HUBUNGAN AIR TANAHHUBUNGAN AIR TANAH

Disarikan oleh: Disarikan oleh:

Prof Soemarno (September 2010)

Tekstur TanahDefinition: relative proportions of various sizes of individual soil particlesDefinition: relative proportions of various sizes of individual soil particlesUSDA classifications

Sand: 0.05 2.0 mmSilt: 0.002 - 0.05 mmClay: <0.002 mm

Textural triangle: USDA Textural ClassesCoarse vs. Fine, Light vs. HeavyAffects water movement and storage

Struktur TanahStruktur TanahDefinition: how soil particles are grouped or arrangedAffects root penetration and water intake and Affects root penetration and water intake and movement

USDA Textural USDA Textural TriangleTriangle

SEGITIGA TEKSTUR

TANAHTriangleTriangleTANAH

Bulk Density ( b) sb

V

M

b = soil bulk density, g/cm3

M = mass of dry soil, g

bb

V

Ms = mass of dry soil, gVb = volume of soil sample, cm3

Typical values: 1.1 - 1.6 g/cm3Typical values: 1.1 - 1.6 g/cm

Particle Density ( p) psM

VParticle Density ( p)

P = soil particle density, g/cm3

M = mass of dry soil, g

psV

Ms = mass of dry soil, gVs = volume of solids, cm3

Typical values: 2.6 - 2.7 g/cm3Typical values: 2.6 - 2.7 g/cm3

Porosity ( )Porosity ( )

volume of poresvolume of pores

volume of soil

1 100%b

Typical values: 30 - 60%

1 100%p

Typical values: 30 - 60%

Kadar Air TanahKadar Air Tanahw

mM

M

Mass water content ( m)s

mM

m = mass water content (fraction)Mw = mass of water evaporated, g

( 24 hours @ 105oC)w

( 24 hours @ 105oC)Ms = mass of dry soil, g

Kadar air volumetrik ( v)wV

V = volumetric water content (fraction)

vw

b

V

VV = volumetric water content (fraction)

Vw = volume of waterVb = volume of soil sampleAt saturation, V =

V = As m

As = apparent soil specific gravity = b/ wAs = apparent soil specific gravity = b/ w( w = density of water = 1 g/cm3)

As = b numerically when units of g/cm3 are used

Equivalent depth of water (d)Equivalent depth of water (d)d = volume of water per unit land area = ( v A L) / A = v Ld = equivalent depth of water in a soil layerd = equivalent depth of water in a soil layerL = depth (thickness) of the soil layer

Kadar air Kadar air volumetrikvolumetrik & & KedalamanKedalaman ekuivalenekuivalen

(cm3)(cm )

Equivalent DepthEquivalent Depth

(g) (g) (cm3)(g) (g) (cm3)

Kadar air Kadar air volumetrikvolumetrik & & KedalamanKedalaman ekuivalenekuivalenNilai-nilai tipikal untuk tanah pertanianNilai-nilai tipikal untuk tanah pertanian

0.50 in.

Soil Solids (Particles): 50%

1 in.

0.50 in.

Very Large Pores: 15% 0.15 in.

0.20 in.

Total Pore Space: 50%

Very Large Pores: 15% (Gravitational Water)

Medium-sized Pores: 20% 0.20 in.

0.15 in.

Space: 50%(Plant Available Water)

Very Small Pores: 15% (Unavailable Water)

DayaDaya SimpanSimpan Air = Air = WaterWater--Holding Capacity (WHC) TanahHolding Capacity (WHC) TanahWaterWater--Holding Capacity (WHC) TanahHolding Capacity (WHC) Tanah

PengaruhPengaruh TeksturTekstur TanahTanah

Coarse SandCoarse Sand Silty Clay LoamSilty Clay LoamCoarse SandCoarse Sand Silty Clay LoamSilty Clay Loam

Dry SoilDry Soil

Gravitational WaterGravitational Water

Water Holding CapacityWater Holding CapacityWater Holding CapacityWater Holding Capacity

Available WaterAvailable Water

Unavailable WaterUnavailable Water

DESKRIPSIMeasure of the energy status of the soil waterMeasure of the energy status of the soil waterImportant because it reflects how hard plants must work to extract waterwork to extract waterUnits of measure are normally bars or atmospheresatmospheresSoil water potentials are negative pressures (tension or suction)(tension or suction)Water flows from a higher (less negative) potential to a lower (more negative) potentialto a lower (more negative) potential

KomponenKomponen

= total soil water potentialt g m o

t = total soil water potential

g = gravitational potential (force of gravity pulling on the water)pulling on the water)

m = matric potential (force placed on the water by the soil matrix soil water tension )by the soil matrix soil water tension )

o = osmotic potential (due to the difference in salt concentration across a semi-permeable membrane, such as a plant root)membrane, such as a plant root)

Matric potential, m, normally has the greatest effect on release of water from soil to plants

meffect on release of water from soil to plants

Kurva pelepasan air tanah Curve of matric potential (tension) vs. water contentLess water more tensionLess water more tensionAt a given tension, finer-textured soils retain more water (larger number of small pores)

Potential Matriks dan Tekstur TanahThe tension or suction created by small capillary tubes The tension or suction created by small capillary tubes (small soil pores) is greater that that created by large tubes (large soil pores). At any given matric potential coarse soils hold less water than fine-textured soils.coarse soils hold less water than fine-textured soils.

Height of capillary rise inversely related to tube diameterto tube diameter

Kapasitas Lapang= Field Capacity (FC or fc)fcSoil water content where gravity drainage becomes

negligible

Soil is not saturated but still a very wet conditionSoil is not saturated but still a very wet condition

Traditionally defined as the water content corresponding to a soil water potential of -1/10 to -1/3 barcorresponding to a soil water potential of -1/10 to -1/3 bar

Titik Layu Permanen: Permanent Wilting Point (WP or)wp)

Soil water content beyond which plants cannot recover from water stress (dead)recover from water stress (dead)

Still some water in the soil but not enough to be of use to plants

Traditionally defined as the water content Traditionally defined as the water content corresponding to -15 bars of SWP

DefinitionWater held in the soil between field capacity and Water held in the soil between field capacity and permanent wilting point Available for plant useAvailable for plant use

Kapasitas Air Tersedia: Available Water Capacity (AWC)Capacity (AWC)

AWC = fc - wp

Units: depth of available water per unit depth of Units: depth of available water per unit depth of soil, unitless (in/in, or mm/mm)Measured using field or laboratory methods Measured using field or laboratory methods (described in text)

Sifat Hidraulik Tanah & Tekstur Tanah Sifat Hidraulik Tanah & Tekstur Tanah

Fraction available water depleted (fd)

wpfc

vfcdf

( fc - v) = soil water deficit (SWD)

v = current soil volumetric water content

wpfc

v = current soil volumetric water content

Fraction available water remaining (fr)

wpfc

wpvrf

( v - wp) = soil water balance (SWB)

wpfc

Total Air Tersedia: Total Available Water (TAW)

TAW = (AWC) (Rd)TAW = total available water capacity within the plant

root zone, (inches)root zone, (inches)AWC = available water capacity of the soil,

(inches of H2O/inch of soil)R = depth of the plant root zone, (inches)Rd = depth of the plant root zone, (inches)If different soil layers have different AWC s, need to sum up the layer-by-layer TAW s

TAW = (AWC1) (L1) + (AWC2) (L2) + . . . (AWCN) (LN)- L = thickness of soil layer, (inches)- : subscripts represent each successive soil layer- 1, 2, N: subscripts represent each successive soil layer

[Error on page 26 of text: change SWD TAW ]

Gravity vs. CapillarityGravity vs. Capillarity

Horizontal movement Horizontal movement due to capillaritydue to capillarityVertical movement Vertical movement

due largely to gravitydue largely to gravitydue to capillaritydue to capillarity

due largely to gravitydue largely to gravity

DefinisiDefinisi: : masuknyamasuknya air air keke dalamdalam tanahtanahDefinisiDefinisi: : masuknyamasuknya air air keke dalamdalam tanahtanah

FaktorFaktor--faktorfaktor yang yang mempengaruhimempengaruhi::FaktorFaktor--faktorfaktor yang yang mempengaruhimempengaruhi::Soil textureSoil textureInitial soil water contentInitial soil water contentInitial soil water contentInitial soil water contentSurface sealing (structure, etc.)Surface sealing (structure, etc.)Soil crackingSoil crackingTillage practicesTillage practicesTillage practicesTillage practicesMethod of application (e.g., Basin vs. Furrow)Method of application (e.g., Basin vs. Furrow)Water temperatureWater temperatureWater temperatureWater temperature

Infiltrasi Kumulatif vs. Tekstur TanahInfiltrasi Kumulatif vs. Tekstur Tanah

LajuLaju InfiltrasiInfiltrasi vs. vs. WaktuWaktuUntukUntuk beragamberagam teksturtekstur tanahtanahUntukUntuk beragamberagam teksturtekstur tanahtanah

LajuLaju infiltrasiinfiltrasi dandan TeksturTekstur TanahTanahLajuLaju infiltrasiinfiltrasi dandan TeksturTekstur TanahTanah

LajuLaju InfiltrasiInfiltrasi vs. vs. LajuLaju IrigasiIrigasi KonstanKonstan

LajuLaju InfiltrasiInfiltrasi vs. vs. LajuLaju irigasiirigasi VariableVariable

Can be viewed as sequentially filling the Can be viewed as sequentially filling the Can be viewed as sequentially filling the Can be viewed as sequentially filling the soil profile in layerssoil profile in layersDeep percolation: water penetrating Deep percolation: water penetrating Deep percolation: water penetrating Deep percolation: water penetrating deeper than the bottom of the root zonedeeper than the bottom of the root zoneLeaching: transport of chemicals from the Leaching: transport of chemicals from the Leaching: transport of chemicals from the Leaching: transport of chemicals from the root zone due to deep percolationroot zone due to deep percolation

SimpananSimpanan air air dalamdalam profilprofil tanahtanah

GravimetricGravimetricMeasures mass water content ( m)Take field samples weigh oven dry weighAdvantages: accurate; Multiple locationsAdvantages: accurate; Multiple locationsDisadvantages: labor; Time delay

Feel and appearanceFeel and appearanceTake field samples and feel them by handAdvantages: low cost; Multiple locationsAdvantages: low cost; Multiple locationsDisadvantages: experience required; Not highly accurate

Neutron scattering (attenuation)Neutron scattering (attenuation)Pengukuran Air TanahPengukuran Air Tanah

Neutron scattering (attenuation)Neutron scattering (attenuation)Measures volumetric water content (Measures volumetric water content ( vv))Attenuation of highAttenuation of high--energy neutrons by hydrogen nucleusenergy neutrons by hydrogen nucleusAdvantages: Advantages: Advantages: Advantages:

samples a relatively large soil sphere samples a relatively large soil sphere repeatedly sample same site and several depths repeatedly sample same site and several depths accurateaccurateaccurateaccurate

Disadvantages: Disadvantages: high cost instrument high cost instrument radioactive licensing and safety radioactive licensing and safety radioactive licensing and safety radioactive licensing and safety not reliable for shallow measurements near the soil surfacenot reliable for shallow measurements near the soil surface

Dielectric constantDielectric constantA soil s dielectric constant is dependent on soil moistureA soil s dielectric constant is dependent on soil moistureA soil s dielectric constant is dependent on soil moistureA soil s dielectric constant is dependent on soil moistureTime domain reflectometry (TDR)Time domain reflectometry (TDR)Frequency domain reflectometry (FDR)Frequency domain reflectometry (FDR)Frequency domain reflectometry (FDR)Frequency domain reflectometry (FDR)Primarily used for research purposes at this timePrimarily used for research purposes at this time

Pengukuran Aur Tanah: Pengukuran Aur Tanah: Neutron AttenuationNeutron AttenuationNeutron AttenuationNeutron Attenuation

Tensiometers

Pengukuran Air TanahPengukuran Air TanahTensiometers

Measure soil water potential (tension)Practical operating range is about 0 to 0.75 Practical operating range is about 0 to 0.75 bar of tension (this can be a limitation on medium- and fine-textured soils)medium- and fine-textured soils)

Electrical resistance blocksMeasure soil water potential (tension)Measure soil water potential (tension)Tend to work better at higher tensions (lower water contents)(lower water contents)

Thermal dissipation blocksMeasure soil water potential (tension)Measure soil water potential (tension)Require individual calibration

Tensiometer for Measuring Soil Water PotentialTensiometer for Measuring Soil Water Potential

Water ReservoirWater ReservoirWater ReservoirWater Reservoir

Variable Tube Length (12 in- 48 in) Based on Root Zone DepthBased on Root Zone Depth

Porous Ceramic Tip

Vacuum Gauge (0Vacuum Gauge (0--100 centibar)100 centibar)

Electrical Resistance Blocks & MetersElectrical Resistance Blocks & Meters