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Saturday, June 1, 2019

Ultimate guide on Measurement of soil moisture content

Moisture content (MC): is the percentage of water present in a given sample of soil
Mathematically:
MC = Mw/Ms(d) x 100 (1) air = Ma, water = Mw
MC = Mw/Mt x 100 (2) soil solid= Ms
Where: Mw= mass of water expelled from the soil (g)
Ms(d) = mass of soil solid/mass of ovendry soil(g)
Mt = mass of wet soil or total mass (g)
MC = moisture content(%)
Method of measuring MC are:
1. Gravimetric method: this is the most common method for determining MC in the developing
countries (Nigeria). It involves weighing a sample of wet soil on a sensitive weighing balance as
M2, empty can on weighing balance as M1 and the put the wet soil in an electric oven at 105
0c
for 24 hours or until a constant mass is obtained as M3
MC= M2 – M3
M3 – M1
The MC obtained is percentage and must be converted to volumetric moisture content by
multiplying MC with
Pb
/Pw Pb= bulk density (g/cm3)
Pw = density of water (g/cm3)
Pb
/Pw = apparent specific gravity of soil
Bulk density: is the mass of oven dry soil per given volume of the soil sample. A core sampler which is a cylinder is used and pressed through the soil to obtain the soil sample
Volume of core sample = Vc
V= π d
2h
4
Pb = Ms(d)
Vc
Pb = 4MdD = 4(M3-M1)
π d
2h π d
2h
Moisture content in volumetric M.C (m3
/m3) = M.C x (Pb
/Pw) = M.C x
Pb
/Pw unit m3
/m3
Example: A core sampler of internal diameter 5.6cm and 7.5 cm high is forced through the soil profile. The soil sample is put into an evaporating can of 29a. the can and wet soil weighed 331g on a sensitive
weighing balance. The wet soil is then put in an electic oven for 24 hours at 105
0c. the oven dry soil
weighed 281g after 24 hours calculate:
1. Moisture content of the soil.
2. Bulk density
3. Volumetric moisture content
(Take Pw to be 1g/cm3)
Solution:
V= π d
2h 3,142 x (5.6)
2 x 7.5 = 184.75
4 4
1. MC= M3 – M2 331 – 281 = 50 x 100
M3 – M1 281 – 29 252
MC = 19.84%
2. Pb = Md = M3 - M1 = 281 – 29
Vc Vc 184.75 =1.364g/cm3 = 1364kg/m3
Or
Pb = 4(M3 – M1) = 4(281 – 29) = 1.364g/cm3
π d
2h 3.142(5.6)
2 x 7.5
3. To convert MC in % to volumetric
MC in volumetric = MC x
Pb
/Pw Pw = 1g/cm3 = 19.84 x 1.364 = 0.2706m3
/m3
100
Order methods for measuring moisture content:
1. Gamma ray attenuation: Reginato(1974) developed a dual energy gamma ray technique by which
changes in both soil water and bulk density can be measured. 2. Neutron moisture meter: this determine moisture content instantly at in situ. This principles is
based on the measurement of the number of hydrogen nuclei that are present in a unit volume of
soil. Radium-beryllium and americum have been used but recently americum is commonly used. The fast neutrons are entitled into soil and gradually loses energy by collision with various
atomic nuclei of hydrogen. Hydrogen is present almost in the entire soil water and it is effective
element in the soil that can slow down nuetrons. The degree of slowing down of the neutrons is a
measure of the soil water content. Tensiometer: is an instrument for measuring soil water potential. It consists of a porous ceramic cup that
is filled with water and connected to a vacuum gauge (may be connected to a mercury manometer) to
show changes in soil water content by pressure changes. In operation, the instrument is put in the soil to cover the ceramic cup or to a desired depth. Water moves
in and out of the ceramic cup with changes in soil-water. This changes the pressure of the vacuum gauge
which can read on the vacuum gauge. Tensiometer is measured in bar. It is effective only in the wet soil between 0-0.8 bar. Below this
potentials. It is unreliable.

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