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)


Sifat-sifat Koloid.1. Luas-permukaan yang reaktif.2. Nature’s great electrostatic chemical reactor!3. Influence most all soil ecosystem functions.4. Site of cation absorption (loosely held) that

are ‘exchangeable’.5. Mampu mengikat “benda” lain, seperti

pestisida dan virus.







IKATAN OKTAHEDRA-TETRAHEDRA


SUBSTITUSI ISOMORFIK


MODEL LIAT TIPE 2:1. MONTMORILONIT (SMEKTIT)



REAKSI PERTUKARAN KATION

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


Koloid Tanah Tipe Rataan KTK Muatan yang tergantung pH (%)

Bahan Organik amorphous 200 90

Smectite 2:1 100 5

Vermicullite 2:1 150 5

Fine micas 2:1 30 20

Chlorite 2:1:1 30 20

Kaolinite 1:1 8 95

Gibbsite (Al) Non-silicate 4 100

Geothite (Fe) Non-silicate 4 100

Allophane SiNon-cystalline

30 90


Muatan listrik pada beberapa Koloid Tanah

Koloid TanahPermukaan 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 TanahPartikel 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.


Koloid Liat TanahBerdasarkan jumlah dan susunan lembaran tetrahedral (silica) dan octahedral (alumina-magnesia) 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.


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.


Koloid Liat TanahTwo clay groups with 2:1 type structures have expansive type crystals, the smectites and

vermiculites.


Koloid Liat TanahModel 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.


Koloid Tanah: GibbsiteA 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.


Koloid Tanah - HumusA 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 TanahA 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 HPO4- shown) 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.


Koloid TanahPenjerapan 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.


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.


Koloid TanahKTK (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.


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 TanahExchange of cautions between a root and soil colloids.

Sumber: Hi-Fert, Plant Nutrition and Soil Fertility, 1997

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.

Documents

dasar ilmu tanah koloid liat