METHODS TO MEASURE

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• Chapter II
• METHODS TO MEASURE
• SOIL MOISTURE

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Chapter II Methods to Measure Soil Moisture

• INTRODUCTION
• The soil moisture affects crop production. The
  plants require an adequate amount of soil
  moisture that may vary according to the crop
  species and stage of growth or development of a
  plant. The soil can only store a limited amount
  of water, and only a part of this storage is
  available to the plant.
• Is essential to know the soil moisture content
  per unit mass or per unit soil volume, and its
  water potential or availability of the soil
  moisture.

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Chapter II Methods to Measure Soil Moisture
INTRODUCTION

• Different methods have been developed to
  determine the soil moisture. The use of these
  methods depends mainly on the economical
  resources of the operator, his knowledge and a
  desirable degree of precision.

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OBJECTIVES
Chapter II Methods to Measure Soil Moisture

• This chapter discusses basic principles of soil,
  water and plant relations and the use,
  operation, advantages and disadvantages of
  various methods to determine soil moisture. We
  hope that this information can enrich the
  knowledge of the farmers, scientists and
  agricultural technicians.

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Chapter II Methods to Measure Soil Moisture

• PRINCIPLES OF
• SOIL AND WATER RELATIONS

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Chapter II Methods to Measure Soil Moisture
SOIL COMPOSITION

• Minerals
• Pore space
• - water
• - Air
• - Solutes
• Organic matter

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Chapter II Methods to Measure Soil Moisture

• Principal Benefits Organic matter
• To provide source of essential nutrients to the
  plants, particularly nitrogen.
• To improve and to stabilize the soil structure
  when helping to form stable aggregates that
  facilitates plowing.
• To improve aeration and drainage in clayey and
  silty soil.
• To improve the field capacity in sandy soil.

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Chapter II Methods to Measure Soil Moisture
Principal Benefits Organic matter

• To improve the retention of available water to
  the plants, in sandy soil.
• To act as a cushioning agent that reduces the
  chances of abrupt changes in pH.
• To affect the formation of organic-metallic
  compounds. This way, soil nutrients are
  stabilized.

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Chapter II Methods to Measure Soil Moisture

• Figure 1. The soil components tthat affect the
  growth and development of a plant.

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Chapter II Methods to Measure Soil Moisture
SOIL TEXTURE

• Sand-Silt- Clay
• This classification is significant to the plant
  growth.
• The surface area increases significantly as the
  particle size is reduced

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Chapter II Methods to Measure Soil Moisture

• Figure 2. Soil texture classification USDA Soil
  Conservice Service, Washington D.C., USA
• Leyend Fi Fine, Co. Coarse, vfi very
  fine, med. mediunm, v.co. very coarse

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Chapter II Methods to Measure Soil Moisture

• Figure 3a. Effect of soil texture on the
  available water (top).Volumetric content of the
  four principal soil components that are adequate
  for ideal growth of plants (bottom).

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Figure 3a. Effect of soil texture on the
available water (top).Volumetric content of the
four principal soil components that are adequate
for ideal growth of plants (bottom).
Chapter II Methods to Measure Soil Moisture
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Chapter II Methods to Measure Soil Moisture

• Figure 4. Soil texture Classification.

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Chapter II Methods to Measure Soil Moisture

• SOIL STRUCTURE
• Laminar
• Prismatic
• Cuboide
• Spherical

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Chapter II Methods to Measure Soil Moisture

• Figure 5a. Soil Structure

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Chapter II Methods to Measure Soil Moisture

• Figure 5b. Soil Structure

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• Figure 5c. Soil Structure

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• Figure 5d. Soil Structure

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Chapter II Methods to Measure Soil Moisture

• SOIL MOISTURE( SOIL WATER)
• Some soils are very wet. Also may lack sufficient
  moisture available at a desired time, to obtain a
  good crop yield.

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Chapter II Methods to Measure Soil Moisture

• CLASSIFICATION OF SOIL WATER
• If appropriate conditions of water-drainage
  exist, capillary pores begin to drain due to the
  gravitational force.
• When all the macropores have been drained but
  capillary pores continue to be full, this limit
  is called field capacity.
• Gravitational water is a soil water between its
  point of saturation (tension of zero atm.) and
  the soil field capacity (tension of 0.33 atm.).

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Chapter II Methods to Measure Soil Moisture

• Figure 6. Classification of soil Physical and
  biological water.

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Chapter II Methods to Measure Soil Moisture

• SOIL AND WATER POTENTIAL
• The movement and the retention of the soil water
  have been visualized on the basis of concept of
  energy potential. The soil water is present in
  several forms colloidal water, free water
  (frequently in capillary pores of the soil) and
  water vapor.

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Chapter II Methods to Measure Soil Moisture
SOIL AND WATER POTENTIAL

• 2. The laws of capillarity movement of the soil
  do not begin or finish at a given value of soil
  moisture tension or at specific pore size. The
  moisture tension is different from one location
  to another and through an elapsed time.

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SOIL WATER POTENTIAL

• The total potential of the soil water consists
  of a series of individual components that can
  alter the free or potential energy of the soil
  water.

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Chapter II Methods to Measure Soil Moisture

• SOIL WATER POTENTIAL COMPONENTS
• Gravitational potential
• Pressure Potential
• -Capillary Suction (Capillary Potential).
• -Hydrostatic pressure in a static water under a
  aquifer level.
• -Water Pressure Induced by Flow
• Osmatic potential.
• Matrix potential.

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Chapter II Methods to Measure Soil Moisture

• COMPONENTS OF SOIL WATER POTENTIAL
• Gravitational potential of the soil water at a
  given state is determined by elevation of this
  point from a datum line.
• Pressure Potential of water is due to an increase
  or decrease of pressure of the free energy of the
  soil water. The pressure of the soil water
  (liquid phase) can be affected by the following
  factors.

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Chapter II Methods to Measure Soil Moisture

• SOIL WATER POTENTIAL
• Capillary Suction (Capillary Potential) Is an
  energy that is required to move a unit or mass of
  water against the capillary forces from the water
  surface to a desired point. This way, it
  describes the effects that have the capillary
  forces on the free energy of the soil water.
• Hydrostatic pressure in a static water under a
  aquifer level is a potential change in the free
  energy.

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Chapter II Methods to Measure Soil Moisture

• PRESSURE POTENTIAL
• Pressure potential is also affected by the amount
  and rate of flow the soil water.
• Pressure induced potential is a change in free
  energy of the soil water due to any source that
  has not been mentioned so far. For example Local
  compressed air locally in the soil, mechanical
  forces on the soil or the suction (negative
  pressure).

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Chapter II Methods to Measure Soil Moisture

• COMPONENTS OF SOIL WATER POTENTIAL
• Osmatic potential includes the effects on the
  soluble salts in the free energy of the water to
  the soil and the effects on the differences in
  the ion disassociations absorbed on the surface
  of colloidal particles of clay and organic matter.

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Chapter II Methods to Measure Soil Moisture

• COMPONENTS OF THE WATER POTENTIAL IN THE SOIL
• Matrix potential expresses the physical-chemical
  attractions between the water and soil particles.
  It includes the capillary attraction and the
  molecular forces that retain the water of
  hydratation in the soil colloids.

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Chapter II Methods to Measure Soil Moisture

• Soil Moisture Tension (or Suction)
• The soil water is a water film that surrounds the
  soil particles. The film is thick when there is
  enough soil moisture. The effects of external
  forces of absorption (absorption by the plant
  roots and evaporation) reduce the thickness of
  the film.

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Chapter II Methods to Measure Soil Moisture

• Soil Moisture Tension (or Suction)
• The moisture tension is a measurement of a force
  with which the moisture is retained by the soil.
  When the tension increases, the thickness of the
  water film decreases. It is easier to extract
  water from wet thick films while high tension is
  necessary to extract water from thin films.

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Chapter II Methods to Measure Soil Moisture

• Soil Moisture Tension Curves
• The tension and soil moisture percentage are
  inversely related. At low tensions, the soil can
  retain more moisture.

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Chapter II Methods to Measure Soil Moisture

• Figure 7. Soil moisture retention curves for
  different types of soil.

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Chapter II Methods to Measure Soil Moisture

• Figure 8a. Pressure membrane apparatus (commonly
  employed) to find the soil moisture tension for
  different types of soils.

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Chapter II Methods to Measure Soil Moisture

• Figure 8b. Pressure membrane apparatus (commonly
  employed) to find the soil moisture tension for
  different types of soils.

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Chapter II Methods to Measure Soil Moisture

• Availability of Soil Moisture to the Plants
• The available water to the plant is a difference
  in the soil moisture at field capacity (tension
  0.33 atm) and at permanent wilting percentage
  (tension 15 atm).

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Chapter II Methods to Measure Soil Moisture

• Availability of Soil Moisture to the Plants
• 2. If the available water in the soil is not
  enough or the root surface for absorption has
  reduced, then there exists a temporary wilting of
  the plant during the hot and drought periods.

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Chapter II Methods to Measure Soil Moisture

• Figure 9. Soil moisture deficit in the root
  zone at different depths.

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Chapter II Methods to Measure Soil Moisture
METHODS TO MEASURE THE SOIL MOISTURE
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Chapter II Methods to Measure Soil Moisture

• METHODS TO MEASURE THE SOIL MOISTURE
• These methods involve measuring soil properties
  that may change with moisture content.
  Therefore, we can have information on the
  determination of water availability to the
  plants.

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• Visual and Tactile Appearance of the Soil
  Use
• This method is an oldest method to estimate the
  soil moisture.
• It consists of a visual inspection and tactile of
  a soil sample.

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Chapter II Methods to Measure Soil Moisture

• Visual and Tactile Appearance of the Soil
  Use
• Generally, it is used when equipment is not
  available or we cannot wait to know the soil
  moisture conditions. However, the experienced
  farmer can estimate the soil moisture with a good
  precision.

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• Visual and Tactile Appearance of the Soil
• Procedure
• By means a auger (Figure 10), a soil sample is
  extracted.
• A visual and tactile inspection of the sample is
  conducted. Table 1 helps to estimate the soil
  moisture.

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Figure 10. Soil auger (bucket type), commonly
used for taking soil samples at different depths.
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• OVEN METHOD Disadvantages
• One requires laboratory equipment and certain
  degree of precision to obtain the reliable data.
• It requires a time period of more than 24 hours
  to carry out the procedure.
• The determination of the moisture for soils rich
  in organic matter can introduce the error due to
  an oxidation of organic matter.

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• OVEN METHOD Disadvantages
• It is a destructive method, because the soil is
  disturbed and samples are lost. Also the root
  system of the plant is disturbed.
• Several soil simples should be take to have a
  representative sample.

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• OVEN METHOD Advantages
• It is a simple method.
• It does not require use of expensive tools and
  equipment.
• It provides a quick estimation of soil moisture.

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• Gravimetric Method Use
• It is simply a determination of the moisture
  content of a soil sample by drying in an oven.
• The method requires Use of certain laboratory
  equipment to obtain accurate results.
• Skill of the operator to obtain reliable results.

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Chapter II Methods to Measure Soil Moisture

• Gravimetric Method Procedure
• With the use of bucket type anger, a soil sample
  is taken from a desired root depth.
• To have a representative sample, samples are
  taken at several locations.
• Then we take only 100 to 200 grams of soil
  sample.

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Chapter II Methods to Measure Soil Moisture

• Gravimetric Method Procedure
• The sample is identified and its wet weight is
  recorded.
• The weighted sample is left in an oven at a
  constant temperature of 105C for a period of 24
  hours

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• Gravimetric Method Advantages
• It is a precise method to find the soil moisture
  if the samples are taken carefully

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• Tesiometer Use
• Tensiometer is an instrument that indicates the
  tension at which the water is adhered to the soil
  particles.
• It can indicate, if a sufficient soil moisture is
  available for the plant growth.

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Chapter II Management of Drip/Micro or Trickle
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ltwww.idrc.ca/en/ev-42826-201-1-DO_TOPIC.htmlgt
gt
Figure 11a. Location of tensiometer in a fruit
orchard .
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• ltwww.ictinternational.com.au/faqjetfill.htm

Figure 11b. Principal components of a tensiometer
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Chapter II Management of Drip/Micro or Trickle
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ltwww.ictinternational.com.au/faqjetfill.htmgt
Figure 11c. Location of a tensiometer
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Chapter II Management of Drip/Micro or Trickle
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ltwww.ictinternational.com.au/faqjetfill.htmgt

• Figure 11d. Electronic sensors

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ltwww.decagon.com/echo/gt
Figure 11e. Gypsum blocks
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• ltwww.lboro.ac.uk/.../gy/natfor/instruments
  .htmlgt

Figure 11f. Gypsum blocks
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Chapter II Management of Drip/Micro or Trickle
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• Tensiometer Procedure
• Once the tensiometer is installed, the water
  within the stem of a instrument makes contact
  with the water retained in soil, flowing in both
  directions through the porous ceramic tip until
  the balance is established.

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Chapter II Methods to Measure Soil Moisture

• Tensiometer Procedure
• The soil water is lost through transpiration,
  evaporation and absorption by the plants. This
  causes a tension or suction and this tension
  increases as the soil continues to loose
  moisture.

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• Tensiometer Procedure
• This tension is measured by a vacuum gage of a
  tensiometer.
• When the soil is wetted again by rain fall or by
  irrigation, the soil tension reduces due to the
  flow of soil water through the porous ceramic
  tip.

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• Tensiometer Advantages
• This helps to decide when to apply the irrigation
  to the soil.
• The tensiometer can be used to determine vertical
  and horizontal of the moisture. This is
  necessary where there are problems of salt
  accumulation.

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Chapter II Management of Drip/Micro or Trickle
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Tensiometer Advantages 3. The instrument
provides a direct measurement of the soil
moisture suction. 4. Tensiometer is specially
appropriate for light soils that have range of
available water to the plants, within limitations
of 10 to 85 bars of tension.
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• Tensiometer Disadvantages
• Tensiometer can only operate up to 80 cbars al
  sea level. Generally, after 80 cbars of tension,
  air enters the porous ceramic tip and breaks the
  water column. When this has happened,
  tensiometer readings are not correct.

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Chapter II Management of Drip/Micro or Trickle
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Tensiometer Disadvantages 2. Tensiometer is
a delicate instrument that must be protected from
mechanical damages due to agricultural implements
and operations. 3. Tensiometers are placed
generally in a fixed location of the field. It
can not be moved from one place to another during
the period of crop growth.
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• Measurement of Electrical Resistance (Porous
  Ceramic Blocks) Use
• This method estimates soil moisture content by
  using resistance or conductance properties of
  soil.

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Chapter II Management of Drip/Micro or Trickle
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• Measurement of Electrical Resistance (Porous
  Ceramic Blocks) Procedure
• A representative area of the field is selected.
• With the use of proper size drill, a hole is made
  in the soil up to a desired depth.
• Then a porous plaster block with 2 or 3
  electrodes is placed inside the hole. There must
  be a good contact between the soil and porous
  block to allow a perfect seal.

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Chapter II Management of Drip/Micro or Trickle
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Measurement of Electrical Resistance (Porous
Ceramic Blocks) Procedure 4. For this, a
soil paste is prepared and is poured into the
hole. 5. The cables or terminals of the
electrodes must be taken out of the soil surface
(Figure 13).
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Chapter II Management of Drip/Micro or Trickle
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Figure 13. Gypsum blocks, commonly used to
determine the depth of irrigation.
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• Measurement of Electrical Resistance (Porous
  Ceramic Blocks) Advantages
• This method estimates soil the moisture.
• This instrument is especially appropriate to
  measure changes in the soil moisture for tensions
  between 1 to 15 atmospheres.

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• Measurement of Electrical Resistance (Porous
  Ceramic Blocks) Disadvantages
• The useful life of the blocks is limited.
• The original calibration of the porous block
  changes with time, because pores can be clogged
  by salts.
• The plaster blocks are usually ineffective for
  soil tensions of less than one atmosphere.

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• Measurement of Electrical Resistance (Porous
  Ceramic Blocks) Disadvantages
• The soluble salts in the soil solution reduce the
  electrical resistance and give high values of
  soil moisture content than actual values. It
  makes more difficult the process of the
  calibration of a instrument.
• The porous blocks may not be homogenous and this
  results in inaccurate readings.

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• Measurement of Electrical Resistance (Porous
  Ceramic Blocks) Disadvantages
• The precision of this method is reduced due to
  temperature, concentration of salts in the soil
  solution, physical characteristics of plaster.

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• Neutron Scattering Method Procedure
• This method consists of emission of neutron
  radiation of high energy and speed from an
  emitter or a radiation source towards the soil.

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Neutron Scattering Method Procedure
Chapter II Management of Drip/Micro or Trickle
Irrigation

• 2. These fast neutrons travel through the soil
  and gradually hit nuclei of different atoms thus
  reducing kinetic energy and speed.
• 3. The higher loss of energy occurs when these
  neutrons hit neutrons of mass similar to these.

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Chapter II Management of Drip/Micro or Trickle
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Figure 14. Determination of soil moisture by
neutron soldering method.
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• Neutron Scattering Method Advantages
• This system can cover a larger volume of soil and
  is relatively independent of the type of soil.
• It can be used for longest periods without any
  change in the radiation source.

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• Neutron Scattering Method Advantages
• The method does not involve taking of soil
  samples.
• Any range of soil moisture content can be
  analyzed. This avoids limitations of tensiometer
  or electrical resistance methods that can only
  measure the soil moisture within a certain range.

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• Neutron Scattering Method Disadvantages
• The equipment uses a radiation source. The
  technician must be have a basic sills and
  knowledge for the operation. It may be cause
  heath risks.
• 2. The system is expensive.

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Chapter II Management of Drip/Micro or Trickle
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Neutron Scattering Method Disadvantages 3.
The moisture measurement in soils with organic
matter are not precise and reliable because of
presence of excessive hydrogen atoms. The
readings for the surface soil layer are not
precise because of escape of neutrons towards the
surface.
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Chapter II Management of Drip/Micro or Trickle
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• Alternate Methods
• The absorption of gamma rays
• The thermodynamic properties of soil.
• The use of ultrasonic waves
• Radar waves/GPS
• The dielectric properties of soil.

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SUMMARY
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Irrigation

• Plants need a specific amount of soil moisture
  to ensure an adequate growth and development.
  This amount varies with the crop species. A
  limited amount of water can be retained by the
  soil, and a fraction of this water is available
  to the plant.

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SUMMARY
Chapter II Management of Drip/Micro or Trickle
Irrigation

• This chapter discusses methods to measure
  soil moisture visual and tactile appearance of
  the soil, gravimetric method, tensiometer,
  electrical resistance, and the neutron scattering
  method. Advantages and disadvantages of each
  method are presented. This chapter also discusses
  soil structure, soil texture, soil water and soil
  moisture available to the plant.

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• BIBLIOBGRAPHY
• Bidwell, R.G.S., 1979. Plant Physiology. New
  York McMillan Publishing Co. Inc. Pages
  247-50.
• Bonnet, J.A., 1968. La Ciencia del Suelo.
  Colegio de Ingenieros y Agrimensores de Puerto
  Rico. G.P.O. Apartado 3845, San Juan, P. R.
  00936. Páginas 107-124.
• Goyal, M.R., J. A. Santaella y L.E. Rivera, 1982.
  El Tensiómetro Su Uso, Intalación y
  Mantenimiento. Servicio de Extensión Agrícola,
  Universidad de Puerto Rico, RUM. Páginas 1-17.
  IA 73 Serie 1.

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Chapter II Management of Drip/Micro or Trickle
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• BIBLIOBGRAPHY
• Glossary of Soil and Water Terms, 1967. Special
  Publication SP-04-67. St. Joseph - Michigan.
  American Society of Agricultural and Biological
• Glossary of Soil Science Terms, 1979. Soil
  Science Society of America. 677 South Segoe
  Road. Madison- Wisconsin 53711, USA.
• Hillel, D., 1982. Introduction to Soil Physics.
  New York Academic Press, Inc.

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Chapter II Management of Drip/Micro or Trickle
Irrigation

• BIBLIOBGRAPHY
• Israelsen, Oscar y V. E. Hansen, 1964.
  Principios y Aplicaciones del Riego. Editorial
  Revelté, S. A. Barcelona.
• Lugo López, M. A., 1953. Moisture Relationships
  of Puerto Rico Soils. Technical Paper No. 9.
  Agricultural Experiment Station, University of
  Puerto Rico, Río Piedras, Puerto Rico.
• Muñoz, O. y V. R. Rodríguez, 1986. Apreciación y
  Clasificación de Suelos en Puerto Rico. Colegio
  de Ciencias Agrícolas, Universidad de Puerto Rico.

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Chapter II Management of Drip/Micro or Trickle
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• BIBLIOBGRAPHY
• Noi, Y., 1967. Relaciones Suelo-Agua. Servicio
  de Extensión Agrícola, Israel,
  Departamento de Capacitación para el Extranjero,
  Israel.
• Thorne, D. W. and M. D. Thorne, 1979. Soil
  Water and Crop Production. AVI Publishing
  Company, Inc. Wesport -Connecticut.
• Western Fertilizer Hand Book, 1980. Sixth
  Edition. California Fertilizer Association.
  2222 Watt Avenue, Sacramento, California 95825.

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Chapter II Management of Drip/Micro or Trickle
Irrigation

• BIBLIOBGRAPHY
• Wildman, W.E. and K. D. Gowans, 1978. Soil
  Physical Environment and How it Affects Plant
  Production. Leaflet 2280. Division of
  Agricultural Sciences, University of California,
  Berkley - CA
• Withers, B. and S. Vipond, 1980. Irrigation
  Design and Practice. Ithaca, NY Cornell
  University Press.

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Chapter II Methods to Measure Soil Moisture
Soil Moisture Meter lthttp//www.interiorlandscapin
g.co.uk/meters.htmgt
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Chapter II Methods to Measure Soil Moisture
Diagram Position of Soil Moisture
sensors lthttp//www.dpi.vic.gov.au/dpi/nreninf.nsf
/childdocsgt
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Chapter II Methods to Measure Soil Moisture
Principal components of soil. lthttp//fig.cox.miam
i.edu/Faculty/Dana/soil.jpggt
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Chapter II Methods to Measure Soil Moisture
Effect of soil structure on water
movement. lthttp//www.ext.colostate.edu/pubs/garde
n/gardimg/07722F02.gifgt
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Chapter II Methods to Measure Soil Moisture
Pressure membrane apparatus that uses compressed
air. lthttp//www.gaeicc.com/Hankison/Dhunit.jpggt
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Chapter II Methods to Measure Soil Moisture
Flow diagram for a pressure membrane
apparatus. lthttp//www.krug2000.ru/eng/images/ppmn
6.gifgt
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Chapter II Methods to Measure Soil Moisture
Pressure membrane that uses a hanging column of
water. lthttp//www.ukaea.com/wagr/images/vessel.jp
ggt
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Chapter II Methods to Measure Soil Moisture
Distribution of soil moisture in the root
zone. lthttp//meted.ucar.edu/nwp/pcu2/images/av4so
ilyr.gifgt
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Positive ThoughtThe sun was shining in my
eyes, and I could barely see To do the necessary
task that was allotted me.Resentment of the
vivid glow, I started to complain--When all at
once upon the air I heard the blindman's
cane.Earl Musselman
Chapter II Methods to Measure Soil Moisture