Soil moisture

DETERMINATION OF SOIL MOISTURE BY GRAVIMETRIC AND GYPSUM BLOCK METHODS

The principal methods of expressing soil moisture

1.      Amount of water in a given amount / volume of soil

2.      The stress of tension under which the water is held by the soil

Expressing the amount of soil moisture

         The amount of moisture held by a given mass or volume of soil can be expressed as weight or volume basis in percentage. Soil moisture on weight basis is based on the dry weight of the sample.

Soil moisture % by weight =

Soil moisture % by volume =  

A) BY GRAVIMETRIC METHOD

Principle

         Soil moisture content is determined by drying a known quantity of soil sample in an electric oven at 105°C to 110°C and finding out the loss in weight.

Materials required

i)Moisture bottle / aluminum box   

ii) Chemical balance/top pan balance

iii) Desiccator

Procedure  

         Place a clean and empty moisture bottle or aluminum box with lid separately in an electric oven and keep it at 105°C for 15 minutes. Replace the stopper or lid, remove the moisture bottle or aluminum box, cool in a desiccator, weigh accurately and record the weight. Fill the moisture bottle or aluminum box to about two third of its capacity with soil sample. Close with stopper / lid and weigh quickly. Remove the stopper / lid and keep it in the oven at 105°C for about 8 hours. After the expiry of time, remove the moisture bottle / aluminum box, cool it in a desiccator and weigh quickly. Calculate the loss in weight and express the moisture content on oven dry basis.

Calculation

Weight of empty moisture bottle                                    =   a g

Weight of the moisture bottle + moist soil sample  =   b g

Weight of the moisture bottle + soil sample after drying in the oven = c g

Weight of moisture in the soil                                    = (b - c) g

Percentage of moisture in the soil sample on oven dry basis =

Result 

        Percentage of moisture in the given soil       =

B)  ESTIMATION OF SOIL MOISTURE BY GYPSUM BLOCK

Principle

·         Resistance block works on the principle of conductivity of electricity.

·         When two electrodes A and B are placed parallel to each other in a medium and when electric current is passed, the resistance to the flow of electricity is proportional to the moisture content in the medium. Thus, when the block is wet, conductivity is high and the resistance is low.

·         Through this method, the percentage of moisture from the field capacity to the wilting percentage can be easily measured.

·         The readings at fields capacity varies from 400-600 ohms and 50000 to 75000 ohms at wilting point. The readings are taken with a portable wheat stone bridge operated by dry cells.

Installation of gypsum blocks

Materials required

1.      Gypsum or nylon blocks   2. A post hole auger

Procedure

Ø  Calibrate the resistance blocks

Ø  Sink a bore with a post hole auger to the desired depth.

Ø  Place the blocks inside and fill back the bore in small depths by tampering the soil with a metal rod.

Ø  Ensure an intimate contact of the block with the soil.

Ø  There should not be any root pieces, pebbles etc. near the blocks.

Ø  Normally 4-5 blocks can be placed in one bore at a vertical interval of 30 cm.

Ø  Heap the soil to a height of about 3 cm near the surface at the bore spot to prevent any water stagnation.

Ø  Irrigate the field and record the readings.

Ø  Check the resistance reading at field capacity.

Ø  Install the block in a row in between two plants.

Ø  The resistance blocks read low resistance at field capacity and high resistance at wilting point.

C) Soil Moisture Estimation by Neutron Moisture Meter

v  The neutron moisture meter is a device to measure the soil moisture status (volumetric water content) in situ without disturbing the soil system.

Principle

Ø  Hydrogen nuclei have a property for scattering and slowing neutrons. The phenomenon of the loss of maximum kinetic energy when neutron collide with hydrogen nuclei of water forms the basis.

Ø  Soil moisture can be estimated quickly and continuously another with neutron moisture meter without disturbing the soil.

Ø  Another advantage is that soil moisture can be estimated from large volume of soil. This meter scans the soil to about 15 cm. diameter around the neutron probe in wet soil and 50 cm in dry soil. it consists of a probe and a scalar or rate meter.

Ø  The probe contains fast neutron source, which may be a mixture of radium and beryllium or Americium and beryllium.

Ø  The scalar or the rate meter counts the number of slow neutrons, which are directly proportional to water molecules. Moisture content of soil can be known from the calibration curve with counts of slow neutrons.

Equipments and materials

1.      Neutron source ( radium beryllium (5 MC) or americium beryllium (30 MC)

2.      Shield for storage of the neutron source.

3.      Detector of slow neutrons

4.      Counting device (scaler)

5.      Access tube (GI pipe of 50 mm diameter)

Procedure

ü  The access tube is first inserted into the soil after drilling a hole with the help of an auger.

ü  The access tube is kept few inches above the soil.

ü  Insert the neutron probe in the access tube at the desired depth.

ü  While making a measurement, turn on the scaler a few minutes earlier to warm up.

ü  Make several standardization counts with the probe in the shield without altering its position. Then take readings at successive depth intervals.

ü  Divide readings by the standard reading to obtain a count ratio and refer to the instrument calibration curve to obtain water content by volume at various depths.

ü  Cover the access tube with a lid after taking readings to avoid entry of water into the tube.

·         Limitations: The two drawbacks of the instruments are that it is expensive and moisture content from shallow top layers cannot be estimated.

·         The fast neutrons are also slowed down by other source of hydrogen (present in the organic matter).

·          Other atoms such as chlorine, boron and iron also slow down the fast neutrons, thus overestimating the soil moisture content.

   Fig .Diagram of a neutron moisture gauge

Soil Moisture Estimation by Time Domain Reflectometry

New devices and methods are becoming available to growers every year. Two new techniques for soil moisture determination are instruments using Time- Domain Reflectometry (TDR probes) and Capacitance (C-Probes, Frequency-Domain Reflectometers).

·         TDR instruments work on the principle that the presence of water in the soil affects the speed of an electromagnetic wave (slows it down).

·          The TDR sends an electromagnetic wave through a guide (usually a pair of parallel metal spikes) placed into the ground at the desired depth.

·         It then measures the time it takes the wave to travel down the guide and bounce back (reflect back) up the guide.

·         The time is recorded and converted to soil moisture. The wetter the soil, the longer it takes for the electromagnetic wave to travel down the guide and reflect back.