Bahan Kajian MK. Dasar Ilmu Tanah

Smno.agroekotek.fpub.okt2013

REAKSI PERTUKARAN

ION &

KETERSEDIAAN HARA

• Humus » 200 cmolc/kg

• Smectite/Vermiculite » 100 cmolc /kg

• Illite » 25 cmolc /kg

• Kaolinite » 10 cmolc /kg

• Fe and Al oxides » 5 cmolc /kg

Muatan listrik pd Komponen Tanah

ColloidNegativecharge

Positivecharge % constant % variable

Humus 200 0 10 90Vermiculite 120 0 95 5Smectite 100 0 95 5Illite 40 0 80 20Kaolinite 12 4 5 95Fe & Al Oxides 5 5 0 100

Asalnya Muatan Listrik

cmol / Kg

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Penjerapan Ion

Muatan negatif pada partikel tanah

dinetralkan oleh anion dari larutan tanah

Kation Yang Dijerap (Kation-Tukar)

(a) arid region soils = "basic" cations Ca2+, Mg2+, K+, Na+

(b) humid region soils = “acidic” cations as well

Ca2+, Mg2+, H+ and Al3+

(c) Kekuatan Jerapan Al3+> Ca2+ = Mg2+ > K+ = NH4

+ > Na+

Pertukaran Kation

Exchange process Ca2+-colloid + 2 Na+ ¬¾® 2 Na+-colloid + Ca2+

= Na+ replaces Ca+2 adsorbed to soil colloids

Ca-x + 2 Na+ ¬¾® 2 Na-X + Ca2+

X = the soil solid phase

Dispersion

Saline Soils

EC > 4 ds/m = osmotic stress

* salt sensitive plants (EC = 2 ds/m) 3 bean, onion, potato, raspberry, carrot,

dogwood, larch, linden, peach, rose, tomato

* salt tolerant plants (EC = 10 ds/m) 3 sugarbeets, barley, cotton, rosemary,

wheat grass, wild rye

(see table 10.2 - 13th ed. or 10.3 – 12th ed.)

Sodic Soils (ESP > 15)

flocculation

poorwater

infiltrationdispersion

Sodium Ion Effect

flocculation dispersion

è attraction ç

Ca2+ & Mg2+

ç repulsion è

Na+

SAR Parameter

SAR is measured ESP/ESR is estimated in water or extract for soil solids

ESR = 0.015(SAR) - 0.01

Good quality irrigation water: 4 for salt hazard = EC < 2 ds/m 4 for Na+ hazard = SAR < 15

Predict sodium effect from saturated soil extract or irrigation water

Sources of Acidity

á Water: H2O ¬® H+ + OH-

á CO2 from soil respiration CO2 + H2O ¬® H2CO3 ¬® H+ + HCO3

-

carbonic acid

á Organic acids from O.M. decomposition RH ¬® R- + H+

á Oxidation of S and N S ¾® H2SO4 ¾® 2 H+ + SO4

2-

NH3 ¾® HNO3 ¾® H+ + NO3-

Human-Induced Acidity

* Chemical fertilizers ü ammonium-based N materials

NH4+ ¾(O2)® HNO3

ü Ferrous-Fe materials

Fe2+ ® Fe3+ ¬(+ 3 H2O)® Fe(OH)3 + 3 H+

ü Elemental Sulfur 2 So + 3 O2 + 2 H2O ¾® 4 H+ + 2 SO4

2-

ü Acid Rain: N and S gases emitted from combustion processes SO2 ¾(O2, H2O)® H2SO4

NOx ¾(O2, H2O)® HNO3

mining wastes, wetland drainage

- oxidation of sulfide (S2-) mineral

S2- ¾(O2, H2O)® H2SO4

Human-Induced Acidity

http://www.physicalgeography.net/fundamentals/8h.html

Phases of Soil Acidity

bound acidity exchangeable acidity soluble acidity

As acidity is removed from or added to soil solution Ø maintain equilibrium within system Ø must change all forms to change pH

Acid Soils: Role of Aluminum

Al3+ ¨ Al(OH)2+ ¨ Al(OH)2+ ¨ Al(OH)3

|¬ strongly ¾®|¬ moderately ¾¾¾®|¬ alkaline acid soils acid soils soils

Al3+ + H2O Al(OH)2+ + H+

K = 10-4.93

 Al(OH)2

+ + H2O Al(OH)2+ + H+

K = 10-4.97

 Al(OH)2+ + H2O Al(OH)3

o + H+

K = 10-5.7

 Al(OH)3o + H2O Al(OH)4

- + H+

K = 10-7.4

Acid Soils: Role of Aluminum

http://www.landfood.ubc.ca/soil200/interaction/acidity.htm

Al+3 ¨ Al(OH)+2 ¨ Al(OH)2+ ¨ Al(OH)3

Changes in Al Speciation

- - - - - -

- - - - - -

Clay Interlayer Soil Solution

pH 4 pH 6

H+

H+

Why [Al3+] ~ [H+] in Acid Soils

Fe3+ + H2O <--> Fe(OH)2+ + H+ K = 10-2.19

 Fe(OH)2

+ + H2O <--> Fe(OH)2+ + H+ K = 10-3.5

 Fe(OH)2+ + H2O <--> Fe(OH)3

o + H+ K = 10-7.4

 Fe(OH)3

o + H2O <--> Fe(OH)4- + H+ K = 10-8.5

Why Not Iron?

Liming Materials

Carbonate forms(a) "limestone" deposits and

industrial byproducts

(b) calcite = (CaCO3) = calcium carbonate and

dolomite = CaMg(CO3)2

(c) dolomitic limestone maintains Ca:Mg balance

Liming Materials

Oxide and Hydroxide forms(a) oxides formed by heating limestones

CaCO3 ¾(heat)® CaO + CO2calcite gas

burned lime or quicklime

(b) add water to oxides to form hydroxides CaO + H2O ¾® Ca(OH)2

hydrated lime

Reaksi Kapur dalam Tanah1. Neutralize acidity

2 H-X + CaCO3 ¾® Ca-X + H2CO3 + H2O

2. Base Saturation increases

BS = (CEC – [Al3+][H+]) / (CEC) * 100

BS = {[Na]+[K]+[Ca]+[Mg]}/CEC *100

3. Soil pH increases

4. Al solubility decreases Al+3 + 3 OH- ¾® Al(OH)3

soluble insoluble (toxic) (not toxic)

Ciri-ciri Tanah Masam

Wetland (Hydric) Soilsand

Redox Conditions

CH2O

CO2

O2

H2O

NO3-CH2O

CO2 N2

Fe(OH)3CH2O

CO2 Fe2+

CH2O

CO2

SO42-

H2S

Energy YieldsDonor Acceptor

700

400

100

Eh (mV)* Condition

oxic

suboxic

anoxic

*pH 7

MnO2CH2O

CO2 Mn2+

Soil Colors

Yellow -> Orange -> Red Fe(III) minerals

Black (veneer) Mn(IV) minerals

Dark Brown (disseminated) Organic Matter

Aerobic Environments

Gray -> Green -> Black Fe(II) minerals

Dark Brown (disseminated) Organic Matter

Anaerobic Environments

Iron massesRedox depletions

Root linings

MottlingNodules

Gleyed colors

Redoximorphic Features

Histic Horizons

“Rotten Eggs”

Plant Effects on Redox Conditions

FeIII(OH)3

deposit

O2(g)

Pembentukan Plaque pada Akar Tanaman

Fe(OH)3

O2

H2O

Fe2+