Below is a protocol that I want to use to determine PWP.
At the base of a 200 l drum, 8 holes are bored so as to allow drainage. The holes are covered with sieve clothes to prevent loss of soil. In order to represent desired soil profile and characteristics of each layer within the profile, dry soil will be packed according to bulk density and depth of each desired layer. After soil has been packed, drum will be watered to field capacity. An access tube will also be inserted so that soil moisture content can be monitor using a P2/6 profile probe for the duration of the experiment. When excess water as drained from the drum, three seeds of sorghum will be sowed and later thinned to one at two weeks after emergence. After emergence, excess seedlings will be thinned to leave one per drum. Soil in drum will be maintained at 100% field capacity until pre-flowering stage. After which watering will be stopped and wilting will be monitored When wilting is apparent early in the morning (before 8 am), it is assumed that the plant is unable to extract water. To verify, leaf sample will be collected and in a dark humid cool room over night. If recovery is not reached by morning, final soil water content measured is the permanent wilting point (PWP).
is there another method that I can use?
Interesting method, based on the plant response.
The conventional method we use in Brazil is the laboratory analysis, with the Richards chamber apparatus..
"To verify, leaf sample will be collected and in a dark humid cool room over night. If recovery is not reached by morning, final soil water content" Of which soil this final water content is measured? Is it surface soil? Or in between? or the whole drum?
If it is the whole drum, then, the surface soil will be over dried and the soils with no root access can hold more water than the WP.
But i appreciate this method as the appropriate!
KRISHNAMURTH L it will the at different heights along the drum. an alternative method will be inserting access tubes in the drum at the beginning of the experiment and periodically obtain the volumetric water content at different depths
If you could use a capacitive probe, like Diviner, it is possible to calibrate a soil moisture curve and extrapolate it to the wilting point. Maybe it is going to be faster than the methods described above.
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