Chemistry of Variable Charge Soils (Topics in Sustainable Agronomy)

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One important variable to be considered when predicting the improvements possible through organic residue management is whether the organic material was grown in situ or obtained from an exterior location. If imported, the nutrient content of the organic material is contributed to the soil system. If it is grown in situ, the overall benefit is usually less because the nutrients are simply recycled. In some cases, however, recycling and bringing nutrients from deep zones in the soil profile can substantially improve the surface soil—the root zone for most annual food crops.

The role of organic material in reducing aluminum toxicity, often the most detrimental aspect of the soil-acidity syndrome, includes the chemical complexing of the aluminum in solution, thereby reducing its activity. Organic material additions can, in some cases, also alleviate phosphorus deficiency in acid soils by supplying phosphorus directly, by reducing phosphorus sorption Capacity, and by complexing soluble aluminum and iron, thereby increasing soluble phosphate concentrations. In large amounts, organic materials can reduce acidity simply by increasing the soil pH. In smaller amounts, the type of organic material becomes important; for example cowpea is more effective in reducing acidity than leucaena.

The ecology of biological nitrogen fixation BNF , including plant-microbe-climate-soil interactions and improved methodologies for predicting BNF response and for quantifying BNF by trees;. Analysis of agroforestry systems, including rooting patterns, allelopathy, nutrient accumulation, and nutrient cycling;. The role of organic matter in variable charge soils on soil acidity, phytotoxicity, and similar factors; and. Soil process-related management techniques to control soil pathogens and nematodes e. The enhancement and maintenance of biotic inputs for sustainability in low industrial input systems, including synchronization of nutrient release from organic inputs to meet nutrient uptake demand by crops and appropriate biotechnology efforts.

Rarely do the inherent properties of the soil provide an ideal environment for agricultural use. Fortunately, many of the limitations are amenable to improvement through inputs, manipulation, and other management practices. Research has led to substantial progress in identifying the fundamental constraints and basic principles of soil management, although such work has been conducted largely in developed countries in temperate regions. However, for both the developed and developing world, a central weakness is our limited capability to provide optimal site-specific soil and water management practices that can be employed by individual land users within the context of their needs and the prevailing social, economic, and political climate.

Thus, this ability to translate scientific knowledge about soil characteristics and plant growth into useful information for farmers is a major research need for the future. Better management of the chemical and physical characteristics of the soil is critical to sustainability.

Research on better management of soil properties will involve both the chemical and physical characteristics of the soil system, and this area offers special potential in the context of the limited use of capital-intensive input characteristics of many developing countries. For instance, work on sources of nutrient amendments is key because low residual levels of essential elements is a common cause of soil infertility in the tropics. This condition usually can be corrected through amendments. Much of the technology for these types of management practices has been developed in areas where purchased inputs are readily available.

In developing countries, however, the capital for such investments and the managerial capability to deal with the type and level of technology is limited. Therefore, alternative practices should be provided that are compatible with local natural resources and social, cultural, and economic conditions. Special emphasis should focus on biotic amendments. Similarly, research on the efficient use of organic materials is critical.

Organic materials can have multiple benefits in reducing or alleviating many soil chemical problems. Notable among these are providing nitrogen and other essential nutrients and correcting soil acidity. The technology for effective and efficient use of organic materials—for example, nutrient-accumulating species of plants and management of residue—is not available in a form suitable for most of the developing world. This is an area where the blending of scientific knowledge with indigenous knowledge offers great potential benefits.

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Mechanisms to ameliorate soil compaction and crusting are important because these frequently lead to decreased water infiltration, increased runoff, increased erosion, and reduced stand and growth of seedlings. General principles for dealing with these problems are reasonably well understood, but management practices that would be useful in the developing world require better knowledge of fundamental causes and alternative solutions. Given the ever-increasing pressures for production in the developing world, strategies for restoring degraded lands will also prove key over the long term.

Past mismanagement has resulted in the abandonment of extensive areas. The basic problems stem from a variety of causes, both chemical—such as loss of fertility and high soil acidity—or physical—such. The sandfighter is used soon after a rain, when its tines can dig the damp sand into shallow depressions and small, tight clods. The broken surface traps windblown sand, reducing erosion and protecting young crops from sand blast and burial. In many cases, application of limestone and phosphorus will ameliorate the chemical problems to the extent that the land can be used in an economically productive manner.

But where the primary degradation problem is physical, remedial measures are more difficult and time consuming. The practices may require chemical treat. Rainwater harvesting is the practice of collecting precipitation for domestic or agricultural use.

It has been employed in various forms for more than 4, years and in areas that range in annual rainfall from 20 mm to 1, mm. Collection schemes vary from clearing hillsides of rocks and gravel to increase runoff and direct it toward cultivated fields further down the slope to collecting water from rooftops in small impoundments.

Rainwater harvesting is primarily for small-scale use for farms, villages, and livestock. Approaches to rainwater harvesting vary greatly. Some practices rely on alteration of the land surface and require construction, such as the building of water catchments and tied ridges. Another approach, one particularly good for porous or unstable soil, is to cover the soil with a waterproof cover.

Plastic sheeting, butyl rubber, and metal foil are low-cost alternatives for rainfall catchments. Gravel can be placed on top of plastic to protect against wind and sun damage. These catchments, if properly built and maintained, can have an expected useful life of more than 20 years.

Water harvesting has the potential to enhance food production in water-short semiarid and arid Third World countries. It is especially promising for developing countries because it provides water without requiring fuel or power. However, specific techniques will need to be developed to meet site-specific soil, climate, and socioeconomic conditions. Water harvesting schemes have the potential to be especially useful in areas with the following characteristics NRC, :.

Clay soils. On these sites much of the water runs off during rainstorms. Small ponds built in the local watershed can be used to harvest water during the rainy season and store it for use during the dry season for domestic uses and irrigation of food crops. Laterite toposequences.

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Many of these toposequences have impervious layers at the top that allow little or no vegetative growth. Thus rain falling on the impervious caps runs quickly down the slopes, causing erosion and exposing infertile subsoils. A series of check dams or levees can slow or stop the movement of water and store it temporarily for domestic or agricultural use. These check dams may also reduce erosion and make the soils down the toposequence more useful for crop production.

Gravel mulches. One of the primary sources of water losses in semiarid environments is soil water evaporation. An established means to reduce such loss is by gravel mulches. In many areas, for example the Sahel, laterite gravel is abundant. Spreading this gravel over the surface can reduce water evaporation and thus increases the water available for human use.

As with other soil management practices, restoration strategies must be tailored to individual sites and circumstances. The ability to make these site-specific recommendations remains a major challenge for research.

Soil degradation under different management strategies is important because the choice of cropping system can have a major influence on the loss or retention of soil. The challenge is to employ alternative systems that will enable the farmer to use the land in a manner that minimizes soil loss and damage. The option generally is not whether to use the land or not—circumstances often require it; it is the method of use that becomes the point at issue.

On-site degradation is only one of the issues. Determining the effects of soil loss on off-site locations is increasingly important. The impact of erosion on downstream ecological and agricultural systems needs to be assessed. Acquiring a better understanding of the social and economic dimensions of soil degradation and providing incentives to the farmer for erosion control and prevention measures would go a long way toward enhancing land use. It is also essential to keep in mind the whole soil-water system and to conduct research that looks at the dynamic relationships between these critical elements.

Finally, improvements are needed in diagnostic technologies to measure nutrient availability so deficiencies of essential soil nutrients can be corrected more easily. Such corrective practices are expensive, whether achieved by purchased inputs or by organic residues. The cost and efficiency of remedial measures can be improved if specific and quantitative data are available on the prevailing level of the nutrient in the soil. Substantial progress has been made in diagnostic analyses of soils to serve as a guide for nutrient inputs. However, the applicability and adaptation of these techniques to developing countries has been given relatively less attention.

For much of the tropical world, water is the key natural resource, and managing variable, dynamic water supplies thus is a critical challenge if agriculture is to be sustainable. As populations grow and urban and industrial water demands increase, competition for water has intensified. Research must address a spectrum of issues ranging from rain-fed agriculture to irrigation, from the effectiveness of small-scale indigenous techniques to the impacts of large-scale impoundments. It must look at water's role in dynamic agricultural systems and, in particular, its close interrelationship to soil resources.

It must move beyond the technical questions toward questions of how to apply technology in the diverse cultures and ecosystems of the developing world. One fundamental difference between most temperate climatic zones and the tropics is that in temperate areas crops generally are planted in the spring in a water-saturated soil environment, while in most parts of the tropics with a dry season, crops have to be planted in dry ground or in newly moist ground at the beginning of the rainy season.

There is pressure to plant early because the food is needed and the growing season is limited, but there is also a real danger of planting too soon—the first rains may not be substantial and the crops may wither. Planting too late, in turn, cam present other problems. Wet soil is hard to work, and pests start to cause problems. Problems associated with vagaries of rainfall in the tropics may be reduced by selectively breeding crops to withstand early drought, by developing better predictions of the pattern of rainfall in a season and ways to communicate this information to farmers, and by developing transplant techniques for crops that have not traditionally been transplanted.

Also, soil-imprinting techniques that act to concentrate sparse rainfall at the base of each seedling are helpful. All these areas need research. The use of rice nurseries before the onset of monsoon rains is certainly one of the oldest strategies to avoid water stress in the seedling stage. Rice is planted, irrigated, and fertilized in small nurseries at the end of the dry season. When the rains start, and fields are sufficiently, flooded, rice is transplanted.

Other similar examples may be found in the savanna area of Africa where sorghum and corn are planted in small irrigated nurseries, and then transplanted to larger fields once the rains have commenced in earnest. The use of soil imprinting and tied ridges also can help to concentrate moisture at the base of seedlings. Four general areas merit attention: 1 techniques for water capture and impoundment, with attention to indigenous, small-scale techniques; 2 strategies to enhance water conservation so maximum return is gained from each drop of water; 3 methods to reduce irrigation-related soil degradation, such as salinization; and 4 larger-scale approaches for watershed and landscape management.

Some of the greatest potential benefits of improved water management may well be found "at the margins" of the water management field, such as the improvement of little known or innovative approaches and technologies. The challenge is to help farmers optimize their use of available water while maintaining the quality. Developing techniques to help farmers plant and maintain crops during the uncertain, early stages of the rainy season. These might include studies of seedling resistance to the vagaries of water supply, transplanting methods, and strategies to provide more secure environments early in the cropping cycle.

Describing and evaluating the array of existing water-harvesting techniques, particularly indigenous ones, and consider their possible effectiveness in new environments. Investigating the role of aquaculture in farming systems and especially in irrigation systems. Are there ways to combine irrigation and aquaculture in regions where this has not been a tradition? Investigating techniques for making the best combined use of surface and sub-surface water in irrigation and dealing effectively with drainage, including an analysis of indigenous technologies.

Innovative thinking can help surmount the constraints posed by insufficient rain early in the growing season. In Nigeria, local farmers have developed a system where sorghum is started in irrigated nurseries and then transplanted into the fields once the rainy season is under way. Many types of microcatchments are being developed to capture and channel limited or sporadic rainfall to crops. In the Negev Desert of Israel, this microcatchment concentrates water around an almond tree. Its design was modeled on evidence of similar structures discovered during archeological research.

Credit: Mike Austin, University of Hawaii. Conducting comparative institutional analyses of risks and benefits of water management strategies, including local as well as large-scale institutions. This would include attention to indigenous systems of water access and tenure. Analyzing the economic, social, and environmental effects of irrigation at various scales.

There has been a plethora of studies of large irrigation projects and their problems, but much less attention on the smaller systems and on supplementation strategies. Columella 's "Husbandry," circa 60 CE, advocated the use of lime and that clover and alfalfa green manure should be turned under, and was used by 15 generations years under the Roman Empire until its collapse.

During the European Middle Ages , Yahya Ibn al-'Awwam 's handbook, [50] with its emphasis on irrigation, guided the people of North Africa, Spain and the Middle East; a translation of this work was finally carried to the southwest of the United States when under Spanish influence. Experiments into what made plants grow first led to the idea that the ash left behind when plant matter was burned was the essential element but overlooked the role of nitrogen, which is not left on the ground after combustion, a belief which prevailed until the 19th century.

His conclusion came from the fact that the increase in the plant's weight had apparently been produced only by the addition of water, with no reduction in the soil's weight. Others concluded it was humus in the soil that passed some essence to the growing plant. Still others held that the vital growth principal was something passed from dead plants or animals to the new plants.

At the start of the 18th century, Jethro Tull demonstrated that it was beneficial to cultivate stir the soil, but his opinion that the stirring made the fine parts of soil available for plant absorption was erroneous. As chemistry developed, it was applied to the investigation of soil fertility. The French chemist Antoine Lavoisier showed in about that plants and animals must [combust] oxygen internally to live and was able to deduce that most of the pound weight of van Helmont 's willow tree derived from air. The enrichment of soil with guano by the Incas was rediscovered in , by Alexander von Humboldt.

This led to its mining and that of Chilean nitrate and to its application to soil in the United States and Europe after The work of Liebig was a revolution for agriculture, and so other investigators started experimentation based on it. In England John Bennet Lawes and Joseph Henry Gilbert worked in the Rothamsted Experimental Station , founded by the former, and re discovered that plants took nitrogen from the soil, and that salts needed to be in an available state to be absorbed by plants.

Their investigations also produced the " superphosphate ", consisting in the acid treatment of phosphate rock. Ammonia generated by the production of coke was recovered and used as fertiliser. However, the dynamic interaction of soil and its life forms still awaited discovery. In J. Thomas Way discovered that ammonia contained in fertilisers was transformed into nitrates, [64] and twenty years later Robert Warington proved that this transformation was done by living organisms.

It was known that certain legumes could take up nitrogen from the air and fix it to the soil but it took the development of bacteriology towards the end of the 19th century to lead to an understanding of the role played in nitrogen fixation by bacteria. The symbiosis of bacteria and leguminous roots, and the fixation of nitrogen by the bacteria, were simultaneously discovered by the German agronomist Hermann Hellriegel and the Dutch microbiologist Martinus Beijerinck.

Crop rotation , mechanisation, chemical and natural fertilisers led to a doubling of wheat yields in western Europe between and The scientists who studied the soil in connection with agricultural practices had considered it mainly as a static substrate. However, soil is the result of evolution from more ancient geological materials, under the action of biotic and abiotic not associated with life processes. After studies of the improvement of the soil commenced, others began to study soil genesis and as a result also soil types and classifications. In , in Mississippi, Eugene W.

Hilgard studied the relationship among rock material, climate, and vegetation, and the type of soils that were developed. He realised that the soils were dynamic, and considered soil types classification. At about the same time, Friedrich Albert Fallou was describing soil profiles and relating soil characteristics to their formation as part of his professional work evaluating forest and farm land for the principality of Saxony.

Due to language barriers, the work of this team was not communicated to western Europe until through a publication in German by Konstantin Dmitrievich Glinka , a member of the Russian team. Curtis F. Marbut was influenced by the work of the Russian team, translated Glinka's publication into English, [71] and as he was placed in charge of the U. National Cooperative Soil Survey , applied it to a national soil classification system. Soil formation, or pedogenesis , is the combined effect of physical, chemical, biological and anthropogenic processes working on soil parent material.

Soil is said to be formed when organic matter has accumulated and colloids are washed downward, leaving deposits of clay, humus, iron oxide, carbonate, and gypsum, producing a distinct layer called the B horizon. This is a somewhat arbitrary definition as mixtures of sand, silt, clay and humus will support biological and agricultural activity before that time.

These constituents are moved from one level to another by water and animal activity. As a result, layers horizons form in the soil profile. The alteration and movement of materials within a soil causes the formation of distinctive soil horizons. However, more recent definitions of soil embrace soils without any organic matter, such as those regoliths that formed on Mars [72] and analogous conditions in planet Earth deserts. An example of the development of a soil would begin with the weathering of lava flow bedrock, which would produce the purely mineral-based parent material from which the soil texture forms.

Soil development would proceed most rapidly from bare rock of recent flows in a warm climate, under heavy and frequent rainfall. Under such conditions, plants in a first stage nitrogen-fixing lichens and cyanobacteria then epilithic higher plants become established very quickly on basaltic lava, even though there is very little organic material. The plants are supported by the porous rock as it is filled with nutrient -bearing water that carries minerals dissolved from the rocks.

Crevasses and pockets, local topography of the rocks, would hold fine materials and harbour plant roots. The developing plant roots are associated with mineral- weathering mycorrhizal fungi [74] that assist in breaking up the porous lava, and by these means organic matter and a finer mineral soil accumulate with time. Such initial stages of soil development have been described on volcanoes, [75] inselbergs, [76] and glacial moraines.

How soil formation proceeds is influenced by at least five classic factors that are intertwined in the evolution of a soil. They are: parent material, climate, topography relief , organisms, and time. The mineral material from which a soil forms is called parent material. Rock, whether its origin is igneous, sedimentary, or metamorphic, is the source of all soil mineral materials and the origin of all plant nutrients with the exceptions of nitrogen, hydrogen and carbon. As the parent material is chemically and physically weathered, transported, deposited and precipitated, it is transformed into a soil.

Typical soil parent mineral materials are: [80]. Parent materials are classified according to how they came to be deposited. Residual materials are mineral materials that have weathered in place from primary bedrock. Transported materials are those that have been deposited by water, wind, ice or gravity. Cumulose material is organic matter that has grown and accumulates in place. Residual soils are soils that develop from their underlying parent rocks and have the same general chemistry as those rocks.

The soils found on mesas, plateaux, and plains are residual soils. In the United States as little as three percent of the soils are residual. Most soils derive from transported materials that have been moved many miles by wind, water, ice and gravity. Cumulose parent material is not moved but originates from deposited organic material. This includes peat and muck soils and results from preservation of plant residues by the low oxygen content of a high water table.

While peat may form sterile soils, muck soils may be very fertile. The weathering of parent material takes the form of physical weathering disintegration , chemical weathering decomposition and chemical transformation. Generally, minerals that are formed under high temperatures and pressures at great depths within the Earth's mantle are less resistant to weathering, while minerals formed at low temperature and pressure environment of the surface are more resistant to weathering.

Rocks that will decompose in a few years in tropical climates will remain unaltered for millennia in deserts. Chemical weathering mainly results from the excretion of organic acids and chelating compounds by bacteria [84] and fungi, [85] thought to increase under present-day greenhouse effect. Of the above, hydrolysis and carbonation are the most effective, in particular in regions of high rainfall, temperature and physical erosion.

Saprolite is a particular example of a residual soil formed from the transformation of granite, metamorphic and other types of bedrock into clay minerals. Often called [weathered granite], saprolite is the result of weathering processes that include: hydrolysis , chelation from organic compounds, hydration the solution of minerals in water with resulting cation and anion pairs and physical processes that include freezing and thawing.

The mineralogical and chemical composition of the primary bedrock material, its physical features, including grain size and degree of consolidation, and the rate and type of weathering transforms the parent material into a different mineral. The texture, pH and mineral constituents of saprolite are inherited from its parent material. This process is also called arenization , resulting in the formation of sandy soils granitic arenas , thanks to the much higher resistance of quartz compared to other mineral components of granite micas , amphiboles , feldspars. The principal climatic variables influencing soil formation are effective precipitation i.

Temperature and moisture both influence the organic matter content of soil through their effects on the balance between primary production and decomposition : the colder or drier the climate the lesser atmospheric carbon is fixed as organic matter while the lesser organic matter is decomposed. Climate is the dominant factor in soil formation , and soils show the distinctive characteristics of the climate zones in which they form, with a feedback to climate through transfer of carbon stocked in soil horizons back to the atmosphere. According to the climatic determination of biomes , humid climates favor the growth of trees.

In contrast, grasses are the dominant native vegetation in subhumid and semiarid regions, while shrubs and brush of various kinds dominate in arid areas. Water is essential for all the major chemical weathering reactions. To be effective in soil formation, water must penetrate the regolith. The seasonal rainfall distribution, evaporative losses, site topography , and soil permeability interact to determine how effectively precipitation can influence soil formation.

The greater the depth of water penetration, the greater the depth of weathering of the soil and its development. Surplus water percolating through the soil profile transports soluble and suspended materials from the upper layers eluviation to the lower layers illuviation , including clay particles [] and dissolved organic matter. Thus, percolating water stimulates weathering reactions and helps differentiate soil horizons. Likewise, a deficiency of water is a major factor in determining the characteristics of soils of dry regions.

Soluble salts are not leached from these soils, and in some cases they build up to levels that curtail plant [] and microbial growth. The direct influences of climate include: []. Climate directly affects the rate of weathering and leaching. Wind moves sand and smaller particles dust , especially in arid regions where there is little plant cover, depositing it close [] or far from the entrainment source. Soil profiles are more distinct in wet and cool climates, where organic materials may accumulate, than in wet and warm climates, where organic materials are rapidly consumed.


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Climate also indirectly influences soil formation through the effects of vegetation cover and biological activity, which modify the rates of chemical reactions in the soil. The topography , or relief , is characterized by the inclination slope , elevation , and orientation of the terrain. Topography determines the rate of precipitation or runoff and rate of formation or erosion of the surface soil profile.

The topographical setting may either hasten or retard the work of climatic forces. Steep slopes encourage rapid soil loss by erosion and allow less rainfall to enter the soil before running off and hence, little mineral deposition in lower profiles. In semiarid regions, the lower effective rainfall on steeper slopes also results in less complete vegetative cover, so there is less plant contribution to soil formation.

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For all of these reasons, steep slopes prevent the formation of soil from getting very far ahead of soil destruction. Therefore, soils on steep terrain tend to have rather shallow, poorly developed profiles in comparison to soils on nearby, more level sites. In swales and depressions where runoff water tends to concentrate, the regolith is usually more deeply weathered and soil profile development is more advanced.

However, in the lowest landscape positions, water may saturate the regolith to such a degree that drainage and aeration are restricted. Here, the weathering of some minerals and the decomposition of organic matter are retarded, while the loss of iron and manganese is accelerated. In such low-lying topography, special profile features characteristic of wetland soils may develop. Depressions allow the accumulation of water, minerals and organic matter and in the extreme, the resulting soils will be saline marshes or peat bogs.

Intermediate topography affords the best conditions for the formation of an agriculturally productive soil. Soil is the most abundant ecosystem on Earth, but the vast majority of organisms in soil are microbes , a great many of which have not been described. Plants, animals , fungi, bacteria and humans affect soil formation see soil biomantle and stonelayer. Soil animals, including soil macrofauna and soil mesofauna , mix soils as they form burrows and pores , allowing moisture and gases to move about, a process called bioturbation.

Humans impact soil formation by removing vegetation cover with erosion , waterlogging , lateritization or podzolization according to climate and topography as the result. Earthworms , ants , termites , moles , gophers , as well as some millipedes and tenebrionid beetles mix the soil as they burrow, significantly affecting soil formation. In general, the mixing of the soil by the activities of animals, sometimes called pedoturbation , tends to undo or counteract the tendency of other soil-forming processes that create distinct horizons.

In localized areas, they enhance mixing of the lower and upper horizons by creating, and later refilling, underground tunnels. Old animal burrows in the lower horizons often become filled with soil material from the overlying A horizon, creating profile features known as crotovinas. Vegetation impacts soils in numerous ways. It can prevent erosion caused by excessive rain that might result from surface runoff. Dead plants and fallen leaves and stems begin their decomposition on the surface. There, organisms feed on them and mix the organic material with the upper soil layers; these added organic compounds become part of the soil formation process.

Human activities widely influence soil formation.


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Time is a factor in the interactions of all the above. Over time the soil will develop a profile that depends on the intensities of biota and climate. While a soil can achieve relative stability of its properties for extended periods, [] the soil life cycle ultimately ends in soil conditions that leave it vulnerable to erosion. Soil-forming factors continue to affect soils during their existence, even on "stable" landscapes that are long-enduring, some for millions of years.

Whether these are slow or rapid changes depends on climate, topography and biological activity. Time as a soil-forming factor may be investigated by studying soil chronosequences , in which soils of different ages but with minor differences in other soil-forming factors can be compared. The physical properties of soils, in order of decreasing importance for ecosystem services such as crop production , are texture , structure , bulk density , porosity , consistency, temperature, colour and resistivity. At the next larger scale, soil structures called peds or more commonly soil aggregates are created from the soil separates when iron oxides , carbonates , clay, silica and humus , coat particles and cause them to adhere into larger, relatively stable secondary structures.

Soil consistency is the ability of soil materials to stick together. Soil temperature and colour are self-defining. Resistivity refers to the resistance to conduction of electric currents and affects the rate of corrosion of metal and concrete structures which are buried in soil. Most of these properties determine the aeration of the soil and the ability of water to infiltrate and to be held within the soil.

The mineral components of soil are sand , silt and clay , and their relative proportions determine a soil's texture. Properties that are influenced by soil texture include porosity , permeability , infiltration , shrink-swell rate , water-holding capacity , and susceptibility to erosion.

In the illustrated USDA textural classification triangle, the only soil in which neither sand, silt nor clay predominates is called loam. While even pure sand, silt or clay may be considered a soil, from the perspective of conventional agriculture a loam soil with a small amount of organic material is considered "ideal", inasmuch as fertilizers or manure are currently used to mitigate nutrient losses due to crop yields in the long term.

Soil texture affects soil behaviour, in particular, its retention capacity for nutrients e. Sand and silt are the products of physical and chemical weathering of the parent rock ; [78] clay, on the other hand, is most often the product of the precipitation of the dissolved parent rock as a secondary mineral, except when derived from the weathering of mica.

Sand's greatest benefit to soil is that it resists compaction and increases soil porosity, although this property stands only for pure sand, not for sand mixed with smaller minerals which fill the voids among sand grains. But it is the clay content of soil, with its very high specific surface area and generally large number of negative charges, that gives a soil its high retention capacity for water and nutrients.

Sand is the most stable of the mineral components of soil; it consists of rock fragments, primarily quartz particles, ranging in size from 2. Silt ranges in size from 0. Clay cannot be resolved by optical microscopes as its particles are 0. There is no clear relationship between the size of soil mineral components and their mineralogical nature: sand and silt particles can be calcareous as well as siliceous , [] while textural clay 0. Soil components larger than 2. When the organic component of a soil is substantial, the soil is called organic soil rather than mineral soil. A soil is called organic if:.

The clumping of the soil textural components of sand, silt and clay causes aggregates to form and the further association of those aggregates into larger units creates soil structures called peds a contraction of the word pedolith. The adhesion of the soil textural components by organic substances, iron oxides, carbonates, clays, and silica, the breakage of those aggregates from expansion-contraction caused by freezing-thawing and wetting-drying cycles, [] and the build-up of aggregates by soil animals, microbial colonies and root tips [] shape soil into distinct geometric forms.

Soil structure affects aeration , water movement, conduction of heat, plant root growth and resistance to erosion. Soil structure often gives clues to its texture, organic matter content, biological activity, past soil evolution, human use, and the chemical and mineralogical conditions under which the soil formed. While texture is defined by the mineral component of a soil and is an innate property of the soil that does not change with agricultural activities, soil structure can be improved or destroyed by the choice and timing of farming practices.

Soil structural classes: []. At the largest scale, the forces that shape a soil's structure result from swelling and shrinkage that initially tend to act horizontally, causing vertically oriented prismatic peds. This mechanical process is mainly exemplified in the development of vertisols.

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At a smaller scale, plant roots extend into voids macropores and remove water [] causing macroporosity to increase and microporosity to decrease, [] thereby decreasing aggregate size. At an even smaller scale, soil aggregation continues as bacteria and fungi exude sticky polysaccharides which bind soil into smaller peds. At the lowest scale, the soil chemistry affects the aggregation or dispersal of soil particles.

The clay particles contain polyvalent cations which give the faces of clay layers localized negative charges. This leaves negative charge on the clay faces that repel other clay, causing the particles to push apart, and by doing so deflocculate clay suspensions. In this way the open structure of the soil is destroyed and the soil is made impenetrable to air and water.

Soil particle density is typically 2. Thereby soil bulk density is always less than soil particle density and is a good indicator of soil compaction. Pore space is that part of the bulk volume of soil that is not occupied by either mineral or organic matter but is open space occupied by either gases or water.

Soil texture determines total volume of the smallest pores; [] clay soils have smaller pores, but more total pore space than sands, [] despite of a much lower permeability. The pore size distribution affects the ability of plants and other organisms to access water and oxygen; large, continuous pores allow rapid transmission of air, water and dissolved nutrients through soil, and small pores store water between rainfall or irrigation events.

Consistency is the ability of soil to stick to itself or to other objects cohesion and adhesion , respectively and its ability to resist deformation and rupture. It is of approximate use in predicting cultivation problems [] and the engineering of foundations. In the wet state, the two qualities of stickiness and plasticity are assessed. A soil's resistance to fragmentation and crumbling is assessed in the dry state by rubbing the sample. Its resistance to shearing forces is assessed in the moist state by thumb and finger pressure.

Additionally, the cemented consistency depends on cementation by substances other than clay, such as calcium carbonate, silica, oxides and salts; moisture content has little effect on its assessment. The measures of consistency border on subjective compared to other measures such as pH, since they employ the apparent feel of the soil in those states. The terms used to describe the soil consistency in three moisture states and a last not affected by the amount of moisture are as follows:. Soil consistency is useful in estimating the ability of soil to support buildings and roads.

More precise measures of soil strength are often made prior to construction. Soil temperature depends on the ratio of the energy absorbed to that lost. Soil temperatures can be raised by drying soils [] or the use of clear plastic mulches. There are various factors that affect soil temperature, such as water content, [] soil color, [] and relief slope, orientation, and elevation , [] and soil cover shading and insulation , in addition to air temperature.

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The specific heat of soil increases as water content increases, since the heat capacity of water is greater than that of dry soil. Soil heat flux refers to the rate at which heat energy moves through the soil in response to a temperature difference between two points in the soil. The heat flux density is the amount of energy that flows through soil per unit area per unit time and has both magnitude and direction. For the simple case of conduction into or out of the soil in the vertical direction, which is most often applicable the heat flux density is:. Heat flux is in the direction opposite the temperature gradient, hence the minus sign.

That is to say, if the temperature of the surface is higher than at depth x the negative sign will result in a positive value for the heat flux q, and which is interpreted as the heat being conducted into the soil. Soil temperature is important for the survival and early growth of seedlings. Soil temperatures are increasing worldwide under the influence of present-day global climate warming , with opposing views about expected effects on carbon capture and storage and feedback loops to climate change [] Most threats are about permafrost thawing and attended effects on carbon destocking [] and ecosystem collapse.

Soil colour is often the first impression one has when viewing soil. Striking colours and contrasting patterns are especially noticeable. The Yellow River in China carries yellow sediment from eroding loess soils. Mollisols in the Great Plains of North America are darkened and enriched by organic matter. Podsols in boreal forests have highly contrasting layers due to acidity and leaching. In general, color is determined by the organic matter content, drainage conditions, and degree of oxidation.

Soil color, while easily discerned, has little use in predicting soil characteristics. Munsell color dimensions hue, value and chroma can be averaged among samples and treated as quantitative parameters, displaying significant correlations with various soil [] and vegetation properties. Soil color is primarily influenced by soil mineralogy.

Many soil colours are due to various iron minerals. Iron forms secondary minerals of a yellow or red colour, [] organic matter decomposes into black and brown humic compounds, [] and manganese [] and sulfur [] can form black mineral deposits. These pigments can produce various colour patterns within a soil.

Aerobic conditions produce uniform or gradual colour changes, while reducing environments anaerobic result in rapid colour flow with complex, mottled patterns and points of colour concentration. Soil resistivity is a measure of a soil's ability to retard the conduction of an electric current. The electrical resistivity of soil can affect the rate of galvanic corrosion of metallic structures in contact with the soil.

Water that enters a field is removed from a field by runoff , drainage , evaporation or transpiration. Water affects soil formation , structure , stability and erosion but is of primary concern with respect to plant growth. In addition, water alters the soil profile by dissolving and re-depositing minerals, often at lower levels. A flooded field will drain the gravitational water under the influence of gravity until water's adhesive and cohesive forces resist further drainage at which point it is said to have reached field capacity.

The water that plants may draw from the soil is called the available water. Water moves in soil under the influence of gravity , osmosis and capillarity. The rate at which a soil can absorb water depends on the soil and its other conditions. As a plant grows, its roots remove water from the largest pores macropores first. Soon the larger pores hold only air, and the remaining water is found only in the intermediate- and smallest-sized pores micropores. The water in the smallest pores is so strongly held to particle surfaces that plant roots cannot pull it away.

Consequently, not all soil water is available to plants, with a strong dependence on texture. Soil water is also important for climate modeling and numerical weather prediction. Each method exhibits pros and cons, and hence, the integration of different techniques may decrease the drawbacks of a single given method. Water is retained in a soil when the adhesive force of attraction that water's hydrogen atoms have for the oxygen of soil particles is stronger than the cohesive forces that water's hydrogen feels for other water oxygen atoms.

Solute Movement in the Rhizosphere Peter B. Table of contents Introduction ; 1. Surface Charge ; 2. Electrostatic Adsorption of Cations ; 3. Electrostatic Adsorption of Anions ; 4. Specific Adsorption of Cations ; 5. Coordination Adsorption of Anions ; 6.

Electrokinetic Properties ; 7. Electric Conductance ; 8. Ion Diffusion ; 9. Reactions with Hydrogen Ions ; Acidity ; Lime Potential ;

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