Methods of Soil Moisture Estimation Laboratory & Field Methods

By measuring soil moisture at regular interval and at several depths within the root zones, information can be obtained as to the rate at which moisture is being used by the crops at different depths. This provides the base for determining when to irrigate and how much water to be applied.

For practical purpose, irrigation should be given when about 50 percent of available moisture in the root zone is depleted.  The amount of water to be applied is directly related to the water already present in the soil. The methods of measuring soil moisture are divided in to:

A)  Direct method:  Measurement of moisture content in the soil (wetness)

B)  Indirect methods: Measurement of water potential or stress or tension under which water is held by the soil.

A) Direct methods:

I) Gravimetric methods: In the gravimetric method, basic measurement of soil moisture is made on soil samples of known weight or volume. Soil sample from the desired depths are collected with a soil auger. Soil sample are taken from desired depth at several locations of each soil type. They are collected in air tight aluminum containers. The soil samples are weighed and they are dried in an oven at 105 oC for about 24 hours until all the moisture is driven off. After removing from oven, they are cooled slowly to room temperature and weighed again. the difference in weight is amount of moisture in the soil. The moisture content in the soil is calculated by the following formula:-

Moisture content           Wet weight –Dry weight
On weight basis    = —————————– X 100
Dry weight

PROBLEMS: Wet weight of a soil sample with can is 210 gms and weight with
can is 180 gms weight of empty container is 40 gms calculated moisture content of
soils sample?

Solution:
Weight of wet soil sample = wet weight – weight of empty can
= 210-40
= 170
Dry weight of soil sample = Dry weight – weight of can
=180-40
=140

Wet weight of soil- Dry weight of soil
Moisture content (%) = ————————————— X 100
Dry weight of soil

170-140
= —————————– X100
140

30
=   ——   X 100
140

= 21.4%

II) Volumetric Method: Soil sample is taken with a core sampler or with a tube auger whose volume is known. The amount of water present in soil sample is estimated by drying it in the oven and calculating by following formula.

Moisture content = Moisture content (%) by weight x Bulk Density (%) by volume.

PROBLEM: Undisturbed soil sample was collected from a field, two days after irrigation when the soil moisture was near field capacity. The inside dimension of core sampler was 7.5 cm diameter and 15 cm deep. Weight of core sampling cylinder weight of the core-sampling cylinder was 1.56 kg. Determine the available moisture holding capacity of soil and the water depth in centimeter per meter depth of soil.

Solution:
Weight of moist soil            = 2.76-1.56 = 1.20kg
Weight of oven dry soil       = 2.61-2.56 = 1.05 kg

1.20-1.05
Moisture content                 =    ————- X 100
1.05
=    14.28%

Volume of core sampler      =   —————————-X d2 x h
=   ————X7.5X7.5X15
4
=   662 cu. Cm

Wt. of dry soil in grams
Apparent specific gravity     =    ——————————–
Volume of soil in cu. Cm

1.05
=    ——   = 1.58
662

Available moisture               = Ap. Sp. Gr. X moisture content
= 1.58 X 14.28
= 22.56 cm / m depth of soil

The method is though accurate and simple it is used mainly for experimental purpose. Sampling, transporting & repeated weighing give errors. It is also laborious and time consuming. The errors of the gravimetric method can be reduced by increasing the size and number of samples. however the sampling disturbs the experimental plots and hence many workers prefer indirect methods.

III) Using Methyl Alcohol: Soil sample is mixed with a known volume of methyl alcohol and then measure the change in specific gravity of school with a hydrometer. This is a shot cut procedure but it is no in common use.

IV) Using calcium chloride: Soil sample is mixed with a known amount of calcium chloride. calcium chloride reacts with water and removes it in the form of acetylene gas. The moisture is determined has come in common use.

B) Indirect methods:

In those methods, no water content in the soil is directly measured but the water potential or stress or tension under which the water is held by the soil is measured. The most common instrument used for estimating soil moisture by indirect method is:
1)  Tensiometer
2)  Gypsum block
3)  Neutron probe
4)  Pressure plate and pressure membrane apparatus

In all these methods, the reading from above instruments and corresponding soil moisture content is determined by oven drying method are plotted on a graph. Subsequently, these calibration curves are used to know soil moisture content from the reading of these instruments.

1) Tensiometer: Tensiometer is also called irrometers since they are used in irrigation scheduling. Tensionmeters provide a direct measure of tenacity (tension) with which water is held by soil. It consist of 7.5 cm porous ceramic or clay cup, a protective metallic tube, a vacuum gauge and a hollow metallic tube holding all parts together. At the time of installation, the system is filled with water from the opening at the top and rubber corked when set up in the soil. moisture from cup moves out with drying of soil, creating a vacuum in the tube which is measured with the gauge. Care should be taken to install tensiometer in the active root zone of the crop. When desired tension is reached, the soil is irrigated. The vacuum gauge is graduated to indicate tension values up to one atmosphere and is divided in to fifty divisions each of 0.2 atmosphere value. The tensiometer works satisfactory up to 0.85 bars of atmosphere.

Merits of tensiometer:

1.  It is very simple and easy to read soil moisture in situ.
2.  It is very useful instrument for scheduling irrigation to crops which require frequent irrigations at low tension.

Limitations:

Sensitivity of a tensiometer is only up to 0.85 atmospheres while available soil moisture range is up to atmosphere and hence is useful more on sandy soils wherein about 80% of available water is held within 0.85 ranges.

2) Gypsum Blocks: Gypsum blocks or plaster of Paris resistance units are used for measurement of soil moisture is situ. These were first invented by Bouycos and Mick in 940. the blocks are made of various materials like gypsum, nylon fiber, glass, plaster of Paris or combination of these materials. The blocks are generally rectangular shaped. A pair or electronics is usually made of 20 mesh stainless steel wire screen soldered to copper lead wire. The common dimensions of screen electrodes are 33.75 cm long and 0.25 cm wide. The usual spacing between the electrodes is 2 cm. A similar block is 5.5 cm long, 3.75 cm wide and 2 cm thick.

Principal of working: It works on principal of conductance of electricity. When two electrodes A and B are placed parallel to each other in a medium and then 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 resistance is low. Generally these read about 400 to 600 ohms resistance at field capacity and 50,000 at wilting point. the readings are taken with portable Wheatstone Bridge Bouycos water Bridge operated by dry cells.

While placing the gypsum block in soil, care should be taken that the blocks must have close contact with undisturbed soil. After placing, the blocks get wetted with soil moisture due to capillary movement. Pure gypsum block sets in about 30 minutes. The gypsum block is sensitive to soil to moisture from 1.0 atm tension to 20.0 atm. How ever, the gypsum blocks are not reliable in wet soils.

3) Pressure membrane and pressure plate apparatus: Pressure membrane and pressure plate apparatus (developed primarily by Richards) is generally used to estimate field capacity, permanent wilting point and moisture content at different pressures. The apparatus consists of air tight metallic chamber in which porous ceramic pressure plate is placed. The pressure plate and soil samples are saturated and are placed in the metallic chamber. The required pressure, say 0.33 bar or 15 bars is applied through a compressor. The water from the soil sample which is held at less than the pressure, Applied trickles out of the outlet till equilibrium against applied pressure is achieved after that, the soil samples are taken out and oven dried for determining the moisture content.

4) Neutron meter (neutron scattering method): Soil moisture can be estimated quickly and continuously another with neutron moisture meter without disturbing the soil. 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. The probe contains fast neutron source, which may be a mixture of radium and beryllium or Americium and beryllium. Access tubes are aluminum tubes of 50 to 100 cm length and are placed in the field where moisture to be estimated.

Neutron probe is lowered into access tube to the desired depth. fast neutrons are released from the probes, which scatter into the soil. When neutrons encounter nuclei of hydrogen atom of water, their speed is reduced. 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.

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.

5) Gama Ray absorption method: it is the technique of measurement of changes in soil water content by change in amount of gamma radiation absorbed. The amount of radiation passing through soil depends on soil destiny which varies chiefly with change in water content. This is suitable where change in bulk destiny is very small.

6) Feel and appearance method:  A practical estimate of moisture content is obtained by the feel and appearance of soil samples taken from the desired depths. the soil sample is squeezed in the hand and its feel and appearance are taken into consideration. In this method, actual moisture content is not determined.

7) Soil moisture characteristic curve: The energy status of water and amount of water in the soil are related with the soil moisture characteristic curve. As the energy status of water decreases (moisture towards more negative values) soil water content also decreases. In other words, as soil moisture content deceases, more energy has to be applied to extract moisture from the soil. the relation between suction (externally applied force) and water content of the soil are represented graphically by a curve which is known as a soil are moisture characteristic curve.

Hysteresis:

The relation between energy status and moisture content can be obtained in two ways, (i) in absorption by taking an initially saturated soil sample and applying increasing suction to dry the soil gradually and (ii) in absorption by gradually wetting an initially dry soil. The measurement of energy status and moisture content during this process are taken and plotted on graph. The curves obtained through desorption and sorption is different for the same soil sample. the moisture content at a given suction is greater in desorption than in absorption and this phenomenon is known as hysteresis.