Oksida-Oksida Fe dan Al - Pada tanah tua daerah tropik - Kristalin dan amorf
- KTK rendah - Sering bermuatan positif Dapat melepaskan fiksasi P scr kuat melalui pertukaran anion
KOLOID ORGANIK
• 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 ENORMOUS external surface area! (but no internal surface – all edges) Central unit of a humus colloid (mostly C and H)
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Charges on soil colloids Colloid type
Negative charge
Positive charge
Humus (O.M.)
200 cmolc/kg
0 cmolc/kg
Silicate clays
100 cmolc/kg
0 cmolc/kg
Oxides of Al and Fe
4 cmolc/kg
5 cmolc/kg
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So what will those negative charges adsorb?
Perbedaan Koloid Liat dan Humus
KOLOID LIAT
• Misel tersusun dari Si, Al, O • Kristalin • Daya jerap kation lebih rendah • Mantap
KOLOID HUMUS
• C, H, O • Amorf • Daya jerap kation lebih tinggi • Tidak semantap liat
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Types of charge
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Permanent charge
Octahedral sheet neutral 5/31/2015
Net negative charge
pH-dependent charge: on edges
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 5/31/2015
Influence of pH on the CEC of smectite and humus
Edge charge
= Internal charge
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Ion exchange The substitution of one ion for another on the surface or in the interstitial spaces of a crystal
Anion exchange (e.g., H2PO4- for NO3-) 5/31/2015
Cation exchange (e.g., Ca2+ for K+)
Definitions 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 5/31/2015
Controls on ion exchange 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
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Relative concentration in soil solution
Cation Exchange Capacity 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 Ca2+ • number of charges, not number of ions, what matters 5/31/2015
cmolc = centimole of unbalanced charge
Exchange affinity Held more strongly
Held more weakly
H+ Al3+ > Ca2+ > Mg2+ > NH4+ = K+ > Na+ This is referred to as the “Lyotropic series”
Strength of adsorption proportional to valence ÷ hydrated radius
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Ion exchange vs. CEC VERY acidic soil
Sandy loam Ca2+ NO3Mg2+ ClH+ NH4+ K+
H+ NO3HSO4NO3H+
How many charges are there to fill???
NO3-
Crystal edge
H+ 5/31/2015
CEC = 7; AEC = 2
Examples of cation exchange Ca2+
K+ K+
+
+ Ca2+
2K+
K+ + Al3+
3K+
K+
+ Al3+
K+
The interchange between a cation in solution and one on a colloid must be CHARGE balanced The reactions are reversible, unless… 5/31/2015
Kapasitas Tukar Kation(KTK) Banyaknya kation (dalam miliekivalen) yang dapat dijerap tanah per satuan berat tanah (100 gram)
KTK tinggi kesuburan tinggi.
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Menggambarkan kation yang dijerap + dipertukarkan antara kompleks jerapan dan larutan tanah.
KTK tinggi : 1. bila BO tinggi ,2. liat tinggi
Nilai KTK digunakan untuk klasifikasi tanah
CEC and pH high
Na+ binds loosely, exchanges readily
CEC
H+ binds tightly, doesn’t exchange low
3 5/31/2015
Soil pH
8
Hubungan KTK - BO
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OM has highest CEC
2:1 clays
1:1 clays Non-clayey soils 5/31/2015
CEC and weathering intensity
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Soil Order CEC (cmolc/kg) Soil order
CEC
Key factor
Oxisols
Low
High Al/Fe oxides
Ultisols
3.5
1:1 clay
Alfisols
9.0
Mollisols
18.7
Vertisols
35.6
2:1 clays
Histosols
128.0
O.M.
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low
Low pH
high
Kapasitas Tukar Anion 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 5/31/2015
KEJENUHAN BASA Perbandingan antara jumlah kation basa dengan jumlah semua kation (kation basa + kation asam)
Kation basa : unsur hara yang diperlukan tanaman Tanah dengan kejenuhan basa tinggi : 1) belum tercuci, 2) subur 5/31/2015
Kation Basa : Ca++, Mg++, K+, Na+ Kation Asam : H+, Al3+
Tanah dengan kejenuhan basa rendah : Kompleks jerapan diisi kation asam (H+, Al3+ ) Kejenuhan basa maks pada pH netral.
Adsorbed cations: area Humid region soil
H+
Arid region soil
NH4+
K+
H+ K+
Mg2+
Ca2+
H+ Al3+ H+ Mg2+ Low pH (acidic) High pH (basic) 5/31/2015
Soil Order Base Saturation (%) Soil order
Base Sat
Oxisols
Low
Ultisols
<35%
Alfisols
Medium-High
Mollisols
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High >50%
low
high
Unsur Hara
Unsur makro Diperlukan dalam jumlah banyak 5/31/2015
Unsur mikro : Diperlukan dalam jumlah sedikit
Mekanisme Penyerapan Hara 1.Aliran Masa
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• 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%)
2. Difusi
• Perpindahan suatu zat (unsur hara) dari konsentrasi tinggi ke konsentrasi rendah. • P (90,9%), K (77,7%)
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3.Intersepsi Akar
• Perpanjangan akar memperpendek jarak tempuh unsur hara mendekati akar tanaman •Ca (28,6%)
MEKANISME SERAPAN ION Absorpsi ion secara aktif difusi pasif
A+
AB
C B-
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BC
C+
MEKANISME TRANSPOR HARA DALAM TANAMAN APOPLAS
SIMPLAS
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Ekstraseluler/transpor dinding sel
Transpor dari sel ke sel melalui plasmodesmata
MEKANISME PELEPASAN ION KE DALAM XYLEM LEAKAGE (BOCORAN) Jaringan stele permeabel dan tidak mampu menyerap ion secara aktif
SECRETED Sel parenchym memegang peran penting dalam sekresi ion 5/31/2015
Faktor-Faktor yang Mempengaruhi Penyerapan Ion (Hara)
Oksigen : rendah serapan rendah
Q10 = 2 X
Cahaya : fotosintesis serapan
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Karbohidrat : rendah serapan rendah
Temperatur
pH : rendah menguran gi serapan kation
T rendah : membran fluiditas rendah/resistensi membran tinggi
