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by: Arne Stoltenberg
Arne Stoltenberg
GPA 3.62


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Class Notes
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This 13 page Class Notes was uploaded by Arne Stoltenberg on Tuesday October 13, 2015. The Class Notes belongs to AGRO 2051 at Louisiana State University taught by Staff in Fall. Since its upload, it has received 6 views. For similar materials see /class/223176/agro-2051-louisiana-state-university in Agricultural & Resource Econ at Louisiana State University.


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Date Created: 10/13/15
rnnulrients Ca Cu Fe Mn NthDawns B CMMHanmns Yhese ave revevved a as mmmnmnems because mew cancemvmmn m wants 5 ahmAH MEI m 55 than wry22mmer m macvanumems nz uency Ind Ynxlc y De cren y sumcrenQand numbEwes Causes aidefmencres ngh was vemave gamma amaums m a ana vsxsvemhmvs mmam 92W vmes enev undevstandmg mmam nmvman ana mpmved methads anam ana sau ana vss a 3 Causes u uxrcmes summery2v mnge s navmvv W21 Vedaan aanamans ana mpr mBV mcvease Fe ana Mn mmxu canuemvatmns nudema apphmtmns Mk2 ca ana Zn m s udge m a ana Ma m w gmmn v atev Rnlesm Plum Nmmmn Vavmusmndmnsx enlvme svstems Nme mm smug mmmnumems are mmah e m P am de uenw Wmmams appeav an Wungemssue Manvanese mew m canary SnurcesMMlcrnnmnems lnurgamc suumes ncmde We stvu uve av anavy ana semnr aary mneva s ana sanaae summed m aundmns mam cumbmahuns GenevaW ess mpanam am memes am ava ahmw Soil Conditions Associated with Micronutrient Deficiencies Acid sandy soils Low levels present Organic soils Levels may be low High pH Cations less soluble Intensiver farmed Depleted Availability of Cations pH Fe3 OH39 ltgt FeOH2 FeOH2 OH39 ltgt FeOH2 FeOH2 OH39 ltgt FeOH3 precipitate Overliming may induce deficiencies Redox conditions aeration Reduced forms are more soluble Fe2 gt Fe3 and Mn2gt Mn Toxicity is possible if pH is sufficiently low Conversely under well aerated oxidizing and high pH conditions metal deficiencies are possible Anions Cl39 is present in soils in relatively large amounts It is deposited from the atmosphere and applied in KCI fertilizer B deficiency is common Its availability greatest at low pH but readily leached from acid sandy soil H3BO3 ltgt H H2BO3 It is bound to mineral and organic colloids and its availability lowest pH 7 to 9 Mo shows increasing availability increases with pH H2MOO4 39 H HMOO4 HMoO439 H M004239 M004239 is the dominant form at pH gt 6 Liming increases its availability Chelates Organometallic complexes with 2 or more sites capable of bonding to metal These include natural products released by plants or components of soil humus There are also synthetic chelate amendments Chelates may increase the solubility and availability of micronutrients by increasing the solution in chelated form concen tration for example FeOH3 EDTA439 FeEDTA39 3OH39 Effectiveness of added metal chelate depends on its relative stability constant the greater the tendency for formation of the chelated form the greater the solubility of the chelated metal Problems may arise when a metal chelate is added to soil if the stability constant for added micronutrient is lower than the stability constant for another metal in soil In this case the added micronutrient tends to be replaced For example the stability constant for FeEDTA is greater than that for ZnEDTA so that ZnEDTAz39 Fe3 39FeEDTA39 Zn2 Also a chelated micronutrient added to the soil may also be displaced by a metal with a lower stability constant if the concentration of the second metal like Ca2 is large Despite these limitations chelates are used with Cu Mn Zn and Fe Often the problem with replacement is offset by slow replace ment kinetics Also the chelate may be applied as a foliar spray or banded Che lates are expensive but used Micronutrient Management A few points Important to maintain balance among micro nutrients Slightly acid pH 6 to 7 is generally optimal Moisture and aeration affects availability of Fe and Mn Banded application to soil more efficient than broadcast Foliar sprays avoid fixation problem Incorporation into fritted glass provides low concentration of micronutrients as glass weathers Soil Colloids General Properties Soil colloids are minute and therefore have a large surface area per unit mass Soil colloids also carry electrostatic charges and that are balanced by adsorbed cations and anions Four general types Layer aluminosilicates which consist of thin layers of repeated structural units These are the dominant clay minerals in temperate regions Amorphous aluminosilicates that form from volcanic ash AI and Fe oxides which may be crystalline or amorphous These are common in subtropical and tropical regions Organic humus which are 3D non crystalline polymers present in all soils Layer Silicates General structure Composed of alternating sheets of Si tetrahedra and Al or Mg octahedra The octahedral sheet is called dioctahedral if Al is the central metal atom or trioctahedral if it is Mg The Si tetrahedral sheet is chemically bonded to the one or two adjacent Al or Mg octahedral sheets via shared oxygen atoms Basic units are the Si tetrahedron and the Al or M Many tetrahedra are linked together to form a Si tetrahedral sheet and many octahedra are similarly linked to f rm octahedral sheet In turn these different sheets are bonded together to form crystalline units Source of electrostatic charge Isomorphr39c substitution Al for Si in the tetrahedral sheet and MgP for Al in the dioctahedral sheet These substitutions occur during formation of the clay mineral and are permanent to the structure Both lead to net negative charge within the crystal lattice that is balanced by adsorbed cations pH dependent charge Loss of ionizable H from certain sites on mineral colloids or from certain functional groups in humus leads to negatively charged sites Protonation of other sites leads to positively charged sites pH dependent negative charge increases with increasing pH but pHdependent positive charge increases with decreasing pH Types of layer silicates 11 21 2 1 1 1 Orientation of tetrahedral and octahedral sheets in 11 21 and 211layer silicates 1 1 layer silicates These have one Si tetrahedral and one Al octahedral sheet per crystalline un t Adjacent layers units are Hbonded together via sharing of H from octahedral OH with 0 adjacent layer of the tetrahedral sheet of 11 layer silicates do not expand Since adjacent layers are Hbonded together these minerals are nonexpanding and exhibit only an external surface area These exhibit little plasticity cohesion or swelling Also there is little isomorphic substitution and the capacity to adsorb cations cation exchange capacity CEC is low Kaolinite halloysite and dickite are 11 layer silicates 21 layer silicates These have the Al or Mg octahedral sheet bonded to Si tetrahedral sheets on top and bottom Unlike the type minerals certain 21 types of minerals may expand by adsorption of water between adjacent 21 units There are three types of 21 minerals Smectite Vermiculite lllite Smectite T octahedral sheet in smectites is dioctahedral Adjacent 21 units are weakly held together by cations mutually adsorbed by each layer Accordingly smectites minerals are highly plastic cohesive and swelling The CEC is large due to a high extent of isomorphic substitution especially in the dioctahedral sheet Sections of two units of a smectite Water and catio may enter and leave the interlayer region Smectites shrink when dry and swell when wet Smectites include montmorillonite beidellite and nontronite Vermiculite The octahedral sheet may be dioctahedral or trioctahedral Extensive isomorphic substitution of Al for Si in the tetrahedral layers leads to an even larger CEC than in the smectites There is strong affinity for cations especially adsorbed Mg bridging tetrahedral sheets of adjacent 21 layers leading to limitedexpansion lllite or finegrained mica These are chemically altered micas There is extensive isomorphic substitution in the Si tetrahedral sheet Due to geometry of the substituted tetrahedral sheet adsorption of K at interlayer positions holds adjacent 21 units tightly together lnterlayer K holds adjacent units of illite tightly together It does not expand Therefore illite is nonexpanding and exhibits a much smaller total surface area er unit mass than smectites or ver miculites The CEO is much less than other 21 minerals 2 1 1 layer silicates These are also 211 type minerals that consist of a Mg octahedral sheet between adjacent 21 units There is little Al in octahedral sheet Fe and Mg instead These minerals are nonexpanding and exhibit a fairly low CEC similar to illite Formation of Soil Colloids Layer Silicate Clays These may develop from chemical alterations of primary minerals such as micas or feldspars Alternatively these may precipitate from soil solution containing dissolved Al and SiO2 The particular layer silicate that precipitates depends on relative stability in the prevailing soil chemical environment In general the 21 minerals are less stable under hot and wet conditions than are the 11 minerals Al and Fe oxides In a general sequence these minerals are the end products of a sequence leading from primary minerals to 21 clays to 11 clays Therefore the Al and Fe oxides are stable mineral colloids Examples include AOH3 gibbsite FeOOH goethite and Fe203 hematite Amorphous colloids Weathering of volcanic ash releases substantial quantities of dissolved Al and SiOz which precipitate as amorphous allophane Organic colloids Complex organic molecules formed by microbial transformation of biomolecules Distribution of Clay Minerals Varies within the profile as well as geographically depending upon climate internal and external and parent material The different soil orders therefore tend to differ in clay mineralogy Order Dominant Clay Minerals Aridisols 21 Vertisols 21 smectites Mollisols 21 gt11 Alfisols 21 11 Ultisols 11 gt Al Fe oxides gt 21 Spodosols Al Fe oxides 11 Oxisols Al Fe oxides gt 11 Note that the mineralogy of the sequence of increasingly weathered soil orders Alfisols Ultisols and Oxisols is reflected in the dominant clay mineralogy of these orders More on Electrostatic Charges Permanent Negative charges arise from isomorphic substitution in tetrahedral or octahedral layers Limited positive charges may also arise from substitution of Al3 for Mg2 in trioctahedral sheet of chlorite and some vermiculites The difference between permanent negative and positive charges gives net permanent charge pHdependent Negative charge Other than permanent charge the magnitude and sign of electrostatic charge on soil colloidal particles is pHdependent Negative pHdependent charge arises from the ionization of H from OH groups on surfaces or at edges of silicate clays and Al and Fe oxides Ionization of H from OH COOH and aromatic OH of humic colloids also generates localized negative sites Neutralization of positive charge associated with adsorbed Al3 or hydrolyzed species also effectively increases CEC Positive charge Protonation of OH to give OH2 leads to positive charges Common for Al and Fe oxides and 11 silicate clays Electrostatic charge on the mixture of inorganic and organic colloids in soil includes negative and positive charges Saullt1 mun pHdependent negative charge increases ith increasing pH but pHdependent positive charge decreases with increasing pH Cation Exchange There is thermodynamic equilibrium between the concentration of cations in solution and adsorbed on soil colloids The below example show stoichiometric exchange of solution phase K for adsorbed Ca2 represented as XCaP XCa 2K ltgtCa2 2XK The distribution of Ca2 and K between solution and adsorbed phases depends on the exchange selectivity coef cient K Ca1xmZ XCaZKZ Note that if the concentration of K in solution were increased the concentration of adsorbed K would increase such that equilibrium was maintained and visa versa Also the colloid typically exhibits greater preference for one of the pair of adsorbed cations In general the order of af nity for cation adsorption follows Ala H gt Ca2 gt Mgp gt K gt Na In soil there are many such binary cation pairs and binary equilibrium relations The distribution of the various types of cations between solution and adsorbed phase depends on many different exchange equilibria one for each different pair of cations like the one above These include cation exchange with acidic cations 3XCa 2AI3 ltgt 30a 2XA3 XCa 2H ltgt Ca 2XH The natural source of solution and adsorbed basic cations is chemical weathering of primary minerals The supply of these primary minerals is limited On the other hand there is a continuous supply of H from H2003 and organic acids Thus if there is suf cient rainfall for leaching conditions to prevail basic cations tend to be depleted and replaced by acidic cations Therefore the longterm tendency is toward soil acidification loss of basic cations This is ag ravated by the fact that exchange equilibria involving and Al favor replacement of basic with acidic cations Also coupled this with the weathering of clay minerals to those of lower and lower CECs increased weathering leads to infertile acidic soil Al Ca2 and H are the commonly adsorbed cations in humid regions This reflects the longterm leaching loss of basic cations and their replacement by acidic cations In contrast Ca M 2 and Na are the commonly adsorbed cations in arid regions To raise or maintain fertility that otherwise is reduced by leaching losses of basic cations d the remov of asic cations in crop harvest fertilizer and lime are added Cation Exchange Capacity CEC is moles of positive charge adsorbed per unit mass of sell It is expressed in cmolE kg It includes acidic and basic cations CEC varies with Types of colloids present Amounts of these colloids pH Charge at pH7 Colloid Permanent pHdependent Total Humus 20 180 200 Vermiculite 140 10 150 Smectite 95 5 100 Illite 24 6 30 Kaolinite 04 76 8 AIOH3 0 4 4 Exchangeable Basic Cations Sum of adsorbed charges due to Ca Mg K and Na per kg of soil divided by the CEC cmolclkg is called the percentage base saturation BS The higher the soil pH the higher the percentage base saturation and the lower the pH the lower the percentage base saturation Anion Exchange Analogous to cation exchange XSOf39 2CI39 804239 2XCI39 Common with 11 type silicate clays and Al and Fe oxides at low pH The sum of exchangeable anions per unit mass of soil is called the anion exchange capacity AEC It is expressed in units of cmolc l kg In addition to anion adsorption at positively charged exchange sites certain anions may also be specifically adsorbed ie bonded to the colloid surface rather than simply attracted by electrostatic force This is common for phosphate sulfate and molybdate anions Nitrogen and Sulfur Nitrogen Influence of N on Plant Growth and Development N is taken up as NH4 or N031 It is essential as a component of many different biomolecules such as proteins nucleic acids and chlorophyll Deficiency results in chlorosis and poor growth However oversupply causes rank but abnormal growth and poor quality Origin and Distribution of Nitrogen Most N is atmospheric There is 10 to 20 x as much N in soil as in vegetative cover Most soil N is in organic combinations and N comprises about 5 of soil organic matter Only about 1 or 2 of the soil N is inorganic Immobilization and Mineralization Mineralization is the conversion of organic to inorganicN About 25 of the organicN in soil mineralized annually This represents a major source of N for plant growth Immobilization is the incorporation of inorganicN into organic compounds When the C N ratio is high immobilization gt mineralization NH4 Fixation Entrapment of NH4 between adjacent tetrahedral sheets of neighboring layers of 21 minerals The tendency for fixation follows the sequence vermiculite gt illite gt smectites Fixed NH4 is slowly released NH3 Volatilization NH4 OH39 39 le NH3 Conditions favoring Opposing volatilization volatilization High pH Low pH Low CEC High CEC Surface application Incorporation in soil Dry soil Moist soil High temperature Cool temperature Nitrification Microbial oxidation of NH4 2NH4 302 39 39ZNOZ39 4H 2H20 E 2N0239 02 39 392N03 E Nitrification is carried out by autotrophic bacteria Step 1 Nitrosomonas Step 2 Nitrobacter Clearly nitrification acidifies soil SUM ehwuhmehtat uhmtmhs affectmg mtrmeanuh rhetuue hmh NH EDHEEHUEUEIH wmch rhhrprts the preeess o2 requrreu SWEE mthheatmh rs aerppre Mmst Eundmuns favur mtrmeatmh puthet se Wet as te affect O avauapmty Optrmum temperature range rs 25 7 35 0 Gem suH remhty atseravers mthheatmh There are ehemreat rhhrprters that reduce the aetwrty er Nrtrosomohas a d therefure stew mthheatmh and the truss at N as N03 by Eacmng eruemthheatmh Nmmcauun tstypteauy rapm NO Leaching Tms rs uhuesrrapte wrth respeet te mam grewth SWEE rt rs a truss at N frum the sum Nrtrate muvemen te greuhu ahu surraee Water atse peses heatth ahu ehvrrehmehtat HSKS Methamogoorhemra mus baby syndrume rs uue te reuuetmh er N03 te NOZ wmch reduces the apamty er hemegtepm te carry 02 Ehhehmeht er su aee Waters wrth N03 may head te euzrophrcatroh EspemaHy mama systems Facturs m uenmng N03 Eacmng rhetuue Seuwaterurarhage N03 eeheertratmh Demmncauon Reuuetmh er N03 te NO N20 er N2 03 quotNO NO39Nzo39Nz W DErHWymg urgamsms rhetuue raeuttatwe ahaerepes such as eudomonas ahu sawus that are hetemtrupm ahu autur rupm Throoacwus denmmcans Facturs affectmg uemtrmeatmh ee er N03 oxmrzapte supstrates rer heteretrpphs Ahaerppre Eundmuns Optrmum temperature 25 7 35 0 Law ph lt 5 rhhrprts uemtrmeatmh Examp e utuemhmeahuh hhehes Demthheatmh eeeurs m Wet sehs We hpahah ZDHES WEUands ahu hee hems EVEN m set areas at uptahu agheutturat suHs spatrauy ahu tempuraHy vamab e up te en Kg ha ahhuauy Nitri cation and denitrilication in a ddy soil Denitrifcation contributes to acid deposition from HN03 formed from N0 and N20 Also N20 is a greenhouse gas Biological N Fixation N2 6H 6e 39 392NH3 The NH3 is incorporated into amino acids Biological N xation is carried out by certain acteria actinomycetes and cyanobacteria About 139000000 Mg N is annually xed in terrestrial systems Nitmgenase is the enzyme complex res ponsible It consists of two proteins The smaller one supplies es and larger traps N2 and the larger supplie electrons for reduction Since the reaction requires nodules that contain the N xing bacteria is inhibited by soil NOa39 On the other hand go Mo Fe P and S fertility is needed for N xation Growth with and without Ntixing organisms Symbiotic Fixation with Legumes Rhizobium and Bradyrhizobium are the genera of bacteria involved These form nodules on roots of legumes The symbiosis is speci c between legume and bacteria species To ensure root nodulation one can inoculate ifthe right species is not present Nodulated root Symbiotic Fixation with Nonlegumes ra Frankia The s nonnodulated associations like the association of Anabaena Within leaves of Azolla Another nonnodulated symbiosis involves N xing organisms living in close but external as ociation with plant roots in the rhizosphere Sulfur This elemental is a component of certain amino acids and vitamins Deficiencies in 8 result in chlorosis and stunted growth Sources of 8 include organic 8 soil min erals such as CaSO4 arid regions FeS formed under reducing conditions and most commonly SO439 adsorbed to colloids and atmospheric forms 8 Oxidation and Reduction Reactions Mineralization of organically bound 8 re leases incompletely oxidized forms of S Oxidation to 80439 occurs chemically but is largely a biological process H28 202 39 H2804 28 302 2H20 39 39ZHZSO4 The autotrophic Thiobacillus does this Reduction is anaerobic so2 8H 8e 52 4Hzo It is carried out by Desulfovibrio and is coupled with oxidation of organic matter Sulfide is subject to precipitation 8239 Fe2 39 39FeS Environmental Acidification Problems due to Inorganic Sulfur Acid sulfate soils Mined soils Oxidation of FeS and FeSz leads to very low soil pH Once soil containing reduced 8 is drained and aerated or minerals containing reduced 8 are excavated reduced 8 is subject to oxidation 4FeS 902 4H20 39 39 2Fe203 4303 8H Acid deposition on forest soils H2804 HN03 These loadings of H in addition to car bonic acid and organic and mineral acids from organic matter decomposition accel erate natural leaching loss of nutrients


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