NH4+ replaces Na+ If one cation is added in large amounts, it may replace the others by the sheer force of its numbers (mass action). This is largely what occurs with the addition of fertiliser. As the soil solution becomes more acidic (an abundance of H+), the other cations bound to colloids are pushed into solution. This is caused by the ionisation of hydroxyl groups on the surface of soil colloids in what is described as …

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Carbon and terra preta

In the extreme environment of heavy rain and high temperatures of tropical rain forests, the clay and organic colloids are largely destroyed. The heavy rain washes the alumino-silicate clays from the soil leaving only sesquioxide clays of low CEC. The high temperatures and humidity allow bacteria and fungi to virtually dissolve any organic matter on the rain-forest floor overnight and much of the nutrients are volatilized or leached from the soil and lost. Carbon, however, is far more stable than soil colloids and is capable of performing many of the functions of the soil colloids of sub-tropical soils. Research into terra-preta is still young but is promising. Fallow periods “on the Amazonian Dark Earths can be as short as 6 months, whereas fallow periods on Oxisols are usually 8 to 10 years long”

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Organic colloids

Humus is the penultimate state of decomposition of organic matter; while it may linger for a thousand years, on the larger scale of the age of the other soil components, it is temporary. It is composed of the very stable lignins (30%) and complex sugars (polyuronides, 30%). Its chemical assay is 60% carbon, 5% nitrogen, some oxygen and the remainder hydrogen, sulfur, and phosphorus. On a dry weight basis, the CEC of humus is many times greater than that of clay. Plant roots also have cation exchange sites.

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Sesquioxide clays

Sesquioxide clays are a product of heavy rainfall that has leached most of the silica and alumina from alumino-silica clay, leaving the less soluble oxides of iron Fe2O3 and iron hydroxide (Fe(OH)3) and aluminium hydroxides (Al(OH)3). It takes hundreds of thousands of years of leaching to create sesquioxide clays. Sesqui is Latin for “one and one-half”: there are three parts oxygen to two parts iron or aluminium; hence the ratio is one and one-half. They are hydrated and act as either amorphous or crystalline. They are not sticky and do not swell, and soils high in them behave much like sand and can rapidly pass water. They are able to hold large quantities of phosphates. Sesquioxides have low CEC. Such soils range from yellow to red in color. Such clays tend to hold phosphorus tightly rendering them unavailable for absorption by plants.

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Amorphous clays

Amorphous clays are young, and commonly found in volcanic ash. They are mixtures of alumina and silica which have not formed the ordered crystal shape of alumino-silica clays which time would provide. The majority of their negative charges originates from hydroxyl ions, which can gain or lose a hydrogen ion (H+) in response to soil pH, and hence buffer the soil pH. They may have either a negative charge provided by the attached hydroxyl ion (OH-), which can attract a cation, or lose the hydrogen of the hydroxyl to solution and display a positive charge which can attract anions. As a result they may display either high CEC, in an acid soil solution, or high anion exchange capacity, in a basic soil solution.

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Silica clays

Alumino-silica clays are characterised by their regular crystalline structure. Oxygen in ionic bonds with silicon forms a tetrahedral coordination which in turn forms sheets of silica. Two sheets of silica are bonded together by a plane of aluminium which forms an octahedral coordination, called alumina, with the oxygens of the silica sheet above and that below it. Hydroxyl ions (OH-) sometimes substitute for oxygen. As much as one fourth of the aluminium Al3+ may be substituted by Zn2+, Mg2+ or Fe2+, and Si4+ may be substituted by Al3+. The substitution of lower-valence cations for higher-valence cations (isomorphic substitution) gives clay a net negative charge that attracts and holds soil solution cations, some of which are of value for plant growth. Isomorphic substitution occurs during the clay’s formation and does not change with time. Montmorillonite clay is made of four planes of oxygen with two silicon and one central aluminium plane intervening. The alumino-silicate montmorillonite clay is said to have a 2:1 ratio of silicon to aluminium. The seven planes together form a single layer of montmorillonite. The layers are weakly held together and water may intervene, causing the clay to swell up to ten times its dry volume. It occurs in soils which have had little leaching, hence it is found in arid regions. The entire surface is exposed and available for surface reactions and it has a high cation exchange capacity (CEC). Illite is a 2:1 clay similar in structure to montmorillonite but has potassium bridges between the clay layers and the degree of swelling depends on the degree of weathering of the potassium. The active surface area is reduced due to the potassium bonds. Illite originates from the modification of mica, a primary mineral. It is often found together with montmorillonite and its primary minerals. It has moderate …

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Soil clays

Due to its high specific surface area and its unbalanced negative charges, clay is the most active mineral component of soil. It is a colloidal and most often a crystalline material. In soils, clay is defined in a physical sense as any mineral particle less than two microns (2×10−6 inches) in effective diameter. Chemically, clay is a range of minerals with certain reactive properties. Clay is also a soil textural class. Many soil minerals, such as gypsum, carbonates, or quartz, are small enough to be classified physically as clay but chemically do not afford the same utility as do clay minerals. Clay was once thought to be very small particles of quartz, feldspar, mica, hornblende or augite, but it is now known to be (with the exception of mica-based clays) a precipitate with a mineralogical composition that is dependent on but different from its parent materials and is classed as a secondary mineral. The type of clay that is formed is a function of the parent material and the composition of the minerals in solution. Mica-based clays result from a modification of the primary mica mineral in such a way that it behaves and is classed as a clay. Most clays are crystalline, but some are amorphous. The clays of a soil are a mixture of the various types of clay, but one type predominates. Most clays are crystalline and most are made up of three or four planes of oxygen held together by planes of aluminium and silicon by way of ionic bonds that together form a single layer of clay. The spatial arrangement of the oxygen atoms determines clay’s structure. Half of the weight of clay is oxygen, but on a volume basis oxygen is ninety percent. The layers of clay are sometimes held together through hydrogen bonds or potassium bridges and …

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Chemical and colloidal properties

The chemistry of soil determines the availability of nutrients, the health of microbial populations, and its physical properties. In addition, soil chemistry also determines its corrosivity, stability, and ability to absorb pollutants and to filter water. It is the surface chemistry of clays and humus colloids that determines soil’s chemical properties. The very high specific surface area of colloids and their net negative charges, gives soil its great ability to hold and release cations in what is referred to as cation exchange. Cation-exchange capacity (CEC) is the amount of exchangeable cations per unit weight of dry soil and is expressed in terms of milliequivalents of hydrogen ion per 100 grams of soil. “A colloid is a small, insoluble, nondiffusible particle larger than a molecule but small enough to remain suspended in a fluid medium without settling. Most soils contain organic colloidal particles as well as the inorganic colloidal particles of clays.

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Soil atmosphere

The atmosphere of soil is radically different from the atmosphere above. The consumption of oxygen, by microbes and plant roots and their release of carbon dioxide, decrease oxygen and increase carbon dioxide concentration. Atmospheric CO2 concentration is 0.03%, but in the soil pore space it may range from 10 to 100 times that level. At extreme levels CO2 is toxic. In addition, the soil voids are saturated with water vapour. Adequate porosity is necessary not just to allow the penetration of water but also to allow gases to diffuse in and out. Movement of gases is by diffusion from high concentrations to lower. Oxygen diffuses in and is consumed and excess levels of carbon dioxide, diffuse out with other gases as well as water. Soil texture and structure strongly affect soil porosity and gas diffusion. Platy and compacted soils impede gas flow, and a deficiency of oxygen may encourage anaerobic bacteria to reduce nitrate to the gases N2, N2O, and NO, which are then lost to the atmosphere. Aerated soil is also a net sink of methane CH4 but a net producer of greenhouse gases when soils are depleted of oxygen and subject to elevated temperatures.

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Consumptive use and water efficiency

Only a small fraction (0.1% to 1%) of the water used by a plant is held within the plant. The majority is ultimately lost via transpiration, while evaporation from the soil surface is also substantial. Transpiration plus evaporative soil moisture loss is called evapotranspiration. Evapotranspiration plus water held in the plant totals consumptive use, which is nearly identical to evapotranspiration. The total water used in an agricultural field includes runoff, drainage and consumptive use. The use of loose mulches will reduce evaporative losses for a period after a field is irrigated, but in the end the total evaporative loss will approach that of an uncovered soil. The benefit from mulch is to keep the moisture available during the seedling stage. Water use efficiency is measured by transpiration ratio, which is the ratio of the total water transpired by a plant to the dry weight of the harvested plant. Transpiration ratios for crops range from 300 to 700. For example alfalfa may have a transpiration ratio of 500 and as a result 500 kilograms of water will produce one kilogram of dry alfalfa.

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