dasar ilmu tanah koloid liat

SOIL COLLOIDS:
Menentukan Sifat Kimia
dan Fisika Tanah
Sumber: www.uamont.edu/.../colliodintro.pptUniversity of Arkansas at Monticello
Apakah Koloid Tanah itu?
Partikel mineral (liat) dan
bahan organik (humus)
yangterdispersi dalam matiks
tanah, ukurannya sangat kecil
(sangat halus)
Sumber: www.uamont.edu/.../colliodintro.pptUniversity of Arkansas at Monticello
Sifat-sifat Koloid.
1.
2.
3.
4.
Luas-permukaan yang reaktif.
Nature’s great electrostatic chemical reactor!
Influence most all soil ecosystem functions.
Site of cation absorption (loosely held) that
are ‘exchangeable’.
5. Mampu mengikat “benda” lain, seperti
pestisida dan virus.
Sumber: www.uamont.edu/.../colliodintro.pptUniversity of Arkansas at Monticello
Sumber: www.uamont.edu/.../colliodintro.pptUniversity of Arkansas at Monticello
Sumber: www.uamont.edu/.../colliodintro.pptUniversity of Arkansas at Monticello
Sumber: www.uamont.edu/.../colliodintro.pptUniversity of Arkansas at Monticello
Sumber: www.uamont.edu/.../colliodintro.pptUniversity of Arkansas at Monticello
IKATAN OKTAHEDRA-TETRAHEDRA
Sumber: www.uamont.edu/.../colliodintro.pptUniversity of Arkansas at Monticello
SUBSTITUSI ISOMORFIK
Sumber: www.uamont.edu/.../colliodintro.pptUniversity of Arkansas at Monticello
MODEL LIAT TIPE 2:1.
MONTMORILONIT (SMEKTIT)
Sumber: www.uamont.edu/.../colliodintro.pptUniversity of Arkansas at Monticello
Sumber: www.uamont.edu/.../colliodintro.pptUniversity of Arkansas at Monticello
REAKSI PERTUKARAN KATION
Sumber: www.uamont.edu/.../colliodintro.pptUniversity of Arkansas at Monticello
Cation Exchange Capacity (CEC)
KAPASITAS TUKAR KATION (KTK)
CEC = the sum of exchangeable cations that
a soil can absorb.
CEC is expressed as the number of moles of
positive charge adsorbed per unit mass.
Units:
Cmol (+) kg-1
or
Meq per 100 g soil
Sumber: www.uamont.edu/.../colliodintro.pptUniversity of Arkansas at Monticello
Muatan listrik pada beberapa Koloid Tanah
Koloid Tanah
Muatan yang
tergantung pH (%)
Tipe
Rataan KTK
Bahan Organik
amorphous
200
90
Smectite
Vermicullite
Fine micas
Chlorite
Kaolinite
Gibbsite (Al)
Geothite (Fe)
Allophane
2:1
100
5
2:1
150
5
2:1
30
20
2:1:1
30
20
1:1
8
95
Non-silicate
4
100
Non-silicate
4
100
Si
Non-cystalline
30
90
Sumber: www.uamont.edu/.../colliodintro.pptUniversity of Arkansas at Monticello
Koloid Tanah
Permukaan koloid
tanah bermuatan
negatif atau positif,
yang dapat menarik
atau menolak ion-ion
yang muatannya
berlawanan bersama
dnegan molekul air.
Ion-ion yang tertarik
(terjerap) pd
permukaan koloid ini
(Ion-tukar) dapat
diserap oleh akar
tanaman.
Sumber: http://faculty.yc.edu/ycfaculty/ags105/week08/soil_colloids/soil_colloids_print.html
Koloid Tanah
Partikel liat Silicate adalah kristalin, tersusun atas dua lembaran. Salah satu lembaran ini
adalah bidang atom Si yang dikelilingi oleh atok oksigen. Atom Oksigen ini bergfungsi sebgaai
jembatan penghubung untuk bergabung dnegan lembaran aluminium dan/atau magnesium
yang dikelilingi oleh atom oksigen dan gugusan hidroksil.
Sumber: http://faculty.yc.edu/ycfaculty/ags105/week08/soil_colloids/soil_colloids_print.html
Koloid Liat Tanah
Berdasarkan jumlah dan susunan lembaran tetrahedral (silica) dan octahedral (aluminamagnesia) yg terkandung dalam unit kristal; liat silikat dikelompokkan menjadi dua: (a)
mineral liat tipe 1:1 (satu lembar tetrahedral dan satu lembar octahedral) dan (b) liat tipe
2:1. Kristal Tipe 1:1 (mis. kaolinite) ukurannya lebih besar dibanding tipe lainnya dan
strukturnya kaku (fixed) tidak mempunyai permukaan-internal dan sedikit sekali substitusi
isomorfiknya. Oleh karena itu, mineral ini mempunyai luas-permukaan yang relatif rendah
dan kapasitasnya untuk menarik kation juga rendah. Mineral ini tidak mengembang kalau
basah dan tidak mengkerut kalau kering.
Sumber: http://faculty.yc.edu/ycfaculty/ags105/week08/soil_colloids/soil_colloids_print.html
Koloid Liat Tanah
Model of two crystal layers and an interlayer characteristic of montmorillonite, a
smectite expanding-lattice 2:1-type clay mineral.
Each layer is made up of an octahedral sheet sandwiched between two tetrahedral
sheets with shared apical oxygen atoms.
There is little attraction between oxygen atoms in the bottom tetrahedral sheet of
one unit and those in the top tetrahedral sheet of another. This permits a variable
space between layers, which is occupied by water and exchangeable cations.
The internal surface area thus exposed far exceeds the surface around the outside of
the crystal.
Note that magnesium has replaced aluminum in some sites of the octahedral sheet.
Likewise, some silicon atoms in the tetrahedral sheet may be replaced by aluminum
(not shown).
These substitutions give rise to a negative charge, which accounts for the high cation
exchange capacity of this clay mineral. A ball-andstick model of the atoms and
chemical bonds is at the right.
Sumber: http://faculty.yc.edu/ycfaculty/ags105/week08/soil_colloids/soil_colloids_print.html
Koloid Liat Tanah
Two clay groups with 2:1 type structures have expansive type crystals, the smectites and
vermiculites.
Sumber: http://faculty.yc.edu/ycfaculty/ags105/week08/soil_colloids/soil_colloids_print.html
Koloid Liat Tanah
Model of a 2:1-type
nonexpanding lattice mineral
of the fine-grained mica group.
The general constitution of the
layers is similar to that in the
smectites, one octahedral
sheet between two tetrahedral
sheets. However, potassium
ions are tightly held between
layers, giving the mineral a
more or less rigid type of
structure that prevents the
movement of water and
cations into the space
between layers. The internal
surface and cation exchange
capacity of fine-grained micas
are thus far below those of the
smectites.
Sumber: http://faculty.yc.edu/ycfaculty/ags105/week08/soil_colloids/soil_colloids_print.html
Koloid Tanah: Gibbsite
A simplified diagram showing
the structure of gibbsite, an
aluminum oxide clay common
in highly weathered soils. This
clay consists of dioctahedral
sheets (two are shown) that are
hydrogen-bonded together.
Other oxide-type clays have
iron instead of aluminum in the
octahedral positions, and their
structures are somewhat less
regular and crystalline than that
shown for gibbsite. The surface
plane of covalently bonded
hydroxyls gives this, and similar
clays, the capacity to strongly
adsorb certain anions.
Sumber: http://faculty.yc.edu/ycfaculty/ags105/week08/soil_colloids/soil_colloids_print.html
Koloid Tanah - Humus
A simplified diagram showing the
principal chemical groups
responsible for the high amount
of negative charge on humus
colloids. The three groups
highlighted all include –OH that
can lose its hydrogen ion by
dissociation and thus become
negatively charged. Note that the
carboxylic, phenolic, and
alcoholic groups on the right side
of the diagram are shown in their
disassociated state, while those
on the left side still have their
associated hydrogen ions. Note
also that association with a
second hydrogen ion causes a
site to exhibit a net positive
charge.
Sumber: http://faculty.yc.edu/ycfaculty/ags105/week08/soil_colloids/soil_colloids_print.html
Koloid Tanah
A diagrammatic representation of
the adsorption of ions on a colloid
by the formation of outer-sphere
and inner-sphere complexes.
(1) Water molecules surround
diffuse cations and anions (such
as the Mg2+, Cl-, and HPO4shown) in the soil solution.
(2) In an outer-sphere complex
(such as the adsorbed Ca2+ ion
shown), water molecules form a
bridge between the adsorbed
cation and the charged colloid
surface.
(3) In the case of an inner-sphere
complex (such as the adsorbed
H2PO4— anion shown), no
water molecules intervene, and
the cation or anion binds
directly with the metal atom
(aluminum in this case) in the
colloid structure.
Sumber: http://faculty.yc.edu/ycfaculty/ags105/week08/soil_colloids/soil_colloids_print.html
Koloid Tanah
Penjerapan Kation dan Anion
Colloids attract and hold a complex swarm of cations and anions. Hydrated
ions from the soil solution are surrounded by a shell of water molecules.
These hydrated ions are attracted to sites on the colloid surface that bear a
charge of the opposite sign to the charge of the hydrated ion. As they
approach the surface of the colloid, they are loosely held by mutual
attraction of the charge colloid site and the ion for opposite ends of the
intervening water molecules.
This type of sorption is termed outer sphere complexation, and is readily
reversible. Exchangeable ions are examples. Other ions may bond directly
with the atoms of the clay surface, forming much stronger, more difficult to
reverse relationships known as inner sphere complexes. Phosphate ions
strongly sorbed or fixed to the hydroxyl plane of the octahedral sheet and
metal ions strongly sorbed to the oxygen plane of tetrahedral sheets are
examples.
Sumber: http://faculty.yc.edu/ycfaculty/ags105/week08/soil_colloids/soil_colloids_print.html
Koloid Tanah
The ions attracted to and adsorbed on the surface of
the soil colloids are subject to exchange with ions in the
soil solution or on plant roots.
Such exchangeable ions serve as valuable storage assets
for plants, the ions being held from loss by leaching or
runoff but remaining available for later uptake by
plants.
H+ ions created by root respiration will exchange for K+,
Mg2+ and Ca2+ for example, that can then be taken up
by plants.
Sumber: http://faculty.yc.edu/ycfaculty/ags105/week08/soil_colloids/soil_colloids_print.html
Koloid Tanah
KTK (CEC = Cation Exchange Capacity)
The cation exchange capacity (CEC) of a soil is determined
primarily by the amount and nature of the soil colloids and
by the pH.
Soils high in smectite and vermiculite clays have high CEC
values at all pH levels, those high in fine-grained micas and
chlorites being intermediate.
The CEC of kaolinite and the Fe, Al oxides is quite low. In
very acid soils the CEC of humus is also quite low, but under
neutral to alkaline conditions humus may have a CEC that,
on a weight basis, several fold exceeds that of smectite.
Sumber: http://faculty.yc.edu/ycfaculty/ags105/week08/soil_colloids/soil_colloids_print.html
Koloid Tanah- Pertukaran ion
Kemampuan
tanah
mendukung
produktivitas
tanaman -
ditentukan oleh
karakteristik
koloid tanah
dasn KTK tanah
Sumber: http://clasfaculty.ucdenver.edu/callen/1202/Soils/Soils.html
Koloid Tanah
Exchange of cautions between a root and soil colloids.
Bahan organik
mempunyai permukaan
yang sangat luas,
sehingga banyak
mempunyai tapak-tapak
pertukaran ion.
Semakin banyak
kandungan BOT suatu
tanah, semakin banyak
hara tersedia bagi
tanaman.
Sumber: Hi-Fert, Plant Nutrition and Soil Fertility, 1997