HANDOUT 1

GEOGRAFI TANAH

PROGRAM STUDI PENDIDIKAN GEOGRAFIFAKULTAS KEGURUAN DAN ILMU PENDIDIKANUNIVERSITAS MUHAMMADIYAH SURAKARTA

What is soil?

Soil is the unconsolidated cover on the surface of the earth.

Soil is made up of mineral particles,

organic particles, air, and water.

Soil is capable of supporting plant growth.

Soil ComponentsThe 4 parts of soil

MineralMatter45%

SoilWater25%

SoilAir

25%

OrganicMatter

5%

About ½ of the soil volume is solid particles

About ½ of the soil volume is pore space

Soil TextureThe mineral part of soil consists of sand, silt, and clay particles

The amounts of each size particle determines the textural property of the soil

Coarse textured, loose (more sand, less clay)Fine textured, heavy (more clay, less sand)Loamy (more even mix of sand, silt and clay

Sand0.1 – 0.002 in2 – 0.05 mm

Silt0.002 – 0.0001 in0.05 - 0.002 mm

ClayLess than 0.0001 inLess than 0.002 mm

1/100 in

Soil StructureThe arrangement of sand, silt, and clay particles to form larger

aggregates.

Organic matter is the glue that holds the aggregates together

Large pores (spaces) between aggregates are filled with air in a moist soil.

Small pores are filled with water in a moist soil. Even smaller pores inside the aggregates (not shown) are also filled with water.

1/10 inch

Supplying Plant Nutrients

Macronutrients:(needed in large amounts)

Nitrogen (N)

Phosphorus (P)

Potassium (K)

Calcium (Ca)

Magnesium (Mg)

Sulfur (S)

Micronutrients:(needed in small

amounts)Chlorine (Cl)

Cobalt (Co)

Copper (Cu)

Iron (Fe)

Manganese (Mn)

Molybdenum (Mo)

Nickel (Ni)

Zinc (Zn)

Nutrients that plants obtain from the soil

Where do plant nutrients come from?

Decaying plant litter

Breakdown of soil minerals

Addition by humansCommercial fertilizerManureLimeOther

Recycling plant nutrients

Breakdown of soil minerals

Acid

Ca

Mg

KCuNi

Zn

Water

Nutrient additions by humans

Commercial fertilizersNutrients are in a form that is available to plantsDissolve quickly and nutrients go into soil water

Lime Dissolves slowly as it neutralizes soil acidityReleases calcium and magnesium

Organic nutrient sources Manure, compost, sewage sludgeDecay and nutrient release is similar to crop litter

The soil solution

Soil water is a complex solution that contains

Many types of nutrientsOther trace elementsComplex organic molecules

Nutrients in the soil solution can be readily taken up by plant roots

If nutrients remained in solution they could all be quickly lost from the soil.

P

Ni Ca Mg

Cu

KN

Zn

AdsorptionAdsorption refers to the ability of an object to attract and hold particles on its surface.

Solid particles in soil have the ability to adsorb

WaterNutrients and other chemicals

The most important adsorbers in soil are

Clays Organic matter

+ -

Surface area of clay

¼ cup ¼ cup of clay has more surface area than a football field

The large surface area of clay allows it to

• Adsorb a lot of water• Retain nutrients• Stick to other soil

particles

Chemical bondingpHNutrient AvailabilityCation Exchange Capacity (CEC)

Soil Chemistry

What is “chemistry?” Elements: the simplest kind of matter. They cannot be broken down into anything simpler. Elements can exist alone. Nitrogen (N), Phosphorus (P), Potassium (K), Oxygen (O), are elements and they can be made to exist alone, but in nature they seldom do. Elements tend to combine with each other. These are called “compounds.” When they combine, it’s called a “chemical reaction.”

What is “chemistry?” Chemistry is the study of the “how” and “why” elements combine, and break apart, through chemical reactions.

Soil Chemistry is simply studying how and why compounds are formed and broken apart in the soil, and how these chemical reactions affect plants.

Chemistry Made Simple: The Atom

Hydrogen

(+)-

The Nucleus = positive chargeThe Electron = negative charge

Chemistry Made Simple: The Atom

Hydrogen

(+)-

The Nucleus = positive charge

A Water Molecule

H2O(+)

(+)

(+)

Hydrogen Hydrogen

Oxygen

Anion ChemicalSymbol

Ionic Form

Chloride Cl Cl-

Nitrate N NO3-

Magnesium Mg H2PO4-

Cation ChemicalSymbol

Ionic Form

Hydrogen H H+

Potassium K K+

Sodium Na Na+

Magnesium Mg Mg++

Calcium Ca Ca++

Common soil cations and anions, their chemical symbols and ionic forms

The Chemistry of Clay:

Clay Particle

- - --

--------------

-- - - - - - -

Clay particles carry negative charges

Water ionizes: it falls apart into ions.

H2O H+ + OH-

Called the self ionization of water.Only a small amount.

The Strange Properties of Water:

It’s simply a measure of the relative amount of H+ ionsIn the soil, it is driven by the ionization of water: H2O H+ + OH-

We us pH to measure the acidity or the alkalinity (basicity) of a solution (a soil solution)

What is pH?:

Possible pH Ranges Under Natural Soil Conditions

black walnut: 6.0-8.0

Most desirable

carrot: 5.5-7.0cucumber: 5.5-7.0

spinach: 6.0-7.5tomato: 5.5-7.5

white pine: 4.5-6.0

Verystrong Strong Moderate Slight Slight Moderate Strong

Verystrong

NeutralAcid Basic

3 4 5 6 7 8 9 10 111 2 12 13 14

Most agricultural soils

Extreme pH range for most mineral soils

cranberry:4.2-5.0

apple: 5.0-6.5

Soil Clays

•Clays are usually crystalline in nature•Some may be amorphous (e.g., allophane)•Three general definitions;

•particle < 2 µm in dia•name of a group of minerals•soil textural class

Soil Clays

•Crystalline clays have layers of Si and Al oxides held together by ionic bonds (attraction of positive and negative charges; no electron sharing)•Three or four planes of O layers with intervening Si and Al make up a layer.•One clay particle is made up of many layers and is called a micelle.

--

-

-Mg+2

H+ Mg+2

K+

H+ Ca+2

Ca+2

PO4-3

-

- -

--

-

-

-

--

-

---

-Al+3

-

--

K+

Clay Particle

Soil reactions in neutral to high (alkaline) pH conditions

Clay and organic particles have a negative charge

--

-

-H+

H+ Al+3

K+

H+ Al+3

Al+3

PO4-3

-

- -

--

-

-

-

--

-

---

-Al+3

-

--

K+

Clay Particle

Soil reactions in low (acid) pH conditions

Clay and organic particles have a negative charge

Pertukaran Kation

KPK atau Cation Exchange Capacity (CEC) merupakan kapasitas tanah untuk menjerap atau menukar kation. Biasanya dinyatakan dalam miliekuivalen/100 g tanah atau me %, tetapi sekarang diubah menjadi cmolc/kg tanah (centimoles of charge per kilogram of dry soil Permukaan aktif terdapat pada permukaan suatu koloid.Koloid tanah yang menjadi merupakan pusat terjadinya reaksi kimia adalah koloid liat dan koloid humus.

•Colloid: solid substance whose particles are very

small (few micrometers, 0.001 cm) .•Predominant colloids in soil are clays and humus. •Colloids have a net negative charge (different reason

for clays vs humus) and impart the soil cation

exchange capacity (CEC): the ability to adsorb and exchange cations. •Soil pH is determined by the nature of the

exchangeable cations on colloids (H+ + Al3+ vs Ca2+,

K+, Mg2+, Na+).

Kejenuhan basa berhubungan erat dengan KPK tanah: 

% Kejenuhan basa =  [Jumlah Kation Tertukar (dlm me %) / KPK] x 100

Contoh : Kation Tertukarme  % Ca 10

Mg 5K 10Na 5Jumlah 30 

Jika KPK tanah = 50 me %, maka  % kejenuhan basa = 30/50 x 100 = 60 % Ada korelasi positif antara pH tanah dan persen kejenuhan basa. Secara umum jika pH tinggi, kejenuhan basa akan tinggi. Kejenuhan basa yang rendah berarti kandungan H+ yang tinggi. Kejenuhan basa biasanya dapat digunakan sebagai indikasi kesuburan tanah.  Tanah sangat subur à derajat kejenuhan basa ≥ 80%,Tanah kesuburan sedang à derajat kejenuhan basa 50 % - 80 %Tanah tidak subur à derajat kejenuhan basa ≤ 50 %