Soil Physical Properties

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Soil Physical Properties
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Why measure soil physical properties?

• Have impact on planning and design of projects.
• Have impact on soil-air-water relationship,
• Have impact on soil water holding capacity, root
  zone development, and the general soil health.
• Soil physical properties determine how much the
  soil can allow water to infiltrate move laterally
  and/or store in the soil and be available to the
  plants.

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Knowing the soil physical properties should
improve our capacity to better plan demand
driven agricultural development activities
(EPLAUA), design adaptive Research to achieve
them (ARARI), implement the demand driven plans
(BoWRD), and execute the demand driven
agricultural development project (BoARD).
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What to measure in-situ?

• Core sampling for bulk density and soil moisture
  characteristic curve.
• Infiltration rate.
• Permeability (hydraulic conductivity).
• Soil moisture content.

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Bulk Density (Db)

• The overall density of a soil (mass of dry
  mineral soil divided by overall volume occupied
  by solids and the pore space).

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Why measure Db?

• Bulk density values represent status of soil
  compaction and porosity.
• The higher the Db (at the same moisture content),
  the lower the soil porosity, and the higher the
  soil compaction.
• Higher Db, lower root penetration, and poorer
  into the soil horizon and poorer aeration.

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Why measure Db? (cont.)

• Higher Db, lower water holding capacity, lower
  permeability.
• Db can be used to convert soil water content by
  weight to soil water content by volume (allowing
  estimation of soil void ratio and porosity).
• Db can also be used to estimate the volume of a
  soil too large to sample.

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Db measurement methods

• Clod method.
• Auger hole method.
• Replacement method
• Use of undisturbed core samples.

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Undisturbed Core Sampling

• Dig to the level that needs to be sampled
• Make a flat surface (bench)
• Drive the cylinder vertically into the soil
• Carefully remove the cylinder after carefully
  digging around the core sampler
• Trim the sample flush with the ends of the
  cylinder
• Cap the cores and
• Take sample to the laboratory to be dried and
  weighed.

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How to calculate Db

• Db (g cm-3) Oven dry weight (g) / Cylinder
  volume (cm3)

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Typical Db values for different textures
Soil Textural Class Db (g cm-3)
Clay, clay loam, and silt loam (topsoil) 1.0-1.6
Sand and Sandy loam 1.2-1.8
Recently cultivated soils 0.9-1.2
Mineral soils, not recently cultivated 1.1-1.4
Soils that might show restriction to root development Soils that might show restriction to root development
Sand, sandy loam, loam gt1.7
Silt, silt loam, silty clay loam 1.4-1.6
Clay, silty clay gt1.3
Compact subsoils gt2.0
Source Taylor et al., 1966 De Geus, 1973
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Infiltration

• Measurement of vertical intake of water into a
  soil from the soil surface.
• Used in determining the most efficient irrigation
  water application method.
• Used in making run-off calculation
• Usually measured by using two concentric rings
  (double ring infiltrometer) and measuring the
  water drop rate in the inner ring.

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Equipment

• Three steel cylinder sets, 40 cm high.
• One steel plate (15 cm by 15 cm)
• Sledge hammer or a heavy weight with handle
• Drums (50 gallons) or water trailers and buckets
  to transport and store water
• Three floats with scales

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Equipment (cont.)

• Auger and shovel
• Knife/machete for cutting vegetation
• Burlap cloth or a plastic sheet to reduce the
  impact of water and the resulting turbidity and
• Standard observation form.

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Procedure

• Select a representative site
• Remove all vegetation
• Record soil surface information (cracks, litter,
  plowing, etc)
• Pre-wet the area (soil surface) about two days
  before sampling date
• Select three test sites in a triangular format,
  about 10 meters apart

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Procedure (Cont.)

• Take a soil sample from outside the ring to
  determine the initial soil moisture content
• Drive infiltrometer rings into soil to
  approximately 15 cm depth
• Tap the soil firmly next to the inside and
  outside of the rings
• Place a piece of cloth or plastic sheet over the
  soil to dissipate the force of water and reduce
  the turbidity

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Procedure (Cont.)

• Fill both rings to a depth of about 15 to 20 cm,
  and record the time and height of water in the
  inner ring
• Repeat the measurement, first every minute, and
  as the rate of infiltration reduces every 5, 10,
  15, and 30 minute intervals until the rate of
  infiltration becomes steady for at least two
  consecutive readings

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Procedure (Cont.)

• Record water level immediately before and after
  each refill
• Make sure that the water level in the inner and
  outer ring stays almost at the same level
• Repeat the same procedure for each replicate
• Record readings on a standard form and calculate
  the infiltration rate

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Procedure (Cont.)

• Curve of infiltration versus time should be
  plotted to calculate the cumulative and basal
  infiltration rates.
• After completion of the test, remove the
  cylinders and dig a cross section through the
  center of the ring and describe the soil
  morphology and extent of wetting front.

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Calculations

• After field observations are recorded, cumulative
  infiltration, F, the average infiltration rate,
  IRave, the instantaneous intake rate, IR, and the
  basal intake rate, IRbas, should be obtained.

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Evaluation of results
Infiltration Class Infiltration Category IR (cm hr-1)
Slow Slow Slow
1 Very slow lt0.1
2 Slow 0.1 to 0.5
Moderate Moderate Moderate
3 Moderately slow 0.5 to 2.0
4 Moderate 2.0 to 6.3
5 Moderately rapid 6.3 to 12.7
Rapid Rapid Rapid
6 Rapid 12.7 to 25.0
7 Very rapid gt25.0
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Infiltration rates
Soil Texture Representative IR Normal IR range
Soil Texture (cm hr-1) (cm hr-1)
Sand 5 2 to 5
Sandy loam 2 1 to 8
Loam 1 1 to 2
Clay loam 0.8 0.2 to 1.5
Silty clay 0.2 0.03 to 0.5
Clay 0.05 lt0.01 to 0.8
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General infiltration rating
IR (cm hr-1) Suitability for surface irrigation
lt0.1 Unsuitable (too slow), but suitable for rice
0.1-0.3 Marginally suitable, ponding could be a problem
0.3-0.7 Suitable, unsuitable for rice
0.7-3.5 Optimum infiltration rate
3.5-6.5 Suitable for gravity irrigation
6.5-12.5 Marginally suitable, deep percolation losses
12.5-25.0 Unsuitable, recommended only for overhead (sprinkler) irrigation
gt25.0 Unsuitable, recommended only for drip irrigation
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Hydraulic conductivity (Permeability)

• Hydraulic conductivity is the volume of water
  that passes through a unit cross-sectional area
  of soil in a unit time, given a unit difference
  in water potential.
• Refers to the subsurface movement of water within
  the soil, both vertically and horizontally.

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Why measure Kfs?

• To compare permeability rates of different
  horizons as a guide to water movement
• To determine possible drainage problems within
  the soil profile and
• As a basis for in-field drainage system design.

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Theory

• Derived from an empirical formula (Darcys law)
  developed in 1856.
• Developed based on the relationship between the
  rates of flow of water through saturated columns
  of sand and the hydraulic head loss.

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Theory (cont.)

• q KA (h/L)
• where
• q rate of flow (volume)
• K hydraulic conductivity
• A cross-sectional area through which the flow
  takes place
• h hydraulic head expended in moving water from
  one side of the sample to other
• L length of the sample in the direction of flow

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Above water table measurement of Kfs

• Guelph permeameter, using a constant head well
  permeameter, will be used in SWHISA project in
  areas where water table is deeper than the study
  horizon.
• Only useful in areas where water table is deep
  and the soil horizon to be studied is above the
  water table.

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Methodology for use of GP in the field Hole
Preparation

• The well should be cylindrical and should have a
  reasonably flat bottom
• The hole bottom should be at least 20 cm above
  the water table
• Auger induced smearing/compaction of auger hole
  wall should be removed, using a small spiked
  roller mounted on a handle.

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Methodology for use of GP in the field Assembly
and installation of GP

• Assemble the GP according to instruction provided
  in the Guideline (Page 30)
• Stand the empty GP in the well and support it
  with a tripod
• Fill the well around the permeameter up to the
  measurement zone with gravel or coarse sand (if
  the soil has a very low permeability (Vertisols,
  heavy clay soils).

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Methodology for use of GP in the field Operation
of GP

• Fill the barrel with water (make sure that there
  is no air space, and use local water that is
  planned to be used for irrigation)
• Measure the rate of fall of water level in the
  reservoir until three constant readings is
  obtained.

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Calculations

• Use formulas provided in GP field data sheet
  (page 35 of the manual) to calculate Kfs (Field
  saturated hydraulic conductivity) and ?m (matric
  potential).

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Below water table measurement of Kfs

• Auger-hole method is used in Kfs measurement in
  areas with high water table.
• It is based on measuring the rate of rise of
  water in an 7.5 to 10 cm auger hole with time.
• Auger hole method provides average permeability
  of the soil layers extending from just below the
  water table to a small distance from the bottom
  of the hole.

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Equipment needed

• 7.5 to 10 cm (3 or 4 inch) diameter auger
• Tape measure and holder/pointer
• Stopwatch
• 7 or 9 cm diameter bailer
• Special cutter to make a flat bottom for the
  hole
• Bucket and water supply
• Observation forms
• Side scratcher to remove smearing effects

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Methodology

• Auger to a depth below the water table
• Finish off the hole with a special cutting tool
  to flatten the bottom of the hole
• Remove smearing, if present, from the sides of
  the hole
• Place the tape holder near the auger-hole so that
  the tape, with float attached, hangs vertically
  over the hole
• Lower the float into the hole (after water table
  is stabilized) and record the water level

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Methodology (cont.)

• Lift the float carefully from the hole and bail
  the water from the hole until the water level is
  reduced by about 20 to 40 cm (usually two passes
  of bailer is adequate)
• Quickly return the pointer to its original
  position, and lower the float to the surface of
  the rising groundwater.
• The reading should start as soon as practically
  is possible

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Methodology (cont.)

• Take at least five readings of groundwater level
  and time at equal intervals of about 5 to 30
  seconds
• Measure the depth of the hole from measuring
  point
• Stop measurement before 20 to 25 of the volume
  of water removed from the hole has been replaced
  by inflowing groundwater.

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Calculations

• Hydraulic conductivity can be measured by using
• Kfs C (Ave. ?y/Ave. ?t)
• where
• C Factor to be obtained from provided graphs
  (p. 38 and 39 of the guideline)
• ?y Average incremental rise during ?t
• ?t Incremental time interval between water
  rise measurements

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Evaluation of the Results

• Obtained data should be interpreted by soil
  scientist, irrigation engineer, considering all
  aspects of the project such as irrigation method,
  cropping pattern, previous soil management
  practices, irrigation scheduling, etc.
• Permeability measurements are point measurements
  and data should be used just as an indication in
  order of magnitude.

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FAO classification of Kfs
Hydraulic conductivity class K (cm hr-1) K (m day-1)
Very slow lt 0.8 lt 0.2
Slow 0.8-2.0 0.2-0.5
Moderate 2.0-6.0 0.5-1.4
Moderately rapid 6.0-8.0 1.4-1.9
Rapid 8.0-12.5 1.9-3.0
Very rapid gt12.5 gt 3.0
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Soil Water Measurement

• Soil water affects plant growth through its
  controlling effect on plant water status.
• Two ways to assess soil water availability for
  plant growth
• by measuring the soil water content and
• by measuring how strongly that water is retained
  in the soil (soil water potential).

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Soil Water Content

• saturated soil All soil voids (pore space) are
  filled with water.
• Field Capacity (FC) All readily drainable water
  (by gravity) are vacated macro-pores,
  approximately 0.33 bar (330 cm or pF 2.5).
• Permanent Wilting Point (PWP) The soil moisture
  content at which the leaves of sunflower plants
  wilt permanently and do not recover if water is
  applied, approximately 15 bars (15,000 cm, pF
  4.2). Water is left only in micro-pores.

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Available Water Capacity (AWC)

• Volume of water that is kept in the soil between
  FC and PWP.
• This water is potentially available to the
  plant and the value is generally used for
  determining frequency of irrigation and the depth
  of water that should be applied.
• AWC (mm m-1)(FC by volume-PWP by volume)x(10)

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Readily available water capacity (RAWC)

• Not all the water held between FC and PWP is
  available at the same rate to the plants.
• RAWC, kept at the lower tension (lower pF
  values), is considered a better indicator of soil
  moisture stress and should be used for irrigation
  scheduling.
• Rule of thumb 50 to 75 of AWC is considered as
  RAWC, varying based on crop physiology, rooting
  depth and volume, and moisture extraction pattern
  of each crop.

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Measurement of FC and PWP

• FC and PWP can be measured in the laboratory,
  using appropriately sized pressure plates and
  corresponding pressure membranes.
• PWP measurement Use of pressure plate (at -15
  bars matric potential or pF4.2) is an accepted
  method.
• Many question the validity of laboratory
  measurement of FC, and prefer field measurement.

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AWC calculation

• Soil moisture is determined on a weight basis.
• Using Db values, MC on a weight basis is
  converted to MC on a volume basis
• MC ( by volume v/v) MC ( by weight w/w)x(Db)
• or,
• MC ( v/v) (water weight/dry soil weight) x
  (weight of dry soil/total soil volume)
• Where,
• Db Bulk density, and MC Moisture content

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Soil moisture characteristics curve

• As water content in soil decreases, the matric
  potential decreases (becomes larger negative
  number).
• The functional relationship between matric
  potential (the potential resulting from
  attractive forces between the soil matrix and the
  water) in the soil and changes in soil water
  content is named the soil moisture
  characteristics (retention) curve.

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Moisture retention curve determination

• Moisture content at saturation (water-content at
  pF 0) is an indication of soils total
  pore-volume percentage.
• Retention curve is produced for different soils
  by determining water content at different
  tensions between saturation and PWP.
• Normal tensions applied (vacuum) are 0.05, 0.2,
  0.33 (FC), 1.0, 3.0, 15 bars (PWP) that are
  equivalent to 1.7, 2.0, 2.5, 3.0, 3.5, and 4.2 pF
  values, respectively.
• Moisture content of oven dry soil can be used as
  the equivalent tension of 9,800 bars (pF value of
  7.0).

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Laboratory Procedures for pF Curves

• Saturate the soil cores until a film of water is
  formed on soil surface, letting water to be
  adsorbed from the bottom
• After weighing, place pre-saturated soils on top
  of the ceramic plate
• Make sure that there is a good contact between
  the soil cores and the ceramic plate

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Laboratory Procedures for pF Curves (cont.)

• The outlet tube of the ceramic plate should then
  connected to the outflow tube of the pressure
  chamber
• The chamber should be pressurized to intended
  positive pressure
• The system should stay pressurized until
  equilibrium is reached with the applied water
  pressure. The equilibrium is reached when outflow
  of water has ceased which may even take three to
  four days

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Laboratory Procedures for pF Curves (cont.)

• After reaching the equilibrium, the pressure
  should be released and the core samples should be
  weighed
• This procedure should be repeated for all
  intended matric potentials, until all
  measurements are completed
• After all measurements are completed, soil cores
  should be dried in forced air oven at 105oC.

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Laboratory Procedures for pF Curves (cont.)

• The volumetric water content for each matric
  potential will be calculated using
• Volumetric water content ()Vol. of water
  (cm3)/Core volume (cm3)
• The volume of water at each matric potential (pF
  value) is then determined from
• Vol. of water(Mass of equilibrated soilMass of
  oven dried core)/DbH2O
• Where DbH2O 1
• The soil moisture characteristic curve is then
  produced by plotting the soil water matric
  potential (bar or pF value) against soil
  volumetric water content ().

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Soil water characteristics (retention) curves
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Field measurement

• It is best to directly measure the degree of
  wetness (soil moisture content) or the matrix
  potential, rather than using calibration curves
  for estimating soil water content for irrigation
  scheduling, because of the effect of hysteresis
  caused by wetting and drying of soil samples.

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Non-destructive water content measurementNeutron
Probe

• Neutron probe uses the property of scattering and
  slowing down neutrons (H ions).
• Alpha particles emitted by the decay of the
  americium (241) collide with the light beryllium
  nuclei, producing fast neutron.
• Fast neutrons, encountering hydrogen in the soil,
  lose their energy and are slowed down or
  thermalized.
• The detection of slow neutrons returning to the
  probe allows estimation of the amount of H ions
  present.
• Since most of the H ions in the soil is
  associated with soil water, it provide water
  content estimate.

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Non-destructive water content measurementTime
Domain Reflectometry (TDR)

• TDR measures the spread of an electromagnetic
  wave through the soil.
• The characteristics of this propagation depends
  on soil water content.
• A good agreement exist between the TDR and
  neutron probe measurements.
• The cost of neutron probe and TDR are prohibitive.

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Non-destructive water potential
measurementGypsum block/Granular Matrix Sensors

• Exhibit a wide range relationship between their
  electric conductivity and soil water potential.
• Somewhat unreliable in some soils caused by loss
  of contact with the soil due to dissolving of
  gypsum, inconsistence pore size distribution and
  soil salinity effects.
• GMS works based on Gypsum block technology, but
  reduces the general inherent problems of gypsum
  blocks, using a granular matrix mostly supported
  in a metal or plastic screen.

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Non-destructive water potential
measurementTensiometers

• Another type of instrument that measures the
  energy status (or potential) of soil water.
• Tensiometers are extensively used for irrigation
  scheduling because they provide direct
  measurements of soil moisture status and are easy
  to manage.
• Tensiometers are available at BoWRD.

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Non-destructive water potential
measurementTensiometers (Components)

• A porous ceramic cup and a rigid body tube that
  is connected to a manometer or a vacuum gauge
  with all components filled with water, having an
  air-tied seal.
• A Bourdon tube vacuum gauge is commonly used for
  water potential measurements.

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Non-destructive water potential
measurementTensiometers (Operation Principles)

• Tensiometers are placed with ceramic cup firmly
  in contact with soil in plant root zone.
• Since ceramic cup is porous, water moves through
  it to equilibrate with soil water, causing a
  hydraulic contact between water in the cup and
  soil water.
• Water moving out of the cup develop a suction or
  negative pressure (partial vacuum) that causes a
  reading on the vacuum gauge.
• Gauge reading, an indication of the attractive
  forces between water and soil particles, is a
  measure of the energy that would need to be
  exerted by the plant to extract water from the
  soil.

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Non-destructive water potential
measurementTensiometers (Operation Principles)

• Tensiometer is able to follow changes in the
  matric potential as a result of soil drying out
  due to drainage, evaporation or plant uptake of
  water (transpiration).
• When moisture is replenished by rain or
  irrigation, the matric potential will drop.
• Tensiometer continuously records fluctuations in
  soil water potential under field conditions.

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Non-destructive water potential
measurementTensiometers (Operation Principles)

• Accurate tensiometer response will occur only if
  air does not enter the water column.
• Air expands and contracts with changes in
  pressure and temperature, thus causing inaccurate
  tensiometer readings.
• Air leaks or dissolved air can enter through the
  ceramic cup during normal operation of the
  instrument.
• If a significant amount of air enters the
  instrument, it must be expelled and the
  tensiometer refilled with water before it can
  reliably operate again.

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Non-destructive water potential
measurementTensiometers (Operation Range)

• The useful range of a tensiometer is limited from
  0 (saturation) to as high as 0.85 bar (85 cm
  head).
• Above 0.85 bar the column of water in the tube
  will form water vapor bubbles (cavitate), causing
  instrument to stop functioning.
• In many agricultural soils, the tensiometer range
  accounts for 50 of the soil water that is taken
  up by the plants (almost RAWC)

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Non-destructive water potential
measurementTensiometers (Site selection)

• Tensiometers measure soil water tension in a
  small volume of soil immediately around the
  ceramic cup.
• Should be placement within the root active
  zone(s) of the crop for which irrigation is
  scheduled.
• Depending on crop type and its root distribution,
  one or more tensiometers of variable length may
  be required.

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Non-destructive water potential
measurementTensiometers (Placement in the field)

• Site(s) selected for installation must be
  representative of the surrounding field
  conditions.
• Tensiometers should be placed within the active
  root zone, in the plant canopy in positions,
  receiving typical amounts of rainfall and
  irrigation as the intended crop.
• shallow-rooted crops (vegetables) need only one
  tensiometer, centered in the crop root zone,
  10-15 cm below the surface.
• Deep rooted crops (tree crops, most row crops)
  two tensiometer should be used at each site.

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Non-destructive water potential
measurementTensiometers (Installation)

• Before field installation, each tensiometer
  should be tested to ensure it is working.
• Fill tensiometers with clean water (deionized
  water) and keep vertically for at least 30
  minutes to saturate the ceramic tip.
• After fully wetting the ceramic tip, it can be
  refilled and capped.

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Non-destructive water potential
measurementTensiometers (Installation)

• Tensiometer will not be serviceable immediately
  after filling because of air bubbles in the
  vacuum gauge.
• small vacuum hand pump should be used to remove
  all air bubbles from the tube and vacuum gauge
  and test for air leaks.
• After air bubbles are removed, tensiometers
  should be installed in previously cored holes to
  the appropriate depth in the field.

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Non-destructive water potential
measurementTensiometers (Installation)

• Soil around tensiometer should be tamped at the
  surface.
• After installation, several hours is required,
  before tensiometer can read the correct soil
  water potential value due to installation induced
  disturbance of the soil and the need for water to
  move through the ceramic cup before equilibrium
  is reached.

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Non-destructive water potential
measurementTensiometers (Installation)

• Tensiometers must be periodically serviced in the
  field.
• Under normal operation, air will be extracted
  from water under tension and becomes trapped
  within the tensiometer, reducing response time
  and its operability.
• Tensiometer tube should be inspected each time
  the tensiometer is read.
• If more than 0.5 cm of air is accumulated beneath
  the service cap, the trapped air should be
  removed and the tube refilled with deionized
  water.

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I hope we all enjoyed the five days of office
discussions and we are fully prepared for the
upcoming field work. I certainly did!