This is a good question. The conventional approach to evaluate the Wilting Point of Soils, WPS, is usually conducted until the soil reaches 15 bars of suction. For some soils, this value can be reached before the 24 hours, for some other soils, samples may take longer than 24 hours the attain the 15 bars of suction. It dependes mainly on the soil texture.

Hope this simple explanation clarifies your main concern

Kind Regards

Sincerely

Jose Navar, Ph.D

The measurement you describe (saturating soil and then allowing to drain under gravity for 24 hours) will give you what is commonly referred to as field capacity. After saturation and draining you have the weight of wet soil. You then dry the soil at approx. 105oC until the weight becomes constant (usually within 24 hours). This give you the dry weight. You can calculate the gravimetric moisture content at field capacity by wet weight - dry weight and divide product by dry weight (and multiply by 100 if you need in percentage). The moisture content at permanent Wilting point (PWP) as previous contributor described is usually measured using pressure plate apparatus with suction at 15 bars.  The difference gives an estimate of plant available water (gravimetic).  To convert gravimetric to volumetric MC multiply by soil bulk density. If you don't have pressure plate apparatus and you need to estimate PWP, depending on how critical the accuracy is, there are lost of pedotranfer estimators out there on the web (most based on soil texture) or you could use air dry moisture content but this is likely drier than PWP. Hope this helps regards Andrew

Equilibrium activity of water at the PWP still corresponds to >98% relative humidity (r.h.) at 20°. People use dissicators with salt solutions at the bottom to maintain given relative humitities, also for much higher water tensions. As all the mass transfer goes through the gas phase, it takes very long to reach constant weight. Further, relative humidities close to atmospheric saturation are not easy to maintain.

Further, there exists a filter paper method to build the water retention curve. It might be worth a try.

Drying the soil at 105 °C definitely will produce water tensions much higher than the PWP.

For practical purpose, I would use a pedotransfer function as Andrew proposed.

The concept of  moisture content at permanent Wilting point (PWP) as previous contributor described is usually measured using pressure plate apparatus with suction at 15 bars after that,  in order to obtain the wilting point, should the soil sample be oven dried until weight becomes constant. 

Dear Angel and Kurt

I do not think to oven dry the soil is necessary. One can start with air dry soil.

Abdul you are correct that in preparing the samples for the measurement it does not need to be (and should not be) oven dried first. But you need to oven dry the sample when you want to actually measure the moisture content either at FC or PWP and I believe that was the point of the original question.

Dear Andrew

Yes, you are correct.

Kurt 

Here are some additional thoughts;

As a starting point; Field (water) Capacity can be grossly estimated as multiplying 0.5 by the Saturated soil water content (expressed on a mass or volume basis).  The Permanent Wilt Point can be estimated by multiplying 0.5 by Field  (water) Capacity or by multiplied Saturated soil water content sample by 0.25.

Field Capacity as defined by (Soil Science Society of America) SSSA “field capacity, in situ (field water capacity)-- The content of water, on a mass or volume basis, remaining in a soil 2 or 3 days after having been wetted with water and after free drainage is negligible. Generally assumed to be 0.01 to 0.033 MPa.

Permanent Wilt Point definition from the SSSA-  The largest water content of a soil at which indicator plants, growing in that soil, wilt and fail to recover when placed in a humid chamber. Often estimated by the water content at –1.5 MPa soil matric potential.

Available Water (capacity) The amount of water released between in situ field capacity and the permanent wilting point (usually estimated by water content at soil matric potential of (–1.5 MPa). It is not the portion of water that can be absorbed by plant roots, which is plant specific.

additional info https://nrcca.cals.cornell.edu/soil/CA2/CA0212.1-3.php

Bob

https://nrcca.cals.cornell.edu/soil/CA2/CA0212.1-3.php