Question

Calculate the permanent wilting point if the depth of water in the root zone at the permanent wilting point per meter depth of soil is 0.4m. The dry density of the soil is 16 KN/m³.

• A. 0.025
• B. 0.245
• C. 0.4
• D. 0.64

Answer 1

The correct option is B 0.245

$\text{Given that : Depth of water in the root zone at the permanent wilting point}\left(d_{wp}\right)=0.4\text{m}$

$\text{Dry density of soil}\left({\gamma }_d\right)=16{\text{KN/m}}^3$

$\text{Root Zone Depth}\left(D\right)=1\text{m}$

$\therefore d_{WP}=\frac{{\gamma }_d}{{\gamma }_w}\times D\times PWP$

$\Rightarrow 0.4=\left(\frac{16}{9.81}\right)\times 1\times PWP[\because {\gamma }_w=9.81{\text{KN/m}}^3]$

$\text{Then,}PWP=0.24525$

$\text{i.e.}PWP=24.525\%$

Hence, the permanent wilting point (PWP) is 24.525%


Note: Permanent wilting point is the moisture content at which the film of water around the soil particles are held so tightly that the plant roots cannot extract enough moisture at a sufficiently rapid rate to satisfy transpiration requirements thus resulting in the wilting of the plants.