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Smol and 2 more. Push-Pull Tests for Site Characterization. Applied Limnology. Comprehensive View from Watershed to Lake. Studies of Cave Sediments. Physical and Chemical Records of Paleoclimate. Sasowsky and 1 more. Springs and Bottled Waters of the World. LaMoreaux and 1 more. Last and 1 more. Continue shopping. Item s unavailable for purchase. Please review your cart. You can remove the unavailable item s now or we'll automatically remove it at Checkout.

Remove FREE. Unavailable for purchase. Continue shopping Checkout Continue shopping. Sort By: Bestsellers. WE21A06 full text url Paasche, H. WE21A01 full text url Sauer, U. Collection of Abstracts. EGU to index. Publications Index: Articles in ISI listed journals 29 Articles in other journals not listed in ISI 3 Book chapters 3 Qualification assignments 1 Conference contributions 7 You could use our publication index for further requests. Eiceman, Z. Karpas and H.

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Hill, Jr. September, p. Proteomics 94 , - full text doi Brauchler, R. Reply to the commentary by Fuchs et al. Acta , 48 - 55 full text doi Zschornack, L. Berichtsband zur 7. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, 4. September , Split, Croatia p. Plant Nutr. Soil Sci. Direct Push — mit Nadelstichen dem Untergrund auf der Spur. Acupuncture for the Ground? Direct push - tracking the subsurface with pinholes Grundwasser 17 1 , 1 - 1 full text doi Matthes, K.

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Effects of measuring inaccuracy during grain size analyses on the determination of hydraulic conductivity Grundwasser 17 2 , - full text doi Paasche, H. Remote Sens. Application of Direct Push techniques for delineation of vertical variations in hydrochemistry at remediation sites—case study Elsterwerda Brandenburg Grundwasser 17 1 , 7 - 17 full text doi Schmelzbach, C. Berichte der Geologischen Bundesanstalt 93 , 82 - 88 Weickhardt, C.

Hydrogeology of arid environments : proceedings ; [results of the Conference "Hydrogeology of Arid Environments", March in Hannover Germany ] Borntraeger, Stuttgart, p. RS23 Hausmann, J. Sandy soil will retain very little water, while clay will hold the maximum amount. Water moves through soil due to the force of gravity , osmosis and capillarity. At zero to 33 kPa suction field capacity , water is pushed through soil from the point of its application under the force of gravity and the pressure gradient created by the pressure of the water; this is called saturated flow.

At higher suction, water movement is pulled by capillarity from wetter toward drier soil. This is caused by water's adhesion to soil solids, and is called unsaturated flow. Water infiltration rates range from 0. Tree roots, whether living or dead, create preferential channels for rainwater flow through soil, [] magnifying infiltration rates of water up to 27 times. Flooding temporarily increases soil permeability in river beds , helping to recharge aquifers.

Water applied to a soil is pushed by pressure gradients from the point of its application where it is saturated locally, to less saturated areas, such as the vadose zone. In the United States percolation water due to rainfall ranges from almost zero centimeters just east of the Rocky Mountains to fifty or more centimeters per day in the Appalachian Mountains and the north coast of the Gulf of Mexico.

Water is pulled by capillary action due to the adhesion force of water to the soil solids, producing a suction gradient from wet towards drier soil [] and from macropores to micropores. Preferential flow occurs along interconnected macropores, crevices, root and worm channels, which drain water under gravity. Of equal importance to the storage and movement of water in soil is the means by which plants acquire it and their nutrients. Most soil water is taken up by plants as passive absorption caused by the pulling force of water evaporating transpiring from the long column of water xylem sap flow that leads from the plant's roots to its leaves, according to the cohesion-tension theory.

It is these process that cause guttation and wilting , respectively. Root extension is vital for plant survival.

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A study of a single winter rye plant grown for four months in one cubic foot 0. The total surface area of the loam soil was estimated to be 52, square meters. However, root extension should be viewed as a dynamic process, allowing new roots to explore a new volume of soil each day, increasing dramatically the total volume of soil explored over a given growth period, and thus the volume of water taken up by the root system over this period. Roots must seek out water as the unsaturated flow of water in soil can move only at a rate of up to 2.

Only a small fraction 0. The majority is ultimately lost via transpiration , while evaporation from the soil surface is also substantial, the transpiration:evaporation ratio varying according to vegetation type and climate, peaking in tropical rainforests and dipping in steppes and deserts.

Evapotranspiration plus water held in the plant totals to consumptive use, which is nearly identical to evapotranspiration. The total water used in an agricultural field includes surface 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 plant plus soil will approach that of an uncovered soil, while more water is immediately available for plant growth.

Transpiration ratios for crops range from to The atmosphere of soil, or soil gas , is very 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 CO 2 concentration is 0. Movement of gases is by diffusion from high concentrations to lower, the diffusion coefficient decreasing with soil compaction. It is the total pore space porosity of soil, not the pore size, and the degree of pore interconnection or conversely pore sealing , together with water content, air turbulence and temperature, that determine the rate of diffusion of gases into and out of soil.

Soil atmosphere is also the seat of emissions of volatiles other than carbon and nitrogen oxides from various soil organisms, e.

We humans can get some idea of the soil atmosphere through the well-known 'after-the-rain' scent, when infiltering rainwater flushes out the whole soil atmosphere after a drought period, or when soil is excavated, [] a bulk property attributed in a reductionist manner to particular biochemical compounds such as petrichor or geosmin. Soil particles can be classified by their chemical composition mineralogy as well as their size. The particle size distribution of a soil, its texture , determines many of the properties of that soil, in particular hydraulic conductivity and water potential , [] but the mineralogy of those particles can strongly modify those properties.

The mineralogy of the finest soil particles, clay, is especially important. Gravel , sand and silt are the larger soil particles , and their mineralogy is often inherited from the parent material of the soil, but may include products of weathering such as concretions of calcium carbonate or iron oxide , or residues of plant and animal life such as silica phytoliths.

Due to its high specific surface area and its unbalanced negative electric charges , clay is the most active mineral component of soil. Many soil minerals, such as gypsum, carbonates, or quartz, are small enough to be classified as clay based on their physical size, but chemically they do not afford the same utility as do mineralogically-defined clay minerals.

Before the advent of X-ray diffraction clay was 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. Typically there are four main groups of clay minerals: kaolinite , montmorillonite - smectite , illite , and chlorite.

The spatial arrangement of the oxygen atoms determines clay's structure. Alumino-silica clays or aluminosilicate clays are characterized by their regular crystalline or quasi-crystalline structure. 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.

The substitution of lower- valence cations for higher-valence cations isomorphous substitution gives clay a local negative charge on an oxygen atom [] that attracts and holds water and positively charged soil cations, some of which are of value for plant growth. The carbonate and sulfate clay minerals are much more soluble and hence are found primarily in desert soils where leaching is less active.

Amorphous clays are young, and commonly found in recent volcanic ash deposits such as tephra. As a result, they may display either high CEC in an acid soil solution, or high anion exchange capacity in a basic soil solution. Sesquioxide clays are a product of heavy rainfall that has leached most of the silica from alumino-silica clay, leaving the less soluble oxides iron hematite Fe 2 O 3 , iron hydroxide Fe OH 3 , aluminium hydroxide gibbsite Al OH 3 , hydrated manganese birnessite MnO 2 , as can be observed in most lateritic weathering profiles of tropical soils.

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, a sorptive process which can at lest partly inhibited in the presence of decomposed humified organic matter. Such clays tend to hold phosphorus so tightly that it is unavailable for absorption by plants. Humus is one of the two final stages of decomposition of organic matter.

It remains in the soil as the organic component of the soil matrix while the other stage, carbon dioxide , is freely liberated in the atmosphere or reacts with calcium to form the soluble calcium bicarbonate. While humus may linger for a thousand years, [] on the larger scale of the age of the mineral soil components, it is temporary, being finally released as CO 2.

On a dry weight basis, the CEC of humus is many times greater than that of clay.


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Humus plays a major role in the regulation of atmospheric carbon , through carbon sequestration in the soil profile, more especially in deeper horizons with reduced biological activity. In the extreme environment of high temperatures and the leaching caused by the heavy rain of tropical rain forests , the clay and organic colloids are largely destroyed. The heavy rains wash 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 decay any organic matter on the rain-forest floor overnight and much of the nutrients are volatilized or leached from the soil and lost, [] leaving only a thin root mat lying directly on the mineral soil. In Amazonia it testifies for the agronomic knowledge of past Amerindian civilizations.

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" [] The incorporation of charcoal to agricultural soil for improving water and nutrient retention has been called biochar , being extended to other charred or carbon-rich by-products, and is now increasingly used in sustainable tropical agriculture.

The chemistry of a soil determines its ability to supply available plant nutrients and affects its physical properties and the health of its living population. In addition, a soil's chemistry also determines its corrosivity , stability, and ability to absorb pollutants and to filter water. It is the surface chemistry of mineral and organic colloids that determines soil's chemical properties. The very high specific surface area of colloids and their net electrical charges give soil its ability to hold and release ions.

Negatively charged sites on colloids attract and release cations in what is referred to as cation exchange. Similarly, positively charged sites on colloids can attract and release anions in the soil giving the soil anion exchange capacity AEC. The cation exchange, that takes place 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 or positive charges on colloid particles make them able to hold cations or anions, respectively, to their surfaces. The charges result from four sources. 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 by the colloid and hence their ability to replace one another ion exchange.

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If present in equal amounts in the soil water solution:. If one cation is added in large amounts, it may replace the others by the sheer force of its numbers. This is called law of mass action. This is largely what occurs with the addition of cationic fertilisers potash , lime. A low pH may cause hydrogen of hydroxyl groups to be pulled into solution, leaving charged sites on the colloid available to be occupied by other cations.

This ionisation of hydroxyl groups on the surface of soil colloids creates what is described as pH-dependent surface charges. Cation exchange capacity should be thought of as the soil's ability to remove cations from the soil water solution and sequester those to be exchanged later as the plant roots release hydrogen ions to the solution. Most of the soil's CEC occurs on clay and humus colloids, and the lack of those in hot, humid, wet climates, due to leaching and decomposition, respectively, explains the apparent sterility of tropical soils.

www.perfectpostage.com/wordpress-3.1/wordpress/wp-includes Anion exchange capacity should be thought of as the soil's ability to remove anions e. Amorphous and sesquioxide clays have the highest AEC, [] followed by the iron oxides. Levels of AEC are much lower than for CEC, because of the generally higher rate of positively versus negatively charged surfaces on soil colloids, to the exception of variable-charge soils. Soil reactivity is expressed in terms of pH and is a measure of the acidity or alkalinity of the soil. More precisely, it is a measure of hydrogen ion concentration in an aqueous solution and ranges in values from 0 to 14 acidic to basic but practically speaking for soils, pH ranges from 3.

The effect of pH on a soil is to remove from the soil or to make available certain ions. Soils with high acidity tend to have toxic amounts of aluminium and manganese. In high rainfall areas, soils tend to acidity as the basic cations are forced off the soil colloids by the mass action of hydrogen ions from the rain against those attached to the colloids. High rainfall rates can then wash the nutrients out, leaving the soil inhabited only by those organisms which are particularly efficient to uptake nutrients in very acid conditions, like in tropical rainforests.

There are acid-forming cations e. The fraction of the negatively-charged soil colloid exchange sites CEC that are occupied by base-forming cations is called base saturation. Base saturation is almost in direct proportion to pH it increases with increasing pH. The amount of lime needed to neutralize a soil must take account of the amount of acid forming ions on the colloids exchangeable acidity , not just those in the soil water solution free acidity.

The resistance of soil to change in pH, as a result of the addition of acid or basic material, is a measure of the buffering capacity of a soil and for a particular soil type increases as the CEC increases. Hence, pure sand has almost no buffering ability, while soils high in colloids whether mineral or organic have high buffering capacity.

However, colloids are not the only regulators of soil pH. The role of carbonates should be underlined, too. The addition of a small amount of highly basic aqueous ammonia to a soil will cause the ammonium to displace hydrogen ions from the colloids, and the end product is water and colloidally fixed ammonium, but little permanent change overall in soil pH. The addition of a small amount of lime , Ca OH 2 , will displace hydrogen ions from the soil colloids, causing the fixation of calcium to colloids and the evolution of CO 2 and water, with little permanent change in soil pH.

The above are examples of the buffering of soil pH. The general principal is that an increase in a particular cation in the soil water solution will cause that cation to be fixed to colloids buffered and a decrease in solution of that cation will cause it to be withdrawn from the colloid and moved into solution buffered.

The degree of buffering is often related to the CEC of the soil; the greater the CEC, the greater the buffering capacity of the soil. Seventeen elements or nutrients are essential for plant growth and reproduction. Nutrients that enhance the growth of plants but are not necessary to complete the plant's life cycle are considered non-essential.

With the exception of carbon, hydrogen and oxygen, which are supplied by carbon dioxide and water, and nitrogen, provided through nitrogen fixation , [] the nutrients derive originally from the mineral component of the soil. The Law of the Minimum expresses that when the available form of a nutrient is not in enough proportion in the soil solution, then other nutrients cannot be taken up at an optimum rate by a plant.

Plant uptake of nutrients can only proceed when they are present in a plant-available form. In most situations, nutrients are absorbed in an ionic form from or together with soil water. Although minerals are the origin of most nutrients, and the bulk of most nutrient elements in the soil is held in crystalline form within primary and secondary minerals, they weather too slowly to support rapid plant growth.

For example, the application of finely ground minerals, feldspar and apatite , to soil seldom provides the necessary amounts of potassium and phosphorus at a rate sufficient for good plant growth, as most of the nutrients remain bound in the crystals of those minerals. The nutrients adsorbed onto the surfaces of clay colloids and soil organic matter provide a more accessible reservoir of many plant nutrients e. K, Ca, Mg, P, Zn. As plants absorb the nutrients from the soil water, the soluble pool is replenished from the surface-bound pool. The decomposition of soil organic matter by microorganisms is another mechanism whereby the soluble pool of nutrients is replenished — this is important for the supply of plant-available N, S, P, and B from soil.

Gram for gram, the capacity of humus to hold nutrients and water is far greater than that of clay minerals, most of the soil cation exchange capacity arising from charged carboxylic groups on organic matter. Nutrients in the soil are taken up by the plant through its roots, and in particular its root hairs. To be taken up by a plant, a nutrient element must be located near the root surface; however, the supply of nutrients in contact with the root is rapidly depleted within a distance of ca.

All three mechanisms operate simultaneously, but one mechanism or another may be most important for a particular nutrient. However, in the case of phosphorus, diffusion is needed to supplement mass flow. For the most part, nutrient ions must travel some distance in the soil solution to reach the root surface. This movement can take place by mass flow, as when dissolved nutrients are carried along with the soil water flowing toward a root that is actively drawing water from the soil. In this type of movement, the nutrient ions are somewhat analogous to leaves floating down a stream.

In addition, nutrient ions continually move by diffusion from areas of greater concentration toward the nutrient-depleted areas of lower concentration around the root surface. That process is due to random motion, also called Brownian motion , of molecules within a gradient of decreasing concentration. Finally, root interception comes into play as roots continually grow into new, undepleted soil.

By this way roots are also able to absorb nanomaterials such as nanoparticulate organic matter. In the above table, phosphorus and potassium nutrients move more by diffusion than they do by mass flow in the soil water solution, as they are rapidly taken up by the roots creating a concentration of almost zero near the roots the plants cannot transpire enough water to draw more of those nutrients near the roots.

The very steep concentration gradient is of greater influence in the movement of those ions than is the movement of those by mass flow. Plants move ions out of their roots in an effort to move nutrients in from the soil, an exchange process which occurs in the root apoplast. However, the rate at which plant roots remove nutrients may not cope with the rate at which they are replenished in the soil solution, stemming in nutrient limitation to plant growth.

Where crops are produced, the replenishment of nutrients in the soil must usually be augmented by the addition of fertilizer or organic matter. Because nutrient uptake is an active metabolic process, conditions that inhibit root metabolism may also inhibit nutrient uptake. Plants obtain their carbon from atmospheric carbon dioxide through photosynthetic carboxylation , to which must be added the uptake of dissolved carbon from the soil solution [] and carbon transfer through mycorrhizal networks.

Nitrogen turnover mostly involved in protein turnover is lesser than that of carbon mostly involved in respiration in the living, then dead matter of decomposers , which are always richer in nitrogen than plant litter , and so it builds up in the soil.

In a field of maize on a still day during high light conditions in the growing season, the CO 2 concentration drops very low, but under such conditions the crop could use up to 20 times the normal concentration. The respiration of CO 2 by soil micro-organisms decomposing soil organic matter and the CO 2 respired by roots contribute an important amount of CO 2 to the photosynthesising plants, to which must be added the CO 2 respired by aboveground plant tissues. Nitrogen is the most critical element obtained by plants from the soil, to the exception of moist tropical forests where phosphorus is the limiting soil nutrient , [] and nitrogen deficiency often limits plant growth.

Plants are commonly classified as ammonium or nitrate plants according to their preferential nitrogen nutrition. However, symbiosis with mycorrhizal fungi allow plants to get access to the organic nitrogen pool where and when mineral forms of nitrogen are poorly available. Usually, grassland soils contain more soil nitrogen than forest soils, because of a higher turnover rate of grassland organic matter. Some micro-organisms are able to metabolise organic matter and release ammonium in a process called mineralisation. Others take free ammonium and oxidise it to nitrate.

Nitrogen-fixing bacteria are capable of metabolising N 2 into the form of ammonia in a process called nitrogen fixation. Both ammonium and nitrate can be immobilized by their incorporation into the microbes' living cells, where it is temporarily sequestered in the form of amino acids and protein. Nitrate may also be lost from the soil when bacteria metabolise it to the gases N 2 and N 2 O. The loss of gaseous forms of nitrogen to the atmosphere due to microbial action is called denitrification.

Nitrogen may also be leached from the soil if it is in the form of nitrate or lost to the atmosphere as ammonia due to a chemical reaction of ammonium with alkaline soil by way of a process called volatilisation. Ammonium may also be sequestered in clay by fixation. A small amount of nitrogen is added to soil by rainfall. In that form the nitrogen is said to be immobilised. Later, when such bacteria die, they too are mineralised and some of the nitrogen is released as ammonium and nitrate. It occurs fastest in warm, moist, well aerated soil.

In nitrogen fixation , rhizobium bacteria convert N 2 to ammonia NH 3. Rhizobia share a symbiotic relationship with host plants, since rhizobia supply the host with nitrogen and the host provides rhizobia with nutrients and a safe environment. Other, free-living nitrogen-fixing bacteria and blue-green algae live independently in the soil and release nitrate when their dead bodies are converted by way of mineralisation. Ammonia, NH 3 , previously released from the soil or from combustion, may fall with precipitation as nitric acid at a rate of about five pounds nitrogen per acre per year.

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When bacteria feed on soluble forms of nitrogen ammonium and nitrate , they temporarily sequester that nitrogen in their bodies in a process called immobilisation. At a later time when those bacteria die, their nitrogen may be released as ammonium by the processes of mineralisation. Protein material is easily broken down, but the rate of its decomposition is slowed by its attachment to the crystalline structure of clay and when trapped between the clay layers. The layers are small enough that bacteria cannot enter.

Some organisms can exude extracellular enzymes that can act on the sequestered proteins. However, those enzymes too may be trapped on the clay crystals. Ammonium fixation occurs when ammonium pushes potassium ions from between the layers of clay such as illite or montmorillonite. Only a small fraction of soil nitrogen is held this way. Usable nitrogen may be lost from soils when it is in the form of nitrate, as it is easily leached. This occurs when poor soil aeration limits free oxygen, forcing bacteria to use the oxygen in nitrate for their respiratory process.

Denitrification increases when oxidisable organic material is available and when soils are warm and slightly acidic. Denitrification may vary throughout a soil as the aeration varies from place to place. The application of ammonium fertiliser to such a field can result in volatilisation losses of as much as 30 percent. After nitrogen, phosphorus is probably the element most likely to be deficient in soils. The soil mineral apatite is the most common mineral source of phosphorus. Total phosphorus is about 0. Of the part available, more than half comes from the mineralisation of organic matter.

Agricultural fields may need to be fertilised to make up for the phosphorus that has been removed in the crop. Phosphorus is largely immobile in the soil and is not leached but actually builds up in the surface layer if not cropped. The application of soluble fertilisers to soils may result in zinc deficiencies as zinc phosphates form. Conversely, the application of zinc to soils may immobilise phosphorus again as zinc phosphate. Lack of phosphorus may interfere with the normal opening of the plant leaf stomata, resulting in plant temperatures 10 percent higher than normal.

Phosphorus is most available when soil pH is 6. Common mineral sources of potassium are the mica biotite and potassium feldspar, KAlSi 3 O 8. When solubilised, half will be held as exchangeable cations on clay while the other half is in the soil water solution. Potassium fixation often occurs when soils dry and the potassium is bonded between layers of illite clay. Potassium may be leached from soils low in clay.

Calcium is one percent by weight of soils and is generally available but may be low as it is soluble and can be leached. It is thus low in sandy and heavily leached soil or strongly acidic mineral soil. Calcium is supplied to the plant in the form of exchangeable ions and moderately soluble minerals. Calcium is more available on the soil colloids than is potassium because the common mineral calcite, CaCO 3 , is more soluble than potassium-bearing minerals.

Magnesium is one of the dominant exchangeable cations in most soils as are calcium and potassium. Primary minerals that weather to release magnesium include hornblende , biotite and vermiculite. Soil magnesium concentrations are generally sufficient for optimal plant growth, but highly weathered and sandy soils may be magnesium deficient due to leaching by heavy precipitation.

Most sulfur is made available to plants, like phosphorus, by its release from decomposing organic matter. The application of large quantities of nitrogen to fields that have marginal amounts of sulfur may cause sulfur deficiency in the rapidly growing plants by the plant's growth outpacing the supply of sulfur. Sulfur abundance varies with depth. The micronutrients essential in plant life, in their order of importance, include iron , [] manganese , [] zinc , [] copper , [] boron , [] chlorine [] and molybdenum.

They are required in very small amounts but are essential to plant health in that most are required parts of some enzyme system which speeds up plants' metabolisms. They are generally available in the mineral component of the soil, but the heavy application of phosphates can cause a deficiency in zinc and iron by the formation of insoluble zinc and iron phosphates.


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  • Iron deficiency may also result from excessive amounts of heavy metals or calcium minerals lime in the soil. Excess amounts of soluble boron, molybdenum and chloride are toxic. Nutrients which enhance the health but whose deficiency does not stop the life cycle of plants include: cobalt , strontium , vanadium , silicon and nickel. Soil organic matter is made up of organic compounds and includes plant, animal and microbial material, both living and dead. A small part of the organic matter consists of the living cells such as bacteria, molds, and actinomycetes that work to break down the dead organic matter.

    Were it not for the action of these micro-organisms, the entire carbon dioxide part of the atmosphere would be sequestered as organic matter in the soil. Soils have organic compounds in varying degrees of decomposition which rate is dependent on the temperature, soil moisture, and aeration. Bacteria and fungi feed on the raw organic matter, which are fed upon by amoebas, which in turn are fed upon by nematodes and arthropods. Organic matter holds soils open, allowing the infiltration of air and water, and may hold as much as twice its weight in water.

    Many soils, including desert and rocky-gravel soils, have little or no organic matter. Soils that are all organic matter, such as peat histosols , are infertile. The final stage of decomposition is called humus. In grassland, much of the organic matter added to the soil is from the deep, fibrous, grass root systems. By contrast, tree leaves falling on the forest floor are the principal source of soil organic matter in the forest.