Please find this pdf file

Please find this pdf file

The matric potential corresponding to the wilting point of the soil sample is -15 bar ( 1 bar = 1000 cm of water column = 0.987 atm) or pF of 4.18. Water retained by soil at -15 bar matric potential is generally estimated in the laboratory using either a pressure plate or a pressure membrane apparatus.The wilting point is affected by both the plant species and stage of the plant growth. It is the soil moisture content at which plant roots cannot extract water at a rate sufficient to meet the transpirational needs.

Thanks @ all.

I want to assume that the person asking this question does not have access to soil moisture equipment needed to determine the moisture content of soils at 15-bar pressure. And that could be the reason why he used the word 'estimate'. My suggestion is as follows; pack the ready-for-analysis air-dry soil samples in a 100-cm3 core with its base covered with a calico cloth or any other semi-permeable membrane. Maintain a depth of soil inside the tube of 3.0 cm; this will be done by gently tapping on the workbench. This will give a bulk density close to typical values for the soil. Then saturate the soil by sorption from beneath and allow to drain. For a period of about three weeks, maintain the soil at about field capacity at two- or three-day intervals. At this of this period, to estimate the moisture content at permanent wilting point, bring the soil to field capacity and allow to stand for 10 days without any further addition of water. The moisture content of the sample after 10 days represents an estimate of the moisture content at permanent wilting point of the soil.  

Thank you @ Sunday Ewele Obalum. Is there any literature available for this method? Yes, actually I do not have pressure plate equipment.

The water content in soils (grav or vol base) at different points is variable according the type of soil. Even the energy linked to those points could change according the sand/clay content and the type of plants. Your question is interesting .

Not having the plate equipment.... "who say" the amount of "enough" water content on soil for living? .... what would happend if you use diferent pots simulating various gradients of water content in the soil and using an indicator (plant)?..

a crazy idea...

Dear Dr. Kaini,

In laboratory the traditional method is by using the Richards pressure chamber. You can use both disturbed and undisturbed soil samples. The choice of the sample type is related to the long time necessary for the samples atain the thermodynamic equilibrium. Normally, the matric potential of -1500 kPa is selected for the water retained in the wilting point. However, this value varies from soil to soil. Another possibility to estimate the water in the wilting point is the WP4-T psychrometer. If you have measurements of water content for different matric potentials you can determine the water content at the wilting point by using the SWRC mathematical adjustment. Another possibility is the use of PTFs, if you have basic information about soil texture, bulk density, OM, etc.

http://manuals.decagon.com/Application%20Notes/13388_Determining%20Perm%20Wilt%20WC%20of%20Soils%20using%20WP4C_Print.pdf

There is two term, one is wilting point and another is permanent wilting point. Moisture percent (volumetric) at 15 bar of soil suction is generally considered as the permanent wilting point which we can be determined using pressure plate and pressure membrane apparatus in laboratory. Roughly, one third moisture content  at field capacity is considered at moisture content at wilting point. 

Interesting discussion , please find enclosed the desired information estimation wilting point soil moisture content. Dr Medhi has already provided some clues... Hope , you find it useful....