Oksida-Oksida Fe dan Al

Dapat melepaskan fiksasi P scr kuat melalui pertukaran anion

- Pada tanah tua daerah tropik - Kristalin dan amorf

- KTK rendah - Sering bermuatan positif

KO

LOID

OR

GA

NIK

• Humus • Susunan : C,H,O

Amorf

KTK tinggi (> monmorilonit)

Lebih mudah hancur drp liat

Sumber muatan negatif : gugus

karboksil dan fenol

Muatan tergantung pH

Negative charges on humus

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Central unit of a

humus colloid

(mostly C and H)

ENORMOUS external surface area!

(but no internal surface – all edges)

Charges on soil colloids

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Colloid type

Humus (O.M.)

Silicate clays

Oxides of Al and Fe

Negative charge

200 cmolc/kg

100 cmolc/kg

4 cmolc/kg

Positive charge

0 cmolc/kg

0 cmolc/kg

5 cmolc/kg

So what will those negative charges adsorb?

Perbedaan Koloid Liat dan Humus

• Misel tersusun dari Si, Al, O • Kristalin • Daya jerap kation lebih rendah • Mantap

KOLOID LIAT

• C, H, O • Amorf • Daya jerap kation lebih tinggi • Tidak semantap liat

KOLOID HUMUS

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Types of charge

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Permanent charge

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Octahedral sheet neutral Net negative charge

pH-dependent charge: on edges

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H+ bound tightly, so

the lower the pH,

the less exchange

there is (i.e., lower

nutrient availability)

Especially important in kaolinite, humus, where no

internal charge imbalance

Influence of pH on the CEC of smectite and humus

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Edge charge

= Internal charge

Ion exchange

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The substitution of one ion for another on the surface or in the interstitial spaces of a crystal

Anion exchange (e.g.,

H2PO

4

- for NO

3

-)

Cation exchange (e.g.,

Ca2+

for K+)

Definitions

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cation: An ion that carries

a positive charge

cation exchange: A process - cations

in solution exchanged with cations on exchange sites of minerals and

OM

cation exchange capacity (CEC): The total amount of exchangeable cations that a

particular material or soil can adsorb at a given pH

Controls on ion exchange

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Strength of adsorption

-Related to hydrated ionic radius and valence -The smaller the radius and greater the valence, the more closely and strongly the ion is adsorbed. Strength valence/radius

Relative concentration

in soil solution

Cation Exchange Capacity

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The sum total of all exchangeable cations that a

soil can adsorb

Expressed in terms of positive charge adsorbed per unit mass

If CEC =10 cmolc/kg

soil adsorbs 10 cmol of H+

can exchange it with 10

cmol K+, or 5 cmol Ca

2+

• number of charges, not

number of ions, what matters

cmolc = centimole of unbalanced

charge

Exchange affinity

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Held more strongly

H+ Al3+ > Ca2+ > Mg2+ > NH4+ = K+ > Na+

Held more weakly

This is referred to as the “Lyotropic series”

Strength of adsorption proportional to

valence ÷ hydrated radius

Ion exchange vs. CEC

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Sandy loam

NH4+

Ca2+

H+

Mg2+

K+

NO3-

Cl-

Crystal edge

H+

VERY

acidic soil

H+

NO3-

NO3-

NO3-

H+

HSO4-

How many

charges are

there to fill???

CEC = 7;

AEC = 2

Examples of cation exchange

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K+

K+

Al3+

+ 3K+

+ 2K+

Ca2+

K+

K+

K+ + Al3+

+ Ca2+

The interchange between a cation in solution and one on a colloid must be CHARGE balanced The reactions are reversible, unless…

Kapasitas Tukar Kation(KTK)

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Banyaknya kation

(dalam miliekivalen)

yang dapat dijerap

tanah per satuan berat

tanah (100 gram)

Menggambarkan kation

yang dijerap +

dipertukarkan antara

kompleks jerapan dan

larutan tanah.

KTK tinggi kesuburan

tinggi.

Nilai KTK digunakan

untuk klasifikasi

tanah

KTK tinggi : 1. bila BO tinggi ,2. liat tinggi

CEC and pH

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CEC

low

high

3 Soil pH

H+ binds

tightly, doesn’t

exchange

Na+ binds loosely,

exchanges readily

8

Hubungan KTK - BO

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OM has highest CEC

2:1 clays

1:1 clays

Non-clayey soils

CEC and weathering intensity

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Soil Order CEC (cmolc/kg)

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Soil order CEC

Oxisols Low

Ultisols 1:1 clay

Alfisols 9.0

Mollisols 18.7

Vertisols 35.6

Histosols 128.0

low

high

2:1 clays

O.M.

Low pH

3.5

High Al/Fe oxides

Key factor

Kapasitas Tukar Anion

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Kapasitas tukar anion

(KTA) banyak ditemukan

pada mineral liat amorf

dan liat Al dan Fe oksida

(seskuioksida)

Tanah yang mempunyai muatan positif berakibat :

a.Terjadi penjerapan anion seperti nitrat dan klor b.Kation-kation tidak dijerap oleh tanah sehingga mudah tercuci c.P dan S difiksasi sehingga menurunkan tingkat ketersediaannya

KEJENUHAN BASA

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Perbandingan antara jumlah kation basa dengan jumlah semua kation (kation basa + kation asam)

Kation Basa : Ca++, Mg++, K+, Na+ Kation Asam : H+, Al3+

Kation basa : unsur hara yang diperlukan tanaman Tanah dengan kejenuhan basa tinggi : 1) belum tercuci, 2) subur

Tanah dengan kejenuhan basa rendah : Kompleks jerapan diisi kation asam (H+, Al3+ ) Kejenuhan basa maks pada pH netral.

Adsorbed cations: area

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Humid region soil Arid region soil

H+

H+

H+

Al3+

K+

K+

Ca2+

Mg2+

H+

Mg2+

NH4+

Low pH (acidic)

High pH (basic)

Soil Order Base Saturation (%)

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Soil order Base Sat

Oxisols Low

Ultisols

Alfisols Medium-High

Mollisols High >50%

<35%

low

high

Unsur Hara

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Unsur makro Diperlukan

dalam jumlah banyak

Unsur mikro : Diperlukan

dalam jumlah sedikit

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Mekanisme Penyerapan Hara 1.Aliran Masa • Gerakan unsur hara dalam tanah

menuju permukaan akar tanaman bersama-sama gerakan air. • Penarik : transpirasi • Perlu energi • Sebagian besar unsur hara : N (98,8%), Ca (71,4%), S(95%), Mo (95,2%)

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2. Difusi

3.Intersepsi

Akar

• Perpindahan suatu zat (unsur hara) dari konsentrasi tinggi ke konsentrasi rendah. • P (90,9%), K

(77,7%)

• Perpanjangan akar memperpendek jarak tempuh unsur hara mendekati akar tanaman •Ca (28,6%)

MEKANISME SERAPAN ION

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difusi pasif

C BC C+

Absorpsi ion secara aktif

AB

A+

B-

MEKANISME TRANSPOR HARA DALAM TANAMAN

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Ekstraseluler/transpor dinding sel

Transpor dari sel ke sel melalui plasmodesmata

APOPLAS

SIMPLAS

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LEAKAGE (BOCORAN)

SECRETED

Jaringan stele permeabel dan tidak mampu

menyerap ion secara aktif

Sel parenchym memegang peran penting dalam

sekresi ion

MEKANISME PELEPASAN ION KE DALAM XYLEM

Faktor-Faktor yang Mempengaruhi Penyerapan Ion (Hara)

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Oksigen : rendah serapan rendah Temperatur

T rendah : membran fluiditas rendah/resistensi membran tinggi

Q10 = 2 X

Karbohidrat : rendah serapan rendah

Cahaya : fotosintesis serapan

pH : rendah mengurangi serapan kation