Soil Moisture content

• The soil water may exist in the soil in various forms and the weight of the water to the weight of the solid contents in a given mass of soil is called water content or moisture content of the soil. i.e.

Moisture content (w) = Ww*100%/Ws

Where Ww= weight of water

Ws=weight of solid mass

Determination of soil moisture

a. Oven drying method:

• This is the commonly adopted and simplest method for the determination of moisture content of soil sample in the laboratory.
• The method basically consists of drying a weighted moist soil sample in a oven at a controlled temperature (110-109 ⁰C).
• For a period of 24 hours after which the day weight of the sample is taken. The water content is then calculated as shown as below.

W1=Weight of the container

W2= Weight of container with moist sample

W3=Weight of the container with oven dried sample

Weight of the water (Ww)=W2-W3

Weight of the soild (Ws)=W3-W1

Thus, water content =Ww *100 %/Ws=(W2-W3)x100%/(W3-W1)

b. Pycnometer method:

It is an approximately 900 ml capacity glass bottle provided with a conical cap.

i. A conical cap provided with a 6 mm diameter hole at the top can be used. To avoid leakage of water, a rubber washer is provided between the conical cap and the rim of bottle. The method consists of weighing the empty dried pycnometer bottle with conical cap and washer. Let its weight be W1.

ii. Next 200 to 400 gm of moist soil is taken in pycnometer with cap washer and soil is weighed again. Let it be W2.

iii. The bottle is filled to about half with water and the contents are stirred thoroughly by glass rod. The pycnometer is now filled with water up to conical top & washer and cap are fixed up. Sometimes a vacuum is used to expel the air entrapped in the soil before filling the bottle with water. The pycnometer is dried from outside and weighed (W3)

iv. Next, pycnometer is emptied, cleaned thoroughly and weighed after filling up with water up to the top (W4)

Water content (w)=Ww*100%/Ws

Weight of water (Ww)=(W2-W1)-Ws

• It can be seen that if from W3, the weight of solid Ws could be removed and replaced by weight of equivalent volume of water, the weight W4 would be obtained.

i.e. W3-Ws+Ws/Gs. Yw. Yw=W4

or W3-W4=Ws(1-1/Grw)

Where, Gs=sp. Gravity of soil sample

Yw= sp. Gravity of water

So, Ws= (W3-W4)(Gs/(Gs-1))

Finally,

W=Ww/Ws= {(W2-W1)-Ws}/Ws={(W2-W1)/Ws}-1

Water content W= [(W2-W1)/(W3-W4)] x[(Gs-1)/(Gs)] *100