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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
5/31/2015
Central unit of a
humus colloid
(mostly C and H)
ENORMOUS external surface area!
(but no internal surface – all edges)
Charges on soil colloids
5/31/2015
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
5/31/2015
pH-dependent charge: on edges
5/31/2015
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
Ion exchange
5/31/2015
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
5/31/2015
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
5/31/2015
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
5/31/2015
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
5/31/2015
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
5/31/2015
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
5/31/2015
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)
5/31/2015
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
5/31/2015
CEC
low
high
3 Soil pH
H+ binds
tightly, doesn’t
exchange
Na+ binds loosely,
exchanges readily
8
Soil Order CEC (cmolc/kg)
5/31/2015
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
5/31/2015
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
5/31/2015
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
5/31/2015
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 (%)
5/31/2015
Soil order Base Sat
Oxisols Low
Ultisols
Alfisols Medium-High
Mollisols High >50%
<35%
low
high
Unsur Hara
5/31/2015
Unsur makro Diperlukan
dalam jumlah banyak
Unsur mikro : Diperlukan
dalam jumlah sedikit
5/31/2015
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%)
5/31/2015
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 TRANSPOR HARA DALAM TANAMAN
5/31/2015
Ekstraseluler/transpor dinding sel
Transpor dari sel ke sel melalui plasmodesmata
APOPLAS
SIMPLAS
5/31/2015
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