SOIL Chemistry Notes (2ND PART) PDF

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Summary

Anion Exchange It’s the interchange between anions in soil solution and other anions on the surfaceof soil colloids.In the tropics, many highly weathered soils can have an anion exchange capacity.The soils attract and retain anions, rather than cations. The anions held and retainedby soil particles ...

Description

4.1. Anion Exchange It’s the interchange between anions in soil solution and other anions on the surface of soil colloids. In the tropics, many highly weathered soils can have an anion exchange capacity. The soils attract and retain anions, rather than cations. The anions held and retained by soil particles include phosphate, sulfate, nitrate and chlorine (in order of decreasing strength). In comparison to soils with cation exchange capacity, soils with an anion capacity have net positive charge. Soils that have an anion exchange capacity typically contain weathered kaolin minerals, iron and aluminum oxides, and amorphous materials. Anion exchange capacity is dependent upon the pH of the soil and increases as the pH of the soil decreases. The OH groups associated with the surface of iron and aluminum oxides and hydroxides and with 1:1 clays can be a source of positive charges under acid conditions.

Phosphorus reacts with soluble iron, aluminum and manganese and hydrous oxides of these minerals are held strongly held

Nitrates and chlorides are held very little and are leached by soil water. The pH of most productive soils is too high for development of anion exchange capacities. Anions with the exception of phosphate and to a lesser degree sulphate will not be retained. 21

4.2. Nutrient retention and release Soils have the capacity to both retain and release plant nutrient. Of major importance in

tropical soils is soil P. Phosphorus Sorption and Desorption Sorption/fixation: The process through which ions or molecules are removed from solution and accumulate within existing solid constituents Desorption: The release of ions or molecules from solids into solution. P-sorption occurs when the orthophosphates, H2PO4- and HPO42-, bind tightly to soil particles. Since phosphate is an anion, particles that generate an anion exchange capacity will form strong bonds with phosphate. P sorption occurs on positively charged colloids such as:  Aluminum and iron oxides  Highly weathered kaolin clays (under acidic conditions)  Amorphous materials.  Calcareous soils with high calcium levels These particles are commonly found in many of the most highly weathered soils and high weathered volcanic soils. Since P-sorption results in a decrease of plant available phosphorus, P-sorption can become a major issue in Hawaii soils. Additionally, in calcareous soils P-sorption may occur as phosphates sorb to impurities such as aluminium and iron hydroxides or displace carbonates in calcium carbonate minerals. Factors that affect P-sorption i.

Soil Mineral Type: Mineralogy of the soil has a great effect on P-sorption.  Volcanic soils tend to have the greatest P-sorption of all soils since volcanic soils contain large amounts of amorphous material.  Following volcanic soils, highly weathered soils (such as Oxisols/ferralsols) and Ultisols) have the next greatest P-sorption capacities. This is due to the presence of large amounts of aluminum and iron oxides and highly weathered kaolin clays.  On the other end of the spectrum, less weathered soils and organic soils have low P-sorption capacities.

ii. iii.

Amount of clay: As the amount of clay increases in the soil, the P-sorption capacity increases as well. This is because clay particles have a tremendous amount of surface area for which phosphate sorption can take place. pH: At low pH, soils have greater amounts of aluminum in the soil solution, which forms very strong bonds with phosphate. In fact, a soil binds twice the amount of

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phosphorus under acidic conditions, and these bonds are five times stronger. At high pH there are high levels of Ca and Mg that precipitate P. Temperature: Generally, P-sorption increases as temperature increases.

iv.

Factors that reduces P-sorption i. Soil anions Other anions, such as silicates, carbonates, sulfates, molybdate etc compete with phosphate for a position on the anion exchange site. As a result, these anions can cause the displacement, or desorption, of phosphate from the soil exchange site. Desorption causes phosphate availability in the soil solution to increase. ii.

Organic matter It increases P availability in four ways: First, organic matter forms complexes with organic phosphate which increases phosphate uptake by plants.  Second, organic anions can also displace sorbed phosphate.  Third, humus coats aluminum and iron oxides, which reduces P sorption.  Finally, organic matter is also a source of phosphorus through mineralization reactions.

iii. Flooding. Flooding the soil reduces P-sorption by increasing the solubility of phosphates that are bound to aluminum and iron oxides and amorphous minerals. Phosphate Precipitation and Dissolution 

Phosphate precipitation is a process in which phosphorus reacts with another substance to form a solid mineral.



In contrast, dissolution of phosphate minerals occurs when the mineral dissolves and releases phosphorus.

Solubility of Phosphate Minerals The solubility of phosphate minerals is very dependent upon soil pH.  The soil pH for optimum phosphorus availability is 6.5  At high or neutral pH, phosphate reacts with calcium to form minerals, such as apatite.  Under acidic conditions, phosphorus may react with aluminum and iron to form Al/Fe-phosphate minerals which are not soluble. 4.3. Nutrient supply processes in soils Almost all nutrients absorbed by plants are absorbed through the roots in inorganic form. There are three ways by which nutrient ions in the soil may reach the root surfaces. These are root interception, diffusion and mass flow. Root interception: Describes nutrients at the root-soil interface to the root surface that do not have to move to be available for absorption. As the root system develops and exploits the soil more completely, soil solution ions are exposed to the root surface for absorption. The quantity of nutrients supplied by root interception is the quantity present in a volume of 23

soil equal to the root volume. Root interception therefore is affected primarily by root growth rate and the amount of nutrients present in the soil. Mass flow: Is the movement of nutrients through the soil in the convective flow of water caused by plant water absorption. The amount of nutrient movement by mass flow is related to the water used and nutrient concentration of that water. The amount of nutrient reaching the roots by mass flow (M) can be calculated as M= (water use per plant)* concentration in soil solution The amount of nutrient supplied by this mechanism will vary with crop, climate, moisture conditions, and concentration of the nutrient in solution. The rate of ion movement to the root by mass flow depends on the rate of water uptake. Diffusion: Occurs when an ion moves from an area of high concentration to one of low concentration by random thermal motion (Brownian motion). When root interception and mass flow do not supply the root with sufficient quantities of a particular nutrient, continued absorption of nutrients from the surrounding soil reduces the concentration of available nutrients in the soil at the root surface. This therefore causes a concentration gradient perpendicular to the root surface, with nutrients subsequently diffusing along the gradient toward the root surface. Because nutrients are continuously being absorbed by the plant roots, equilibrium will not occur, and nutrients will continue to diffuse a long a concentration gradient towards the roots. A higher requirement for a nutrient or a high root absorbing power results in a strong sink for nutrients. 4.4. Soil Nutrient Pools The soil primary nutrients pools are:

    

Soil solution Exchangeable cations and anions Sorbed ions Organic Matter Primary and secondary minerals

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Soil nutrient pools  Soil solution: Soluble nutrients are readily available for plant uptake and it is believed that most elements must be released to the soil solution before they are available for biological uptake.  Exchangeable cations and anions: Exchangeable cations and anions form outer-sphere complexes with the charged surfaces. The negative and positive charge associated with clay minerals and organic matter are balanced by electrostatic attraction of cations and anions, respectively. These bonds are relatively weak resulting in rapid replacement of one ion with that of another.  Sorbed ions: A stronger type of interaction of cations and anions with soil minerals occurs through the sorption process, often termed inner-sphere complexation . In this case, the cation or anion forms a bond with the mineral surface in which the ion actually enters the coordination shell of the mineral structure. This bond is much stronger than electrostatic bonds associated with exchangeable ions.  Organic Matter: As fresh litter is decomposed to humus, the plant nutrients contained within the organic matter are slowly released making them available for biological uptake. Soil organic matter is a very important nutrient pool in forest ecosystems, especially for nitrogen, sulfur, and phosphorus. The pool of nutrients contained in organic matter is made slowly available through decomposition and mineralization by microorganisms over a period of months to years. The mean residence time of soil organic matter ranges widely (0.4 to 353 years) depending on the litter quality and the environmental conditions.  Primary and secondary minerals: Primary minerals are formed due to crystallization of the molten magma and have not undergone any chemical change, While Secondary minerals are formed by pre-existing minerals that have undergone chemical changes in their composition. In weak to moderately weathered soils, a very large

pool of nutrients remains locked within primary minerals. These nutrients are 25

not available for uptake until weathering reactions release the nutrient elements to the soil solution.

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5. SOIL REACTION Soil reaction refers the degree of acidity, alkalinity or neutrality of a soil solution. There are three types of soil reactions i.e. acidic, alkaline and neutral. Soil reaction is important because: In determines the availability for both plants and micro-organisms nutrients.  It influences the amount of toxic substance in the soil.  It controls the chemical environment in which both plants and micro-organisms exists  It’s used in diagnosing the fertility and productivity of soils. Soil pH is used to determine soil reaction. Soil pH is a measure of the acidity or basicity in soils. Its measures of H+ ions in soil solution. pH is defined as the negative logarithm to base 10 of H+ ions concentration. It is expresses as: pH = Log 1/[H+] or = - Log [H+] In water, soil pH ranges from 1 to 14, with: i) a pH below 7 being acidic, ii) ii) 7 being neutral and iii) iii) above 7 is basic. Soil pH is considered a master variable in soils as it controls many chemical processes that take place.

5.1.

Determination of pH in soils

Procedure:   

The pH meter is calibrated using pH 7 buffer solution. Then the meter is adjusted with known pH of buffer solutions 4.0 and 7.0. 20 g of soil is weighed and transferred into 100 mL beaker. 27

  

50 mL distilled water is added and stirred well with a glass rod. This is allowed to stand for half an hour with intermittent stirring. To the soil water suspension in the beaker, the electrode is immersed and pH value is determined from the automatic display of the pH meter.

Note that sometime soil pH is measure in 1: 1 (soil: water) or by dissolving a salt such as KCl in water.

5.2.

Soil acidity

Acid soils, by definition, are those with pH below 7.0. The lower the pH, the more acid is the soil. Each unit pH drop indicates ten times more acidity. For example, pH 5.0 has 10 times more acidity than pH 6.0, and 100 times more acidity than pH 7.0 etc. Strong and weak acids: Acids are classified according to the extent by which they dissociate in water; if dissociation is great, the acid is said to be strong e.g, nitric, sulfuric and hydrochloric acid. Acids that dissociate to only a slight extent, examples of which are acetic, carbonic and boric are termed as weak acids. 5.3. Sources of soil acidification Acidity in soils comes from H+ and Al3+ ions in the soil solution and sorbed to soil surfaces. While pH is the measure of H+ in solution, Al3+ is important in acid soils because between pH 4 and 6, Al3+ reacts with water (H2O) forming AlOH2+, and Al(OH)2+, releasing extra H+ ions. 1. Leaching of bases by heavy rainfall. Rainfall leaches basic cations such as Ca, Mg, K and Na beyond the roots. When these soluble bases are lost H+ ions developed during rains replace them at the colloidal complex. As exchangeable bases become depleted of the base the soil becomes more acidic. 2. Acids in rainfall The atmosphere contains large amounts of gases such as CO 2, SO2, NO2 etc which originates from industries. When it rains, snows or during a fog, the these gases dissolve in water to produce weak acids (carbonic acid) and other strong acids (H2SO4 and HNO3) which end up in the soil. The acids then dissociate to produce H+ as per the reactions belows i) C02 + H20

H2C03

Dissociation

CO3- + H+

H2C03 ii) S02 + H20 2H2S03 + O2

H2S03, 2H2S04,

Dissociation

H2SO4 SO42- + 2H+ iii) 2N02 + 2H20 HNO3 NO3- + H+ 2H2S03 + O2

2H2S04, 28

2HN03

Dissociation

H2SO4 SO42- + 2H+ 3. Continuous application of acid forming fertilizers. Use of ammonium based fertilizers such as, ammonium nitrate (NH4NO3), Ammonium sulphate ((NH4)2SO4), urea (NH2)2CO, Diammonium phosphate (DAP) (NH4)2 H2PO4 etc. leads soil acidification. These fertilizers acidify soils in through two ways: i) Nitrification of ammonium ions which leads to release of H+ ions which acidifies the soil as shown in the reactions below.  Ammonium nitrate NH4NO3 + 2O2 => 2NO3- + 2H+ + H2O  Urea (NH2)2CO + 4O2 => 2NO3- + 2H+ + CO2 + H2O  Ammonium phosphate NH4H2PO4 + 2O2 => NO3- + H2PO4- + H2O + 2H+  Ammonium sulphate (NH4)2SO4 + 4O2 => 2NO3- + SO42- + 4H+ + 2H2O ii) The ammonium ions displaces adsorbed basic cations at the sorption complex which are leached down leading to soil acidification. Eventaully NH4+ ions are nitrified leading to more soil acidity 2NH4+ + Ca2+

NH4+ NH4+

+ Ca2+ (Leached)

4. Weathering of minerals: Both primary and secondary minerals that compose soil contain Al. As these minerals weather, some components such as Mg, Ca, and K, are taken up by plants, others such as Si are leached from the soil, but due to chemical properties, Fe and Al remain in the soil profile. Highly weathered soils are often characterized by having high concentrations of Fe and Al oxides. 5. Plant root activity: Plants normally absorb more cations especially base cations than anions. Since plants must maintain a neutral charge in their roots to compensate for the extra positive charge, they release H+ ions from the root. Some plants will also exude organic acids into the soil to acidify the zone around their roots to help solubilize metal nutrients that are insoluble at neutral pH, such as iron (Fe). This activities acidify the soil. Organic matter (OM) Decomposition: During OM decomposition C02 is released, which reacts with H20 to form weak carbonic acid which acidifies the soil. C02 + H20 H2C03

6.

Dissociation

H2C03

CO3- + H+

7.

Accumulation of organic matter Accumulation of organic matter acidifies the soil in three ways:29

i. Organic matter forms complexes with base cations such as Ca2+ and Mg2+ thus facilitating the loss of these cations via leaching. ii. OM is a source of H+ because it contains numerous acid functional groups from which these ions can dissociate. Different groups dissociate at different pH levels. If pH is increased more functional groups undergo dissociation of H+ ions, leaving behind an increasing number of negatively charged sites on the molecule. It is these pH dependant negative charges that give rise to the large cation exchange capacity for which humus is known. Organic matter (R)

−OH −COOH

Organic matter (R)

−O + H+ −COO + H+

iii) The decomposition of plant residues commonly involves the oxidation of organic-SH groups to yield sulfuric acid (H2SO4). This is a strong acid. 8. Acid sulphate soils Acid s sulphate soils are naturally occurring soils, sediments or organic substrates (e.g. peat) that are formed under waterlogged conditions. These soils contain iron sulphide (pyrite) minerals. When the water is drained, excavated or exposed to air by a lowering of the water table, the sulfides react with oxygen to form sulfuric acid. FeSO4 + 2H+ + SO42-

FeS2 + 3 ½ O2 + H2O Pyrite

ferrous sulfate

dissociated sulfuric acid

This and related reactions are responsible for producing large amounts of acidity in certain soils in which reduced sulfur is plentiful and oxygen levels are increased during excavation or drainage. 5.4. Role of aluminium in soil acidity Al plays a central role in soil acidity since it is a major constituent of most soil minerals (aluminosilicates and aluminium oxides), including clays. The exchangeable and soluble Al3+ ions play two critical roles in soil acidity. Al is highly toxic to most organisms. High Al alter the permeability of cell membrane and therefore inhibits the uptake of many cations including Ca 2+, Mg2+, K+ and NH4+. Al toxicity inhibits root growth, such that the injured roots become short, swollen and have reduced root hair development resulting to limited mineral and water uptake.

High aluminium

No aluminium

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Plate: Effect of Al toxicity on plant root growth.  Al3+ ions react with water (hydrolyze) to form H + ions. The Al combines with the OH ions, leaving the H+ ions in solution which contribute to increasing soil acidity. Al3+ + H2O===> Al(OH)3 + 3H+ 5.5.

Pools of soil acidity

There are three general pools, or sources, of acidity: i) active, ii) exchangeable and iii) residual. 

Active acidity is the quantity of hydrogen ions that are present in the soil water solution. Its small compared to exchangeable and residual acidity. This pool most readily affects plant growth.



Exchangeable is also called salt replaceable acidity. It refers to the amount of acid cations, Al3+ and H+ ion on the exchange site and can easily be exchanged by other cations in a simple unbuffered salt solution such as KCl. K+ K+

Al

3+

H

+

Soil Soil solid

Soil solution



K+

+ 4KCl

K+

+ AlCl3 + HCl

solid Soil solution

Residual acidity comprises of all bound aluminium and hydrogen ion (including aluminium hydroxyl ions) nonexchangeable forms on soil colloids. It’s the least available form of acidity. When the increases, H bound dissociates and Al ions are released and precipitates as Al(OH)3. Ca2+ Ca2+ + Al(OH)3 + H 2O

Al3+ H+ + 2Ca(OH)2 Soil colloid solution

Soil solution

Soil solid

Soil

Thus, total acidity = Active acidity + Exchangeable acidity + Residual acidity The fourth pool of acidity is Potential acidity, which is less common but sometimes very important. It arises from the oxidation of sulfur compounds in certain acid sulfate soils.

5.6.

Importance of soil pH

Soil pH play important role on:  Soil nutrient availability,  Biological activities and processes and  Soil physical characteristics.

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i)

Nutrient availability 

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