5. Chemical Properties of Soils. ENVS 334: Applied Soil Science and Land Management INSTR.: R.M. Bajracharya

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1 5. Chemical Properties of Soils ENVS 334: Applied Soil Science and Land Management INSTR.: R.M. Bajracharya 1

2 Soil Reaction (ph) Definition of acid: Arrhenius: acids produce H+ (H 3 O + ) ion in aqueous solution; bases produce OH - (hydroxyl) ion in aqueous solution. Bronsted-Lowry: an acid is a proton donor; base is a proton acceptor Generalized acid-base reaction: HA (aq) + H 2 O (l) H 3 O + (aq) + A - (aq) 2H 2 O (l) H 3 O + (aq) + OH - (aq) Equilibrium expression: K a = [H] [A] For pure water: K w = [H 3 O + ] [OH - ] = 1 e -14 [HA] i.e., -log K w = 14, or, pk w = 14 ph + poh = 14 ph = -log[h + ] 2

3 The ph scale and examples of the ph of some common fluids Acidic Neutral Alkaline M HCl Stomach acid Vinegar Beer Milk Blood Sea water 1M NaOH Lime juice 3

4 Sources of Soil Acidity 1. From inorganic minerals exchangeable acidity H +, Al 3+, Fe From organic matter exchangeable acidity Carboxylic acids: R COOH Phenlic acids: R OH 3. Acids in solution Carbonic acid from atmospheric CO 2 (equilibrium) H 2 CO 3 (aq) Anthropogenic: fertilizers ammonium salts, eg., (NH 4 ) 2 SO 4 ; (NH 4 )NO - 3 phosphate salts, eg., Ca(H 2 PO 4 ) 2 4

5 ph - a master soil variable ph is an important soil variable as it influences many soil processes: Ion exchange: ]Ca + 2H + ]2H + Ca 2+ Precipitation: Fe 3+ (aq) + 3OH - (aq) Pb 2+ (aq) + 2OH - (aq) Fe(OH) 3 (s) Pb(OH) 2 (s) Reduction-oxidation: Fe(OH) 3 (s) + 3H + + e- Fe 2+ (aq) + 3H 2 O (l) ; E h º = 1.06V Adsorption: ]FeOH + Pb 2+ FeOPb + + H + Complexation: Al 3+ + H 2 O Al(OH) 2+ + H + 5

6 Effects of soil acidity Direct toxicity of H + to plants; when ph <4 Toxicity of Al 3+, Mn 2+ to plants Deficiencies of basic cations that are plant nutrients, eg., Ca 2+, Mg 2+, Cu 2+, Zn 2+ Adverse effects to soil organisms, such as, nitrifying bacteria Soil acidity may be reduced by liming (applying agricultural lime): CaCO 3, CaO, Ca(OH) 2, CaMg(CO 3 ) 2 Reactions: ]2H + CaCO 3 ]Ca + H 2 O + CO 2 2Al CaCO 3 + 3H 2 O 2Al(OH) 3 + 3Ca CO 3 2- Lime is applied to neutralize not only soil solution, but also potential acidity (H + and Al 3+ ) on exchange sites. 6

7 Characteristics of soil colloids Colloids are very small (< mm) particles that have high surface area:volume ratios, are electrically charged (net negative in case of clays), and chemically highly reactive Clay colloids (layered silicates) consist of alternating layers of silica tetrahedra and alumina octahedra sheets arranged in 1:1, 2:1, or 2:2 (Mg-silicates) basic units: 1:1 basic unit consists of one Si and Al layer forming a crystal lattice; e.g. kaolinite; rigid H-bonding, hence, these clay minerals have low charge, surface area, cohesion, plasticity, shrink/swell and cation adsorption capacity compared to other minerals 2:1 (non-expanding) are called hydrous micas, e.g. illite; fix K ions between adjacent units, thus, low shrink/swell and moderate adsorption 2:1 (expanding): Al layer sandwiched by 2 Si layers; eg. Montmorillonites and Vermiculites; basic crystal units very loosely held together by weak O O linkages, therefore, high shrink/swell, SA, adsoption capacity, etc. 2:2 group are Mg-silicates with alternating layers of talc and brucite {Mg(OH) 2 }; silimar properties as illite. 7

8 Clay mineral structure Silica Tetrahedron Alumina Octahedron Silicon Oxygen Hydroxyl Al OH Al 1:1 O, OH Si O 8

9 2:1 mineral lattice structure OH Si O, OH 2:1 Al, Mg Mg O, OH Negative charge in clay minerals arise from isomorphic substitution of Mg for Al (in Al-layer) and Al for Si (in the Silayer); also from unsatisfied valences at the broken edges 9 of the Al and Si sheets. Si O

10 Structures of some common clay minerals Lattice structure of Kaolinite Structure of Smectite with cations adsorbed within interlayer space 10

11 Comparison of silicate clays and organic colloids Property Type of Clay Mineral Kaolinite Illite Montmorillon. Organic colloid/humus Size (λm) Shape Hex. crystals Irreg. flakes Irreg. flakes Non-crystaline Sp. SA,sq.m/g ? External SA Low Medium High High Internal SA None Medium Very high High Cohesion/plas. Low Medium High -- Swelling cap. Low Medium High -- CEC,cmolc/kg Apart from silicates other mineral colloids include: hydrous oxides of Fe and Al, e.g., gibbsite {Al 2 O 3 3H 2 O} and goethite {Fe 2 O 3 H 2 O}; and 11 amorphous minerals such as allophane {Al 2 O 3 2SiO 2 H 2 O} volcanic ash

12 ph dependence of colloidal charge Essentially all of the charge of organic colloids is ph dependent Charge of clay mineral increases with ph beyond 6 At ph below about 6 only the permanent charge of clay minerals in available for exchange At higher ph, H, OH and other functional group dissociation leads to increased charge PH OC Mont Illite CEC (cmoc/kg) Permanent charge OC Mont Illite ph ph-dependent charge 12

13 Nitrogen in Soils Nitrogen is an integral component of many compounds E.g., chlorophyll, enzymes, amino acids It is an essential nutrient for plant growth and development Often it is the most limiting plant nutrient N makes up ~0.1 to 0.3% of most mineral soils (by weight) It comprises ~78% of the atmosphere MINERALIZATION +H 2 O +O 2 +[O] R-NH 2 R-OH + NH 4 + NO H + NO 3 - -H 2 O -O 2 nitrification -[O] Organic-N Nitrosomonas Nitrobacter IMMOBILIZATION 13

14 Soil Nitrogen Pools Plant residues Animal wastes Atmosphere Fertilizer nitrogen Organic Nitrogen pool mineralization immobilization Inorganic pool {NO 3- ; NH 4 + } 14

15 Fate of Nitrate in soil In the mineralization process the transformation of nitrite [NO 2- ] to nitrate is very rapid; hence no accumulation in soil, which would be toxic to plants & mammals. Nitrate [NO 3- ] is the most readily available form of N for plants But it is easily lost from the soil system: Very mobile; hence readily washed away or leached & leads to surface and groundwater contamination Denitrification = biochemical reduction of nitrate-n to gaseous forms: -2[O] -2[O] -[O] -[O] 2NO 3-2NO 2-2NO N 2 O N 2 Nitrate ions Nitrite ions Nitric oxide Nitrous oxide Elemental nitrogen (+5) (+3) (+2) (+1) (0) 15

16 Fate of Ammonia in soils Ammonium nitrogen may move in five directions: 1. Immobilization = tying up of ammonia compounds in the bodies of organisms. 2. Plant uptake = higher plants use this form of N along with nitrate. 3. Fixation = binding of ammonium ion in interlayer spaces of clay (vermiculite); also to some extent by micas and OM. 4. Volatilization = gaseous losses of ammonia to the atmosphere. 5. Nitrification = oxidization of ammonium ion by bacteria to produce nitrite and nitrate. Clay micelles NH 4 + NH 4 + (aq) (In solution) Exchangeable ammonium NH 4 + NH Fixed ammonium ions

17 Nitrogen Fixation Symbiotic N-fixation: mutually beneficial association between plants and bacteria (Rhizobium spp.) Leguminous plants [nodule-forming]: Soybean (Glycine max), clover (Trifolium spp.), bean (Phaseolus vulgaris), peas (Pisum sp.), cowpeas (Vigna sp.) Non-legumes [nodulated]: Alder (Alnus sp.) associated with actinomycets (Frankia sp.) Gunnera spp. associated with blue-green algae (Nostoc) Non-legumes [non-nodulated]: Pangola grass bacteria (Azospirillium) Bahia grass bacteria (Azotobacter) Azolla Blue-green algae (Anabaena) Non-symbiotic N-fixation: free-living soil microorganisms Azotobacter, Clostridium; various blue-green algae 17

18 Sulfur in Soils Sulfur is essential of plant and animal growth: Constituent of amino acids (methionine, cysteine & cystine) Constituent of vitamins (biotin, thiamine & B 1 ) Important in structure of proteins and protein enzymes In the past, crop S needs were inadvertently met by: SO2 (atmospheric) from combustion of coal & fuel oils (S-rich) S-bearing fertilizer [CaSO4; (NH4)2SO4] & pesticides In recent years, change in fertilizer/pesticide types and replacement of coal & fuelwood by natural gas & electricity has led to decline in S in soils & deficiency. 18

19 Sources of sulfur Natural sources of sulfur are: Organic matter (plant & animal origin): proteins & amino acids Minerals: sulfates, sulfides, iron & aluminum oxides, clays {these tend to be slowly available or available in very small quantities}. Anthropogenic sulfur: Atmospheric sulfur: dry and wet deposition, esp. near industrial areas (due to combustion of S-rich fuels; ironore smelting) Elemental S: used for manufacture of fertilizers 19

20 Transformations of sulfur Crop residue & manure Commercial fertilizer S-bearing minerals SOM Immobilization (microbial activity) Crop removal Available soil S leaching rain volatilization Erosion losses Atmosphere H 2 S, CS 2, COS, CH 3 SH more under anaerobic conditions 20

21 Soil Phosphorus Next to nitrogen in importance for plant nutrition Essential for plant growth & maturation: Component of: ADP, ATP, DNA & RNA Important for : photosynthesis, N-fixation, crop maturation, root development, straw strength and crop quality. In most soils, available P is usually less than 0.01% (by weight. Sources of P in soils: Inorganic: primary minerals (apatites) unavailable form Secondary minerals: 2 groups calcium phosphates; iron/aluminum phosphates Organic forms: inositol phosphates; nucleic acids; phospholipids 21

22 ph dependence of P OH - OH - H 2 PO - 4 H HPO 2-4 H 2 O + PO 3-4 H + H + {very acid solutions} {very alkaline solutions} Fate of phosphates in soil: Precipitation by Fe, Al & Mn: Al 3+ + H 2 PO H 2 0 2H + + Al(OH) 2 H 2 PO 4 (insoluble) Reaction with hydrous oxides: Al(OH) 3 + H 2 PO - 4 Al(OH) 2 H 2 PO 4 + OH - Fixation by silicate clays: [Al] + H 2 PO H 2 0 2H + + Al(OH) 2 H 2 PO 4 (in silicate clays) Anion exchange: [AlOH 2+ ] OH- + H 2 PO - 4 [AlOH 2+ ] H 2 PO OH- 22

23 Phosphorus transformations in soil Crop residue & manure Commercial fertilizer P-bearing minerals SOM Available soil P Crop removal Leaching losses Erosion losses Fixation 23

24 Potassium in soil Potassium is the third most important plant nutrient. Plays many essential roles in plants: Activation of numerous enzymes Catalyst in enzymatic reactions/processes Photosynthesis, protein synthesis, starch formation, translocation of sugars, etc. Sodium (Na) partially replaces K in cases of deficiency. Sources of soil K: Primary minerals: feldspars, micas (released through weathering) Exchangeable K (ions adsorbed to soil colloids) Fixed K in clay minerals (not readily available) Organic matter and bodies of soil organisms (released through mineralization upon decomposition). 24

25 Distribution of soil K in various forms Relatively unavailable potassium (feldspars, micas, etc.) 90-98% to total K Slowly available potassium (fixed or non-exchangeable) 1-10% of total K Readily available potassium K on exchange sites & in solution 1-2% of total K 90% 10% Non-exchangeable K Exchangeable K Soil solution K 25

26 Potassium transformations in soil Crop residue & manure Commercial fertilizer Slowly avail. K minerals Available soil K Crop removal Leaching losses Erosion losses Fixation Clay micelles K + (aq) (In solution) K + K + K + Exchangeable potassium 26 Fixed potassium ions

27 Micronutrients elements in soil Of the 17 essential plant elements, 8 are required in very small amounts, hence, called micronutrients. These include: Iron, manganese, zinc, copper, boron, molybdenum, cobalt, chlorine Toxic when present in quantities greater than tolerated by plants Important in plant and microbial growth processes In enzyme systems (e.g., electron carriers in oxidation-reduction reactions) Plant development and reproduction In enzyme systems necessary for metabolism Nitrogen transfer (by Mo and Mn) 27

28 Range of micronutrient levels in plants Mn Zn B Cu Mo Level in plants (mg/kg) Deficiency Sufficient Toxic 28

29 Forms and ranges of micronutrients in soils Element Major forms in nature Range in soils (mg/kg) Dominant forms in Soil solution Fe Oxides, sulfides & silicates g/kg Fe 2+, Fe(OH) 2+, Fe(OH) 2+, Fe 3+ Mn Oxides, silicates, carbonates Mn 2+ Zn Cu Sulfides, carbonates & silicates Sulfides, hydroxy carbonates & oxides Zn 2+, Zn(OH) Cu 2+, Cu(OH) + B Borosilicates, borates H 3 BO 3, H 2 BO 3 - Mo Sulfides, oxides, molybdates MoO 4 2-, HMoO 4 - Cl Chlorides 7-50 Cl - Co Silicates 1-40 Co 2+ 29

30 Chelates Chelate is a term applied to compounds formed from binding or complexing of certain metallic cations with organic molecules or anions. Zn, Cu, Fe & Mn ions tend to form chelates with organic molecules in soil humus or synthetic (man-made). In the chelated form, these ions are readily plant available; unchelated they tend to react with inorganic substances, making them unavailable. O O CO O CO CH 2 Fe O CO CH 2 CO CH 2 N CH2 FeEDTA ferric ethylenediaminetetra-acetate N CH 2 CH 2 30

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