• soil components
• mineral fraction; 45-50%. Mineral particles from 95-99% of solid fraction.
• organic matter: 0.5-5%, made up of different substances that are gradually broken down by microorganisms. Includes carbohydrates, proteins, lignins, fats, waxes. Many of these compounds do not decompose completely and are transformed to humus, a dark, complex, non-defined colloidal material.
• water: 25% of soil volume.
• air: 25% of soil volume.
• organisms: a small but important fraction of soil.
• macro organisms: insects, grubs, earthworms. Earthworms help decompose organic matter, releasing plant nutrients, aerating soils and improving drainage. The insect-like organisms have hard skins and hard jaws, helpful for dissecting woody substances.
• microorganisms: protozoa and nematodes, plant parasitic nematodes as well as harmless ones present in most soils.
• bacteria: (heterotrophic, autotrophic, aerobic, anaerobic, facultative) oxidise S and N. Decompose organic matter and may cause some plant diseases. Rhyzobium and Bradyrhyzobium bacteria form symbiosis with legume plants to fix nitrogen from the air, and are particularly important in tropical soils.
• fungi: (mycorrhyzal, damping-off, stem fungi, crown fungi, root fungi, rot) important in decomposition of organic matter, but may cause some plant diseases. Mycorrhyza form symbiosis with plants to facilitate absorption of P, S, Zn and perhaps water too. They are important in soils low in phosphorus (P) but can't extract P where it does not exist.
• actinomycetes: important in the decomposition of organic mater.
• algae: autotrophic organisms. Blue-green algae fix some nitrogen. they need sunlight and can live only on the surface.
• soil texture
• unweathered primary materials: have little capacity to hold water and nutrients and are relatively chemically unreactive.
• gravel: 2 - 4mm
• sand: 0.05 - 2.0 mm
• silt: 0.002 - 0.05 mm
• weathered secondary materials
• clay: is a secondary mineral less than 0.002 mm in diameter. It is formed as a result of weathering. Silica and alumina sheets are formed by recrystallisation. Amorphous clays in warm climates form oxides of iron and aluminium. Clays have a high cation exchange capacity (CEC) because they are negatively charged and can attract, retain and exchange cations. Their water holding capacity is very high because of their large surface area per unit mass.
• three-layer clays (Si-Al-Si lattice) montmorillonite and illite, have high CEC.
• two-layer clays (Si-Al lattice) like kaolinite have low CEC.
• amorphous clays are composed of oxides of Fe and Al and have very low CEC.
• textural classification:
• infiltration of water: rapid in sands, slow in clays.
• drainage: rapid in sands, slow in clays.
• aeration: sand has rapid gas exchange; clay slow.
• fertility: sand has low fertility. Clay high, depending on type.
• soil structure
• appearance: granular, blocky, platy, prismatic, columnar.
• larger-sized aggregates: held together by divalent cations, organic residues, other cementing agents.
• influences on structure:
• aeration: good soil structure is very important to provide for both god aeration and a high water holding capacity. Poor structure can result in high water holding capacity but poor aeration. Structure can be influenced by tillage. Puddling occurs when soil is tilled when too wet, or by compaction.
• infiltration of water: the capacity of water to enter soil.
• percolation of water downward: water seeps downward through the soil profile.
• soil pore space: soil is 40-60% pores. The pore size influences water holding capacity and aeration.
• capillary pores: retain water against the pull of gravity
• noncapillary pores: contribute to soil aeration
• soil profile: a vertical cut through the soil shows its layering, characteristic of its mineralogy and history.
• O horizon: organic matter, leaf litter.
• A horizon: zone of leaching. Plow layer in agricultural soils
• B horizon: zone of accumulation: weathering from below + leaching from above.
• C horizon: zone of weathering, between B and bedrock R
• R horizon: the bedrock.
• soil moisture: Soil moisture is expressed as osmotic pressure, required to extract it from the soil. Commonly quoted in bar or in MPa pressure. To convert: 10 bar= 1 MPa (megapascal). The Permanent Wilting Percentage (PWP) is the water potential below which plants are unable to draw sufficient water for life, let alone growth. Each plant species or group of plants has its own specific PWP. Conventionally a PWP of -1.5 MPa is an average for most plants, but xerophilous (dry-loving) plants wilt at PWP=-3 MPa, whereas hygrophilous (moisture-loving) plants wilt at PWP=0.7 MPa.
• chemically combined water: part of rock composition, hydrated minerals. It is unavailable to plants.
• hygroscopic water: a thin layer of water that coats soil particles. It is unavailable to plants. (-5MPa)
• capillary water:
• held in soil capillaries: water retained agains the pull of gravity because off cohesive and adhesive forces. (Cohesive= attraction of unlike charged polar water molecules to each other) (Adhesive= attraction between polar water molecules and other polar materials such as glass, soil particles, cellulose)
• plant available water: soil water available to plants, water potential ranging from -0.03 to -1.5 MPa.
• -0.03 MPa: soil is at field capacity, saturated. The upper limit of soil water.
• -0.2 MPa: most soil water gone.
• -1.5 MPa: there is virtually no plant-available water below this pressure.
• gravitational water: free water that moves downward through the soil profile, by percolation. Little of it is available to plants.
• movement of soil moisture:
• infiltration: movement of water into a soil. Enhanced by good soil structure, coarse texture, presence of organic matter, but little is held in mulches. Decreased by soil compaction, poor soil structure, high clay content, high soil water content.
• percolation: movement of water through the soil profile. Good structure and coarse texture results in rapid movement. Movement decreases by poor soil structure, high clay content, high soil water content.
• capillary movement: slow redistribution of water in soil capillaries; important where subsurface irrigation is used and as crops withdraw water. Water can move towards roots by capillary action.
• soil pH: The pH value measures the number of H+ ions in solution as inverse powers of ten. Thus a solution with pH=8 has 10 times less H+ ions than one with pH=7. At pH=7 (pure water), a solution is neutral with as many acidic H+ ions as basic OH- ions. A pH greater than 7 is basic (alkaline), whereas a pH less than 7 is acidic.  Soil pH ranges from acidic 3.5 in high rainfall areas to basic 8.0 in low rainfall areas. Most plants thrive in slightly acidic soils pH=5.5-7.0, which also promotes the formation of new soil and the availability of nutrients. Positively charged ions like H+ are cations, whereas negatively charged ions like OH- are anions.
• causes of acidity:
• hydrogen ions H+: common acidity.
• aluminium ions AL+++: react with water to form H+ : Al + H2O = Al(OH)3 + 3H+
• soil acidification:
• leaching of cations: Ca++, Mg++, K+ by water.
• crop removal of cations: crops use cations as nutrients.
• use of acid-forming fertilisers such as ammonium sulfate. These change the pH by introducint H+ ions.
• acid rain: nitrates (NO3-) and sulphates (SO4--) rain down, which causes aluminium to leach from clay, forming poisonous compounds. Forests leach excessive amounts of acid. Soils, rivers and lakes acidify.
• significance of soil pH
• effect on plants: most plants grow best at PH 5.5-7.0 but plants vary in their requirements. pH less than 4.0 or greater than 9.0 can be toxic to roots.
• influence on nutrient availability: pH 4-5 and 8-9 influences the availability of minerals to plants.
• acidic soil (pH <5.5): reduced availability of Ca, Mg. Fixation of PO4. Reduced availability of B, Mo. The levels of soluble Al, Mn, Zn, Fe increase to the point where they may be toxic to plants.
• alkaline soil: reduces availability of B, Fe, P, Mn, Zn, Cu. If the Na concentration is high, then Ca and Mg are reduced too.
• amending soil pH: depends on soil texture.
• raising pH: use ground lime, coral, dolomite (contains both Ca & Mg). Finely ground lime reacts more quickly than coarse material. Addition of CaCO3 (lime) replaces H+ by mass action, raising soil pH.
• lowering pH: usually by adding elemental sulfur (S), which is oxidised to sulphuric acid H2SO4 by soil bacteria.
• cation exchange capacity (CEC): a measure of the negatively charged sites in a soil that can attract, hold and exchange positively charged ions (cations) Ca, K, Mg, NH4, H, Na. The more negatively charged sites a clay contains, the more cations it can hold and the higher its CEC.
• units: expressed as centimoles of negative charge in 1.0 kg of dry soil (centimol/kg). The CEC is equivalent to the centimoles of H+ that will combine with 1.0 kg of dry soil.
• percent base saturation: the proportion of CEC satisfied by Ca, Mg, K and Na, but H and Al are excluded.
• cation exchange: exchange of cations between the soil solution and exchange sites are based on two phenomena:
• the lyotropic series: the relative capacities for cations to replace one another if present in equivalent quantities. The order is Al > H > Ca > Mg > K > Na, where the weakest-bound ones (left) are the easiest to replace. Sodium (Na) is usually very strongly bound.
• law of mass action: adding large amounts of one cation will replace others, regardless of their relative capacity for replacement. Since H will replace Ca, when they are present in equivalent amounts, excess Ca will replace H, and large amounts of it must be added to soil in order to raise its pH.
• importance of soil CEC:
• High CEC increases soil buffering capacity, the resistance to the change in concentration of a nutrient or pH.
• High CEC enhances nutrient retention in soils.
• range of soil CEC: zero to over 100 centimole/kg; kaolinite clay= 10; montmorillonite clay= 100; organic matter = 150-300.
• problem soils:
• saline soils: soils having less than 15% of CEC satisfied by Na, and a pH 7.0-8.5, and an excess of Ca, Mg, and Na as salt (NaCl) or as sulphate (Na2SO4). It has a detrimental effect on plants due to high concetration of salts. The concentration can be reduced by leaching. Salts can accumulate also due to too much fertiliser, poor drainage and salty irrigation water, and soil water evaporation.
• sodic soils: soils having more than 15% of CEC satisfied by Na, and a pH 8.5-10 cause detrimental effects to plants due to high pH and Na concentration but not because of salt (NaCl). It can adversely affect soil structure. Soils can be reclaimed by applying CaSO4 (gypsum), wich displaces Na to produce soluble Na2SO4. (sodium sulfate).
• soil formation factors:
• mechanical weathering: base rock is broken into smaller pieces but does not change chemically.
• chemical weathering: base rock falls apart in parts that are chemically different, some being soluble.
• rainfall: slightly acidic rainfall dissolves and leaches minerals fom parent material. Water acts as a catalyst.
• temperature: higher temperature hastens the rate of all chemical reactions, so the parent material weathers more rapidly.
• time: weathering of parent material is a very slow process.
• biological weathering: organisms acidify their environment and hasten the rate of dissolution of parent material. Near roots, the concentration of ions can be very high.

• loss in quantity
• gravity
• creep: Soil slowly creeps down-hill. Particularly clayey soils do this because clay, especially three-layer clays (Montmorillonite) can hold water up to equal their volume (100%). During droughts, the clay shrinks and during rains it expands again. This causes cracks in summer and also moves the soil slowly down-hill.
• slip: A slip is usually a small area suddenly sliding down-hill, leaving a scar behind and producing large amounts of loose soil on top of  down-hill soil. Slips are natural but occur more so on agricultural land. Clay soils may slip over sloping bedrock after long periods of gradual rainfall. This softens the clay, which lubricates the bedrock. A slight disturbance like an earth shock can then set off masses of slips. Trees reduce slips considerably.
• slump: very large slips where whole hill sides move, are called slumps.
• water
• raindrop impact damage: Water drops hit bare soil and loosen clay and sand particles. This is the largest source of clay runoff. Raindrop impact can be eliminated by covering the soil with vegetation or mulches.
• sheetwash: Water runs over the soil like a sheet. It was thought that this caused most runoff, but at this stage, the speed and pressure of the water is low. Sheetwash can be stopped by dense planting and soil cover.
• rilling: As water collects downhill, small streams or rills are formed. They can cut through a cropland, transporting loose soil particles. At this stage, the water flows fast and has sufficient volume to exert some pressure on the land. Sand, silt and clay are transported. Rilling can be prevented by contour ploughing and planting, and by reducing the uphill size of the field. Allowing steeper uphill ground to revegetate naturally can serve as a water trap.
• gullying: Surface gullies are formed in steep terrain where water flows fast, eroding the soil underneath until the bedrock is reached. Gullies normally form outside the fields. They can be controlled by fencing and revegetating their sides and by constructing check dams.
• tunnelling: Water runs through cracks to the bedrock and scours out tunnels between bedrock and soil. Eventually such tunnels slump, causing gullies that can erode quickly. This form of erosion is hard to control because it happens out of reach and out of sight. Fencing, revegetating and planting spaced trees uphill prevents further tunnelling.
• river bank erosion: Swollen rivers exert pressure and friction on river banks, particularly when saturated with mud. Most river banks were deposited by earlier rain storms and are easily eroded. Rivers need to flow freely during rain storms. Tree roots cannot prevent this. Riparian (riverside) planting has little effect and attention must be paid to the uphill sources of erosion. Riparian fencing helps to keep cattle out.
• wind: Wind without rain is surprisingly erosive. Clay particles become air-borne and can blow vast distances. The soil selectively loses its most fertile components. Clay disappears and sand remains. Leaving stubble on the field helps. Planting shelter belts is very effective over a large distance.
• ice
• glaciers: Glaciers are rivers of ice, formed from snow. Under pressure, snow compacts to ice. Glaciers exert enormous pressure but move slowly. They grind rocks to finer particles, which are deposited where they end, producing a sil (bar) in the valley. Because their pressure increases with depth, glaciers scour deep U-shaped valleys.
• frost-heave: Under special conditions of moisture and repetitive frost, the soil can expand suddenly, pushing its surface up and damaging roads.
• rock cracks: When water freezes, it expands with enormous force, enough to break large rocks. On mountains, erosion is highest where snow becomes water, then freezes again.
• drought
• deep cracks: Deep cracks occur particularly in clay soils. Clay absorbs water and expands. During droughts, it contracts and forms cracks. Cracks dry the land more quickly. Water from the first rains, runs into these cracks, causing erosion.
• creep: Consecutive expansion and contraction of the soil causes land to creep down-hill.
• down-hill degradation
• nutrients/chemicals in rivers: Depending on the amount of rainwater and the amounts of fertiliser applied, rivers can exceed safe levels of nutrients, causing mortality to fish and other river life. Biocides in low concentrations are sufficient to also cause damage.
• nutrients/chemicals in aquifers: Farm nutrients and persistent chemicals often end up in aquifers where they may remain and accumulate for hundreds of years. Aquifers also provide drinking water, becoming poisonous in the process.
• rivers silting up: As water flows down-hill, it meets more water. Rills become torrents, then rivers. Rivers flow swiftly in some places, slower in others, allowing sediment to settle out. This may change the downslope profile of the river, causing repeated flooding.
• eutrophication of lakes: Nutrients from farms and fertilisers can wash out into rivers, ending up in lakes where they cause dense plant and plankton blooms. When plant matter sinks to the bottom, decomposing bacteria use up all oxygen and the lake becomes poisonous to life.
• eutrophication of the sea: Nutrients from agriculture wash into the sea, fertilising the waters and causing excessive plankton blooms. Poisonous plankton species take over, posioning coastal fisheries and killing marine species. Decomposing bacteria take over, spreading disease and death.
• pollution of the sea: Clay particles soil the sea, causing filter-feeding animals to suffocate; Clay settles on plants and shades their leaves, while inhibiting their life processes. Clay clouds the water and shades deeper plants, who die. Clay forms pans on the bottom, killing bottom life and changing vast areas of sea bottom habitats. Persistent biocides are concentrated in marine animals, disrupting food chains.
• disappearing beaches: when beaches become polluted by nutrients (plankton), sewage (bacteria) or fine soil particles (mud), they won't dry out anymore between high tides. As a result, their self-repair mechanism is lost and they gradually retreat and disappear.
• lowering of groundwater levels: By pumping groundwater for irrigation, its level gradually drops, affecting areas down-hill and around. It may affect natural stands of vegetation and wetlands.
• loss in quality
• rain: Soluble nutrients are dissolved in water and transported. Carbondioxide CO2, dissolved in rain forms a weak carbonic acid H2CO3 (H.HCO3) which can dissolve a number of elements like calcium (Ca.(HCO3)2). Sulphur from the atmosphere forms sulphuric acid H2SO4 which binds strongly with cations like Ca++ to gypsum CaSO4 and others.
• leaching out (eluviation): The leaching out of nutrients and minerals is greatly accelerated by ploughing. Soils lose their fertility.
• leaching in (illuviation):  Dissolved nutrients can be transported through the soil into the groundwater but can also react with the soil minerals in the B horizon. Impenetrable iron pans can be formed or layers with carbonate compounds (CO3--) ('effervescence'). Acidity can prevent the formation of three-layer clays.
• drought
• loss of soil biota: During drought, the soil biota shrivel and die. The soil can lose its fertility easily.
• ferrugination:  Long droughts prevent clays from forming. Instead, iron sesquioxides are formed, which adhere firmly to sand and gravel, giving them a red colour, and may cement them to form a subsurface iron pan. It forms a soil with low fertility because nutrients are leached out and downward. Ferrugination may occur after deforestation or because of poor farming practices.
• rubification: when soil is thoroughly dried from time to time, precipitates of iron and organic matter cannot accumulate and the organic matter disappears by decay, causing irreversibly dehydrated iron sesquioxides to form. Soils are called cinnamonic (red).
• farming practices affecting soil
• burning: Almost all biomass (90%) is lost to the atmosphere, including all nitrogen and carbon compounds to feed the soil biota. In addition, soil biota are cooked and the top layer baked, losing its valuable humus and water-binding structure. The charcoaled wood won't decompose.
• ploughing: Deep ploughing and shallow harrowing serve to render soil more friable and porous. It removes weeds and produces a fine structure suitable for seeding and planting. Harrowing is also used to 'mulch' soil to prevent excessive evaporation.
• nutrient loss: Water can flow more freely through the A horizon into the B and C horizons and into the water table, taking nutrients with it. Only about 15-50% of nitrogenous fertiliser is taken up by plants. Some evaporates, but as much as 40-50% washes into groundwater and aquifers.
• humus loss: Organic matter is lost because soil biota decompose it much faster since the loose soil brings all the oxygen they need. Even persistent humus is lost this way.
• soil biota loss: Soil biota are lost because not enough organic matter remains to feed them. In this way fertility is lost with them. The fertility of a natural soil is kept inside the bodies of the soil organisms!
• waterlogging: Ploughed soil opens the path for fine particles to be washed downward where they clog together, blocking the natural drainage of water.
• compaction: Heavy machinery (in 1948 averaging 2.7t, in 1990 averaging 7-22t) is used to work the soil, causing compaction. Because the soil has lost its natural structure with roots and tunnels made by soil organisms, it is easily compacted, leading to water logging or reducing the soil's ability to absorb water. Even without compaction, ploughed soils have a porosity equal to a density of 1.4, whereas no-till or natural soil has density 1.0 near the surface. Deeper than plough depth, they become equally dense.
• acidity pH: Compared to no-till cropping, the ploughed soil becomes acidic pH=4.5 at 25cm, whereas no-till soil stays less acidic pH=6.5 to twice that depth.
• irrigation
• eluviation of nutrients: Excessive irrigation can cause loss of nutrients.
• pan-forming: Hard pans of iron oxide can form between B and C horizons when the soil is unusually acid, such as under pine forests and other forests producing resinous leaf litter. The resins decompose to acids that leach minerals down into the B horizon. Here they react with newly weathered soil to form a hard pan, a thin layer, which is impenetrable to water and roots. (note that this happens mainly in cold pine forests). Acids may originate from fertiliser, acid rain and lack of calcium. Poor drainage is also a factor.
• salinisation: When land is irrigated with water from lakes, water that ran off other land first, cropland can become salty from the salts dissolved in the water. Normally, salt (NaCl) is not held by the soil (but enough is retained in its organisms), and it is leached downward through the soil profile and groundwater. Salinisation thus happens in arid climates with high evaporation and too little rain to wash excessive salt away. It can also be caused by a drainage problem.
• chemical application: Herbicides (plant killers), fungicides (mould killers) and pesticides (insect killers)
• loss of soil biota: The soil contains bacteria, fungi, worms and insects. Like their above-ground cousins, these are also sensitive to biocides. They all work together and if one group is affected, it affects the entire functioning of the soil. Manufacturers' claims of chemicals 'neutralising' in the soil, must be treated with suspicion. How do they neutralise? By killing soil biota?
• permanent unsuitability: Persistent biocides may cause long-term damage to soil biota and thus soil fertility.
• grazing: Grasslands can be sustainable but many are not. Because most meadowland has no trees, it is sensitive to erosion (see above) but other dangers arise:
• overgrazing: Overgrazing won't leave enough organic matter for the soil organisms, causing soils to become less fertile. Overgrazing does not leave enough leaf coverage so that rain drops cause erosion. Lack of leaf cover also unnecessarily dries the land out. Lost nutrients are not replaced, causing soils to degrade. The slope of the land worsens all overgrazing effects. Overgrazing also reduces plant biomass because the small part left above ground cannot maintain a large part underground.
• pugging/camping/tramping: Cattle left to range over the land freely, compact the soil by their small hooves, which concentrate their weight over a small surface. The soil recovers slowly but now that the grassland has been made far more productive than natural prairie or steppe, many more hooves tread it per acre. Pugged areas can become waterlogged.
• atmospheric pollution: Farms do contribute to atmospheric pollution and global warming.
• fossil fuel use: Modern farming uses some 7 Gcal/ha = 830 l/ha fuel each year.
• CO2: Carbondioxide is produced by farm machinery.
• biogenic gases:
• NH3: Ammonia is produced by soil bacteria when converting wastes and fertilisers.
• N2O: Laughing gas and other nitrous oxides are produced when nitrogen is in excess, due to fertilising.
• CH4: Natural gas or methane is produced by grazing animals and by rice padis.
• CO2: Carbon dioxide escapes from soil as its organic matter degrades, being burnt up by soil organisms. In the process of soil degradation and deforestation, large amounts of CO2 enter the atmosphere.
• CO: Carbonmonoxide is produced by agricultural soil as part of organic matter degradation and burning.

• tectonic movement and replenishment: 50-200 million years
• soil profile formation: 10,000 years. Soil and terrestrial ecosystem have to evolve together and go through many thousands of years of succession until finally a stable community with its soil profile is formed. Soil without a ground cover of vegetation will never form a profile, since erosion exceeds soil formation. Soil formation: 1mm per 10 years. 1m = 10,000 years. New Zealand soils developed under very slow metabolising forests and therefore took much longer to form. Erosion rates were much lower too and NZ coastal seas very clear.
• soil slip repair: 50-300 years. Slips repair relatively quickly because the soil is essentially still there and plants and micro organisms have to adjust to its new place. Some forests take a long time to recover because trees over 300 years old may need to be replaced.
• soil degradation through careful farming: 50-200 years, depending on the slope. Much farming has been a hit-and-miss affair by trial and error. Modern scientific farming aims to change this time scale.
• soil degradation through careless farming: 5-50 years. Depending on slope, bad soil can be lost very rapidly through bad farming practices.
• Nitrogen cycling in the soil from bacterium to other microorganism: days
• Nitrogen cycling between soil and plant: months
• Nitrogen cycling from litter to soil: 1-200 years
• Nitrogen cycling between soil and atmosphere: ?

Acidic -  intermediate - Basic = grouping according to the ratio of metal to oxygen atoms. Basic= high ratio (less than 50% silica). Acidic = low (more than 50% silica). It is also the order of mineral formation from a magma melt.

Solid solution = composed of various components such that the chemical formula of the rock is not unique and any combination is possible. Two or more elements can substitute for each other completely. For example, the anions Mg++ and Ca++ , which are similar in size and function, can combine CaSiO3 and MgSiO3 to (Ca,Mg)SiO3 as if the rock components were dissolved into one another. The fact that silicate rocks allow for substitution makes them easy to take apart through weathering.

Bowen Series: the Bowen Series orders igneous minerals by how soon they condensate out of a magma melt, as it cools. First 'ultra-basic' minerals are formed. These have a high content of heavy elements and are correspondingly low in silica content. Likewise, the last minerals to condensate are 'acidic', having high silica content and low heavy elements. A very rough rule is that the darker or denser the rock type, the more basic it is. Crustal minerals and rocks tend to be siliceous. Erupted lavas tend to be basic, and deep-seated minerals and rocks tend to be ultrabasic. The bowen series is:

• ultrabasic
• olivine
• pyroxene
• Ca-feldspar, plagioclase
• basic
• amphibole
• biotite mica
• intermediate
• Na-feldspar, orthoclase
• K-feldspar, orthoclase
• muscovite mica
• acidic
• quartz

Lyotropic replacement series = the relative capacity for cations to replace one another if present in equivalent quantities. The order of preference is: Al+++ > H+ > Ca++ > Mg++ > K+ > Na+. Thus Al has the weakest bond whereas Na the strongest.

Law of mass action: adding large amounts of one cation will replace others, regardless of their relative replacement. Since H+ will replace Ca++, an excess in Ca++ must be added to soil in order to raise the pH (make it less acidic).

Cation Exchange Capacity = a measure of soil, particularly the charged clay particles, to attract, hold and eschange cations Ca++, K+, Mg++, NH4+, H+ and Na+. The more negatively charged sites a clay contains, the more cations it can hold and the higher its CEC. A high CEC increases the soil's buffering capacity (its resistance to changes in pH or changes in nutrient concentrations). A high CEC enhances nutrient retention in soils so that they can hold more. Soil CEC ranges from 0 to over 1 mole/kg. Note that 'organic' soil (the soil biota) has a CEC of nearly two orders of magnitude (100x) larger than soils without soil biota.
(See CEC table above)

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