Bab 8 Soil Reaction

8/13/2019 Bab 8 Soil Reaction

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Soil Acidity and Alkalinity

PRODI AGROTEK


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INDIKATOR KOMPETENSI

MAHASISWA DAPAT MENJELASKAN

TENTANG REAKSI TANAH, SUMBER-SUMBER KEMASAMAN,KAPASITAS

PENYANGGAAN, HUBUNGAN pH

DENGAN TANAMAN DANPENGAPURAN


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Soil pH* pH - the negative log of the hydrogen ion (H+)concentration in the soil water solution.pH = - log [ H+]

* the pH scale is how we measure acidity andalkalinity of solutions.at neutral (pH =7) the number of H+ = OH-

Remember :

at pH of 6 there are 10x more H+ ions than ata pH 7 and there are 100x more H+ ionsbetween pH 7 & 5


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Soil pH

0-7 = acidic; 7-14 = basic.Higher concentration of H + ions = highacidity, higher concentration of OH -

ions = high basicity.In general, the ideal pH for plantgrowth is about 5.5 in organic soils and

about 6.5 in mineral soils.


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Soil pH Ranges

76543 8 9 10 11

NeutralAcidity Alkalinity

Common pH range for soils

10x 100x 1,000x 10,000x10x100x1,000x10,000x 0


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Tanah masam : ion H +>ion OH -

Kation-kation dapat dipertukarkan sebagai sumber H + dan OH -

yang penting :Ion hidrogen (H +) yang terdapat bebas pada larutan tanahIon aluminium (Al +++) penyebab tak langsung, berasal dari Alpada struktur oktahedral Al, yang jika beraksi dengan airmembentukAl+++ + 3H2O Al(OH +)3 + 3H+

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

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

Al dijerap pada bidang pertukaran

Al+3H2O Al(OH)3+ H+MISEL MISEL

Penyebab kemasaman tanah


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Sumber-sumber penyebab kemasaman tanah

1. Humus atau bahan organikGugus-gugus karboksilgugus-gugus phenolik

gugus-gugus aminoProses dekomposisiH2CO 3 mencuci basa-basa terus-menerusH2SO4, HNO 3 tambahan ion H +

Timbunanion H+


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2. Garam-garam yang larutPemupukan

Pelapukan mineralDekomposisi BOmenambah kation-kation

menggantikan Al teradsorbsi

Al masuk ke larutan tanah

penyebab tambahnya H+


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3. Intensitas pencucianTanah di daerah humid : basa-basa tercuci,

tertinggal H+ dan Al+++ Al++++H2O Al(OH) +++H+

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

Al(OH) +2+H2O Al(OH) 3+H+

4. Mineral lempung (liat) alumino silikat olehpelapukan dari oktahedral Al membebaskan Aldengan 2 kemungkinan :

Al teradsorbsi Al+3H2O Al(OH) 3+ H+

Al dalam larutan

MISEL


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5. Karbon dioksida (CO 2)

CO2dihasilkan oleh adanya respirasi akar dan jasad

hidup dalam tanah,

CO2+ H2O H 2CO3 mencuci basa-basa


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The Colloidal Control of Soil ReactionSource of Hydrogen Ions under very acid conditions

AlSoil Solution

Al+++ + H2O Al(OH) 3 + 3H+

The Al(OH) 3 is not ionized so the H ions thus released give low pH value inthe soil solution

HH Soil Solution Hydrogen

The effect of both adsorbed hydrogen and aluminum is to increase the H ionconcentration in the soil solution

Micelle =Al +++

Micelle =H +

Al in solution Hydrolize


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Source of OH ionsCa H

Ca HUnder natural conditions the reaction to furnish H and

OH ions to the soil solution occur simultaneously

The Colloidal Control of Soil Reaction...

Micelle + H 2O Micelle + Ca++

+ 2OH


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factor of Acid forming in Soil

1. Nitrification:Ammonium to Nitrate (oxidation of NH4+)

NH4+ + 2O2 ---> NO3- + H2O + 2 H+

2. O.M. decompositionorganic acids ionized :

R-COOH---> R-COO- + H+respiration:

CO2 + H2O ----> H2CO3 = H+ +HCO3-


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3. Acid rainAcid rain is caused by theburning of fossil fuels.Burning oil, gas and coal in powerstations releases SulfuricDioxide (SO2) into theatmosphere.Burning oil and gasoline in motor

vehicles puts nitrogen oxides(NOX) into the atmosphere.These gases mix with waterdroplets in the atmospherecreating weak solutions of nitricand sulfuric acids.When precipitation occurs thesesolutions fall as acid rain.

SO2 +OH --> H2SO4 --> SO4- + 2 H+NO2 + OH --> HNO3--> NO3- + H+


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4. Uptake of basic cations by plants.

Basic cations are sources of OH- tothe soil solution .Ca++, Mg++, K+ ,

Basic cations that are taken up byplants no longer contribute OH- to thesoil solution.H+ ions are released to the soil

solution.


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5. Leaching of basic cationsas basic cations are removedfrom the soil solution byleaching they no longercontribute the OH- ions toneutralize the ever increasingamounts of H +Ca++ + 2 H20 ---> Ca(OH)2 + 2H+

-----> Ca++ + 2OH-


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Back to Soil pH Active Acidity - due to the H+ ion activity inthe soil solution at any given time

Reserve Acidity - represented by the H+ and Al3+that are easily exchanged by other cations(positively charged ion)H H H H H+ H+

H Ca++ H+Mg Mg++ H+Ca Ca++ H+ H+

H H H Na

soil

Reserve /PotentialAcidity Active Acidity -Soil solution


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Buffer pHOrganic matter

Soil pH


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Pools of acidity in soils

Residualacidity

Exchangeableacidity

Activeacidity


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Soil Buffering Capacity The tendency of soils is to resist changes of the pHof the soil solution.This resistance is termed buffering. Soils have different buffering capacities.Generally, higher CEC = greater buffering capacity.Buffering capacity indicates dynamic equilibrium

of soil solution.Changes of all types tend to be resisted by thesystem.


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Buffering Capacity

The ability of soil to resistchange in pH.

The amount of H+ in the soilsolution is small compared withthe H +, Al 3+ adsorbed on thesoil colloids (reserve)

Neutralization (by the additionof bases) of the solution H+ (H+is removed from the system)results in a rapid replacement ofH+ from the exchangeable H+ onthe soil colloid. CaCO3 when added to soil willneutralize H+.CaCO3 = Lime (dolomitic =MgCO3 & CaCO3


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Why is soil pH important?

Affects solubility of minerals. Affects type, numbers and activity ofmicroorganisms.

Fungi tolerate acidity better thanbacteria. Bacteria often negatively affectedby high acidity (i.e. low pH).Indirectly affects aggregate stability.

Determines what happens to many soilpollutants.CEC increases with soil pH.


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Soil pH affects many chemical and

physical reactions in soil Availability of most essential elements

Activity of microorganisms Ability of soil to hold cationsSolubility of non-essential elements such asheavy metalsHerbicide performance


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pH and nutrient availability

As the soil pH increases from an acidic condition topH 6.5

Macronutrients (N,P,K) increases in solubilitySecondary nutrients (Ca, Mg, S) increases insolubility

Micronutrients (except Molybdenum) decreases insolubility Al decreases in availability (very important)


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pH influence on microorganisms

Bacteria and actinomicete are reduced at low pHNitrification occurs at pH range 6.0 to 9.0,optimum pH 7Denitrification (biological loss of N) occurs at aminimu of pH 5.5, bellow this point chemicaldenitrification occurs

Nitrogen fixation by Rhyzobium (legume-bacteriasymbiosis) optimum occurs between a pH of 6.0 to6.5Organic matter decomposition: optimum pH 7.0


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MASALAH DALAM TANAH MASAM

Aluminum toxicity/keracunan Al

Manganese toxicity/keracunan MnCa, Mg deficiency/kekurangan Ca.MgFe deficiency induced by Al/Kekurangan Fe ygdisebabkan AlMolybdenum deficiency/ kekrngan Mo


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pH 4.4 pH 5.7

Al-Tolerant variety

Breeding toovercome Altoxicity

Photo by C. D. Foy, USDA


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Pengapuran

Bahan penting dari kapur dalam menetralkankemasaman tanah adalah CO 32- dan OH - yangdihasilkan

Ion CO 32- mampu menarik ion H+ dari koloid tanahIon OH - dapat mengusir Al 3+ dari kompleks jerapan


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Bila sumber kemasaman tanah adalah ion H +

CaCO3 Ca2++CO32-

+CO32- +H2CO3+Ca2+

Bila sumber kemasaman adalah ion Al 3+CaCO3 Ca2++CO2-CO32-+H2O HCO 3-+OH -

+OH- +Al(OH)3

+Ca2+

MISEL

MISELMISEL

MISEL

MISEL MISEL

MISEL MISEL


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Why apply lime ?

1. helps nutrients become available to plants2. improves soil structure

3. provides nutrients for plant growth

4. promotes growth of beneficial microorganisms5. overcomes acidifying effects of fertilizers

6. reduces metal toxicity to plants

(solubility vs. pH)


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KESIMPUL N

1. Reaksi tanah (pH) menggambarkan konsentrasi ionH+ dalam tanah

2. pH tanah dapat dibedakan menjadi pH aktual danpH potensial

3. Reaksi tanah berhubungan erat denganketersediaan hara, aktivitas mikroorganisme,efisiensi pemupukan dan reaksi kimia dan fisikayang terjadi dalam tanah

4. Pengapuran dilakukan untuk meningkatkan pHtanah


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TERIM K SIH

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Bab 8 Soil Reaction